No abstract

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.

Proton beam therapy has advantages in comparison to conventional photon radiotherapy due to the physical properties of proton beams (e.g. sharp distal fall off, adjustable range and modulation). In proton therapy, there is the possibility of sparing healthy tissue close to the target volume. This is especially important when tumours are located next to critical organs and while treating cancer in paediatric patients. On the other hand, the interactions of protons with matter result in the production of secondary radiation, mostly neutrons and gamma radiation, which deposit their energy at a distance from the target. The aim of this study was to compare the response of different passive dosimetry systems in mixed radiation field induced by proton pencil beam inside anthropomorphic phantoms representing 5 and 10 years old children. Doses were measured in different organs with thermoluminescent (MTS-7, MTS-6 and MCP-N), radiophotoluminescent (GD-352 M and GD-302M), bubble and poly-allyl-diglycol carbonate (PADC) track detectors. Results show that RPL detectors are the less sensitive for neutrons than LiF TLDs and can be applied for in-phantom dosimetry of gamma component. Neutron doses determined using track detectors, bubble detectors and pairs of MTS-7/MTS-6 are consistent within the uncertainty range. This is the first study dealing with measurements on child anthropomorphic phantoms irradiated by a pencil scanning beam technique.

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.

The purpose of this study was to measure out-of-field organ doses in clinical conditions in anthropomorphic paediatric phantoms which received a simulated treatment of a brain tumour with intensity modulated radiotherapy (IMRT) and 3D conformal radiotherapy (3D CRT). Organ doses measured with radiophotoluminescent and thermoluminescent dosemeters were on average 1.6 and 3.0 times higher for the 5 y-old than for the 10 y-old phantom for IMRT and 3D CRT, respectively. A larger 5-y to 10-y organ dose ratio for 3D CRT can be explained because the use of a mechanical wedge for the 5-y-old 3D CRT phantom treatment increased out-of-field doses. Due to different configurations of the radiation fields, for both phantoms, the IMRT technique resulted in a higher non-target brain dose and higher eye doses but lower thyroid doses compared to 3D CRT. For 3D CRT (which used a non-coplanar field configuration), eye doses were 3-6% and for IMRT (which used a coplanar field configuration) 27-30% of the treatment dose, respectively. For thyroid and more distant organs, doses were less than 1% of the treatment dose. Comparison of measured doses and doses calculated by the treatment planning system (TPS) showed that the TPS underestimated out-of-field doses both for IMRT and 3D CRT.