Mathias Pfaender’s research while affiliated with Humboldt-Universität zu Berlin and other places

Mittels der Polymergele ist man in der Lage dreidimensionale Dosisverteilungen zu messen. BANG 1 und BANG 3 Gele wurden mit dem 68 MeV Protonenstrahl am Augentumortherapie platz des Hahn Meitner Instituts in Berlin bestrahlt. In einem Phantom konnten bis zu zwölf Be strahlungsfelder appliziert werden. Es wurden monoenergetische und ausgedehnte Bragg Peaks sowie Patientenfelder untersucht. Die Gele wurden mittels Magnetresonanztomographie ausge wertet. Die Ergebnisse wurden mit Messungen einer Ionisationskammer im Wasserphantom ver glichen. Die BANG Polymergele zeigten im Vergleich zur Ionisationskammermessung eine Unterdrü ckung des Bragg Peaks. Die BANG 3 Gele stellten sich als ungeeignet für weitere Untersuchun gen mit 68 MeV Protonen heraus. Anhand der am BANG 1 durchgeführten Messungen konnten Keilfilterneigungen und irreguläre Feldformen verifiziert werden. Aufgrund unserer Erfahrungen eignen sich die Polymergele zur Qualitätssicherung in der Protonentherapie.

The use of polymer gel makes it possible to measure three-dimensional dose distributions of ionizing radiation. Phantoms with BANG-1 and BANG-3 gel were irradiated using a 68 MeV proton beam at the eye tumour therapy beam line of the Hahn Meitner Institute in Berlin. Up to twelve treatment fields could be applied to one gel phantom. The investigations consisted of mono-energetic Bragg curves, spread-out Bragg curves of circular fields (diameter 20 mm), and spread-out Bragg curves of patient fields. Magnetic resonance imaging was used to obtain the gel dose distributions. The results were compared to measurements of a water-phantom ionization chamber. The BANG polymer gels showed a significant quenching of Bragg peaks compared to ionization chamber measurements. The BANG-3 gel was found to be unsuitable for further investigations with 68 MeV protons. The use of BANG-1 allowed the verification of wedge slopes and irregular field forms. On the basis of our experience, polymer gels are well suited for quality assurance in proton therapy in principle.

Purpose Polymer gels are used for the verification of dose distributions in intensity modulated radiotherapy IRMT , stereotatic radiotherapy and brachytherapy. First experiments in proton therapy of eye tumours were done by us with BANG polymer gels. We analysed patient collimators and wedge angles. The BANG gel handling is very complicated and they show a pronounced quenching of the Bragg Peak. The resolution of the MR images 0.3 mm Pixel should be improved. Methods and Materials Now we use two different polymer gels MAGIC gel proposed by S. Scheib et al. and THPC gel proposed by C. Bayreder et al. . Both are normoxic gels and can be easily produced by ourselves within two hours. The gels were filled in poly propylene PP or Barex containers and were irradiated with a 68 MeV proton beam at the Hahn Meitner Institut Berlin. In order to achieve a better resolution and a better signal to noise ratio we used two different 3 T MR scanners, a clinical MR scanner at the Charit in Berlin and a research MR scanner with high field gradients at Medical University of Vienna. Results In the PP containers we observed an area of reduced polymerisation near the wall of the containers. Both gels show a linear dose response THPC gel up to 4 Gy; MAGIC gel up to 30 Gy. The Quenching of the Bragg Peak can be equally parametrised but it is not as prominent as in the BANG gels. Discussion and Conclusion With the 3 T MR scanners we achieve the needed resolution for gel dosismetry in eye tumour therapy. Due to its great linear dose range the MAGIC gel should be preferred.

BANG polymer gel dosimetry using magnetic resonance imaging (MRI) was applied to an ophthalmologic 68 MeV proton beam. The object was to examine the use of BANG gel for the verification of proton fields in eye tumor therapy and to explore the applicability of polymer gel dosimetry in proton therapy under practical aspects. The gel phantoms were irradiated with monoenergetic and modulated proton beams. MRI analysis was carried out at clinical 1.5 and 3 T MR scanners. At constant LET, results show a linear relationship between spin–spin relaxation rates and dose. However, depth dose curves in BANG gel reveal a quenching of the Bragg maximum due to LET effects. The dose response of the gel for monoenergetic protons and spread‐out depth dose distributions can be calculated based on ionization chamber measurements. Experiment and calculations show good agreement and indicate that BANG polymer gels might become a valuable tool in proton therapy quality assurance.

Conformal stereotactic radiosurgery and radiotherapy with linear accelerators and hole collimators yield a dose concentration in the target volume by rotation of the gantry. For small target volumes collimators with isocentre diameters of 4-45 mm are used. In this paper dosimetric measurements with a commercial high doped p-type silicon detector are demonstrated and compared to measurements with diamond detector and ionisation chamber. The properties of the silicon detector SFD from Scanditronix were investigated with the radiation of a Gammatron S and a Varian 2100 CD at 6 MV. The results were compared with those of a calibrated ionisation chamber (0.3 cm3) and a diamond detector. At the beginning the reproductibility of the registered dose and dose rate and the temperature dependence of the Si-detector were investigated at the Gammatron S. For the comparison the absorbed dose was measured with the ionisation chamber in air. The sensitivity decreases slightly with dose and dose rate. After a period of several days without radiation again higher doses were registered. The temperature dependence causes deviations of 0.25%/K. The signal-to-noise ratio and the spatial resolution were investigated with the linear accelerator. The signal-to-noise ratio is clearly lower compared with that of the diamond detector, whereas the resolution is nearly the same. The Si-detector is qualified for dosimetry of very small fields because of the insignificant dose and dose rate dependence and in spite of some disadvantages regarding dosimetric properties compared with the diamond detector. The advantage is the availability and the cost. Measurement with ionisation chambers are not useful for collimator diameters below 20 mm.

Conformal stereotactic radiosurgery and radiotherapy with linear accelerators and hole collimators yield a dose concentration in the target volume by rotation of the gantry. For small target volumes collimators with isocentre diameters of 4–45 mm are used. In this paper dosimetric measurements with a commercial high doped p-type silicon detector are demonstrated and compared to measurements with diamond detector and ionisation chamber. Material and Methods: The properties of the silicon detector SFD® from Scanditronix were investigated with the radiation of a Gammatron®S and a Varian 2100 CD® at 6 MV. The results were compared with those of a calibrated ionisation chamber (0.3 cm3) and a diamond detector. Measurements and Results: At the beginning the reproducibility of the registered dose and dose rate and the temperature dependence of the Si-detector were investigated at the Gammatron S. For the comparison the absorbed dose was measured with the ionisation chamber in air. The sensitivity decreases slightly with dose and dose rate. After a period of several days without radiation again higher doses were registered. The temperature dependence causes deviations of 0.25%/K. The signal-to-noise ratio and the spatial resolution were investigated with the linear accelerator. The signal-to-noise ratio is clearly lower compared with that of the diamond detector, whereas the resolution is nearly the same. Conclusions: The Si-detector is qualified for dosimetry of very small fields because of the insignificant dose and dose rate dependence and in spite of some disadvantages regarding dosimetric properties compared with the diamond detector. The advantage is the availability and the cost. Measurement with ionisation chambers are not useful for collimator diameters below 20 mm.

Conformal stereotactic radiosurgery and radiotherapy using a linear accelerator and a micromultileaf collimator (mMLC) offer the possibility of irradiating irregularly shaped target volumes. Dynamic arc radiosurgery and radiotherapy, i.e., stereotactic radiation therapy combining a moving gantry with a dynamic mMLC, enable the radiation even of lesions with concave structures. The dynamic arc method requires additional tools for quality assurance (QA) and three-dimensional verification at a high spatial resolution. A QA program was developed. Dose distributions of planning target volumes with concavities were investigated in polymer gel phantoms. The radiation-induced change of the relaxation rate R(2) was measured by magnetic resonance imaging. The distributions were compared with image processing tools. Using the therapy-planning software BrainSCAN 4.0 (and 4.1 beta) in combination with the mMLC m3, deviations between the planned and measured 90% isodoses of about 2 mm were registered in the isocenter plane. Three-dimensional verification was feasible in the range of accuracy achieved in planning and dose measurement. Dynamic arc radiosurgery and radiotherapy offer excellent conformation even for complicated planning target volumes with concavities. The dose distribution calculated with the treatment-planning software used can be accomplished with the available equipment. Patients can be treated by dynamic arc radiosurgery and radiotherapy.

Conformal stereotactic radiosurgery (SRS) and radiotherapy (SRT) using a linear accelerator and a micro-multileaf collimator (mMLC) with fixed gantry angles enables the irradiation of highly irregularly shaped target volumes. This requires a three-dimensional verification with high spatial resolution of the planned dose distribution. Dose distributions of several planning target volumes were investigated in polymer gel phantoms. The radiation induced change of the relaxation rate R2 is measured by MRI. The distributions were compared using image processing tools. Using the therapy-planning software BrainSCAN 3.53 and the mMLC m3 deviations between the planned and measured 90 % isodoses in the isocenter plane of about 2 mm were registered.

In radiation oncology more and more effort is taken to include the dose conformity to the target as well as the dose sparing of risk organs (OAR) and healthy tissue to the treatment planing. Conformal stereotactic radiosurgery and radiotherapy (CSRS and CSRT) offer the potential to optimise these parameters according to the demands of the physician and the physicist. The study compares treatment plans using different techniques for dose delivery as static beams, arcs or multiple isocentres. Also different beam shaping devices for CSRS and CSRT as conformal blocks, circular cones or multileaf collimators have been considered.