Trigger-Based Adaptive Planning to Reduce Bowel Dose in Patients Receiving Radiotherapy for Anal Cancer

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<h3>Purpose/Objective(s)</h3> Standard of care for locally advanced anal carcinoma is definitive chemoradiation with IMRT. Currently, target and organ-at-risk (OAR) volumes are created using a pre-treatment CT-simulation scan. However, during the course of radiotherapy, there is often change in the size and/or position of the tumor and surrounding normal structures. Therefore, we hypothesize that the implementation of an adaptive radiation therapy treatment protocol in anal cancer may reduce radiation dose to critical surrounding normal structures, such as the bowel, through the use of anatomy-adapted replanning. <h3>Materials/Methods</h3> Retrospective daily adaptive CTs were created for 4 patients (54 Gy, 30 fraction VMAT) in an oncology imaging informatics system using the Adaptive Calculation and Tracking for Offline Plan Review (ACTOR) toolkit. Each adaptive CT was created by deformable image registration (dir) of the planning CT to the daily CBCT. The structure sets from the planning CT were deformed onto each adaptive CT and ultimately corrected and revised by two qualified radiation oncologists. The adaptive CTs were transferred to a treatment planning system for recalculation. The original treatment plan was recalculated on each fraction adaptive CT to establish the real dose received during treatment. A two-sample t-test was used to compare the average real dose received dosimetric values to the values obtained on trigger-based adaptive planning. <h3>Results</h3> Using dose prescriptions for target volumes and normal tissue dose constraints as defined in RTOG 0529 (Kachnic et al 2013), normal bowel dose metrics, including bowel V45 < 20cc, were tracked as possible triggers for adaptation. Using bowel V45 > 20cc as trigger 1, there was a significant reduction in bowel V45 for each patient (table), with an average reduction in bowel V45 of 86.7% (<i>P</i> = 0.0001) across all patients. Adapting only the 5 fractions with the highest values for bowel V45 (trigger 2), we also found a significant reduction for each patient (table), with an average reduction of 20.8 % (<i>P</i> = 0.0011) across all patients. <h3>Conclusion</h3> Our study demonstrated that triggered adaptive re-planning using bowel V45 > 20cc could likely achieve reductions in normal tissue dose. Adaptive planning during radiation treatment of anal cancer may therefore be beneficial for reduction of treatment-related GI toxicity. These results can help inform a future randomized trial of adaptive therapy vs. standard planning.