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Craniospinal Irradiation by Volumetric Modulated Arc Therapy Technique on Halcyon
Abstract
Aims
Craniospinal irradiation (CSI) is a challenging task on halcyon due to its field size constraint (28 cm × 28 cm). CSI was planned by volumetric modulated arc therapy (VMAT) technique on Halcyon (6MV) linac with no junction shift with multiple arcs and numerous isocenter depending on the length of the patients.
Methods and Materials
Planning CSI was achieved on Eclipse treatment planning system version 15.6 with anisotropic analytical algorithm and was optimized using autofeathering technique. Positioning accuracy was ensured by obtaining daily kvCBCT before radiation which ensured accurate field placement and avoidance of junctional errors. Pretreatment portal dosimetry was done to ensure the dose distribution calculated by the treatment planning system matches the dose delivered to the patient.
Results
All VMAT CSI plans produced outstanding planning target volume (PTV) coverage with V95% >98% and gave acceptable doses to organ at risk in all CSI cases. Furthermore, the dose distributions were highly uniform, with homogeneity index values ≤0.1 and target conformity was equally excellent with values more than 0.95. In portal dosimetry, all of the composite images of CSI plans were evaluated, yielding good passing criteria of >98%.
Conclusions
The remedy was straightforward to plan and deliver, thanks to autofeathering optimization. CSI plan was created with no junction shift which resulted in homogeneous and conformal doses to the PTV. The gamma analysis in the portal dosimetry composite image, which was utilized as a pretreatment verification, met all of the requirements and revealed a homogeneous and uniform junction dose.
... In addition, Halcyon offers translation but no rotational table correction and has no manual table shift option, so the localization accuracy of the entire craniospinal axis is of the highest importance. To combat this, a planning technique in which no junction shifts are utilized and the shifts between the isocenters are only along the longitudinal direction has been shown to be feasible and able to provide outstanding coverage with acceptable doses to OARs [17,18]. Compared with TrueBeam's VMAT plans, Halcyon demonstrated superiority in terms of lower OAR doses and lower dose spillage [19]. ...
... We utilized this multi-isocenter approach routinely in our clinic on the TrueBeam LINAC. Similar to our VMAT CSI results, comparisons between the Halcyon and TrueBeam plans have demonstrated identical dosimetric results as previous studies [17][18][19]. ...
Currently, there is a lack of methods and tools that efficiently evaluate the auto-feathering junctions created by multileaf collimator (MLCs) for supine volumetric modulated arc therapy (VMAT) craniospinal irradiation (CSI) plans. We have investigated the feasibility of stitching together multi-isocenter fluence maps to then analyze the feathered junctions for patient-specific quality assurance (QA). Furthermore, we investigated the capability of Halcyon for the treatment of CSI patients. Three patients, who previously underwent VMAT CSI treatment on TrueBeam (6-MV flattening filter-free (FFF)) for 36 Gy in 20 fractions were replanned for Halcyon. A multi-isocenter approach with only translational superior-inferior shifts was used for both platforms. Each isocenter consists of two full arcs with anterior avoidance sectors, ±5° collimator rotations between arcs, and 5-8 cm of overlapping MLC auto-feathering junctions. All plans were QA’d via electronic portal imaging device (EPID) portal dosimetry and analyzed with a gamma criteria of 3%/3 mm. A variety of plan quality metrics were analyzed to evaluate dose distributions to the target, doses to organs at risk (OARs), and integral dose to the patient. A MATLAB script was developed to stitch the calculated and measured fluence maps in order to perform patient-specific QA for the composite fluence. The Halcyon plans provided highly conformal and homogenous dose distributions to the entire CSI target, superior to the clinical TrueBeam plans, while sparing critical organs with significantly lower values of V10Gy and V18Gy by up to 2% and 2.5%, respectively. Qualitative depictions of vertical dose profiles from the stitched DICOM of the entire CSI target for both planned and delivered fluence maps demonstrated equivalency, with slightly lower average pass rates with Halcyon (97%) compared to TrueBeam (99.9%). This approach to stitch multiple measured versus calculated EPID fluence maps has shown to be a feasible and accurate method and will be helpful for comprehensive VMAT CSI QA on both platforms. Further implementation of this script will be used in examining dosimetric impacts of daily patient positioning errors at MLC auto-feathering junctions.
Purpose: To carry out a comparative analysis of the dosimetric characteristics of plans for craniospinal irradiation (CSI) performed by the VMAT technique on Varian TrueBeam and Halcyon electron accelerators. Material and methods: Comparison of dose distributions was carried out for a group of 5 patients with tumors of the central nervous system. Contouring and planning were in Eclipse version 15.6. The following dose characteristics were calculated and evaluated: homogeneity index (HI), conformity index (CI), target coverage (CO) and constraints (lenses, esophagus, lungs, heart, liver, bowel bag and kidneys). Patient-Specific QA was performed to verify the consistency of the dose distribution calculated in the planning system and obtained at the accelerators. Results: For all CSI plans the target coverage metric was D98>95 % and constraints for critical organs were obtained. Homogeneity index was HI<=0.12 for two accelerators. The mean value of the CI conformity index was 1.09 for the TrueBeam accelerator, and CI=1.12 for the Halcyon. Gamma analysis showed results within tolerance both plans at TrueBeam and Halcyon. Conclusions have been made about the advantages and disadvantages of CSI on TrueBeam and Halcyon electron accelerators. Conclusion: It is possible to treat patients with the CSI method on the Halcyon accelerator using the VMAT method with the same quality as on the TrueBeam accelerator.
The accuracy of radiotherapy is the subject of continuous discussion, and dosimetry methods, particularly in dynamic techniques, are being developed. At the same time, many oncology centers develop quality procedures, including pretreatment and online dose verification and proper patient tracking methods. This work aims to present the possibility of using portal dosimetry in the assessment of radiotherapy repeatability. The analysis was conducted on 74 cases treated with dynamic techniques. Transit dosimetry was made for each collision-free radiation beam. It allowed the comparison of summary fluence maps, obtained for fractions with the corresponding summary maps from all other treatment fractions. For evaluation of the compatibility in the fluence map pairs (6798), the gamma coefficient was calculated. The results were considered in four groups, depending on the used radiotherapy technique: stereotactic fractionated radiotherapy, breath-hold, free-breathing, and conventionally fractionated other cases. The chi2 or Fisher's exact test was made depending on the size of the analyzed set and also Mann-Whitney U-test was used to compare treatment repeatability of different techniques. The aim was to test whether the null hypothesis of error-free therapy was met. The patient is treated repeatedly if the P-value in all the fluence maps sets is higher than the level of 0.01. The best compatibility between treatment fractions was obtained for the stereotactic technique. The technique with breath-holding gave the lowest percentage of compliance of the analyzed fluence pairs. The results indicate that the repeatability of the treatment is associated with the radiotherapy technique. Treated volume location is also an essential factor found in the evaluation of treatment accuracy. The EPID device is a useful tool in assessing the repeatability of radiotherapy. The proposed method of fluence maps comparison also allows us to assess in which therapeutic session the patient was treated differently from the other fractions.
Purpose
This manuscript describes the experience of two institutions in commissioning the new HalcyonTM platform. Its purpose is to: (a) validate the pre‐defined beam data, (b) compare relevant commissioning data acquired independently by two separate institutions, and (c) report on any significant differences in commissioning between the Halcyon linear accelerator and other medical linear accelerators.
Methods
Extensive beam measurements, testing of mechanical and imaging systems, including the multi‐leaf collimator (MLC), were performed at the two institutions independently. The results were compared with published recommendations as well. When changes in standard practice were necessitated by the design of the new system, the efficacy of such changes was evaluated as compared to published approaches (guidelines or vendor documentation).
Results
Given the proper choice of detectors, good agreement was found between the respective experimental data and the treatment planning system calculations, and between independent measurements by the two institutions. MLC testing, MV imaging, and mechanical system showed unique characteristics that are different from the traditional C‐arm linacs. Although the same methodologies and physics equipment can generally be used for commissioning the Halcyon, some adaptation of previous practices and development of new methods were also necessary.
Conclusions
We have shown that the vendor pre‐loaded data agree well with the independent measured ones during the commission process. This verifies that a data validation instead of a full‐data commissioning process may be a more efficient approach for the Halcyon. Measurement results could be used as a reference for future Halcyon users.
Our purpose is to present a novel technique for delivering craniospinal irradiation in the supine position using a perfect match, field-in-field (FIF) intrafractional feathering, and simple forward-optimization technique. To achieve this purpose, computed tomography simulation was performed with patients in the supine position. Half-beam, blocked, opposed, lateral, cranial fields with a collimator rotation were matched to the divergence of the superior border of an upper-spinal field. Fixed field parameters were used, and the isocenter of the upper-spinal field was placed at the same source-to-axis distance (SAD), 20 cm inferior to the cranial isocenter. For a lower-spinal field, the isocenter was placed 40 cm inferior to the cranial isocenter at a constant SAD. Both gantry and couch rotations for the lower-spinal field were used to achieve perfect divergence match with the inferior border of the upper-spinal field. A FIF technique was used to feather the craniospinal and spinal-spinal junction daily by varying the match line over 2 cm. The dose throughout the target volume was modulated using the FIF simple forward optimization technique to obtain homogenous coverage. Daily, image-guided therapy was used to assure and verify the setup. This supine-position, perfect match craniospinal irradiation technique with FIF intrafractional feathering and dose modulation provides a simple and safe way to deliver treatment while minimizing dose inhomogeneity.
Background
The purpose of this study is to propose a volumetric modulated arc therapy (VMAT) treatment planning technique for supine craniospinal irradiation (CSI) in order to improve dose conformity and homogeneity, as well as the reliability of the matching of abutting fields.
Methods and materials
CT datasets of a 9-year-old child and a 24-year-old adult treated with supine CSI were identified. The planning target volume (PTV) was contoured to include the whole contents of the brain and spinal canal with a uniform margin of 5 mm. RapidArc plans were generated with two partial arcs covering the brain and the superior portion of the spinal cord and a single partial arc covering the remaining inferior portion of the spinal cord. Conformity and heterogeneity indexes (CI and HI) and organs at risk (OAR) dose–volume histograms were evaluated. The effect of treatment inaccuracy was simulated with intentional patient shifts of ±3 mm.
Results
The CI for the pediatric and adult cases was 1.0 and 0.99, respectively, and the HI for the two cases was 12.7 and 11.8 %, respectively. The mean dose to the PTV was 102.5 and 102.1 %, respectively, while the mean doses to the OARs were all low. The simulated ±3-mm shift in patient position generated an error of less than ±10 % of the calculated dose in the spinal cord.
Conclusions
VMAT CSI was able to achieve highly conformal and homogeneous dose distributions, with minor doses to the surrounding normal tissues. Setup errors of ±3 mm were shown to have little effect on the intended dose distribution.
To dosimetrically evaluate the effects of improper patient positioning in the junction area of a VMAT cranio-spinal axis irradiation technique consisting of one superior and one inferior arc and propose a solution to minimize these patient setup errors.
Five (n=5) cranio-spinal axis irradiation patients were planned with 2 arcs: one superior and one inferior. In order to mimic patient setup errors, the plans were recalculated with the inferior isocenter shifted by: 1, 2, 5, and 10mm superiorly, and 1, 2, 5, and 10mm inferiorly. The plans were then compared with the corresponding original, non-shifted arc plans on the grounds of target metrics such as conformity number and homogeneity index, as well as several normal tissue dose descriptors. "Gradient-optimized" plans were then created for each patient in an effort to reduce dose discrepancies due to setup errors.
Percent differences were calculated in order to compare each of the eight shifted plans with the original non-shifted arc plan, which corresponds to the ideal patient setup. The conformity number was on average lower by 0.9%, 2.7%, 5.8%, and 9.1% for the 1, 2, 5, and 10mm inferiorly-shifted plans and 0.4%, 0.8%, 2.8%, and 6.0% for the respective superiorly-shifted plans. The homogeneity indices were, averaged among the five patients and they indicated less homogeneous dose distributions by 0.03%, 0.3%, 1.0%, and 2.8% for the inferior shifts and 0.2%, 1.2%, 6.3%, and 15.3% for the superior shifts. Overall, the mean doses to the organs at risk deviate by less than 2% for the 1, 2, and 5mm shifted plans. The 10mm shifted plans, however, showed average percent differences, over all studied organs, from the original plan of up to 5.6%. Using "gradient-optimized" plans, the average dose differences were reduced by 0.2%, 0.5%, 1.2%, and 2.1% for 1, 2, 5, and 10mm shifts, respectively compared to the originally optimized plans, and the maximum dose differences were reduced by 11.7%, 8.5%, 12.4%, and 13.9% on average for the 1, 2, 5, and 10mm shifted plans.
Setup errors related to isocenter shifting should be minimized in order to provide the patient with the most dosimetrically accurate treatment possible. Errors of 1-2mm can negatively affect the quality of the delivered treatment, most notably in the arc junction area, but the deterioration of the treatment plan accuracy is not as problematic as in the cases of larger errors such as 5-10mm. By employing a new planning technique, the dose differences due to setup errors can be greatly reduced.
To report the treatment of cranio-spinal irradiation (CSI) with volumetric modulated arc therapy (RapidArc) in adults and a child.
Five patients from 5 institutions were treated with CSI using RapidArc technology. Patient age varied between 7 and 45 years. The lengths of the planning target volumes (PTV) and dose prescriptions ranged from 54.6 to 78.4 cm, and from 12 to 36 Gy, respectively. Different arc arrangements were used, with two or three isocentres, from two to six modulated arcs, for patient positioned either in prone (n=3) or supine position (n=2). Doses to PTV, organs at risk and non-target tissue are reported for each of the five patients. Data concerning imaging strategy and pre-treatment quality assurance are also reported.
The mean conformity index CI(95%) was 1.05 ± 0.05 (range: 1.00, 1.13). Mean doses to the lenses, lungs, heart, and kidneys were, respectively, 7.6 ± 1.6, 6.6 ± 3.0, 5.7 ± 2.3, and 6.1 ± 2.1 Gy, keeping those value acceptably low. Mean non-target tissue dose was 21.0 ± 3.8% (5.6 ± 1.8 Gy, range: 3.0-8.1 Gy). All patients were successfully planned and treated with RapidArc.
RapidArc techniques achieved highly conformal treatment plans for 5 adults or paediatric patients. Beam-on time was short, and RapidArc plans were satisfactorily delivered to all presented patients.
It is well established that elective cranio-spinal irradiation is beneficial in the treatment of medulloblastoma. However, because of the geometry of the brain and the spinal column, a simple treatment procedure which results in a uniform dose over the entire target volume while maintaining a low dose to other tissues is not easy to perform. This report describes a relatively simple treatment technique which can be administered with radiotherapy apparatus that is available in most treatment centers. Special consideration is given to the dosimetry in the junction regions of the contiguous â¶Â°Co fields. Computer dosage calculations were made to check the effectiveness of this technique. This calculated data was verified experimentally with thermoluminescent dosimetry (TLD) in both a rectangular polystyrene phantom as well as the human like Alderson Rando phantom. The dose to critical organs such as the eyes was found to be less than 15 percent of the tumor dose, while the male gonadal dose was less than 10 percent. Detailed consideration is given to the factors affecting the overall variation of dose in the entire target volume. It has been estimated that using this technique, the overall dosage variation to the cranio-spinal axis is within +-10 percent of the prescribed tumor dose for the average adult, while for the average child it is better than +-5 percent.
The commissioning of a three-dimensional treatment planning system requires comparisons of measured and calculated dose distributions. Techniques have been developed to facilitate quantitative comparisons, including superimposed isodoses, dose-difference, and distance-to-agreement (DTA) distributions. The criterion for acceptable calculation performance is generally defined as a tolerance of the dose and DTA in regions of low and high dose gradients, respectively. The dose difference and DTA distributions complement each other in their useful regions. A composite distribution has recently been developed that presents the dose difference in regions that fail both dose-difference and DTA comparison criteria. Although the composite distribution identifies locations where the calculation fails the preselected criteria, no numerical quality measure is provided for display or analysis. A technique is developed to unify dose distribution comparisons using the acceptance criteria. The measure of acceptability is the multidimensional distance between the measurement and calculation points in both the dose and the physical distance, scaled as a fraction of the acceptance criteria. In a space composed of dose and spatial coordinates, the acceptance criteria form an ellipsoid surface, the major axis scales of which are determined by individual acceptance criteria and the center of which is located at the measurement point in question. When the calculated dose distribution surface passes through the ellipsoid, the calculation passes the acceptance test for the measurement point. The minimum radial distance between the measurement point and the calculation points (expressed as a surface in the dose-distance space) is termed the gamma index. Regions where gamma > 1 correspond to locations where the calculation does not meet the acceptance criteria. The determination of gamma throughout the measured dose distribution provides a presentation that quantitatively indicates the calculation accuracy. Examples of a 6 MV beam penumbra are used to illustrate the gamma index.
Purpose:
Craniospinal irradiation poses technical difficulties that may be addressed with the use of the newer technologies that have become available over the past decade. The use of CT simulation allows improved target localisation and beam geometry definition while significantly reducing the treatment simulation time. We have developed a CT-based technique for whole CNS irradiation in the supine position that uses fixed field parameters, asymmetric jaws for field matching and drastically reduces simulation and treatment times.
Methods:
The patient is CT scanned and treated in the supine position. The clinical target volume and relevant critical structures are outlined on a planning CT scan. Half beam blocked lateral fields with a collimator rotation are used to match the beam divergence from the superior border of the spinal field at the C2 vertebral body. The shielding for the cranial fields is generated automatically, and the dose distribution is calculated using a 3D treatment planning system. Fixed field parameters are used for the planning and treatment. The position of the isocenter of the spine field is always a fixed longitudinal distance from the isocenter of the brain fields. If multiple posterior fields are required, the isocenter of the second spine field is always a fixed longitudinal distance from that of the first and the gap between the fields is determined using virtual simulation and feathered during treatment using the asymmetric jaws of the linear accelerator. All treatment portals are filmed daily during the first week of treatment, and after each junction change thereafter.
Results and conclusion:
The supine position provides numerous advantages. Patients are more comfortable, the treatment position is more reproducible, and access to the airway is possible, if necessary, for patient sedation. The use of CT simulation decreases the simulation time, allows for increased planning accuracy, and enables the use of multimodality image registration, and 3D treatment planning. The use of asymmetric jaws allows for junction feathering without changing the patient setup or using a couch angle.
To plan craniospinal irradiation with "field-in-field" (FIF) homogenization in combination with daily, intrafractional modulation of the field junctions, to minimize the possibility of spinal cord overdose.
Lateral cranial fields and posterior spinal fields were planned using a forward-planned, step-and-shoot FIF technique. Field junctions were automatically modulated and custom-weighted for maximal homogeneity within each treatment fraction. Dose-volume histogram analyses and film dosimetry were used to assess results.
Plan inhomogeneity improved with FIF. Planning with daily modulated junction shifts provided consistent dose delivery during each fraction of treatment across the junctions. Modulation minimized the impact of a 5-mm setup error at the junction. Film dosimetry confirmed that no point in the junction exceeded the anticipated dose.
Field-in-field planning and modulated junction shifts improve the homogeneity and consistency of daily dose delivery, simplify treatment, and reduce the impact of setup errors.
The gamma tool was developed to quantitatively compare dose distributions, either measured or calculated. Before computing gamma, the dose and distance scales of the two distributions, referred to as evaluated and reference, are renormalized by dose and distance criteria, respectively. The renormalization allows the dose distribution comparison to be conducted simultaneously along dose and distance axes. The gamma quantity, calculated independently for each reference point, is the minimum distance in the renormalized multidimensional space between the evaluated distribution and the reference point. The gamma quantity degenerates to the dose-difference and distance-to-agreement tests in shallow and very steep dose gradient regions, respectively. Since being introduced, the gamma quantity has been used by investigators to evaluate dose calculation algorithms, and compare dosimetry measurements. This manuscript examines the gamma distribution behavior in two dimensions and evaluates the gamma distribution in the presence of data noise. Noise in the evaluated distribution causes the gamma distribution to be underestimated relative to the no-noise, condition. Noise in the reference distribution adds noise in the gamma distribution in proportion to the normalized dose noise. In typical clinical use, the fraction of points that exceed 3% and 3 mm can be extensive, so we typically use 5% and 2-3 mm in clinical evaluations. For clinical cases, the calculation time is typically 5 minutes for a 1 x 1 mm2 interpolated resolution on an 800 MHz Pentium 4 for a 14.1 x 15.2 cm2 radiographic film.