Author: Davide Brivio, Ivan M. Buzurovic, Thomas C. Harris, Desmond A. O'Farrell 👨🔬
Affiliation: Department of Radiation Oncology, Brigham and Women’s Hospital, Dana-Farber Cancer Institute, Harvard Medical School, Brigham and Women's Hospital, Harvard Medial School, Dana-Farber Cancer Institute, Department of Radiation Oncology 🌍
Purpose: Cervical cancer can be treated with a brachytherapy boost following external beam radiotherapy. One applicator option is the Advanced Gynecological Applicator "Venezia" (Elekta) hybrid, consisting of a tandem and two lunar ovoids with channels for interstitial needles and capacity for an optional interstitial template. The Venezia is comprised of polysulfone (1.29g/cc) and the applicator cavities may introduce air gaps. Plans are traditionally calculated using T-43 formalism, which assumes all material is water. This study investigates the effect of these heterogeneities on dose distributions.
Methods: 12 previously treated cervical brachytherapy plans were evaluated, 11 of which used MRI for segmentation. Physicians contoured the residual GTV (GTVres), high-risk CTV (HR-CTV), intermediate-risk CTV (IR-CTV), bladder, rectum, sigmoid, and bowel. Treatment planning was performed using Oncentra Brachy 4.5.3. The plans were recalculated using Oncentra’s Advanced Collapsed-cone Engine (ACE) algorithm based on TG-186’s model-based dose calculation algorithm recommendations. The applicator, organs, soft tissue, and air pockets were assigned appropriate densities, and the plans were recalculated with unchanged dwell times. A paired Wilcoxon signed-rank test compared dose differences between the two calculations.
Results: GTVres D98%, HR-CTV D90%, IR-CTV D98%, and OAR D2cc were evaluated. Comparing the target volumes in ACE to TG-43, there was no statistically significant change in reported dose. For the OAR D2cc’s, rectum dose was 2.6% lower (p=0.002) and bowel dose was 2.1% lower (p=0.035) with ACE. Although not commonly used for clinical evaluation, the OAR D0.1cc doses demonstrated significant and larger magnitude differences.
Conclusion: 12 cervical brachytherapy plans using the Venezia applicator were recalculated using TG-186 formalism. Calculations using the TG-43 model overestimated doses to rectum and bowel but were statistically unchanged for other clinically relevant volumes. These results suggest that converting to TG-186 for treatment planning utilizing the Venezia applicator may be achieved without major modifications to current planning methods.