High Risk CTV Asymmetry Predicts Dosimetric Benefit with Use of Combined Intracavitary/Interstitial Brachytherapy Applicators for Cervical Cancer

Abstract

<h3>Purpose/Objective(s)</h3> Compared to intracavitary (IC) brachytherapy applicators, combined intracavitary/interstitial (IC/IS) applicators provide greater dose to the High-risk CTV (CTV_HR) with similar doses to organ at risk (OAR) structures. Despite increasing use, there is little data to guide clinicians as to which patients are most likely to benefit from use of IC/IS applicators. Recent data from retroEMBRACE suggests CTV_HR volume greater than 30 cm³(cc) is associated with increased likelihood of benefit. In previous work, we provided initial evidence that asymmetry of the CTV_HR may correlate with benefit to use of IC/IS applicators. Here, we provide an expanded analysis, hypothesizing that the degree of asymmetry of the CTV_HR with respect to location of the tandem rod may better correlate with benefit than CTV_HR volume. To test this hypothesis, we performed a dosimetric study of image guided adaptive brachytherapy treatment plans modeled with IC versus IC/IS applicators. <h3>Materials/Methods</h3> Treatment planning MRI or CT datasets for thirty patients treated with IC or IC/IS brachytherapy applicators as part of definitive therapy for FIGO 2018 stage IB3 to IIIC cervical cancer were acquired. IC and IC/IS HDR brachytherapy treatment plans were generated for each dataset using a radiotherapy treatment planning system software from a precision radiation medicine company. For patients treated with IC applicators, needles were virtually digitized and dwell positions modeled based on ovoid location to create IC/IS plans. For patients treated with IC/IS applicators, needle dwell positions were removed and plans re-optimized based on tandem and ovoid dwell positions only. All plans were optimized to minimize dose to the D_2cc volume of OARs while reaching an equivalent dose to the D₉₀ of the CTV_HR. OAR (bladder, rectum, and sigmoid colon), and CTV_HR volumes were not modified from the original administered treatments. The maximum distance (d_max) between the tandem rod and distal edge of the CTV_HR in the axial plan at the level of greatest CTV_HR asymmetry, as determined visually by the author (DS), was obtained for each dataset. Conformity index (CI) was defined as the ratio of the prescription isodose volume to the CTV_HR, and was computed for each plan. Spearman correlation coefficients were used to compare the CI improvement (%CI) in IC/IS versus IC plans, OAR dose reductions, and to measure the correlation between the %CI improvement and the asymmetry surrogate d_max. <h3>Results</h3> Mean %CI improvement in IC/IS plans compared to IC plans was 20.6%. The %CI improvement in IC/IS plans demonstrated a significant correlation with D_2cc dose reduction for bladder (p<0.01), rectum (p<0.01), and sigmoid (p<0.01). The %CI improvement in IC/IS plans demonstrated a significant correlation with d_max (p<0.01). We found no correlation between %CI improvement and volume of the CTV_HR. <h3>Conclusion</h3> During cervical cancer brachytherapy, greater CTV_HR asymmetry, as measured by d_max, correlates with likelihood of benefit from use of IC/IS versus IC brachytherapy applicators.