Spindle Cell Predominant Trichodiscoma or Spindle Cell Lipoma With Adnexal Induction? A Study of 25 Cases, Revealing a Subset of Cases With RB1 Heterozygous Deletion in the Spindle Cell Stroma.

In our routine and consultative pathology practices, we have noticed that a relatively high proportion of spindle cell predominant trichodiscomas demonstrate a remarkable stromal admixture of adipose tissue, which along with spindle cells, prominent collagen bundles and myxoid change closely resembles spindle cell lipoma (SCL). To clarify their possible relationship to SCL, 25 cases of trichodiscoma and fibrofolliculoma with stromal "lipomatous metaplasia" were collected and examined using immunohistochemical stains [CD34 and retinoblastoma-1 (RB1) protein] and fluorescence in situ hybridization (RB1 deletion). The patients ranged in age from 35 to 81 years (median 64 years). The male to female ratio was almost equal (14:11). All tumors with a known location were situated on the face with a special predilection for the nose. All cases were sporadic, with all patients having a single lesion and showing no clinical features of Birt-Hogg-Dubé syndrome. No case with available follow-up presented with a recurrence or an otherwise aggressive clinical course. Spindle cell stroma was immunohistochemically positive for CD34 in 16 of 20 cases, and 18 of 19 cases showed loss of RB1 staining in lesional spindle cells. Fluorescence in situ hybridization analysis detected RB1 gene heterozygous deletion in 6 of 20 cases. We conclude that despite the SCL-like appearance of the investigated cases, the majority of them supposedly represent genuine spindle cell predominant trichodiscomas with adipose tissue admixture. However, there was a subset of histopathologically indistinguishable cases with proved RB1 deletion, which likely represent SCL with trichodiscoma/fibrofolliculoma-like epithelial/adnexal induction rather than spindle cell predominant variant of trichodiscoma.