Endocrine Abstracts

Background: PET/CT scan with ¹⁸F-fluoro-dihydroxyphenylalanine (¹⁸F–DOPA) is an emerging useful tool in medullary thyroid cancer (MTC) patients with high calcitonin (CTN) values. The aim of this study is to evaluate the potential usefulness of ¹⁸F-DOPA-PET/CT in MTC patients with biochemical incomplete response (BiR) (detectable CTN values and negative/indeterminate findings at CT scan with i.v. contrast).

Materials and Methods: Fifty-three MTC patients with BiR performed ¹⁸F-DOPA-PET/CT scan between May 2021 and December 2022 after having performed CT scan with i.v. contrast.

Results: Most of patients were females (60.4%). The median age at the diagnosis was 47 years. Sixteen (30.2%) patients were positive for RET germline mutation. At the time of ¹⁸F-DOPA-PET/CT [median 37 months (IQR: 102-196.5) from diagnosis], the median serum CTN value was 399 (165-1135) pg/ml. ¹⁸F-DOPA-PET/CT was negative in 15 (28.3%) cases, while an uptake of the radiotracer was detected in 38 (71.7%). Conversely, CT scan was negative in 21 (39.6%) while in the remaining 32 (60.4%) patients, indeterminate findings were highlighted. When comparing the two imaging methods, both were negative in 11 (20.8%) cases. In the remaining 10 (18.9%) cases of negative CT scan; ¹⁸F-DOPA-PET/CT showed pathologic uptake mainly in neck. However, in 2 patients the uptake was in liver. Among 32 patients with indeterminate findings at CT scan, 28 showed an uptake at ¹⁸F-DOPA-PET/CT while 4 did not. When we analyzed the concordance between the two methods (uptake of ¹⁸F-DOPA in the same sites of indeterminate lesions described at CT scan), 9 patients showed a concordance while 4 patients a discordance (uptake of ¹⁸F-DOPA in other sites than those described at CT scan). The other 15 patients showed a partial concordance, most of whom (9/15 – 60%) had more indeterminate lesions at CT scan than ¹⁸F-DOPA uptake.

Conclusions: In most of the cases of MTC patients with BiR and negative or indeterminate findings at CT scan; ¹⁸F-DOPA-PET/CT scan showed radiotracer-enhancing lesions. Therefore, in this setting; ¹⁸F-DOPA-PET/CT can be useful to clarify and define as metastases those lesions with indeterminate features at CT scan. However, although these findings are relevant and useful to better follow-up the disease, the clinical management rarely has been changed.