Purpose: To evaluate the dosimetric consequence caused by the inter-fractional variations in bladder during the prostate radiation therapy using deformable registration of simulation CT to Daily Cone-Beam CT (CBCT).
Methods: Four patients who underwent volumetric modulated arc therapy (VMAT) for localized prostate cancer under RTOG 0126 protocol were chosen in this study, with total prescription dose of 7920 cGy delivered in 44 fractions in two stages. Patients were under daily CBCT guidance through the treatment course. To simulate the effect on consistent bladder under-filling, nine daily CBCT instances with smaller bladder volumes were selected for each patient, with seven selected for initial plan, and two selected for boost plan. For each instance, the simulation CT was deformably registered to daily CBCT image. During the registration, the shape of prostate and target region was preserved to stabilize the original planning ISO-center. The deformed simulation CT was then used to generate fractional dose distribution using original beam configuration. Finally, dose distributions from all nine instances were then deformed to the original simulation CT and summed up to generate the final accumulated dose distribution through the radiation course.
Results: The average reduction of the bladder volume among the four patients was 39.9% ±12% when compared with simulation CT. An increase was observed for bladder dose-volume parameters: for bladder V65Gy, the increase was 17.2%±11.6%; for bladder V75Gy, the increase was 15.8%±13%.
Conclusion: A workflow has been developed to evaluate the accumulative dose through treatment course accounting for inter-fractional organ shape and position variations. Inconsistent bladder filling leads to increased bladder doses. It can be used to evaluate the dose-volume parameters variation due to inter-fractional bladder volume change and has the potential to guide future analysis of dose-volume relationship for bladder toxicity.