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Occurrence of radiation-induced secondary cancer risk following mantle field radiotherapy for Hodgkin's lymphoma (HL) patients with long survival demands well-established radiotherapy strategy. Organs doses and resulted secondary cancer risk due to out-of field photons were calculated during mantle field radiotherapy for Hl patient. The male and female mathematical phantom of the Oak Ridge National Laboratory (ORNL) and validated 6MV photon beam of a Varian 2300 C/D were modeled by MCNPX 2.4.0 MC code. Using suitable lungs and thyroid shields for AP and PA fields, the organ specific absorbed doses, effective dose, and secondary cancer risk were calculated following to mantle field radiotherapy for HL. Among the out-of-field organs, the nose, eyes, head and neck's skins and sinuses have the higher received doses. The total effective doses and secondary cancer risk for a male and female were estimated to be 199, 234 mSv and 1.72, 1.87 respectively. During mantle field radiotherapy for Hodgkin's lymphoma, accurately estimations of organs dose near to the field's edge and suitable shielding of critical in-field organs are crucial factor to establish an optimal treatment plan.

In some linear accelerators (Linac), the collected charges in beam monitor chamber (BMC) is partly caused by the backscattered particles from the accelerator components downstream the BMC that influence the Linac output factors. In the intensity modulated radiation therapy technique, the desired dose distribution can be achieved through an unflattened beam. Although removing the flattening filter provides some advantages, the amount of backscatter radiation into BMC can be changed. In this study, contribution of backscattered particles into the BMC response of a Varian 2300 C/D Linac with and without a flattening filter was determined for 6, 18 MV photon beams. The experimental procedure included telescopic method and calculation procedure consisted of Monte Carlo simulation (MCNPX, version 2.4.0), were used to investigate the contribution of backscattered particles into the BMC performance. Our results showed a 2.3 and 3 increase in backscatter for a 0.5 x 0.5 cm2 field compared to a 40 x 40 cm2 field for 6 MV and 18 MV, respectively. The energy deposition from backscattered radiation is mainly caused by backscattered electrons. Removing the flattening filter did not change the BMC performance for a conventional Linac with a flattening filter. However, this result was not valid for small fields (e.g. 0.5 x 0.5 cm2, 18 MV). The corrected backscatter factors is necessary to taking into account the contribution of backscattered radiation in the monitor chamber response for small fields in the case of the free flattening filter Linacs (18 MV). © 2015, Oriental Scientific Publishing Company. All rights reserved.