Lymph node metastasis occurs in a subset of papillary microcarcinoma patients. We aimed to analyze the differences between metastatic and non-metastatic papillary microcarcinomas in order to identify a high-risk subgroup that is likely to require more aggressive treatment.
126 thyroidectomies with lymph node dissections (central ± lateral), diagnosed as papillary microcarcinoma, were reviewed.
Mean age of 126 patients (F/M = 3.3) was 42 years. Mean size of the largest tumor was 7 mm. Classical was the most frequently (89%) encountered subtype. Multiple histologic subtypes co-occurred in 19%. Lymphovascular invasion was present in 16% (n = 20). 55 (44%) and 71 (56%) cases were unifocal and multifocal, respectively. 90 cases (71%) were non-encapsulated with overall infiltrative tumor borders, whereas in 36 cases (29%), the tumor had a well-defined capsule. Among those, 23 (64%) had tumor capsule invasion. 47 (37%) cases had metastasis in lymph nodes. In univariate analysis, metastasis was associated with tumor size of >5 mm (p = 0.02), tumor burden of >5 mm (p = 0.03), lymphovascular invasion (p = 0.02) and non-encapsulation (p = 0.01). No associations were found regarding sex, age, histologic subtype, lymphocytic thyroiditis, tumor capsule invasion (in capsulated tumors), laterality and multifocality (p > 0.05). In multivariate analysis, lymphovascular invasion (p = 0.01, OR = 3.97, 95% CI 1.35-11.67), tumor size >0.5 cm (p = 0.031, OR = 2.92, 95% CI 1.10-7.71) and non-encapsulation (p = 0.033, OR = 2.85, 95% CI 1.08-7.51) were independent risk factors.
Size (largest tumor or sum of all foci) of >5 mm, non-encapsulation and lymphovascular invasion were independent predictors of LNM in PMs. Unifocal tumors metastasize the same as multifocal tumors, suggestive of the contribution of other factors. Patients with sporadically resected microcarcinomas should be carefully followed-up, especially those that harbor risk factors in histology.

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