MC estimation of out-of-field organ doses from scattered photons, photoneutrons, and capture gamma rays in prostate radiation therapy

In the current study, the out-of-field organ and effective dose for external radiation therapy of prostate cancer was estimated by the MCNPX Monte Carlo code and a mathematical phantom. Organ doses from scattered photons, photoneutrons, and capture gamma rays were calculated for an 18 MV photon beam. Our results show that scattered photons are the main contributors in out-of-field patient doses. The resulting effective doses from scattered photons, neutrons, and capture gamma rays amounted to 723 mSv, 134 mSv, and 45 mSv per a 72 Gy prostate dose, respectively. In conventional treatment, the total effective dose from the three radiations in the current study was 902 mSv per a 72 Gy tumor dose. Taking into account that the risk factor for a secondary cancer of an adult male patient is 2% per Sv, the probability of secondary cancer risks of 1.8% and 6.3% were obtained for the conventional and intensity-modulated treatment of the prostate, respectively. Our study suggests that, taking into account all contributors to organ doses - including scattered photons, neutrons, and capture gamma rays - out-of-field dose calculations can provide a more realistic estimation for secondary cancer risk analysis, as well as a wider range of therapeutic techniques for comparison. (author)