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The range of a proton beam in a patient can be determined by scanning the distribution of the prompt gammas emitted from the beam passage. However, this method suffers from a high level of background gammas, especially in the case of high energy proton beams. The present study determined the optimal energy window for selective measurement of the prompt gammas, effectively discriminating background gammas, for a prompt gamma scanning system. To that end, the energy spectra of the prompt and background gammas were calculated by transporting the protons and other secondary particles with MCNPX. A detailed analysis of these spectra revealed that the optimal energy window is 4-10 MeV. The application of the energy window to simulated and measured data confirmed that the range of a proton beam in a patient can be determined much more accurately by employing the optimal energy window.

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The risk of secondary cancers after radiation therapy has become an important issue, mainly because advanced treatment techniques such as 3-D conformal radiation therapy (CRT) and intensity-modulated radiation therapy (IMRT) tend to increase the radiation dose to out-of-field organs. Moreover, cancer patients' average survival period, following those advances in treatment techniques, also has increased, effectively raising the risks for the same levels of radiation exposure. In the present study, the secondary cancer risks to out-of-field organs were determined for some representative cases of 3-D CRT by measuring organ doses with LiF thermoluminescence (TL) dosimeters and an ATOM TM male phantom. Our results show that the secondary cancer risks are similar (difference: < 20%) for 6 and 10 MV X-ray beams for the same prostate treatment case. The total secondary cancer risks of the out-of-field organs were on the order of 2-4% for the treatment cases considered in the present study.