A case report of image-based dosimetry of bone metastases with Alpharadin (223Ra-dichloride) therapy: inter-fraction variability of absorbed dose and follow-up

A 70-year-old man affected by bone metastases from castration resistant prostate cancer underwent Alpharadin ( 223 Ra-dichloride) therapy (6 administrations of 50 kBq per kg i.v., once every 4 weeks). The inter-fraction variability of the absorbed dose to lesions was evaluated for four injections. Dosimetric assessments were performed following the MIRD approach and a recently published methodology. The mean absorbed dose and standard deviation for 4 lesions [mean ( σ %)] were: 434 mGy (15 %) and 516 mGy (21 %) for the right and left humeral head, 1205 mGy (14 %) and 781 mGy (8 %) for the right and left glenoid. The estimated total absorbed dose after the whole treatment, considering also the relative-biological effectiveness of alpha particles (RBE = 5), yielded a D RBE range of 13–36 Gy. A good correlation between 99m Tc and 223 Ra uptake was obtained ( R 2 = 0.7613). The tumour–non-tumour (TNT) ratio of 8 lesions (those above, plus 4 additional), monitored by six 99m Tc-MDP bone scans over a period of about 10 months, evidenced a TNT reduction in two lesions (−42 and −48 %), but in most lesions the TNT remained fairly constant, evidencing that 223 Ra-dichloride therapy tends to prevent further progression of osseous disease, leading to chronicity of the metastatic status.