Context: differentiated carcinomas of the thyroid are divided into follicular thyroid carcinoma (FTC) and papillary thyroid carcinoma (PTC), based on their propension to invade and on their cytologic features (papillary carcinoma type nuclear changes, PTC-NC). PTC typically exhibits a diploid karyotype sometimes with inv10(q11.2q21.2) leading to rearranged RET gene. FTC are often aneuploid and may exhibit t(2;3)(q13;p25) resulting in PAX8-PPAR1 gene fusion. Isolated trisomy 17 has rarely been reported in thyroid lesions and its significance is unknown.

Objective/Design: to determine whether isolated trisomy 17 corresponds to a specific histologic or molecular thyroid tumor subset. Nine cases with isolated trisomy 17 were critically reviewed, investigated for RAS and BRAF mutations, and for RET and PAX8-PPAR1 rearrangements.

Results: all nine cases were non invasive, exhibited follicular growth pattern and showed PTC-NC focally defined within the nodule: four were PTCs follicular variant within larger tumors, five were follicular patterned nodules with incomplete cytologic features of papillary carcinoma (variable proportion of cells with PTC-NC scattered inside the lesion). RAS, BRAF^V600E mutation, RET or PAX8-PPAR1 rearrangements were not identified. One case had BRAF^K601E mutation. Only two of the 53 control cases showed focal PTC-NC changes.

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.