A Treatment Planning Method for Sequentially Combining Radiopharmaceutical Therapy and External Radiation Therapy

Purpose: Effective cancer treatment generally requires combination therapy. The combination of external beam therapy (XRT) with radiopharmaceutical therapy (RPT) requires accurate three-dimensional dose calculations to avoid toxicity and evaluate efficacy. We have developed and tested a treatment planning method, using the patient-specific three-dimensional dosimetry package 3D-RD, for sequentially combined RPT/XRT therapy designed to limit toxicity to organs at risk. Methods and Materials: The biologic effective dose (BED) was used to translate voxelized RPT absorbed dose (DRPT) values into a normalized total dose (or equivalent 2-Gy-fraction XRT absorbed dose), NTDRPT map. The BED was calculated numerically using an algorithmic approach, which enabled a more accurate calculation of BED and NTDRPT. A treatment plan from the combined Samarium-153 and external beam was designed that would deliver a tumoricidal dose while delivering no more than 50 Gy of NTDsum to the spinal cord of a patient with a paraspinal tumor. Results: The average voxel NTDRPT to tumor from RPT was 22.6 Gy (range, 1-85 Gy); the maximum spinal cord voxel NTDRPT from RPT was 6.8 Gy. The combined therapy NTDsum to tumor was 71.5 Gy (range, 40-135 Gy) for a maximum voxel spinal cord NTDsum equal to the maximum tolerated dose of 50 Gy. Conclusions: A method that enables real-time treatment planning of combined RPT-XRT has been developed. By implementing a more generalized conversion between the dose values from the two modalities and an activity-based treatment of partial volume effects, the reliability of combination therapy treatment planning has been expanded.