SU-E-T-742: The Risk of Secondary Malignancies as An Evaluation Factor in the Comparison of Prostate Cancer Intensity Modulated Radiotherapy (IMRT) and Conformal Radiotherapy (CRT) Treatment Plans

Purpose: To examine the clinical efficacy of both MLC‐based IMRT, and 3D‐Conformal Radiation Therapy modalities (CRT) to treat prostate cancer using radiobiological measures. Furthermore, to investigate the risk for developing secondary malignancies in bladder and rectum due to radiotherapy from the corresponding modalities. Methods: Treatment plans for prostate cancer were developed using IMRT and CRT for ten patients. For the IMRT plans, two different treatment protocols were used (RTOG and FCCC). For the evaluation of these plans, the complication‐free tumor control probability, the total probability of injury, the total probability of control/benefit, and the biologically effective uniform dose were employed. Furthermore, based on the dosimetric data of IMRT and CRT, the risk for secondary malignancies was calculated for bladder and rectum. Results: The average risk for secondary malignancy was lower for the bladder (0.37%) compared to the rectum (0.81%) based on all the treatment plans of the ten prostate cancer patients. The highest average risk for secondary malignancy for bladder and rectum was for the 6X 3D modality (0.46% and 1.12%, respectively) and the lowest was for the 18X IMRT RTOG modality (0.33% and 0.56%, respectively). The response probability was lower for the bladder than for the rectum in all the plans. For the bladder the highest value was for the 18X IMRT FCCC (0.03%) and the lowest was for the 18X 3D modality (0.0%). For the rectum, the highest value was for the 6X IMRT RTOG (3.52%) and the lowest was for the 18X IMRT FCCC modality (0.41%). Conclusion: By using radiobiological measures it is shown that the IMRT FCCC plans had the lowest risks for normal tissue complications, whereas the IMRT RTOG had the highest. Regarding the risk for secondary malignancies, the CRT plans showed the highest values both in bladder and rectum.