Voxel-Level Dosimetry for Combined Iodine 131 Radiopharmaceutical Therapy and External Beam Radiation Therapy Treatment Paradigms for Head and Neck Cancer.

PurposeTargeted radiopharmaceutical therapy (RPT) in combination with external beam radiotherapy (EBRT) shows promise as a method to increase tumor control and mitigate potential high-grade toxicities associated with re-treatment for patients with recurrent head and neck cancer. This work establishes a patient-specific dosimetry framework that combines Monte Carlo based dosimetry from the two radiation modalities at the voxel level using deformable image registration (DIR) and radiobiological constructs for patients enrolled in a phase I clinical trial combining EBRT and RPT.MethodsSerial SPECT/CT patient scans performed at approximately 24, 48, 72, and 168 hours post-injection of 577.2 MBq/m2 (15.6 mCi/m2) iodine-131 containing RPT agent called XXX 131. Clinical EBRT treatment plans were created on a treatment planning CT (TPCT) using RayStation; SPECT/CT images were deformably registered to the TPCT using the Elastix DIR module in 3D Slicer and assessed by measuring mean activity concentrations and absorbed doses. Monte Carlo EBRT dosimetry was computed using EGSnrc. RPT dosimetry was conducted using a GEANT4 based RPT dosimetry platform named XXX. Radiobiological metrics (BED, EQD2) were utilized to combine the two radiation modalities.ResultsThe DIR method provided good agreement for the activity concentrations and calculated absorbed dose in the tumor volumes for the SPECT/CT and TPCT; the maximum mean absorbed dose difference was -11.2%. Based on the RPT absorbed dose calculations, two to four EBRT fractions were removed from patients’ EBRT treatments. From the combined treatment, the absorbed dose to target volumes ranged from 57.14 – 75.02 Gy. When including partial volume corrections (PVC), the mean EQD2 to the PTV from EBRT+RPT was -3.11% to 1.40% different compared to EBRT alone.ConclusionThis work demonstrated the clinical feasibility of performing combined EBRT+RPT dosimetry on TPCTs. Dosimetry guides treatment decisions for EBRT and this work provides a bridge for the same paradigm to be implemented within the rapidly emerging clinical RPT space.