SU‐E‐J‐88: Dose Summation Between Multimodality Treatments for Cervical Cancer

Abstract

Purpose: 1) To explore the feasibility of volumetric dose summation between external beam radiation (EBRT) and high dose rate (HDR) brachytherapy treatments for cervical cancer, and 2) to investigate the differences between two deformable registration platforms: MIM Software (M) and VelocityAI (V). Methods: Five patients treated with combined EBRT (45 Gy) and HDR (5 x 5.5 Gy/FX, T&R) were selected in this study. The doses were converted to EQD2 (α/β of 10 for early and 3 for late effects). The HDR CT sets were registered using rigid (r), based on applicator, and deformable (d) registration. To quantify geometrical similarities between the deformed secondary and the original primary structures, the Dice Similarity Coefficient (DSC) was calculated for HRCTV (for HDR sums only), bladder, and rectum. Each HDR fractional dose was resampled to a primary set. The registration between EBRT and HDR was performed using the HDR primary CT. The metrics HRCTV D90, and bladder/rectum D2cc were extracted from total dose DVHs and compared to the current clinical point dose summation protocol. Results: The average DSC for HRCTV, bladder and rectum was 0.72, 0.71, and 0.56 for MIM and 0.71, 0.70, and 0.51 for Velocity. Volumetric dose summation between the two modalities lead to differences from the point summation of 2.1±1.6% (r, M), 3.2±2.0% (r, V), 2.6±1.9% (d, M), and 3.9±1.5% (d, V) for HRCTV D90, 8.9±6.7% (r, M), 9.9±6.5% (r, V), 10.2± 6.2 (d, M), and 9.4± 4.5% (d, V) for bladder, and 5.2±3.4% (r, M), 5.0± 1.1%(r, V), 5.2± 3.3% (d, M), and 5.3± 3.8% (d, V) for rectum. Conclusion: The two algorithms produced similar results, with expected better DSC for HRCTV and bladder than for rectum. With understanding of the limitations of current deformable registration algorithms, 3D dose summation can be accomplished and composite dose estimates can be improved.