Stromal tissue as an adjunct tool in the diagnosis of follicular thyroid lesions by fine-needle aspiration biopsy.

Background: The stroma in fine-needle aspiration biopsy (FNAB) of thyroid lesions has not been well investigated.

Design: We studied 256 consecutive cases of thyroid FNAB prepared with traditional smear technique. The stroma was categorized: Type 1a consisted of long (more than 3 mm), broad bands composed of mesh containing collagen fibrils thickened by entrapped blood components and follicular cells. Type 1b consisted of dense strands/bands. Type 2 was similar to Type 1a but with shorter (<2 mm) and looser stromal strands.

Results: Types 1a and b showed straight/curved/circular branching patterns suggestive of incomplete frameworks of nodular/papillary architectures or fragments of capsule. Type 1b stroma likely represented thick/collagenized fibrous septae. Incomplete or complete rings of small encapsulated tumor were occasionally identified. These frameworks of stroma were frequently associated with multinodular goiters (MNGs) which are often hypocellular and follicular neoplasms/papillary thyroid carcinoma with increased cellularity. Type 2 was associated with microfollicles in encapsulated neoplasms or with macrofollicles in MNG. Follicular lesions of unknown significance (n = 41) either negative (n = 26) or positive (n = 15) for carcinoma in subsequent follow-up were frequently associated with stroma characteristic of MNG and carcinoma, respectively.

Conclusion: The preservation of the in vivo architecture of Type 1 is likely due to its elasticity. Recognition of the stromal architecture will likely facilitate the diagnosis.