A retrospective study on the potential of 99mTc‐HDP imaging before therapy for individualizing treatments with 223Ra‐Cl2 for metastatic castration resistant prostate cancer

Purpose Research on dose‐effect correlation is necessary to move toward an individualization of treatments of metastatic castration resistant prostate cancer (mCRPC) with 223Ra‐Cl2. We first looked for a possible correlation of 99mTc‐HDP lesion uptake in pretreatment whole‐body scans (WBSs) with lesion absorbed dose. Moreover, we looked for a possible correlation of 99mTc‐HDP lesion uptake in pretreatment WBSs and of lesion absorbed dose with relative change in the 99mTc‐HDP lesion uptake obtained from pre‐ and post‐treatment WBSs in patients treated for mCRPC with six cycles of 223Ra‐Cl2. Methods Eleven patients received six cycles of 55 kBq/kg of 223Ra‐Cl2 separated by 4 weeks. In addition, one patient received concomitant treatment with abiraterone and two patients with enzalutamide. The 99mTc‐HDP WBSs were acquired before the first cycle and after the sixth cycle of the treatment. For the lesions with the higher 99mTc‐HDP uptake, the absorbed dose was calculated for the first cycle. Lesion volume was determined from 99mTc‐HDP SPECT/CT images before the first cycle and 223Ra‐Cl2 activity in the lesions was determined from 223Ra‐Cl2 planar images after the first cycle. The effect of the treatment was evaluated from the relative change of the mean and the maximum counts in the lesions, both estimated from the WBSs acquired before the first cycle and after the sixth cycle. Results The absorbed dose was calculated for 30 lesions, with values ranging between 0.4 and 3.8 Gy (mean 1.5 Gy). A significant (P