Extended-range BSS, TEPCs, and the WENDI-II enable accurate measurements of stray neutrons while other rem-counters are not appropriate considering the high-energy range of neutrons involved in proton therapy.

Systematic 3D mapping of out-of-field doses induced by a therapeutic proton pencil scanning beam in a 300  ×  300  ×  600 mm water phantom was performed using a set of thermoluminescence detectors (TLDs): MTS-7 (LiF:Mg,Ti), MTS-6 (LiF:Mg,Ti), MTS-N (LiF:Mg,Ti) and TLD-700 (LiF:Mg,Ti), radiophotoluminescent (RPL) detectors GD-352M and GD-302M, and polyallyldiglycol carbonate (PADC)-based (CHO) track-etched detectors. Neutron and gamma-ray doses, as well as linear energy transfer distributions, were experimentally determined at 200 points within the phantom. In parallel, the Geant4 Monte Carlo code was applied to calculate neutron and gamma radiation spectra at the position of each detector. For the cubic proton target volume of 100  ×  100  ×  100 mm (spread out Bragg peak with a modulation of 100 mm) the scattered photon doses along the main axis of the phantom perpendicular to the primary beam were approximately 0.5 mGy Gy at a distance of 100 mm and 0.02 mGy Gy at 300 mm from the center of the target. For the neutrons, the corresponding values of dose equivalent were found to be ~0.7 and ~0.06 mSv Gy, respectively. The measured neutron doses were comparable with the out-of-field neutron doses from a similar experiment with 20 MV x-rays, whereas photon doses for the scanning proton beam were up to three orders of magnitude lower.

Purpose: To evaluate the effect on charge collection in the ionization chamber (IC) in proton pencil beam scanning (PBS), where the local dose rate may exceed the dose rates encountered in conventional MV therapy by up to three orders of magnitude. Methods:We measured values of the ion recombination (k s ) and polarity (k pol ) correction factors in water, for a plane-parallel Markus TM23343 IC, using the cyclotron-based Proteus-235 therapy system with an active proton PBS of energies 30-230 MeV. Values of k s were determined from extrapolation of the saturation curve and the Two-Voltage Method (TVM), for planar fields. We compared our experimental results with those obtained from theoretical calculations. The PBS dose rates were estimated by combining direct IC measurements with results of simulations performed using the FLUKA MC code. Values of k s were also determined by the TVM for uniformly irradiated volumes over different ranges and modulation depths of the proton PBS, with or without range shifter. Results: By measuring charge collection efficiency versus applied IC voltage, we confirmed that, with respect to ion recombination, our proton PBS represents a continuous beam. For a given chamber parameter, e.g., nominal voltage, the value of k s depends on the energy and the dose rate of the proton PBS, reaching c. 0.5% for the TVM, at the dose rate of 13.4 Gy/s. For uniformly irradiated regular volumes, the k s value was significantly smaller, within 0.2% or 0.3% for irradiations with or without range shifter, respectively. Within measurement uncertainty, the average value of k pol , for the Markus TM23343 IC, was close to unity over the whole investigated range of clinical proton beam energies. Conclusion: While no polarity effect was observed for the Markus TM23343 IC in our pencil scanning proton beam system, the effect of volume recombination cannot be ignored.

No abstract

Irradiation, delivered by a synchrotron facility, using a set of highly collimated, narrow and parallel photon beams spaced by 1 mm or less, has been termed Microbeam Radiation Therapy (MRT). The tolerance of healthy tissue after MRT was found to be better than after standard broad X-ray beams, together with a more pronounced response of malignant tissue. The microbeam spacing and transverse peak-to-valley dose ratio (PVDR) are considered to be relevant biological MRT parameters. We investigated the MRT concept for proton microbeams, where we expected different depth-dose profiles and PVDR dependences, resulting in skin sparing and homogeneous dose distributions at larger beam depths, due to differences between interactions of proton and photon beams in tissue. Using the FLUKA Monte Carlo code we simulated PVDR distributions for differently spaced 0.1 mm (sigma) pencil-beams of entrance energies 60, 80, 100 and 120 MeV irradiating a cylindrical water phantom with and without a bone layer, representing human head. We calculated PVDR distributions and evaluated uniformity of target irradiation at distal beam ranges of 60-120 MeV microbeams. We also calculated PVDR distributions for a 60 MeV spread-out Bragg peak microbeam configuration. Application of optimised proton MRT in terms of spot size, pencil-beam distribution, entrance beam energy, multiport irradiation, combined with relevant radiobiological investigations, could pave the way for hypofractionation scenarios where tissue sparing at the entrance, better malignant tissue response and better dose conformity of target volume irradiation could be achieved, compared with present proton beam radiotherapy configurations.