Morphologic, Immunohistochemical, and Molecular Features of Laser-Ablated Thyroid Nodules: Diagnostic Pitfalls and Differential Diagnosis with Thyroid Carcinoma.

Thermal ablation (TA) is an increasingly adopted minimally invasive treatment for benign thyroid nodules. However, TA induces marked histological alterations that may simulate thyroid malignancy, creating significant diagnostic pitfalls for pathologists. The present study expands our previous institutional series and further characterizes the morphologic, immunohistochemical, and molecular features of thermally ablated thyroid nodules in order to refine the differential diagnosis with thyroid carcinoma. Fourteen surgically excised thyroid nodules previously treated with laser thermal ablation were retrospectively analyzed. Histopathological evaluation focused on architectural changes, nuclear atypia, capsule alterations, degenerative phenomena, and evidence of invasion. Immunohistochemical analysis included galectin-3 (Gal-3), HBME-1, BRAF V600E, p53, and Ki-67. In addition, molecular profiling for the principal thyroid cancer-related alterations, including BRAF, RAS family genes, TERT promoter mutations, PIK3CA alterations, and RET rearrangements, was performed using targeted next-generation sequencing. All nodules showed treatment-related reactive and degenerative changes, including fibrosis/sclerosis, subcapsular hemorrhage, focal oncocytic metaplasia, and architectural distortion. No true capsular or vascular invasion was identified. Immunohistochemically, all cases were negative for Gal-3 and BRAF V600E, while HBME-1 expression was absent or only focally weak. Ki-67 proliferative activity remained consistently low (<3%) in all cases. Molecular analyses did not identify pathogenic alterations involving BRAF, RAS, TERT promoter, PIK3CA, or RET genes in any case. Thermal ablation induces reproducible reactive and degenerative histologic alterations that may closely mimic follicular or papillary thyroid neoplasms. The absence of malignancy-associated immunohistochemical and molecular alterations strongly supports the benign nature of these lesions and highlights the importance of an integrated morphologic, immunohistochemical, and molecular diagnostic approach in challenging post-ablation specimens. Thermally ablated thyroid nodules may display significant pseudo-neoplastic changes that can lead to overdiagnosis of carcinoma. Awareness of these treatment-related alterations, combined with immunohistochemical and molecular profiling, represents a reliable strategy to distinguish reactive post-ablation changes from true thyroid malignancy and to avoid inappropriate clinical management.