Dosimetry of metastases and whole body in repeated radionuclide therapy of bone metastases from prostate cancer with Rhenium-188 HEDP

1673 Objectives: The repeated radionuclide therapy of bone metastases of the prostate with Rhenium-188 HEDP has been proved to be advantageous. The aim of this study was to determine the irradiation dose in bone metastases and other compartments of the body (soft tissue, normal bone, bone marrow) that underwent multiple treatments with Re-188 HEDP. Methods: 81 patients underwent posttherapeutic whole body scans (immediately p.i., after 1.5 h, 18 h, 24 h, 42 h). Using the ROI-technique several metastases, normal bone, soft tissue and whole body were evaluated. The doses were calculated after single and repeated radionuclide therapy. So far 55 therapies at 19 patients with prostate cancer and multifocal bone metastases (2-6 therapies/patient; 14 patients with more than 2 therapies; mean applied activity/therapy: 3.2 GBq Re-188 HEDP) were analyzed. Results: The mean dose in the metastases per patient was 11.3 Gy after single injection. The cumulative dose after repeated therapy (mean 2.9) was 34.7 Gy. The intraindividual comparison of the doses of the metastases from 1st and 2nd injection gave us the following results: 6 / 13 patients showed a higher / equal dose after the 2nd therapy (no patient with lower dose). The result of the same comparison from 2nd and 3rd injection was: 2 of 7 patients showed higher or equal dose after the 3rd therapy; in 5 patients the dose was lower. The mean soft tissue dose per patient was 0.5 Gy after each injection. Conclusions: Following these results a repetitive therapy of at least 3 injections is meaningful to obtain a higher cumulative dose in the bone metastases. Further evaluations have to show, if up to 6 injections are reasonable.