A retrospective study on the potential of Tc-HDP imaging before therapy for individualizing treatments with Ra-Cl 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 Ra-Cl . We first looked for a possible correlation of Tc-HDP lesion uptake in pretreatment whole-body scans (WBSs) with lesion absorbed dose. Moreover, we looked for a possible correlation of Tc-HDP lesion uptake in pretreatment WBSs and of lesion absorbed dose with relative change in the Tc-HDP lesion uptake obtained from pre- and post-treatment WBSs in patients treated for mCRPC with six cycles of Ra-Cl .

Methods: Eleven patients received six cycles of 55 kBq/kg of Ra-Cl separated by 4 weeks. In addition, one patient received concomitant treatment with abiraterone and two patients with enzalutamide. The Tc-HDP WBSs were acquired before the first cycle and after the sixth cycle of the treatment. For the lesions with the higher Tc-HDP uptake, the absorbed dose was calculated for the first cycle. Lesion volume was determined from Tc-HDP SPECT/CT images before the first cycle and Ra-Cl activity in the lesions was determined from Ra-Cl 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 < 0.05) high positive linear correlation was found between the lesion absorbed dose in the first treatment cycle and the mean and maximum counts in the lesions in the WBSs acquired before the first cycle (R = 0.75 and 0.76, respectively). The relative change of the mean and the maximum counts in the lesions in the Tc-HDP WBSs showed a significant (P < 0.05) high positive logarithmic correlation with the Tc-HDP mean and maximum counts in the lesions before the first cycle (R = 0.79 and 0.78, respectively). Lastly, a significant (P < 0.05) high positive logarithmic correlation was also found between the relative change of the mean and the maximum counts in the lesions in the Tc-HDP WBSs and the lesion absorbed dose (R = 0.86 and 0.85, respectively). For this correlation the influence of the administered activity and of the concomitant treatments was not found to be significant (P > 0.05).

Conclusions: The high correlations found for the Tc-HDP lesion uptake before the first cycle lesion with the relative change in the Tc-HDP lesion uptake after the six cycles of Ra-Cl , and with the lesion absorbed dose in the first cycle show the potential of pretreatment Tc-HDP imaging in order to personalize the performance of these treatments.