Reclassification of papillary thyroid microcarcinoma (pMCT) according to the "Porto proposal" criteria.

Introduction: pMCT is defined as a variant of papillary carcinoma that measures≤1 cm in diameter and which is characterized by an excellent prognosis. Recently, a proposal has been advanced to use the designation of papillary mirotumour (pMT) for pMCTs with no risk factors .

Aim: In this study, we aimed to reclassify pMCTs according to the Porto proposal(Pp) criteria.

Methods: We have retrospectively collected cases of pMCT diagnosed in our pathology department over a period of 10years(2012-2022). Clinical and pathological parameters have been retrieved from the patient's medical records and pathological reports. We have evaluated all cases following the criteria of Pp. Cases that fulfilled all the criteria have been reclassified as pMT. We have briefly compared the clinical outcomes in both groups.

Results: 29 cases of pMCT was found. Mean age of patients was 46,6 years-old (17-67) with a female predominance (sex ratio=0,45). 23 cases of pMCT were incidentally discovered. The tumor was located in the right thyroid lobe in 15cases. The mean size of the tumor was 5,2mm. Multifocality was observed in 5 cases. A total of 17 cases could be classified as pMT according to the Pp. Only one patient developed pulmonary metastasis and local recurrence; however it was related to the papillary carcinoma firstly diagnosed in his contralateral lobe. Clinical outcome was also good in the group of papillary microcarcinoma (pMC) with no recurrence or distant metastasis.

Conclusion: According to the Pp,>50% of pMCT could be reclassified as pMT which could reduce the psychological impact and overtreatment. Further studies with large sample size and molecular analysis are however needed in order to definitively validate and generalize the use of Porto proposal.