Whole-remnant and maximum-voxel SPECT/CT dosimetry in 131 I-NaI treatments of differentiated thyroid cancer

Purpose To investigate the possible differences between SPECT/CT based whole‐remnant and maximum‐voxel dosimetry in patients receiving radio‐iodine ablation treatment of differentiated thyroid cancer (DTC). Methods Eighteen DTC patients were administered 1.11 GBq of 131 I‐NaI after near‐total thyroidectomy and rhTSH stimulation. Two patients had two remnants, so in total dosimetry was performed for 20 sites. Three SPECT/CT scans were performed for each patient at 1, 2, and 3–7 days after administration. The activity, the remnant mass, and the maximum‐voxel activity were determined from these images and from a recovery‐coefficient curve derived from experimental phantom measurements. The cumulated activity was estimated using trapezoidal‐exponential integration. Finally, the absorbed dose was calculated using S ‐values for unit‐density spheres in whole‐remnant dosimetry and S ‐values for voxels in maximum‐voxel dosimetry. Results The mean absorbed dose obtained from whole‐remnant dosimetry was 40 Gy (range 2–176 Gy) and from maximum‐voxel dosimetry 34 Gy (range 2–145 Gy). For any given patient, the activity concentrations for each of the three time‐points were approximately the same for the two methods. The effective half‐lives varied ( R = 0.865), mainly due to discrepancies in estimation of the longer effective half‐lives. On average, absorbed doses obtained from whole‐remnant dosimetry were 1.2 ± 0.2 (1 SD) higher than for maximum‐voxel dosimetry, mainly due to differences in the S ‐values. The method‐related differences were however small in comparison to the wide range of absorbed doses obtained in patients. Conclusions Simple and consistent procedures for SPECT/CT based whole‐volume and maximum‐voxel dosimetry have been described, both based on experimentally determined recovery coefficients. Generally the results from the two approaches are consistent, although there is a small, systematic difference in the absorbed dose due to differences in the S ‐values, and some variability due to differences in the estimated effective half‐lives, especially when the effective half‐life is long. Irrespective of the method used, the patient absorbed doses obtained span over two orders of magnitude.