Papillary Thyroid Carcinoma with Honeycomb-Like Growth: Clinicopathological Characteristics and Diagnostic Significance as a Novel Variant.

Introduction: We aimed to clarify the clinical and pathological characteristics of papillary thyroid carcinoma (PTC) with unique honeycomb-like growth (HLG) and discuss its diagnostic significance.

Methods: Among the 12,745 PTCs that were resected and histologically diagnosed, 28 PTC cases with HLG components (0.2%) were included.

Results: PTC-HLG was subclassified into pure (9 cases), which consisted of only HLG components, and mixed (19 cases), which consisted of conventional PTC and HLG components, types. HLG components were histologically characterized by (1) neoplastic cyst aggregation with intervening normal thyroid follicles, (2) the cyst wall composed of single-layered carcinoma cells, (3) low papillary growth, and (4) ball-like granulation tissues. Compared with the mixed type, the pure type occurred in older people (p < 0.05), had a smaller tumor size (p < 0.0001), was more interpreted as being benign by ultrasound (p < 0.05), and had a lower lymph node metastasis rate (p < 0.005). In the mixed type, 44.4% of conventional PTCs showed a Ki-67 labeling index of >5%. All and 10.5% of the mixed type showed lymph node and lung metastases, respectively.

Conclusion: The pure type could be a nonaggressive variant of PTCs with a unique honeycomb growth pattern and tended to be clinically interpreted as benign. The mixed type is pathogenetically different from the pure type and is slightly aggressive compared with conventional PTCs.