Abstract
Objective: To investigate the effect of 6 and 15 MV photon energies on intensity-modulated radiation therapy (IMRT) prostate cancer treatment plan outcome and to compare the theoretical risks of secondary induced malignancies. Methods: Separate prostate cancer IMRT plans were prepared for 6 and 15 MV beams. Organ equivalent doses were obtained through thermoluminescent dosimeter (TLD) measurements in a Rando phantom. The neutron dose contribution at 15 MV was measured using polyallyl-diglycol-carbonate (PADC) neutron track etch detectors. Risk coefficients from ICRP Report 103 were used to compare the risk of fatal secondary induced malignancies in out-of-field organs and tissues for 6 and 15 MV. For the bladder and the rectum, a comparative evaluation of the risk using three separate models was carried out. Dose-volume parameters for the rectum, bladder and prostate planning-target-volume were evaluated, as well as normal tissue complication probability (NTCP) and tumour control probability (TCP) calculations. Results: There is a small increased theoretical risk of developing a fatal cancer from 6 MV compared with 15 MV, taking into account all the organs. Dose-volume parameters for the rectum and bladder show that 15 MV results in better volume sparing in the regions below 70 Gy, but the volume exposed increases slightly beyond this in comparison to 6 MV, resulting in a higher NTCP for the rectum of 3.6% versus 3.0% (p = 0.166). Conclusion: The choice to treat using IMRT at 15 MV should not be excluded, but should be based on risk versus benefit, considering the age and life expectancy of the patient together with the relative risk of radiation-induced cancer and NTCPs.