Combination Therapy with Radiopharmaceutical and SBRT: Radiation Safety and Dosimetry Assessment
Abstract
Purpose
Combination radiopharmaceutical therapy (RPT) and external beam radiation therapy (EBRT) is an underexplored strategy that may improve outcomes over either modality alone; however, practical implementation requires a clear understanding of dosimetry and radiation safety. This work evaluates feasibility, staff exposure, and tumor dosimetry for sequential Lu-177 RPT followed by EBRT.
Methods
Two patients received sequential RPT and stereotactic body radiation therapy (SBRT). Patient 1 (metastatic neuroendocrine tumor) received 205 mCi Lu-177 DOTATATE, followed by MR-Linac SBRT to a maxillary sinus lesion (35 Gy in 5 fractions) initiated 28 h post-infusion. Patient 2 (mCRPC) received 191.9 mCi Lu-177 vipivotide tetraxetan (Pluvicto™), followed by CT-Linac SBRT to SpineT10 (27 Gy in 3 fractions) initiated 72 h post-infusion. Post-therapy SPECT/CT-based Lu-177 dosimetry was performed using MIM®. Exposure rates were measured at the patient surface and at 1 m prior to SBRT delivery. Radiation safety included a lead-lined waiting room, a dedicated restroom, couch draping with absorbent sheets, and focused education for nursing and radiation therapy teams.
Results
RPT absorbed dose (fx1) to the maxillary tumor and SpineT10 lesion was 0.59 Gy and 22.3 Gy, respectively. For Spine T10, EQD210 (1-fraction equivalent) was ~14.3 Gy for SBRT and ~20 Gy for RPT. Post Lu-177 DOTATATE exposure rates at 10 cm at liver level were 1.6, 1.1, 0.9 mR/h at 24, 96, 144 h. At 1 m, exposure rates were 3.0, 1.0, 0.5, 0.939 mR/h at 2, 24, 96, 144 h. For Pluvicto™, exposure at 72 h was 0.3 mR/h. Contamination surveys measured 0.02 mR/h on couch drapes and staff hands; the bite block measured 0.4 mR/h.
Conclusion
Linac-based SBRT within 72 h of Lu-177 RPT is clinically deliverable with patient-specific dosimetry and standardized radiation safety protocols. An isolated waiting area and dedicated restroom are recommended for treatments within the first 3 days post-infusion.