Bilateral multifocality, a marker for aggressive disease, is not an independent prognostic factor for papillary thyroid microcarcinoma: A propensity score matching analysis.

Context: Multifocality and bilaterality are common in patients with papillary thyroid microcarcinoma (PTMC). However, their clinical behaviours and prognostic implications remain controversial.

Objective: To investigate the relationship between multifocality and classically aggressive characteristics and outcomes in patients with PTMC.

Methods: Clinical data of 3005 patients with PTMC were retrospectively reviewed at a tertiary medical centre. The role of unilateral and bilateral multifocality in aggressive characteristics and clinical outcomes of PTMC was evaluated using propensity score matching (PSM).

Results: A total of 573 patients had bilateral multifocal disease (B-MFD), 272 had unilateral multifocal disease (U-MFD), and 2160 had unifocal disease (UFD). Univariate analysis showed that patients in the multifocal disease (MFD) groups showed significantly different characteristics compared to patients in the UFD group in terms of age, chronic lymphocytic thyroiditis (CLT), follicular variant PTMC, tumour diameter, aggressive growth, including extrathyroidal extension (ETE), central lymph node metastasis (CLNM) and lateral lymph node metastasis (LLNM), and TNM stage, and underwent radioactive iodine (RAI) therapy. Further stratified analysis revealed that patients in the B-MFD group reflected the differences between the MFD and UFD groups. However, those in the U-MFD group showed slight differences only in sex, CLT and cell subtypes, compared to the UFD group. In addition, PSM indicated differences in ETE, CLNM and LLNM between the B-MFD and UFD groups (p < .001), while only ETE differed between the U-MFD and UFD groups (p < .001). After a median follow-up period of 60 months, no difference was observed in recurrence-free survival between the UFD and B-MFD (p = .294) or U-MFD (p = .603) groups using PSM.

Conclusion: This propensity score matching analysis provides strong evidence that bilateral multifocality, rather than unilateral multifocality, should be considered as an aggressive marker at presentation, and neither is an independent prognostic factor for clinical outcome in PTMC.