Discordance in 68Ga-DOTANOC and 177Lu-DOTATATE uptake in diagnostic and post-therapy scans in patients with medullary thyroid cancer

Objectives The aim of this study was to evaluate discordance between pre therapy 68Ga-DOTANOC scans and post therapy 177Lu-DOTATATE scans in patients with Medullary Thyroid Cancer (MTC).

Methods Fifteen MTC patients (M=13; F=2, Range 22-74 yrs) with a positive 68Ga-DOTANOC PET-CT scan were recruited for 177Lu-DOTATATE therapy. 150 mCi of 177Lu-DOTATATE was administered as first therapeutic dose to each patient. Cumulative dose of 600 to 800 mCi of 177Lu-DOTATATE was infused to each post therapy scan positive patients over 5 to 6 cycles (10 to 12 weeks of interval between each cycle).

Results Eight of fifteen (53.3%) patients showed no/poor uptake of 177Lu-DOTATATE while seven (46.7%) of them showed good uptake of 177Lu-DOTATATE and were followed up for next therapy. Mean serum calcitonin was 7136 pg/ml (Range 245 to 29406 pg/ml). Mean percentage reduction in Serum calcitonin of seven DOTATATE positive patients was 36.93 at 3 months after therapy. Of the 8 post therapy scan negative patients only one patient showed fall in serum calcitonin level while others showed a rise at 3 months after therapy. Mean duration between the two scans in the patients with positive post therapy scan was 131 days while that in the negative group was 132 days.

Conclusions Different receptor affinity profile of DOTA-NOC and DOTA-TATE, could be reason for non concentration of 177Lu-DOTATATE in MTC. These findings point towards selective somatostatin receptor type 3 and 5 expression rather than type 2 in a subset of MTC patients. Another speculation, although unlikely could be the occurrence of stunning due to the pre- therapy scan. This issue of discordance may be resolved by using same ligands in the diagnostic scan and therapeutic scan