SAT-374 - When Oncocytic Thyroid Tumor Cells Lead to Misdiagnosis of Poorly Differentiated Thyroid Carcinoma and Overtreatment

Errors in cancer diagnosis are reported to occur in up to 11.8% of all reviewed cytologic-histologic specimen pairs. A second opinion in pathology is intended to expose clinically significant errors that have a direct impact on patient care.

Case Presentation:

A 58-yo man with a h/o anxiety and cigarette smoking presented for evaluation of a thyroid nodule incidentally identified on lung cancer screening. He denied any family h/o thyroid disease or malignancy; no compressive sx.

Thyroid function tests were nl. Thyroid US reported a 2.7 cm L dominant TIRADS 3 nodule and a 1.2 cm R TIRADS 4 nodule. FNA cytology of the R and L nodules reported Bethesda category II & IV, respectively. Afirma molecular testing showed 50% risk of malignancy; no BRAF/V600E/RET/PTC variants. After collecting 3 opinions, the pt underwent L thyroid lobectomy with L modified neck dissection. Histopathology reported a 2.5 cm unifocal PDTC without extrathyroidal extension, tumor necrosis, perineural, lymphatic, or vascular invasion. Mitotic activity was 5 mitoses / mm2. All surgical margins were negative. Resected lymph nodes showed no malignant characteristics. 2 mo postop, thyroglobulin (Tg) was 22 ng / ml, while TSH was 1.8 mIU/L. PET-CT revealed focal increased FDG uptake along the residual left thyroid gland, indeterminate for residual disease. The pt was advised to have surveillance with laboratory and imaging f/u. He preferred to undergo completion thyroidectomy with central compartment lymph node dissection at another (3rd) institution, followed by RAI ablation. Histopathology showed benign (right thyroid) tissue. TSH (>150) stimulated Tg was 3 ng, anti-Tg negative. Post-therapy whole body scan demonstrated uptake confined to the thyroid bed. Discrepancy between initial imaging characteristics, FNA results and histopathology of the left thyroid lobe prompted a 2nd review by another pathologist which showed a 2.5 cm encapsulated oncocytic cell follicular adenoma and lymph nodes negative for malignancy, no evidence of PDTC. At 1-y f/u, Tg levels remain undetectable.



Discussion:

This case underscores the importance of a 2nd pathology review distinguishing oncocytic follicular adenoma from carcinoma to prevent overtreatment and patient harm. Often this can be accomplished by a second FNA bx even without molecular testing. Our pt underwent lobectomy for a Bethesda IV, TI-RADS 3 thyroid nodule with intermediate-risk molecular findings. However, the initial diagnosis of PDTC lacked several defining features, yet completion thyroidectomy and central neck dissection were performed without a 2nd pathology review first. The nodule was later reclassified as an encapsulated oncocytic follicular adenoma with low mitotic activity and proliferative index, findings incompatible with PDTC. As a result, the pt developed permanent postsurgical hypothyroidism following an intervention that might have been avoided.

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