Endocrine Abstracts

Objectives: PET/CT scan with ¹⁸F-fluoro-dihydroxyphenylalanine (¹⁸F-DOPA) is an emerging useful tool in medullary thyroid cancer (MTC) patients. The question whether ¹⁸F-DOPA-PET/CT could be useful in MTC patients with biochemical incomplete response (BiR) is to clarify.

Methods: Fifty-three MTC patients with BiR performed ¹⁸F-DOPA-PET/CT scan between May 2021 and December 2022 after having performed total body CT scan with i.v. contrast, all with negative or indeterminate findings.

Results: Most of patients were females (60.4%). The median age at the diagnosis was 47 years. Sixteen (30.2%) patients had familial form of MTC. When ¹⁸F-DOPA-PET/CT was performed [median 37 months (IQR: 102-196.5) from diagnosis], the median serum calcitonin (CTN) value was 399 (165-1135) pg/mL. ¹⁸F-DOPA-PET/CT result was negative in 15/53 (28.3%) cases. Conversely an uptake of the radiotracer was observed in 38/53 (71.7%) cases. CT scan was negative in 20/53 (37.7%) while in the remaining 33 (62.3%) patients, indeterminate findings were underlined. When comparing the two imaging procedures, they were both negative in 11 (20.8%) cases. In 9 (16.9%) cases of negative CT scan, ¹⁸F-DOPA-PET/CT was positive in the neck and in 2 case in the liver. Among 33 patients with indeterminate findings at CT scan, 29 showed uptake at ¹⁸F-DOPA-PET/CT (neck in 25, mediastinal lymph node in 5, bone in 4, liver in 4 and 2 in lung), while 4 were negative. When we analyzed the concordance between the two procedures (uptake of ¹⁸F-DOPA in the same sites of indeterminate lesions described at CT scan), 9 patients showed a concordance, while 3 patients a discordance (uptake of ¹⁸F-DOPA in other sites than those described at CT scan). The other 17 patients showed a partial concordance, most of whom (11/17 – 64.7%) 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 revealed radiotracer-enhancing lesions. Therefore, in this setting of uncertain diagnostic imaging and detectable CTN values, ¹⁸F-DOPA-PET/CT can be helpful to better define the nature of indeterminate lesion at CT scan. It is worth to note, however, that although these findings are effective in improving the follow-up of the disease, we did not experience any change in the clinical management, because of the small size of the lesions.