Cobalt60: An Old Modality, A Renewed Challenge

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Cobalt-60: An Old Modality, A Renewed Challenge
Jake Van Dyk and Jerry J. Battista
Physics Department
London Regional Cancer Centre
London, Ontario, Canada
KEY WORDS: radiation therapy, cobalt-60, megavoltage x-rays, cost/benefit
Correspondence to: Jake Van Dyk, Head, Clinical Physics,
London Regional Cancer Centre,
790 Commissioners Road East,
London, Ontario, N6A 4L6 Canada.

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Cobalt-60: An Old Modality, A Renewed Challenge
Jake Van Dyk and Jerry J. Battista
Physics Department
London Regional Cancer Centre
London, Ontario, Canada
"Inconsistencies of opinion, arising from changes of circumstances, are often justifiable."
Daniel Webster, 1846
1. INTRODUCTION
The discovery of x-rays and radioactivity 100 years ago has led to revolutionary advances in
diagnosis and therapy. However, it was not until the middle of the twentieth century that
megavoltage photon energies became available through the use of betatrons, cobalt-60 gamma rays
and linear accelerators (linacs). The increased photon penetration and skin sparing provided radiation
oncologists with new opportunities for optimizing patient treatments. In recent years, several reports
have considered various issues which define the "optimum" photon energy for the treatment of
malignant disease10,26,41,44. In many of these articles10,26, cobalt-60 is mentioned although it is
generally not recommended for radiation therapy departments in the western world. Indeed, many
now consider cobalt-60 as an old modality that is only useful for palliative treatments in a large
department or for developing countries36,58 with limited technical resources.
The paper by Glasgow et al.16 published in this issue of Current Oncology reviews the use
and dosimetry of a new, extended distance cobalt-60 therapy machine. The authors not only provide
detailed physical considerations of this new unit but they also provide a brief comparison of the

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clinical use of cobalt-60 versus x-rays produced by accelerators. In this commentary, we extend this
discussion further. We briefly review the arguments that have been presented both for and against
the use of cobalt-60 as well as add some up-to-date insights and perspectives. Undoubtedly, we will
not resolve this debate for all clinical situations. However, we hope that by putting "all the cards on
the table", the cobalt-60 option will be viewed from a fairer perspective than we have seen in recent
years of rapidly advancing accelerator technology. Furthermore, we also make some
recommendations for the designers of cobalt-60 technology so that modernized units can be made
more attractive for today's radiation therapy facility.
2. LINEAR ACCELERATORS VERSUS COBALT-60: ISSUES FOR COMPARISON
Table 1 summarizes issues for comparison for the use of cobalt-60 versus linear accelerators. These
issues are not listed in any order of priority although they are broadly categorized according to
radiation beam characteristics, machine characteristics, technical support, safety considerations and,
finally, cost considerations. Different levels of importance can be assigned to each of these factors
according to the local practice of radiation oncology in a cancer centre.
2.1 Radiation Beam Characteristics
2.1.1 Beam edge sharpness (penumbra)
(a) Issues for Consideration
One of the strongest arguments against cobalt-60 has been the unsharpness of the beam edge or its
large penumbra. This is generally manifested by the distance between the 80% to 20% or the 90 to
10% doses at the edge of the beam. Sample data have been published by various authors16,26,41,46.
It is important to note that there are sizeable differences between penumbras as published in the
literature. These are strongly dependent on both the depth of measurement in water as well as

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dosimeter type and size. It is clear that for cobalt-60, the penumbra widths increase with source
diameter (e.g. 1.0 to 2.0 cm), the distance between the source and the bottom of the field definer, and
the distance between the field definer and the patient. The x-ray beams from linacs, on the other
hand, offer penumbras which are only mildly dependent on geometry due to the small source focal
spots (e.g. 0.1 to 0.3 cm)38. However, with increasing x-ray energies, the beam edge is blurred by
more energetic electrons scattered in tissue over a greater lateral range. The effective penumbra
achieved in the patient is thus significantly enlarged compared with a pure geometric penumbra, and
it cannot be reduced by machine design.
There are at least four other major considerations that should be incorporated in the
penumbra criterion for comparison although little quantitative data exist for these considerations.
The first has to do with the radiation oncologist's ability to define target volumes accurately or
consistently. The need for very precisely defined field edges is based on the assumption that target
volumes and normal tissues can be defined with a high degree of accuracy. For some normal tissues
and with appropriate imaging data, this assumption can be valid. However, for the accuracy of
definition of planning target volumes, very little data exist. A recent study by Leunens et al.27,
comparing the variability of 12 physicians defining target volumes of 5 different patients with brain
tumours, indicated that the estimated tumour and target sizes varied by factors of 1.3-2.6 and 1.3-2.1,
respectively. Maximum variations were of the order of 1.1 to 2.7 cm in the cranio-caudad direction
and 1.4-2.1 cm in the fronto-occipital direction.
The second consideration has to do with patient motion. Our desire for a tight penumbra is
reasonable only if a narrow treatment penumbra can be maintained in clinical practice. However, in
reality, patients undergo 20 to 30 fractions for radical therapy and usually 5 to 10 fractions for

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palliative therapy. As a result, the sharpness of the dose delivered to the patient is strongly dependent
on the reproducibility of the patient setup relative to the beam18,25,57. With the recent developments
in portal imaging, it is now well recognized that setup reproducibility is typically of the order of 0.5
cm for head and neck patients (up to 0.7 cm in Michalski et al.35) while it is about 1.0 cm for pelvic
and thoracic treatments (up to 1.4 cm and 1.2 cm, respectively in the data of Michalski et al.35). The
impact of this will be to blur the edge of the beam with respect to tissue elements near the beam
edge. The net result is that even a "perfect" penumbra (i.e., a step function with "0" penumbra width)
will be smeared out by beam placement uncertainty.
The third consideration has to do with organ motion. Various authors have shown that
prostatic motion of up to 3 cm can occur mostly in the anterior and/or superior direction8,25,48.
Similarly, bladder treatments involve large changes in bladder and rectal diameters52. Thoracic
studies have shown substantial tumour movement ( −1.5 cm) as a result of cardiac and respiratory
cycles45. Similarly, in head and neck treatments, gross tumour volumes can change during a 6 week
course of treatment. The net result is that our ability to reposition the involved tissues is severely
limited by both a "moving target" within the patient as well as our ability to reposition the patient
from day to day.
A fourth consideration takes a different perspective and has to do with the biological
response of the irradiated tissues. The response of the irradiated cells generally have a sigmoidal
dose-response relationship. Tumours and normal tissues behave similarly although with different
dose sensitivity and slopes to the curves. These dose-response curves can be characterized by the
slope at the 50% response level {often parameterized4 by the contrast figure, γ50 = (%change in
response) / (%change in dose)}. While there are large variations in gamma, dependent on tumour or

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normal tissue type4,49, it is not unreasonable to produce a 10% change in response by a 5% change in
dose, i.e., γ50 = 2. The net effect of this is that even a large geometric penumbra of 1.6 cm as might
be found on a conventional cobalt-60 machine, could have a "biological penumbra" that is twice as
steep as the physical penumbra54. Of course, these biological considerations are much more complex
because they depend on the dose level (i.e., in which portion of the dose-response curve) and they
could involve partial volume effects for the tumour and normal tissue compartments.
In summary, our usual simple preference for sharp physical penumbras is intuitive but it
should be extended to consider the reality of non-reproducible patient setups as well as the biological
considerations which greatly accentuate the biological penumbra.
(b) Opportunities for Improvement
While the above considerations question the need for ultra-sharp physical penumbras, there are still
opportunities for improving the beam sharpness for cobalt-60 since the penumbra on a cobalt-60
machine is primarily dependent on geometric considerations. Both source size and source-to-
collimator distances are adjustable parameters. A redesigned modern cobalt unit could incorporate
multileaf collimators and dynamic wedges thus minimizing the need for trays for ancillary devices
and thereby allowing a larger distance between the source and the field defining apparatus.
Furthermore, is it not possible to redesign source capsules such that similar effective activities can be
contained within smaller source diameters? We recognize the simplicity of our comments. However,
there has been very little effort on improving the design of cobalt-60 units since their original
design20-22.
2.1.2 Beam Penetration (Energy)
(a) Issues for Consideration

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The benefits of an increase in energy have been well documented10,15,26,41. Often this is reviewed
from the point of view of depth dose fall off for single fields, or comparing the ratio of the dose at
the depth of maximum dose to the midplane dose for parallel-opposed fields10,41,51,55, or by
considering the integral dose for typical multiple field treatments10,26. However, other factors must
be incorporated into this discussion as well. For example, when there are superficial nodes to be
treated as occur in head and neck7 or Hodgkins disease51, or superficial target volumes as in breast
cancer patients or in total body irradiation for bone marrow transplants56, then it is important to
consider also the build-up depth for a parallel opposed pair of fields. For example, for cobalt-60
(10x10 cm2 field, patient thickness 25 cm), the 95 % depth occurs at about 0.3 cm, for 6 MV x-rays
at 0.7 cm and 25 MV x-rays at 1.8 cm51. These data suggest that the choice of energy is strongly
dependent on the "shallowness" of the target volume relative to the skin surface. Simple
generalizations based only on deeper target volumes could lead to inappropriate conclusions.
Laughlin et al.26 produced a figure indicating their best estimate of optimum choice of energy versus
treatment site. However, Suit47 in an editorial on their paper indicated that appropriately fitted
cobalt-60 units could be "fully acceptable in the treatment of a large majority of the patients
undergoing radiation treatment for carcinoma of the head-neck region, breast, and sarcomas of soft
tissues of the extremities".
The trend today is toward conformal therapy with segmented or moving field techniques.
Generally, this requires multifield irradiation or the use of arc/rotation therapy. As the number of
fields increases, the advantage of higher energies over cobalt-60 radiation decreases as manifested in
the dose distributions and in integral doses. A simple calculation of dose at the depth of dose
maximum compared to the isocentric dose for an increasing number of fields illustrates this issue.

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Table 2 shows a 186% difference in these doses (relative to the prescribed dose at the isocentre) for a
single field technique when comparing cobalt-60 and 18 MV x-rays but demonstrates only a 7%
difference when the number of fields is increased to 20.
(b) Opportunities for Improvement
Today there is a tremendous amount of technological development in making conformal therapy
techniques aimed at linear accelerators5,6. On a lesser scale of developmental activity, related
technology has also been implemented using a cobalt-60 tracking unit11. However, this technology
has never been adequately commercialized to make it readily available. Cobalt-60 units could be
enhanced with the application of multileaf collimators and moving field hardware/software to
provide dose distributions that would be very comparable to those provided with higher energy
radiations.
2.1.3 Scattering conditions/dose uniformity
(a) Issues for Consideration
While rectangular fields generally provide reasonable dose uniformity, fields with a large amount of
shielding will have altered photon scattering conditions resulting in greater dose variation throughout
the volume as a result of photon scatter51. Generally, an increase in photon energy will result in
more forwardly directed scatter, yielding a more uniform dose distribution within a shaped field.
Accelerators generally provide a more uniform field flatness in comparison to cobalt-60 machines,
and uniformity is less prone to changes in scattering conditions.
(b) Opportunities for Improvement
First, the field flatness for cobalt-60 machines could be improved by the incorporation of flattening
filters28. Secondly, complex irregular fields could have their dose uniformity improved by the

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additional use of dose compensators29. Such compensation is complex but can achievable with a
three dimensional dose computation system.
2.1.4 Contour/inhomogeneity corrections
Under conditions of electron equilibrium, the magnitude of both contour and inhomogeneity
corrections decreases with increasing energy51. Thus, from this perspective, higher energies are
advantageous since the beams are less affected by tissue density and air gap. However, with
increasing energies above 10 MV, issues related to electron transport and disequilibrium must also
be considered. It is now well recognized that inhomogeneity corrections for the higher energy photon
beams in low density, lung-type media are strongly affected by the lack of electron
equilibrium2,24,32,41,60. These effects are not computed accurately on most commercial treatment
planning computers. Often, for small fields and low density tissues, where an increase in dose is
predicted, the effects of electron transport actually result in a decrease in dose32. Furthermore, this
effect manifests itself at the edge of any field with an increase in physical penumbra. This was
quantified by Ekstrand et al.12 who showed that the ratio of penumbral width in lung to that in water
magnifies from about 1.0 with 4 MV x-rays to about 2.5 with 18 MV x-rays. Indeed, some
institutions24,60 limit thoracic treatments to machines with less than 10 MV x-rays to minimize the
perturbation effects of the electron disequilibrium.
Interface effects are directly related to the above discussion on inhomogeneity corrections.
These interface effects are manifested at the edge of small air cavities13,14,39, at bone-tissue
interfaces, and at the interfaces of metallic prostheses as might occur in mandible reconstructions or
hip prostheses. Generally, in these situations, cobalt is the preferred choice of energy since the
volume of tissue under-dosed or over-dosed is smaller with cobalt than it is with the higher

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energies13,14,19.
2.1.5 Dose to bone
The dose to bone compared to the dose to soft tissue is often given by fbone/ftissue. For the higher
energies, an average stopping power ratio of tissue to bone is further incorporated into the
numerator26. Usually, these values have been quoted for the primary beam photon spectrum26.
However, Cunningham et al.9 have shown that photon spectra change with depth and field size in the
patient due to multiple scattering of photons. While conventionally, the dose to bone relative to the
dose to tissue is thought to increase from 1.03 in cobalt-60 to 1.07 for 18 MV primary x-rays26,
Rawlinson41 showed that for a 20x20 cm2 field at a depth of 10 cm, the corresponding values are
1.08 to 1.07, respectively. Thus, there is no significant difference in dose to bone relative to dose to
tissue when comparing cobalt-60 to higher energy x-rays at depth for conventional field sizes. For
very large fields, as encountered with total or half body photon irradiation, the increase in multiple
scatter for cobalt-60 could result in a substantial increase (−10%) in the dose to bone compared to
higher energy x-rays. For total body irradiation, where irradiation of blood forming tissues is
intended, the use of cobalt-60 could, indeed, accentuate the dose to bone and serve as an
advantage55,56.
2.2 Machine Characteristics
2.2.1 Dose Rate
(a) Issues for consideration
The dose rate "in air" at the isocentre of a cobalt unit depends on the source activity and the distance

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to the isocentre. With a half-life of 5.26 years, the dose rate also decays slowly with time
(approximately 1 % per month). Maximum initial activities of 13 kCuries (481 TBq) are currently
produced in nuclear reactors for cobalt therapy sources. With the standard source capsules and for a
source-axis-distance (SAD) of 100 cm, a dose rate of approximately 200 cGy/min is obtained for the
modern cobalt unit16. By comparison, the dose rate from a low energy (4 MV) linac is typically 250
cGy/min while higher energy accelerators often operate at 400 cGy/min. High energy accelerator
dose rates are usually limited by safety considerations rather than the maximum electron beam
current on the x-ray target. It should be noted that during a treatment, the cobalt machine dose rate is
essentially continuous and constant, while the accelerator dose rate is pulsed and adjusted
electronically to deliver a set dose. The pulsed nature of the accelerator beams does not seem be of
radiobiological significance although it is of concern for accurate ionization chamber dosimetry due
to the possibility of ion recombination.
A major advantage of an effectively constant output as found on a cobalt-60 machine is that
it reduces some of the uncertainties associated with the delivery of a specified dosage to the patient.
Linacs have uncertainties associated with the reproducibility of the monitor ionization chamber, field
flatness, and possibly a change of energy due to a drift in the electronics. The precision of the
corresponding parameters on cobalt machines is much tighter than it is on linacs. A major benefit of
having at least one cobalt-60 machine in a radiation therapy facility is that it provides a means of
checking radiation detectors for reproducibility in their calibrations.
Practically, the dose rate and daily dose prescription determine the daily treatment time per
patient. Of course, the dose rate achieved "in tumour" depends on the overlying thickness of tissue
and the beam energy. Generally, the cobalt unit is at a disadvantage with respect to dose rate

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achieved at the target volume. For example, as the calibration dose rate is reduced from 400 cGy/min
to 200 cGy/min, the treatment time is prolonged from 0.6 to 1.4 minutes for a 200 cGy dose
fraction. This simplified example assumes an overlying tissue of 10 cm, a single field size of 10 x
10 cm2 and a linac beam of 10 MV X-rays.
The "beam on" time is only a small portion of the overall time allocated per patient
treatment (typically 15 minutes) but it can impact the overall patient throughput per day by
approximately 5%. Assuming an average of 40 patients treated per day, a doubling in dose rate
potentially results in a gain of 32 minutes per day or the equivalent 2 extra dose fractions. With the
increasing pressure on greater utilization of radiotherapy machines, a higher dose rate is clearly
desirable. However, this assumes that the higher dose rate can be achieved without a loss in machine
reliability or the need for additional time for quality assurance. (See section 2.3 below.)
(b) Opportunity for improvement
For cobalt-60, a higher dose rate could be achieved by increasing the source activity, by innovative
redesign of source encapsulation which optimizes source packing and reduces self attenuation, or by
reducing the SAD (e.g. back to 80 or 90 cm). The reduction of treatment distance from 100 to 80 cm
has significant impact since it increases the dose rate by 56% ! However, this is at the expense of a
reduced clearance between the patient and the collimation system. Thus, a revised design must
achieve a compromise between an improved collimation system providing acceptable penumbras
and acceptable dose rates while at the same time allowing sufficient clearance between the head of
the machine and the patient to ensure adequate setups for most techniques.
2.2.2 Patient to Collimator Distance
(a) Issues for consideration

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Practically, a greater patient-collimator distance simplifies the set up of the patient for treatment,
particularly when beam modifiers are appended (e.g wedges, blocks, compensators). Dosimetrically,
a reduction in this distance improves the penumbral width, assuming a constant source-collimator
distance. (see 2.1.1 above). This distance is therefore based on achieving a compromise between
ease of patient setup when beam modifiers are used and penumbral width.
(b) Opportunity for improvement
The clearance between the patient and collimator could be improved for a cobalt machine by the use
of multi-leaf collimator (MLC) technology. For cobalt-60 radiation, the collimator leaves could be
significantly thinner (1-2 cm of lead equivalent thickness) than for higher energy x-rays, and
dynamic control could permit wedge or compensated fields without appended hardware. However,
this should not be achieved with a loss in overall reliability of the machine. (See section 2.3 below.)
2.2.3 Isocentre Height
(a) Issues for consideration
The height of the isocentre above the floor has generally been lower for traditional cobalt units (114
cm) and this has eased the transfer of patients onto the treatment table, particularly for patients who
are less mobile. On the modern cobalt unit, with a SAD of 100 cm, this height is elevated to 132 cm,
approaching or surpassing the isocentric heights available on accelerators and results in difficulty in
patient setup by the therapists.
(b) Opportunity for improvement
The major determinant of the isocentric height is the bulkhead of the rotating gantry which must
clear the floor when the gantry is pointing upward. Thus, a more compact head structure provides
opportunity for a lower isocentre. There are a number of major electromagnetic components within

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the head of an accelerator and it is difficult to visualize how these could be placed into a more
compact structure. However, for a cobalt unit, the major bulk of the head is due to shielding which
could be optimized, particularly on the "back" side of the source. If additional higher density
materials were used as a back attenuator just above the source drawer, the head could be slightly
reduced in height and provide additional floor clearance to achieve a lower isocentric height.
2.2.4 Radioactive Source
From a radiation physics viewpoint, the gamma rays emitted from cobalt are nearly mono-energetic
compared with the polyenergetic x-rays from an accelerator. This simplifies the characterization of
the source energy and thus simplifies dose calibrations and calculations. For example, the primary
beam is uniformly attenuated through absorbers such as wedges. In contrast, an x-ray beam spectrum
is differentially attenuated across a wedge as the beam quality changes. This x-ray beam "hardening
or softening" complicates dose computations and can reduce their accuracy dependent on the dose
algorithm employed34.
2.3 Service/Maintenance Issues
Practical experience at the Princess Margaret Hospital has shown that the average down time for a
cobalt unit is less than 1% while the down time for linear accelerators increases with increasing
complexity from about 3% for a single low energy unit to about 11% for the 25 MV type machine41.
Similar data was quoted by Das and Kase10 who observed a 3% down time for a 4 MV machine
compared to about 5-7% for higher energy units. Our own data, at the London Regional Cancer
Centre, demonstrate similar trends although with lower down times during clinical hours. For the
higher energy machines, the clinical down time is about 3%, for single low energy linacs it is about

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2%, and the cobalt unit has a down time that is less than 0.4%. However, to consider overall costs,
the after hours preventative maintenance time should be added. In our centre, this corresponds to
another 3% and 1% of clinical hours for linacs and cobalt, respectively. Of course, the expertise of
maintenance staff required is directly related to the complexity of the treatment machines and this
affects the maintenance costs accordingly.
2.4 Safety Considerations
2.4.1 Radiation protection
From an ecological viewpoint, radioactive sources pose an environmental hazard while being
transported, while in service, and finally at disposal time. Furthermore, the source is always radiating
whereas the linac x-ray beam is switchable to the "off " state at will. In practice, the cobalt source is
shielded by a bulky head assembly which attenuates the continuous stream of emitted gamma rays.
The advantage is a constant dose rate, but at the risk of a radiation accident as might occur if the
source "gets stuck" in the "on" position. Clearly, the production of x-rays is more controllable,
although there have been serious accidents due to inadequate linac controls1,23,30. With the current
improvements in hardware and a software, the risk from linacs now appears to be smaller. The issue
of source disposal is of substantial concern if inadequate legislation is in place as has been
demonstrated by improper disposal procedures in Mexico and Brazil. The solution for this, however,
is not the avoidance of cobalt but rather the development and implementation of appropriate
regulations for the disposal of cobalt sources.
An additional hazard of the higher energy linacs (>10MV) is the production of neutrons as a
result of photonuclear reactions in the head of the machine. For such machines, the greatest exposure
to the radiation therapists operating the machines is due to residual radioactivity from both the

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machine and treatment room walls40.
2.4.2 Pacemaker concerns
An increased number of patients are seen in radiation therapy departments with implanted
pacemakers31,33 as a result of an aging population combined with increased indications for the
insertion of permanent pacemakers. Pacemaker faults are potentially generated either by interference
due to electromagnetic radiation or by ionizing radiation. The level of concern for both of these is
controversial. Conservative recommendations by an AAPM Task Group33 suggest close monitoring
of patients treated on linear accelerators to observe any potential effects of electromagnetic
interference and maintaining total dose levels to pacemakers to less than 2 Gy. In England, where no
formal recommendations regarding the use of pacemakers have been adopted, a survey indicated that
about one-half of the radiation therapy departments treated their patients with pacemakers on cobalt
units in preference to linear accelerators31.
2.5 Cost Considerations
Cost analyses have been performed by various authors10,41,53. Rawlinson41 has done an analysis in
which he compared the annual operating costs for a cobalt unit, a low energy linac and a high energy
linac including estimates of capital depreciation of the machine and the building as well as
maintenance costs. The results indicate an annual cost in 1986 Canadian dollars of $38,100,
$122,800 and $181,800 for the operation of a cobalt machine, low energy linac and a high energy
linac, respectively. This indicates that a low energy linac costs more than 3 times as much as a cobalt
unit and a high energy linac costs more than 5 times as much to operate. As indicated above, patient
throughput due to a lower dose rate on cobalt differs only by about 5 to 10%. Clearly, there is a
substantially reduced operating cost for treating patients on cobalt-60 machines.

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3. DISCUSSION
The treatment of malignant disease by radiation therapy is performed by a large range of radiation
therapy technologies including cobalt-60 machines and high energy linacs combined with a gamut of
ancillary devices many of which are computer-controlled. Much of this advanced technology is
purchased and implemented with relatively little consideration for actual cost/benefit but an
assumption that higher energies, smaller penumbras, and higher dose rates "obviously" provide
improved therapy. While there are indications that institutions using advanced technologies have
better patient outcomes17, it has never been proven that this was directly due to the machine energy
levels and could well be due to surrogate issues related to staff quality that is associated with
institutions having more sophisticated equipment.
Recently, in times of economic constraint, much more consideration needs to be given to
cost/benefit of radiation therapy3,37,42,43,59. A maxim in today's culture of reducing costs is that we
need to provide the most effective therapy at the lowest possible cost. This commentary has
attempted to provide a comparison of cost and benefit issues associated with cobalt-60 and
megavoltage x-ray utilization. This discussion is summarized in Table 3 which contains a synopsis
of the use of cobalt-60 versus low, medium, and high energy x-rays combined with a list of
opportunities for change. It is clear from this Table that there is still a role for cobalt-60 radiation.
However, this role needs to be defined in the context of individual radiation therapy institutions.
Clearly, a small institution cannot purchase the entire range of radiation therapy equipment and will
have to chose the minimal number of therapy units while providing maximum overall baseline
service to its patients. For such institutions, dual energy linacs with one low and one medium to high
energy x-ray mode along with electron capabilities will provide the maximum flexibility. Larger

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institutions, however, will require more therapy units and, therefore, will have the flexibility to
purchase a greater variety of equipment.
A review of Table 3 indicates that cobalt-60 can still play a significant role in treating those
patients whose target volumes lie near the surface, or those anatomical regions where the patient
separation is small, or those target volumes that can be treated with multiple conformal therapy
beams. If we make some simple assumptions about capabilities and case mix then we can provide a
crude estimate of the percentage of patients that could be treated by cobalt-60 radiation. First, about
20% of patients receiving radiation therapy are treated for cancer of the breast and another 17% are
treated for cancer of the lung50. If we assume that about one quarter of the breast patients have chest
wall separations that are smaller than 18 cm and one third of the lung patients could be treated by
cobalt-60 then this already represents 11% of all the patients treated. Add to that one third of the
head and neck patients, three quarters of the sarcoma patients, and one quarter of the palliative cases
then we can estimate the total percentage that could be treated with cobalt-60. This is summarized in
Table 4 and demonstrates that one out of every 4-5 megavoltage therapy machines could be a cobalt-
60 unit. These estimates are based on today's procedures. With an improved design, cobalt-60 could
possibly be used for about 30% of all patients treated with radiation therapy with a corresponding
reduction in overall operating costs. However, such a benefit is only achievable in an economic and
health care system that rewards minimal cost treatments and where physicians are not rewarded for
the use of more complex technologies. These estimates are also based on resources available in the
western world. Where resources are more constrained, as in developing countries, the fraction of
patients treated by cobalt-60 can be increased substantially and will be strongly dependent on local
resources.

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4. CONCLUSIONS
In conclusion, cobalt-60 can still be regarded as a very viable and cost effective option for the
treatment of a sizeable fraction of cancer patients assuming that the technology is improved. It is
our estimate that, in the western world with readily available financial resources, at least 25% of
cancer patients requiring radiation therapy could be treated with this modality. This is only possible
in departments that use more than two or three therapy machines since higher photon energies and
the use of electrons will still be required for those patients with larger separations and deep seated
tumours and those patients with tumours near the skin surface. In developing countries, where
financial and technical resources are severely restrained, a much larger fraction of patients requiring
treatment can benefit from cobalt-60 therapy. Recognizing its limitations, the "cobalt-60 challenge"
is to redesign existing technology to offer cobalt-60 therapy as a cost/beneficial alternative for a
well-defined fraction of the patients requiring radiation therapy. In times of economic restraint, it
becomes even more important to strike a delicate balance between the use of accelerators and cobalt-
60 machines.

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5. REFERENCES
1. Aldrich, J. Three reports on Zaragoza. Canadian Medical Physics Newsletter. December 1991:
5-6.
2. Battista, JJ, Sharpe, MB, True three-dimensional dose computations for megavoltage x-ray
therapy: A role for the superposition principle. Austral. Phys. Engin. Sciences Med. 15: 1992,
159-178.
3. Barton, MB, Gebski, V, Manderson, C, Langlands, AO. Radiation therapy: Are we getting
value for money? Clin. Oncol. 7: 1995, 287-292.
4. Brahme, A. Dosimetric precision requirements in radiation therapy. Acta Radiol. Oncol. 23:
1984, 379-391.
5. Carol, MP. PeacockTM: A system for planning and rotational delivery of intensity-modulated
fields. Int. J. Imag. Sys. Tech. 6: 1991, 56-61.
6. Carol, MP. Integrated 3-D conformal planning/multivane intensity modulating delivery system
for radiotherapy. In: 3D radiation treatment planning and conformal therapy. Purdy, JA,
Emami, B, eds. Medical Physics Publishing. Madison, Wisconsin, 1993.
7. Chu, JCH, Coia, LR, Aziz, D, Stafford, PM. Dose to superficial node for patients with head
and neck cancer treated with 6 MV and 60Co photons. Radioth. Oncol. 21: 1991, 257-260.
8. Crook, JM, Raymond, Y, Salhani, D, Yang, H, Esche, B. Prostate motion during standard
radiotherapy as assessed by fiducial markers. Radioth. Oncol. 37: 1995, 35-42.
9. Cunningham, JR, Woo, M, Rogers, DWO, Bielajew, AF. The dependence of mass energy
absorption coefficient ratios on beam size and depth in a phantom. Med. Phys. 13: 1986, 496-
502.

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
21
10. Das, IJ, Kase, KR Higher energy: Is it necessary, is it worth the cost of radiation oncology?
Med. Phys., 19: 1992, 917-925.
11. Davy, TJ. Physical aspects of conformation therapy using computer-controlled tracking unit.
In: Orton, CG., ed. Progress in medical radiation physics, Vol. 2. New York: Plenum 1985:
45-94.
12. Ekstrand, KE, Barnes, WH. Pitfalls in the use of high energy x-rays to treat tumours in the
lung. Int. J. Radiat. Oncol. Biol. Phys. 18: 1990, 249-252.
13. Epp, ER, Lougheed, MN, McKay, JW. Ionization build-up in upper respiratory passages
during teletherapy with cobalt-60 radiation. Br. J. Radiol. 31: 1958, 361-367.
14. Epp, ER, Boyer, AL, Doppke, KP. Underdosing of lesions resulting from lack of electronic
equilibrium in upper respiratory air cavities irradiated by 10 MV x-ray beams. Int. J. Radiat.
Oncol. Bio. Phys. 2: 1977, 613-619.
15. Garrison, H, Anderson, J, Laughlin, JS, Harvey, RA. Comparison of dose distributions in
patients treated with x-ray beams of widely different energies. Radiol. 58: 1952, 361-368.
16. Glasgow, GL, Kurup, RG, Leybovich, L, Wang, S, Fatyga, M. Dosimetry and use of 60Co 100
cm source-to-axis distance teletherapy units. Current Oncology, this issue, 1996.
17. Hanks, GE, Diamond, JJ, Kramer, S. The need for complex technology in radiation oncology.
Correlations of facility characteristics and structure with outcome. Cancer 55(9 Suppl): 1985,
2198-2201.
18. Hess, CF, Kortmann, R-D, Jany, R, Hamberger, A, Bamberg, M. Accuracy of field alignment
in radiotherapy of head and neck cancer utilizing individualized face mask immobilization: A
retrospective analysis of clinical practice. Radioth. Oncol. 34: 1995, 69-72.

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
22
19. Jacobs, SW, Gullane, P, Galbraith, D, Cummings, B. Implications of radiation dose
distribution at mandibular prosthesis/tissue interface. Ontario Cancer Institute internal report,
1987.
20. Johns, HE, Bates, LM, Watson, TA. 1000 curie cobalt 60 unit for radiation therapy. I. The
Saskatchewan cobalt 60 unit. Brit. J. Radiol. 25: 1952, 296-302.
21. Johns, HE, Cunningham, JR. A precision cobalt 60 unit for fixed field and rotation therapy.
Amer. J. Roentgen. Rad. Ther. Nucl. Med. 81:1959, 4-12.
22. Johns, HE, MacKay, JA. Collimating device for Co60 teletherapy units. J. Fac. Radiologists, 5:
1954, 239-247.
23. Karzmark, CJ. Procedural and operator error aspects of radiation accidents in radiotherapy. Int.
J. Radiat. Oncol. Biol. Phys. 13: 1987, 1599-1602.
24. Kornelsen, RO, Young, MEJ. Changes in dose profile of a 10 MV x-ray beam within and
beyond low density material. Med. Phys. 9: 1982, 114-116.
25. Kutcher, GJ, Mageras, GS, Leibel, SA. Control, correction, and modelling of setup errors and
organ motion. Semin. In Radiat. Oncol. 5: 1995, 134-145.
26. Laughlin, J, Mohan, R, Kutcher, GJ. Choice of optimum megavoltage energy for accelerators
for photon beam treatment, Int. J. Radiat. Oncol. Biol. Phys. 12: 1986, 1551-1557.
27. Leunens, G, Menten, J, Verstraete, J, van der Schueren, E. Quality assessment of medical
decision making in radiation oncology: variability in target volume delineation for brain
tumours. Radioth. Oncol. 29: 1993, 169-175.
28. Leung, PMK, Rider, WD, Webb, HP, Aget, H, Johns, HE. Cobalt-60 therapy unit for large
field irradiation. Int. J. Radiat. Oncol. Biol. Phys. 7: 1981, 705-712.

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
23
29. Leung, PMK, Van Dyk, J, Robins, J. A method for large irregular field compensation. Br. J.
Radiol. 47: 1974, 805-810.
30. Leveson, NG, Turner, CS. An investigation of the Therac-25 accidents. Computer 26: July
1993, 18-41.
31. Little, FA. Pacemakers in radiotherapy (Editorial). Clin. Oncol. 6: 1994, 211-212.
32. Mackie, TR, El-Khatib, E, Battista, J, Scrimger, J, Van Dyk, J, Cunningham, JR. Lung
corrections for 6 and 15 MV x-rays. Med. Phys., 12: 1985, 327-332.
33. Marbach, JR, Sontag, MR, Van Dyk, J, Wolbarst, AB. Management of radiation oncology
patients with implanted cardiac pacemakers: Report of AAPM task group no. 34. Med. Phys.
21: 1994, 85-90.
34. Metcalfe, PE, Hoban, PW, Murray, DC, Round, WH. Beam hardening of 10 MV radiotherapy
x-rays: analysis using a convolution/superposition method. Phys. Med. Biol. 35: 1990,
1533-1549.
35. Michalski, JM, Wong, JW, Gerber, RL, Yan, D, Cheng, A, Graham, MV, Renna, MA, Sawyer,
PJ, Perez, CA. The use of on-line image verification to estimate the variation in radiation
therapy dose delivery. Int. J. Radiat. Oncol. Biol. Phys. 27: 1993, 707-716.
36. Mould, RF. Letter to editor. Current Oncology 1: 1994 185-186.
37. Munro, AJ, Potter, S. Waiting times for radiotherapy treatment: Not all that mysterious and
certainly preventable. Clin. Oncol. 6: 1995, 314-318.
38. Munro, P, Rawlinson, JA, Fenster A. Therapy imaging: Source sizes of radiotherapy beams.
Med. Phys. 15: 1988, 517-524.
39. Nilsson, B, Schnell, PO. Build-up effects at air cavities measured with thin thermoluminescent

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
24
dosimeters. Acta Radiol. Ther. Phys. Biol. 15: 1976, 427-432.
40. Rawlinson, JA. Personnel exposure at a high energy medical accelerator. Ontario Cancer
Institute internal report, 1978.
41. Rawlinson, J A. The choice of equipment for external beam radiotherapy, Proceedings of the
IAEA Seminar on Organization and Training of Radiotherapy in Africa, Cairo, 11-15
December, 1989.
42. Read, G. Radiotherapy: Effective treatment at low cost! (Editorial) Clin. Oncol. 7: 1995, 275-
276.
43. Read, G. Estimating the cost of radiotherapy. Clin. Oncol. 6: 1994, 35-39.
44. Reaume LL. Rethinking cobalt therapy. Administ. Radiol.: 1987, 95-98.
45. Ross, CS, Hussey, DH, Pennington, EC, et al. Analysis of movement of intrathoracic
neoplasms using ultrafast computerized tomography. Int. J. Radiat. Oncol. Biol. Phys. 18:
1990, 671-677.
46. Sasane, JB, Iyer, PS. Relevance of radiation penumbra in high-energy photon beam therapy.
Strahlentherapie 157: 1981, 685-661.
47. Suit, H. What's the optimum choice? (Editorial) Int. J. Radiat. Oncol. Biol. Phys., 12: 1986,
1711-1712.
48. Ten Haken, RK, Forman, JD, Heimberger, DK, et al. Treatment planning issues related to
prostate movement in response to differential filling of the rectum and bladder. Int. J. Radiat.
Oncol. Biol. Phys. 20: 1991, 1317-1324.
49. Thames, HD, Hendry, JH. Fractionation in radiotherapy. Taylor and Francis, London, 1987.
50. The Ontario Cancer Treatment and Research Foundation, Cancer in Ontario 1992-93,

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
25
Kwiatkowska, K, ed.
51. Timothy, AR, Van Dyk, J, Sutcliffe, SB. Radiation therapy for Hodgkin's disease. In:
Hodgkin's Disease. Selby, P, McElwain TJ (eds.): 1987, 181-249.
52. Turner, SL, Swidell, R, Bowl, N, et al. Bladder movement during radiotherapy for bladder
cancer: Clinical implications for computerized treatment planning. In Hounsell AR, Wilkinson,
JM, Williams, PC (eds) Proceedings of the 11th international conference on the use of
computers in radiation therapy. Manchester, UK, North Western Medical Physics Department,
Christie Hospital: 1994, 206-207.
53. van der Giessen, PH. A comparison of maintenance costs of cobalt machines and linear
accelerators. Radioth. Oncol. 20: 1991, 64-65.
54. Van Dyk, J, Mah, K. Isoresponse distributions versus isodose distributions for pulmonary
tissues. In: The use of computers in radiation therapy. Bruinvis, I.A.D. et al (eds.): 1987, 395-
398.
55. Van Dyk, J, Galvin, JM, Glasgow, GP, Podgorsak, EB. The physical aspects of total and half
body photon irradiation. AAPM Report Number 17, American Institute of Physics, 1986.
56. Van Dyk, J. Dosimetry for total body irradiation. Radioth.Oncol. 9: 1987, 107-118.
57. Verhey, LJ. Immobilization and positioning patients for radiotherapy. Semin. In Radiat. Oncol.
5: 1995, 100-114.
58. Wallace, RP. On the Issue of cobalt teletherapy (Letter to the editor), Current Oncology 1:
1994, 185.
59. Wodinsky, HB, Jenkin, RDT. The cost of radiation treatment at an Ontario regional cancer
centre. CMAJ 137: 1987, 906-909.

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
26
60. Young, MEJ, Kornelsen, RO. Dose corrections for low density tissue inhomogeneities and air
channels for 10-MV x-ray. Med. Phys. 10: 1983, 450-455.

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
27
Table 1. Criteria for Comparing Radiation Therapy Machines
A. Radiation Beam Characteristics
1. Beam edge sharpness (penumbra)
2. Beam penetration (energy)
3. Scattering conditions/dose uniformity
4. Contour/inhomogeneity corrections
5. Dose to bone
B. Machine Characteristics
1. Dose rate
2. Patient collimator distance
3. Isocentre height
4. Radioactive source versus x-rays
C. Service/Maintenance Issues
D. Safety Considerations
1. Radiation protection
2. Pacemaker concerns
E. Cost Considerations

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
28
Table 2. Comparison of the dose at depth of maximum dose (in %) to the dose at the isocentre
(100%) as a function of number of fields. Depth to isocentre = 20 cm (equivalent to a lateral
patient thickness of 40 cm). Field size is 10x10 cm2, SAD = 100 cm.
Number of fields
Cobalt-60
6 MV
18 MV
1
390
284
204
2
206
159
126
4
99
79
63
6
66
53
42
10
40
32
25
20
20
16
13

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
29
Table 3. An evaluation of cobalt versus linacs combined with opportunities for improvement
* low score *** medium score ***** high score
Feature
Cobalt
Low
energy
linac
Medium
energy
linac
High
energy
linac
Opportunity for change
A. Radiation Beam Characteristics
1. Beam edge sharpness (penumbra)
*
****
***
**
Cobalt source redesign and use of MLC
2. Beam penetration (energy)
Parallel pair: Small separations < 14 cm
Medium separations 14 - 20 cm
Large separations >20 cm
3-4 fields: Thorax, Pelvis
Multiple fields, arcs, rotations
****
****
*
*
****
*****
**
**
****
****
****
***
*****
***
*****
*****
****
Development of conformal therapy
techniques using moving fields combined
with MLC.
3. Scattering conditions/uniformity
*
**
****
****
Use of flattening filters and dose
compensators.
4. (a) Contour/inhomogeneity corrections
(b) Build-up/build-down/interface effects
**
****
***
***
**
**
**
*
5. Dose to bone
***
***
***
***
A1. Optimum Energy By Site (refs)
1. Brain
****
*****
***
**
2. Head and neck
*****
****
**
*
3. Breast
****
*****
**
*
4. Lung/oesophagus
**
*****
***
*
5. Lymphoma
**
*****
***
*

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
30
6. Pancreas * * *** *****
Table 2 (continued)
7. Pelvis
*
**
****
*****
8. Extremity soft tissue sarcoma
*****
****
**
*
9. Pediatrics
****
*****
**
*
B. Machine Characteristics
1. Dose rate
*
****
*****
*****
Redesign source encapsulation
2. Patient collimator distance: Co-60, 80 cm
Co-60, 100 cm
*
***
***
***
***
Implementation of MLC
3. Height of isocentre above floor:Co-60, 80 cm
Co-60, 100 cm
****
*
**
**
**
Improved shielding to reduce head size.
4. Gamma rays vs x-rays
**
*
*
*
5. Constancy of output
*****
*
*
*
C. Service/Maintenance
*****
***
*
*
D. Safety Considerations
1. Radiation protection
***
****
**
**
2. Pacemaker concerns
*****
***
*
*
E. Cost Considerations
*****
***
**
*

Cobalt-60: An Old Modality, A Renewed Challenge Van Dyk and Battista
31
Table 4. Clinical sites which can benefit from the use of cobalt-60. Based on data from
reference 50 as well as data from the London Regional Cancer Centre for 1992-93.
(Sites where cobalt is not generally recommended are excluded.)
Clinical site
% of total
patients for
OCTRF1
(LRCC2)
Potential
fraction treated
with
Co-60
% of total
patients who
can potentially
be treated with
Co-60 for
OCTRF1
(LRCC2)
Brain
2.2 (6)
0.75
1.7 (4.5)
Breast
20 (18)
0.25
5.0 (4.5)
Head and neck
5 (15)
0.33
1.7 (5)
Lung/respiratory
17 (9)
0.33
5.6 (3)
Thyroid/sarcomas
2.4 (13)
0.75
1.8 (0.8)
Pediatrics
1.3 (13)
0.75
1.0 (0.8)
Palliative for other
sites
20 (26)
0.25
5 (6.5)
Total
68% (76%)
22% (25%)
1. OCTRF = Ontario Cancer Treatment and Research Foundation. The OCTRF data include all new
cancer cases registered by clinical site for the 1992-93 fiscal year.
2. LRCC= London Regional Cancer Centre. The LRCC data include radiation therapy patients for
the 1992-93 year
3. This is an estimate since detailed data were not available.