(99m)Tc(V)DMSA quantitatively predicts (188)Re(V)DMSA distribution in patients with prostate cancer metastatic to bone.

Rhenium-188 dimercaptosuccinic acid complex [(188)Re(V)DMSA], a potential therapeutic analogue of the tumour imaging agent (99m)Tc(V)DMSA, is selectively taken up in bone metastases in patients with prostate cancer. It would be helpful in planning palliative radionuclide therapy if (99m)Tc(V)DMSA could be used to predict tumour and kidney retention of (188)Re(V)DMSA. The aim of this study was to determine the correlation between tumour-to-normal tissue ratios and kidney-to-soft tissue ratios of (99m)Tc(V)DMSA and (188)Re(V)DMSA. This would determine whether a scan with (99m)Tc(V)DMSA could be used to identify patients for whom (188)Re(V)DMSA treatment would be contra-indicated, and enable prediction of relative kidney and tumour radiation absorbed dose in (188)Re(V)DMSA treatment. Ten patients with prostate carcinoma were recruited following observation of disseminated bone metastases on a recent (99m)Tc-hydroxydiphosphonate bone scan. Whole-body planar scans were obtained at ca. 4 h and 24 h after hydration and injection of 600 MBq (99m)Tc(V)DMSA, and a week later, at similar times after hydration and injection of 370 MBq (188)Re(V)DMSA. A triple-energy window (TEW) scatter correction was applied to the (188)Re scans. Counts per pixel were determined in regions of interest drawn over metastatic sites, kidneys and normal soft tissue. Tumour-to-soft tissue ratios were significantly lower (by a factor of approximately 0.8 after the TEW was applied) on (188)Re scans than on (99m)Tc scans, but the two were highly linearly correlated both in all individual patients and in tumours pooled from all patients together both at 4 h and at 24 h. Kidney-to-soft tissue ratios were similarly correlated and were lower for (188)Re than for (99m)Tc by a similar factor. Both tumour- and kidney-to-soft tissue ratios increased between 4 and 24 h but the latter increased more. In conclusion, only minor differences were seen between (99m)Tc and (188)Re scans, and kidney-to-background ratios on (188)Re scans were not higher than on (99m)Tc scans. These differences are insufficient to infer that they are due to a real difference in biodistribution, and they may be due only to different physical imaging characteristics. Thus (99m)Tc(V)DMSA scans are predictive of (188)Re(V)DMSA biodistribution and could be used to estimate tumour and renal dosimetry and assess suitability of patients for (188)Re(V)DMSA treatment.