Trisomy 17 as a marker for a subset of noninvasive thyroid nodules with focal features of papillary carcinoma: cytogenetic and molecular analysis of 62 cases and correlation with histological findings.

Background: Differentiated carcinomas of the thyroid are divided into follicular thyroid carcinoma and papillary thyroid carcinoma (PTC), based on their propensity to invade and their cytological features [papillary carcinoma-type nuclear changes (PTC-NCs)]. PTC typically exhibits a diploid karyotype sometimes with inv10(q11.2q21.2), leading to rearranged RET gene. Follicular thyroid carcinomas are often aneuploid and may exhibit t(2;3)(q13;p25), resulting in PAX8-PPARgamma1 gene fusion. Isolated trisomy 17 has rarely been reported in thyroid lesions, and its significance is unknown.

Objective: Our objective was to determine whether isolated trisomy 17 corresponds to a specific histological or molecular thyroid tumor subset. Nine cases with isolated trisomy 17 were critically reviewed and investigated for RAS and BRAF mutations and for RET and PAX8-PPARgamma1 rearrangements.

Results: All nine cases were noninvasive, exhibited follicular growth pattern, and showed PTC-NCs focally defined within the nodule: four were PTCs follicular variant within larger tumors, and five were follicular-patterned nodules with incomplete cytological features of papillary carcinoma (variable proportion of cells with PTC-NCs scattered inside the lesion). RAS, BRAF V600E mutation, RET or PAX8-PPARgamma1 rearrangements were not identified. One case had BRAF K601E mutation. Only two of the 53 control cases showed focal PTC-NCs.

Conclusions: Isolated trisomy 17 is associated with focal papillary carcinoma changes in follicular-patterned thyroid nodules and may be a marker for this subset of thyroid lesions that are often difficult to classify.