Dose area product measurements with a novel large area ionization chamber in scanned proton beams

Abstract

Technological developments in radiation oncology have lead to novel treatment modalities such as intensity modulated radiotherapy with the use of high energy photon beams including arc therapy, as well as proton and ion beam therapy by means of scanning technologies. These developments have created a continuous demand on new dosimetric approaches and new detectors suitable to perform accurate dosimetry for the new modalities. One approach is the use of large-area laterally integrating ionization chambers for the dosimetry of narrow beams. This is a promising approach, as current recommendations in proton dosimetry are based on the experience with passively scattered and modulated beams, where calibration can be performed in terms of absorbed dose to water. For scanned beams it is more practical to calibrate a detector in terms of the number of particles or its dosimetric equivalent the dose are product in water. The use of large area detectors with small beams would relieve the common method of very small detectors such as p-types or diamond detectors. Positioning of the detectors with respect to the narrow beam for example is of no concern for dose area product, while it is crucial for small detectors. PTW developed a prototype of a large-area ionization chamber with an active diameter of 147 mm. Within the scope of this thesis the behavior of this detector, which will be referred to as BPC150 was investigated, with respect to proton therapy. The chamber was first calibrated in a 60Co source and further cross-calibrate it in a scanned proton beam at MedAustron, the center for ion therapy and research, Wr. Neustadt. Twodimensional response maps of the chamber, using an X-ray source, were recorded. The homogeneous response over the whole active area of the chamber is crucial for accurate reference dosimetry, but on the other very challenging to realize, as already few m variations between the collecting electrode and the entrance window result in response variations of more than 1%. Dose area product (DAP) measurements using large fields and single beams were conducted in a proton beam at different energies. Further measurements with the chamber included its water equivalent thickness as well as its behavior in carbon beams. The investigation of the water equivalent thickness (WET) of the BPC150 yielded results of 11.24 mm for the WET of the whole chamber and a WET of 4.63 mm for the entrance window. To guarantee adequate conditions for both cross calibration in proton beams and DAP measurements with the broad field approach, a field size of 18 cm 18 cm was found to be sufficient. The dose response map recorded in photon beams revealed an under response of 4-6% in the chamber center. This was confirmed by measurements in proton beams. Measurements in carbon ion beams with the BPC150 revealed a better resolution of the fragmentation tail in comparison with chambers with smaller active area. In summary, the behavior of the BPC150 is very similar to the PTW chambers of type 34070. The real difference between those chamber types are the response behavior. The chambers of type PTW34070 show the highest response in the chamber center and response is gradually decreasing when moving closer to the chamber edge, whereas the BPC150 shows the exact opposite behavior. The major advantage of the larger active area lie in measurements of the fragmentation tail in light ion beams.