Monte Carlo Calculation of Neutron Doses to Organs of a Female Undergoing a Pelvic 18 MV Irradiation

Zahra Shakarami¹,  Mansour Zabihzadeh^{1, 2*}, Seyyed Rabi Mahdavi³, Mohammad Reza Ay⁴

¹ Department of Medical Physics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran. ² Department of Radiotherapy and Radiation Oncology, Golestan Hospital, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran. ³Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran. ⁴Department of Medical Physics and Biomedical Engineering, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran.

Abstract: Applying of high-energy photon beams beside all advantages obstacled by photoneutrons that may cause extra dose to the patient that has not been considered in routine radiotherapy. The purpose of this study is calculation of neutron and gamma doses to a female undergoing a pelvic 18 MV irradiation. A simplified Linac head model as a sphere with 10 cm radius of tungsten and with the total spectrum of an isotropic neutron distribution was located inside a typical bunker. The female anthropomorphic phantom was irradiated with equal weighted four-field pelvic box (18MV). MCNPX (2.4.0) code was used to calculate of absorbed doses. The greatest effective dose, 1.04 mSv Gy-1, was calculated for the AP field while the lowest effective dose, 0.36 mSv Gy-1, was obtained for the RL field. The Percent risk of fatal second malignancy of neutron contamination following a 70 Gy x-ray treatment dose (with equal weights for each field, 17.5 Gy) is 0.152 %, including 0.056 % for the AP field, 0.033 % for the PA field, 0.031 % for the RL field and 0.032 % for the LL field. If this dose delivered only with the AP field, the risk would be 0.224 %, which is 32 % higher than that is in case of 4-field irradiation. Our present analysis shows that this simplified model can be used to estimating of photoneutron doses.

Keywords: Photoneutron dose; The fatal secondary malignancy risk; Monte Carlo simulation

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