SU‐E‐T‐654: Calculation of Cumulative Biological Effective Dose Distributions for Multimodality Treatment Regimes

Abstract

Purpose: Evidence has recently been reported supporting the role of stereotactic body radiotherapy (SBRT) as a supplemental treatment option in the form of a boost or salvage reirradiation following conventional radiotherapy. It may therefore be desirable to augment the process of physical dose summation in the cumulative plan evaluation process by constructing corresponding biological effective dose (BED) distributions that incorporate the biological effects of the different dose fractionation schemes required for multimodality treatments. To that end, we developed a computational process for calculating cumulative BED distributions for patients receiving conventional radiotherapy followed by SBRT. Methods: We developed a program designed to accept two DICOM dose matrices, as calculated in standard treatment planning systems, and calculate the BED at each matrix element obeying the dose per fraction dependence in the BED formalism. It then calculates the cumulative BED distribution for multimodality treatment regimes by matrix summation and outputs the result in DICOM format. Two clinical head and neck IMRT cases (large field (2.13Gy × 33) and small field (2Gy × 32)) were evaluated as part of a pilot study to demonstrate the overall feasibility of the calculation scheme. Following clinical IMRT planning, each case was planned in a mock SBRT scenario (6Gy × 5) where a smaller planning volume was drawn to simulate a boost or salvage reirradiation. The clinical IMRT and mock SBRT dose matrices were then processed to generate cumulative BED distributions. Results: Our computational process was successful in producing cumulative BED distributions. For the large field IMRT case, the overall BED distribution beyond the initial treated volume was dominated by the initial IMRT course, whereas clear additive increase in the BED distribution beyond the initial treated volume was evident in the small field case. Conclusions: Cumulative BED distributions may reveal useful clinical information during plan evaluation.