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Comparison of passive dosimeters for secondary radiation measurements in scanning proton radiotherapy

Proton therapy is used increasingly in cancer treatment because of the possibility of sparing healthy tissue close to the target volume. However, the interactions of protons with matter result in the production of secondary radiation comprised mostly of neutrons and gamma radiation. Unwanted doses, deposited distantly from the target volume, may lead to an increasing probability of late effects of radiotherapy including the generation of secondary cancers. The EURADOS WG9 measurement campaign is designed to investigate the secondary radiation generated by a scanning proton beam. Experiments were carried out in the IBA (230 MeV) active-scanning proton beam therapy facility in Trento, Italy. A volume of 10 x 10 x 10 cm3 inside the water phantom (60 x 30 x 30 cm3) was irradiated uniformly to a dose of 100 Gy. Depth dose distributions along the beam axis and profiles at various depths were measured. The following dosimeters were used: thermoluminescent (TLD-700, MTS-7, MTS-6 and MTS-N), radiophotoluminescent (GD-352M and GD302-M) and CR-39 track detectors. The results obtained within the irradiated volume show good agreement between prescribed and measured dose with RPLs (94.6Gy ± 2.4%). In Figure 1 preliminary results for TLDs and RPLs are shown for measurements outside the radiation field (beyond the range of proton beam). For photons, doses decrease from 22 mGy and 18 mGy at distance aproximately 13 cm from the inner surface of phantom, to 1 mGy and 0.5 mGy at distance aproximately 56 cm for MTS-7 and GD-352M respectively. Higher doses for TLDs compared with RPLs are observed due to higher sensitivity of TLDs to secondary neutrons in comparison to RPLs. A systematic decrease of the γ–equivalent neutron dose (Dn) with distance is observed. Dn is defined as the TL signal induced by neutrons in MTS-6 expressed in terms of the γ-ray dose producing an identical TL signal. Dn varied from 130 mGy at 13 cm from the inner surface of the phantom to 2 mGy at 56 cm. The detailed analysis of results combined with future Monte Carlo calculations will give valuable information about doses from scattered radiation distant from the target volume.

proton radiotherapy ; secondary radiation ; passive dosimeters ; TLD ; RPL

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