SU-E-T-485: Comparison of the Oncogenic Potential for Radiation-Associated, Second Malignant Neoplasms for Several Prostate Radiotherapy Modalities as a Function of Relative OAR & PTV Volumes

Abstract

Purpose: To compare plan quality for a comprehensive set of OAR & PTV geometries considering relative risk estimates for the induction of secondary bladder and rectal cancers due to prostate radiotherapy using several modern delivery techniques. Methods: IMRT / 3D-conformal, volumetric-arc (VMAT), and Tomotherapy (TT) plans were generated using PlanUNC, Pinnacle3 ver. 9.2, and TomoTherapy treatment planning systems, respectively, for 12 prostate cases using, RTOG 0815-derived planning goals. Selected cases were representative of relative (i.e. small (Svol), medium (Mvol), large (Lvol)) PTV, rectal & bladder structure volumes encountered clinically. PlanUNC was used to calculate the elevated relative-risk (ERR) of secondary cancer induction in normal tissues, the bladder and rectal ERR, PTV conformity index (CI), dose heterogeneity index (DHI), and bladder and rectal V5, V15, V35, V50 (Bvx / Rvx) for all cases. All metrics were averaged for each relative tissue / target volume, and normalized to corresponding 3D-conformal cases for comparison. Results: Considering relative rectal volumes, all metrics fell within the standard error for each modality excluding bladder ERR (TT 13%ave, IMRT −9% ave & VMAT −18% ave) and BV15 (VMAT −24% ave) for Lvol rectum cases. Considering relative bladder volumes, bladder ERR (TT 4%, IMRT −10%, VMAT −18%) for Lvol and (VMAT −20%) for Mvol; rectal ERR (TT −40%) for Mvol were observed to be significant in addition to bladder ERR (TT 13%) for Svol PTV cases. Generally, all modalities performed better when compared to 3D conformal delivery excluding nominal (<5%) increases in normal tissue dose and increased bladder ERR for several TT cases. Conclusion: We compared three prostate EBRT modalities considering OAR ERR as well as traditional plan quality metrics for a class of cases defined by relative structure size. Preliminary results indicate comparable improvements in PTV conformity & OAR dosing for each modality but differences in OAR ERR.