Dosimetric Modeling of Intracavitary versus Combined Intracavitary/Interstitial Applicators During High Dose Rate Brachytherapy for Cervical Cancer

Abstract

With increased adoption of image guided adaptive brachytherapy (IGABT) for definitive treatment of cervical cancer, combined intracavitary/interstitial (IC/IS) brachytherapy applicators are increasingly utilized as a means of dose escalation while minimizing exposure to organs at risk (OAR) structures compared to traditional intracavitary (IC) applicators. Aside from high risk CTV volume, little data exists to guide clinicians as to relative OAR dose reductions that might be predicted for a given patient with use of combined IC/IS vs IC applicators and tumor/target volume characteristics that correlate with greatest benefit. We performed pair-wise comparisons of IGABT treatment plans modeled with IC and IC/IS applicators to determine relative dose reductions to bladder, rectum, and sigmoid. In addition, we hypothesized that CTV asymmetry would be associated with a relatively greater degree of OAR dose reduction with use of an IC/IS applicator. Treatment planning MRI or CT images for fifteen consecutive patients treated with IC or IC/IS brachytherapy applicators as part of definitive therapy for FIGO 2018 stage IB3 to IIIC cervical cancer at a tertiary care academic medical center were obtained. For each dataset, high dose rate brachytherapy treatment plans were generated for both IC and IC/IS brachytherapy applicators using a treatment planning software. When interstitial needles were not placed at the time of applicator insertion, needle/dwell positions were modeled based on ovoid position and patient anatomy. Treatment plans were generated to minimize dose to the D2 cc volume of OAR contours while reaching an equivalent dose to the D90 of the high-risk CTV. OAR (bladder, rectum, and sigmoid colon), CTV, and GTV volumes, consistent with GEC-ESTRO guidelines, were obtained from previously administered treatment plans and were not modified. As a surrogate for target volume asymmetry, the ratio of maximum antero/posterior to lateral CTV dimension (AP:L ratio), was calculated for each patient. Paired t-tests were used to identify significant differences between IC and IC/IS treatment plans with respect to OAR doses, while Spearman correlation tests were used to assess correlations between OAR sparing and CTV AP:L ratios. Mean volumes for GTV and CTV were 7.4 (1.1 to 19.4) and 29.7 (10.4 to 52.3) cubic centimeters. Mean doses to D2 cc volumes were 15.9% lower for bladder (p<0.01); 15.3% lower for rectum(p<0.01); and 10.0% lower for sigmoid (p<0.01) with IC/IS plans. The CTV AP:L ratio demonstrated a significant inverse correlation with D2cc dose reduction for bladder (p<0.05), rectum (p<0.05), and sigmoid (p<0.05). For an equivalent prescribed radiation dose, IC/IS applicators are associated with significant reductions in bladder, rectal, and sigmoid D2cc doses. CTV asymmetry, as measured by AP:L ratio, is associated with relatively greater dose reductions to bladder, rectum, and sigmoid with use of IC/IS applicators.