IMPACT OF ADAPTED RADIOTHERAPY SCHEDULES ON BOWEL SPARING IN NODE-POSITIVE CERVICAL CANCER

Definitive radiotherapy for locally-advanced node-positive cervical cancer confers significant toxicity to pelvic organs including small bowel. Gross nodal disease exhibits significant shrinkage during radiotherapy (RT), and yet conventional RT does not account for this change. We evaluated the reduction in absorbed bowel dose using various adaptive radiotherapy (ART) schedules.We obtained 130 evaluable scans (computed tomography (CT) simulation and 25 cone beam CT scans per patient) of 5 patients who had received definitive external beam RT for lymph node positive cervical cancer over 5 weeks daily. Using a single universal volumetric modulated arc therapy (VMAT) plan with predefined optimization priorities, we created adapted RT plans in 4 schedules: Daily, Weekly, Twice, and NoAdapt. The in silico (computer modeled) patients were treated to 45 Gy to primary cervical disease with a simultaneous integrated boost (SIB) to involved lymph nodes to 55 Gy. We evaluated dose metrics including D2cc, D15cc, and V45 to determine the impact of adapted RT schedules on bowel sparing. Statistical tests included Student's t, ANOVA, and Spearman's rank correlation.The quantity of reduced bowel dose was significantly associated with the chosen planning schedule in all evaluated metrics, and was proportional to the frequency of adaptive RT with significant moderate-to-strong monotonicity. Both D2cc and D15cc were reduced an average of 2.7 Gy using Daily replanning compared to a non-adapted approach. A minimally-adapted strategy of only two replans also confers a significant dosimetric benefit over a non-adapted approach. Reduced standard deviations of D2cc and V45 bowel doses over the treatment courses were significantly associated with the choice of planning schedule with strong monotonicity.All adaptive RT schedules evaluated confer significant dosimetric advantages in bowel sparing over a conventional non-adapted technique, with greater sparing seen with more frequent replanning schedules. These findings warrant future trials of adaptive radiotherapy for pelvic malignancies.