Endocrine Abstracts

Introduction: American Thyroid Association (ATA) guidelines suggest considering hemithyroidectomy in patients with unifocal well-differentiated thyroid tumours,smaller than 4 cm, withoutpreoperative evidence of extrathyroidal extension (ETE) nor lymph node metastases. Although multifocal microcarcinomas are considered to increase the risk of structural disease recurrence from 1%–2% to 4%–6% compared to unifocal microcarcinomas, the ATA guidelines do not suggest different risk stratification for multifocal andunifocal tumours; with the exception of BRAF V600E mutated tumours with ETE, such tumours are low risk when they are unifocal and moderate risk when they are multifocal. Our objective was to evaluate whether multifocality influences response to treatment one year after papillary thyroid cancer (PTC) surgery.

Material and Methods: We retrospectively collected every histologically confirmed PTC (non-aggressive variants),larger than 2 mm, operated at our centre between 2001 and 2018. Using the Dynamic Risk Stratification system,we compared responses to therapy at one year of follow-up (1YFU) between multifocal tumours (MFPTC) and unifocal tumours (UFPTC). Using chi square test, we evaluateda possible association between MFPTCand higher risk for incomplete remission (non-excellent: biochemical incomplete, structural incomplete or indeterminate). We also stratified our sample by different variables: biggest focus size, presence of BRAF V600E mutation, treatment with I131 (RAI) and evidence of histological ETE.

Results: We studied 498 patients: 202 MFPTC and 296 UFPTC. Among MFPTC, 149 (73.8%) had an excellent response at 1YFU. Among UFPTC, 264 (89.2%) had an excellent response at 1YFU (P < 0.0001; RR: 2.94). Higher risk for incomplete response to treatment in MFPTC was maintained when stratified by size, including those larger than 10 mm (P < 0.0001; RR: 5.49)or 20 mm (P < 0.0001; RR: 5.39);presence of ETE (n: 121, P = 0.02; RR: 2.47); or including only PTC treated with radioiodine after surgery (n: 422; P < 0.0001; RR: 3.29). A non-significant tendency to more frequent non-excellent response in MFPTC was observed when stratified by detection of the BRAFV600E mutation, with P = 0.06 in wild type (n: 116) and P = 0.13 in mutated PTC (n: 115).

Conclusion: MFPTC are associated with a non-excellent dynamic stratification risk at 1YFU compared with UFPTC, regardless of the size of their largest focus, the finding of ETE,or treatment with RAI. Therefore, we suggest multifocality shouldbe considered by clinicians for decision-making regarding follow-up of PTC patients. Given that total thyroidectomy allows clinicians to assess multifocality, we suggest this procedure for consideration as the routine surgery rather than lobectomy in PTC patients.