Impact of Adapted Radiotherapy Schedules on Bowel Sparing in Node-Positive Cervical Cancer.

Definitive radiation therapy (RT) for locally advanced node-positive cervical cancer confers significant toxicity to pelvic organs including the small bowel. Gross nodal disease exhibits significant shrinkage during RT, and yet conventional RT does not account for this change. We evaluated the reduction in absorbed bowel dose using various adaptive RT schedules. We obtained 130 evaluable scans (computed tomography simulation and 25 cone beam computed tomography scans per patient) of 5 patients who had received definitive external beam RT for lymph node positive cervical cancer daily over 5 weeks. Using a single universal volumetric modulated arc therapy plan with predefined optimization priorities, we created adapted RT plans in 4 schedules: Daily, Weekly, Twice, and NoAdapt (mimicking conventional nonadapted RT). The in silico (computer modeled) patients were treated to 45 Gy to primary cervical disease with a simultaneous integrated boost to 55 Gy to involved lymph nodes. We evaluated dose metrics including D2cc, D15cc, and V45 to determine the impact of adapted RT schedules on bowel sparing. Statistical tests included the Student t test, analysis of variance, and the Spearman 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 with a nonadapted approach. A minimally adapted strategy of only 2 replans also confers a significant dosimetric benefit over a nonadapted 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 nonadapted technique, with greater sparing seen with more frequent replanning schedules. These findings warrant future trials of adaptive RT for pelvic malignancies. [ABSTRACT FROM AUTHOR]