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Secondary radiation doses in paediatric phantoms for proton radiotherapy- EURADOS WG9 intercomparison

Background and Objectives Proton therapy potentially reduces acute and late side effects of treatment in comparison to conventional radiotherapy, which is particularly important when treating paediatric cancer patients. EURADOS WG 9 (Radiation Dosimetry in Radiotherapy) performed a measurement campaign to study secondary radiation generated by a proton scanning beam (PBS). The main aim of this study is to characterize secondary neutron and gamma radiation fields inside child anthropomorphic phantoms with different solid state detectors and compare the results with measured secondary doses (1, 2) during different photon therapy modalities for similar case of brain treatment in children. Materials and Methods Proton irradiations were carried out at the CCB IFJ PAN (Krakow, Poland) using a Pencil Beam Scanning (PBS) technique (IBA, Proteus 235). Measurements of secondary gamma and neutron radiation were performed inside three phantoms which represents 1, 5 and 10- year old children. Unwanted doses were measured with different detectors based on thermoluminescence (TL) MTS-7: 7LiF(Mg, Ti), MTS-6: 6LiF(Mg, Ti) and MCP-n: LiF(Mg, Cu, P) and radiophotoluminescence (RPL): GD-352M and using poly-allyl-diglycol carbonate (PADC) track etched detectors and two types bubble detectors (BD). For all three phantoms treatment of brain tumor was simulated. Target volume was 65 cm3 with the isocentre in the head of the phantom. Doses following intensity modulated proton therapy (IMPT) were compared with different photon radiotherapy techniques (IMRT, 3D-CRT, GK) (1, 2) Results and Discussion For IMPT non-neutron organ doses are higher in the 5-year old phantom compared to the 10-year old phantom due to increased proximity of organs to the target, while measured neutron organ doses were higher in the 10-y old phantom. The mean stray non-neutron doses for the 5-year phantom (treatment dose of 1Gy) measured 15-45 cm from the isocentre ranged from 0.47 mGy to 0.15 µGy. Neutron doses are a factor of 3 lower than secondary non- neutron doses close to the target but become a factor of 3 higher than secondary non-neutron doses further away from the target. IMPT results in lower secondary doses compared to different photon therapy techniques by one order of magnitude close to the target and more than two order of magnitude further away from the target. Conclusion A combination of several detector types was used to study the total secondary organ doses in proton radiotherapy which showed to be significantly lower in comparison to the different photon radiotherapy techniques for the same delivered tumour dose.

secondary radiation ; spot scanning proton therapy ; paediatric phantoms

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