Dosimetric evaluation of adaptive planning for five-fraction gynecologic template-based interstitial brachytherapy

Abstract

Purpose: The purpose of this work was to evaluate whether inter-fraction imaging and replanning enhance treatment delivery adherence to clinical planning objectives in the context of a 5-fraction template-based interstitial brachytherapy (TISB) approach for gynecologic cancer treatment. Methods and Materials: This retrospective study analyzed nineteen patients who underwent five fractions of interstitial brachytherapy over three days using the Syed-Neblett template. A verification CT scan was acquired for applicator assessment and reviewed by a radiation oncologist and medical physicist before each fraction. Eleven patients required replanning at least once during the treatment course. Replanning on the verification CT scan consisted of generating new target and organ-at-risk contours, digitizing catheter positions, and optimizing source dwell times to meet planning objectives. Dwell times and positions from the initial treatment plan were evaluated on the new contours to assess the dose that would have been delivered without replanning (non-adapted). Significance of non-adapted versus adapted dose differences were evaluated using a two-sided Wilcoxon sum rank test. Results: The average (min, max) change in dose (Gy) between the clinically delivered plans and the non-adapted plans were HR-CTV D90%: -6.5 (-0.6, -15.1), HR-CTV D98%: -6.5 (-0.4, -12.6), Bladder D2cc: -0.5 (0.0, -2.8), Bowel D2cc: -0.8 (0.0, -3.2), Rectum D2cc: -1.1 (0.0, -11.5), Sigmoid D2cc: -1.4 (-0.1, -5.4). Dosimetric changes in HR-CTV coverage were significantly improved with replanning while organ-at-risk differences were nonsignificant (p>0.05). Fraction 3 was the most common fraction indicated for replanning. Conclusions: Replanning template-based interstitial brachytherapy can improve target coverage and adherence to planning goals.