LMAP-13 THE IMPACT OF RADIATION TREATMENT DOSE TO THE TUMOR AND NORMAL TISSUES USING POST-OPERATIVE MRI VS. MR-SIMULATION FOR RADIOTHERAPY PLANNING OF HIGH-GRADE GLIOMA

Abstract BACKGROUND Radiotherapy (RT) targets are commonly defined on post-operative (post-op) MRI registered to CT simulation images. However, changes can occur between surgery and simulation due to tumor progression and post-operative healing. In this work, we evaluate the impact of treatment planning using MR images taken at the time of CT simulation (MR-sim) versus post-op MRI. METHODS We evaluated 30 patients with primary HGG who received post-op MRI followed by RT planned using an MR-sim. We retrospectively delineated the gross tumor volume (GTV), clinical target volume (CTV), and normal structures on the post-op MRI and generated an experimental RT plan for these volumes. The RT plans based on post-op and MR-sim images were compared to determine the volume of tumor tissue that would have been undertreated, and the amount of normal brain tissue that would have been exposed to radiation (overtreated) had the plan been based on the post-op imaging. RESULTS The mean length between post-op imaging and MR-sim was 26 days (range: 12-42). A mean of 7.3cc (range 0.36cc–25.23cc) and 32.4cc (0.35–140.14cc) of the GTV and CTV, respectively, were undertreated on the post-op MRI plan compared to the MR-sim plan. The plan based on post-op target volumes resulted in a mean decrease in prescription dose coverage of the MR-sim-based GTV and CTV of 15.8% (max 72%) and 11.7% (max 57%), respectively. A mean of 14.5cc (0.04cc–53.9cc) and 47.5cc (range 2.4–179.2) of non-target brain tissue as identified on the MR-sim were included in the post-op MRI-based GTV and CTV, respectively. On average, the optic chiasm and brainstem would have received 191cGy and 117cGy more dose than intended, respectively. CONCLUSIONS Our findings confirm the valuable impact of RT planning using MR-sim in HGG to optimize treatment targets and reduce the exposure of normal tissue to unnecessary toxicity.