Total Voxel-Wise BED and EQD2 Calculation for Combined External Radiotherapy and Brachytherapy Treatments
Abstract
Purpose
In combined external beam radiation therapy (EBRT) and brachytherapy treatments, the ICRU 89 and GEC-ESTRO guidelines establish the use of EQD2 for radiobiological assessment. However, clinical verification is currently often restricted to volumetric parameters (D2cc or D90) and the analysis of point doses, which can overestimate the dose to organs at risk (OARs) by omitting the spatial distribution of energy deposition. This work aims to develop a tool to calculate full voxel-level BED and EQD2 matrices for subsequent analysis within a Treatment Planning System (TPS).
Methods
EBRT and brachytherapy plans were generated in Eclipse TPS v18.1 using a pelvic phantom, segmenting the rectum, bladder, and a PTV. DICOM files (CT, structures, and dose matrices) were exported. A Python-based script was designed to calculate BED and EQD2 matrices using the linear-quadratic (LQ) model, assuming alpha/beta ratios of 10 for tumor tissue and 3 for OARs. Finally, the resulting matrices were imported back into the TPS for evaluation.
Results
The script is capable of generating BED and EQD2 distributions. Data integrity was verified through DVH analysis generated by the TPS from the imported distributions. This workflow allowed for the first integrated visualization of the spatial distribution of the total biological dose, overcoming the limitations of simple arithmetic summation of physical doses.
Conclusion
The efficacy of the script in transforming physical dose matrices into radiobiological ones was validated. This advancement facilitates the spatial summation of EQD2 distributions, providing a superior clinical verification tool that optimizes decision-making based on actual biological tissue tolerance and tumor control. Elastic deformation of the dose matrix will be performed to achieve a more accurate three-dimensional calculation