Lesion dose in differentiated thyroid carcinoma metastases after rhTSH or thyroid hormone withdrawal: 124I PET/CT dosimetric comparisons.

PURPOSE: Renal radioiodine excretion is ~50% faster during euthyroidism versus hypothyroidism. We therefore sought to assess lesion dose/GBq of administered 131I activity (LDpA) in iodine-avid metastases (IAM) of differentiated thyroid carcinoma (DTC) in athyreotic patients after recombinant human thyroid-stimulating hormone (rhTSH) versus after thyroid hormone withdrawal (THW).

METHODS: We retrospectively compared mean LDpA between groups of consecutive patients (N=63) receiving 124I positron emission tomography/computed tomography (124I PET/CT) aided by rhTSH (n=27) or THW (n=36); we prospectively compared LDpA after these stimulation methods within another individual. Data derived from serial PET scans and one CT scan performed 2-96 h post-124I ingestion. A mixed model analysis of covariance (ANCOVA) calculated the treatment groups' mean LDpAs adjusting for statistically significant baseline intergroup differences: non-IAM were more prevalent, median IAM count/patient lower in cervical lymph nodes and higher in distant sites, median stimulated thyroglobulin higher, mean cumulative radioiodine activity greater and prior diagnostic scintigraphy more frequent in the rhTSH patients.

RESULTS: Mean LDpAs were: rhTSH group (n=71 IAM), 30.6 Gy/GBq; THW group (n=66 IAM), 51.8 Gy/GBq. The difference in group means (rhTSH less THW), -21.2 Gy/GBq, was statistically non-significant (p=0.1667). However, the 95% confidence interval of that difference (-51.4 to  +9 Gy/GBq) suggested a trend favouring THW. The within-patient comparison found 2.9- to 10-fold higher LDpAs under THW.

CONCLUSION: We found some suggestions, but no statistically significant evidence, that rhTSH administration results in a lower radiation dose to DTC metastases than does THW. A large, well-controlled, prospective within-patient study should resolve this issue.