Sci-Fri PM: Delivery - 02: CT image guidance strategies for dose-adaptive IMRT of the prostate

On-line CT imaging in the radiotherapy room has become the norm for targeted intensity-modulated radiotherapy (IMRT), enabling precise adjustments of the daily patient setup based on soft tissue visualization. Corrections for plasticity of the anatomy and dose deformation are within technological reach but will require more on-line resources. We have developed a computer model that allows exploration of "what if" scenarios for assessing the benefits of Image Guidance strategies in terms of the multi-fraction dose distribution and DVH metrics (Target D95 and rectum V70). In this work we report on changes in anatomy and resultant dose distribution as observed in 35 daily megavoltage CT (MVCT) scans of the pelvis during prostate therapy for 13 patients. Our goal is to assess the effectiveness and efficiency of various adaptive strategies involving imaging schedule with and without dose re-planning of 5-field IMRT with 18 MV x-rays. Our research questions are: To what extent do radiation dose distributions delivered to individual patients (in vivo) diverge from the planned dose distributions (in silico)? Is there a robust schedule of CT image guidance, with or without dose re-planning that will mitigate discrepancies? For prostate IMRT, we conclude that image guidance schedule can be relaxed when generous GTV margins (10/7mm) are used. Tighter margins (isotropic 5 mm) reduce the dose to the rectum as expected. However, daily re-planning may be required to maintain adequate target coverage as planned when tighter margins are used.