The value of 18F-FDG PET/CT and 18F-DOPA PET/CT in determining the initial surgical strategy of patients with medullary thyroid cancer : Preoperative PET/CT imaging for medullary thyroid cancer.

Background: While total thyroidectomy with central neck dissection (CND) is standard for medullary thyroid cancer (MTC), performing a lateral neck dissection (LND) depends on locoregional metastatic spread and is usually decided per individual. This study evaluated the utility of preoperative PET/CT in staging patients at diagnosis and guiding the initial surgical plan, while also exploring the value of neck ultrasound, MRI, and CT.

Methods: All MTC patients from two tertiary hospitals (2000 - 2020) were identified from two retrospective databases. All reports of neck ultrasounds, MRIs, CTs and PET/CTs < 8 months prior to primary surgery or < 4 months after MTC diagnosis were reviewed. The sensitivity and specificity of each imaging modality for locating locoregional lymph node metastases (LNM) was determined.

Results: A total of 175 MTC patients were included (91 females and 57 hereditary MTCs). Median age at presentation was 52 years (IQR 38 - 62). Initial treatment included a total thyroidectomy, CND and LND in 155 (89%), 140 (80%) and 59 (33%) patients. Preoperative imaging of the neck included ultrasound (91, 52%), MRI (33, 19%) and CT (31, 18%). PET/CT imaging was performed in 56 (32%) patients (35 ¹⁸F-FDG PET/CTs and 33 ¹⁸F-DOPA PET/CTs). Sensitivity for LNM in the central compartment was 72%, 39%, 6%, 42% and 93% for ¹⁸F-FDG PET/CT, ¹⁸F-DOPA PET/CT, ultrasound, MRI and CT, respectively. Respective specificity rates were 80%, 100%, 100%, 71% and 100%. Sensitivity rates for lateral neck LNM were 89%, 81%, 77%, 76% and 75%, for ¹⁸F-FDG PET/CT, ¹⁸F-DOPA PET/CT, ultrasound, MRI and CT, while specificity rates were 100%, 100%, 75%, 78% and 50%, respectively. Twenty-three patients had distant metastases on imaging. In total, 14 ¹⁸F-FDG PET/CTs and 9 ¹⁸F-DOPA PET/CTs were made in these 23 patients (both in six patients). All but one PET/CT showed distant metastases.

Conclusions: PET/CT is a powerful tool to detect locoregional LNM and can particularly help identify cases where LNDs are required, avoiding reoperation later on. For accurate staging of the central neck, PET may be combined with diagnostic CT. Finally, PET/CT's ability to detect distant metastases may support de-escalation of a surgical intervention when cure is unlikely.