Biologically Equivalent Dose Summation Considering Organ-Specific Radiosensitivity for Reirradiation of Thoracic Cancer Patients Treated with Stereotactic Body Radiation Therapy
Abstract
Purpose
Dose-volume constraints for organs at risk (OARs) in stereotactic body radiation therapy (SBRT) for thoracic cancer patients have been well established through multiple prospective clinical trials. However, during equivalent dose in 2 Gy fractions (EQD2) summation for reirradiation, inconsistencies between physical dose and EQD2-based constraints have been observed for certain OARs when a nominal α/β =3 Gy is used in linear–quadratic (LQ) model–based calculations. We developed an EQD2 summation workflow in MIM that incorporates organ-specific α/β ratios.
Methods
Organ-specific α/β ratios were derived from published dose–volume constraint data used in thoracic treatment planning. A workflow was developed in MIM to deformably map prior physical doses onto retreatment CTs , convert physical doses to EQD2 using the LQ model with organ-specific α/β ratios, and accumulate them to generate a composite dose distribution for plan evaluation . Each dose voxel was assigned an α/β value based on the OAR contour in which it resided. To address resolution mismatches between the dose and contour grids —particularly at structure boundaries where a single dose voxel may intersect multiple contours—a custom Java extension was implemented in MIM to resample the dose grid to the contour resolution using trilinear interpolation, ensuring a one-to-one correspondence between dose and contour voxels. The workflow was evaluated in seven patients undergoing EQD2 summation involving SBRT for thoracic cancer.
Results
Organ-specific α/β ratios were successfully obtained for thoracic OARs and ranged from 2.1-28.1 Gy. The developed MIM workflow eliminated α/β misassignment caused by voxel sharing across adjacent contours. Compared with EQD2 calculations using α/β =3 Gy for all OARs, the proposed workflow produced more consistent and clinically interpretable EQD2 results across all fractionations evaluated.
Conclusion
Organ-specific variations in radiosensitivity should be incorporated into EQD2 conversions and summations to reduce inconsistencies and improve clinical interpretation of composite plans for reirradiation.