Tumor dosimetry on SPECT (186)Re-HEDP scans: variations in the results from the reconstruction methods used.

The aim of this work was to estimate tumor-absorbed doses delivered from the administration of fixed activities of (186)Re-HEDP for the treatment of bone metastases from prostate cancer. The variations and reproducibility in the estimated absorbed dose owing to the reconstruction algorithm used (OSEM vs. FBP) were also analysed. For this aim, correction methods for scatter and attenuation were kept identical, whereas the same calibration data and thresholding techniques were used to obtain quantification. This study was carried out in spinal and pelvic lesions of 7 patients. For comparison, the absorbed doses, based upon the maximum and the mean voxel count, were calculated, resulting in the absorbed dose (maximum): 60 Gy (sigma = 30 Gy) and 33 Gy (sigma = 15 Gy) for OSEM and FBP, respectively, and absorbed dose (mean): 26 Gy (sigma = 12 Gy) and 17 Gy (sigma = 7 Gy) with OSEM and FBP, respectively. We concluded that the administration of fixed activity resulted in a range of absorbed doses, and we showed that, despite using the same approach, the choice of the reconstruction algorithm can result in differences higher than 50% in the estimated tumor-absorbed doses. In conclusion, the need for a standardization of the methodology used for the calculations is emphasized by this work, especially when comparisons between patients and different centers are of interest.