Room: AAPM ePoster Library
Purpose: investigate the dose reduction to organs at risk (OARs) and the improvement of conformity index (CI) in HDR treatments with intracavitary versus combined intracavitary/interstitial (IC/IS) applicators. To identify the correlation of dose to OARs with CI.
Methods: MRI or CT images for fifteen patients treated with IC or IC/IS brachytherapy applicators were obtained. For each patient, one structure set (GTV, HR-CTV and OARs) was delineated and two treatment planning scenarios were modeled for both IC and IC/IS applicators using Oncentra Brachy v.4.5.3. Each IC and IC/IS treatment plan was optimized to reach an equivalent dose to the D90 of HR-CTV while consistent with GEC-ESTRO guidelines for D2cc to OAR (bladder, rectum, and sigmoid). CI, defined as the quotient of treatment volume receiving 90% or more of the prescribed radiation dose, and the HR-CTV volume was computed for each treatment plan. Paired t-tests were used to identify significant differences between IC and IC/IS treatment plans with respect to OAR doses and CI, while Spearman correlation coefficients were used to assess correlations between OAR sparing and CI.
Results: volumes of HR-CTV were 29.7 (range 10.4 to 52.3) cc. Mean doses to D2cc 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. Mean CI for IC and IC/IS plans were 2.95 (range 1.90 to 4.5) and 2.44 (range 1.9 to 3.3) respectively and demonstrated significant improvement for IC/IS plans (p<0.05). CI improvement demonstrated inverse correlation with D2cc dose reduction for bladder (p<0.05), rectum (p<0.05), and sigmoid (p<0.05).
Conclusion: an equivalent prescribed radiation dose, IC/IS applicators are associated with significant reductions in D2cc doses to rectum, sigmoid, and bladder. Significantly improved CI in IC/IS plans is associated with greater dose reductions to bladder, rectum, and sigmoid with use of IC/IS applicators.