EWSR1-ATF1 Fusion in a Myoepithelial Carcinoma of Soft Tissue With Small Round Cell Morphology: A Potential Diagnostic Pitfall

Myoepithelial tumors of soft tissue and bone in children and young adults: A clinicopathologic study of 40 cases occurring in patients ≤ 21 Years of age

Myoepithelial tumors of the soft tissue and bone occurring in patients 21 years of age and younger are rare, and their clinicopathologic features remain incompletely understood. We studied a well-characterized series of 40 such tumors. Cases were retrieved from our archives for the period 2009-2022 and re-reviewed. Available immunohistochemical and molecular genetic data was collected. Clinical information including available follow-up was obtained. The tumors occurred in 18 males and 22 females, ranging from 3 months to 21 years of age (median 11.5 years), and involved a wide variety of soft tissue (n = 36) and bone (n = 4) locations. Histologically benign myoepithelial tumors tended to occur in adolescents (median age 14.5 years; range 5-21 years), whereas myoepithelial carcinomas occurred in younger patients (median age 8.5 years; range 3 months-20 years). Microscopically, the tumors showed a complex admixture of epithelioid, plasmacytoid and spindled cells in a variably hyalinized, myxoid, chondroid or chondromyxoid background. Small subsets of histologically malignant tumors had rhabdoid or "round cell" features. Immunohistochemistry showed 35/40 (88%) cases to be positive with at least one keratin antibody. The 5 keratin-negative tumors were uniformly positive for S100 protein and/or SOX10 and expressed EMA (4 cases) and/or p63 (3 cases). EMA, SMA and GFAP were positive in 21/25 (84%), 13/21 (62%), and 8/21 (38%) tumors, respectively. SMARCB1 and SMARCA4 expression was retained in 29/31 (94%) and 22/22 (100%) of cases, respectively. FISH for EWSR1 gene rearrangement was positive in 6/18 (33%) tested cases. Two EWSR1-negative tumors were also FUS-negative. NGS identified EWSR1::POU5F1, FUS::KLF17, and BRD4::CITED1 gene fusions in 3 tested cases. Clinical follow-up (22 patients; median 23 months; range 1-119 months) showed 3 patients with local recurrences and 5 with distant metastases (lymph nodes, lung, and brain). Three patients died of disease, 3 were alive with recurrent or unresectable disease, and 16 were disease-free. Adverse clinical outcomes were seen only in patients with malignant tumors. We conclude that myoepithelial neoplasms of soft tissue and bone are over-repesented in patients ≤21 years of age, more often histologically malignant, and potentially lethal. Histologic evaluation appears to reliably predict the behavior of these rare tumors.

EWSR1::WT1 Fusions in Neoplasms Other Than Conventional Desmoplastic Small Round Cell Tumor: Three Tumors Occurring Outside the Female Genital Tract

Desmoplastic small round cell tumor (DSRCT) is a high-grade, primitive round cell sarcoma classically associated with prominent desmoplastic stroma, coexpression of keratin and desmin, and a characteristic EWSR1::WT1 gene fusion. DSRCT typically arises in the abdominopelvic cavity of young males with diffuse peritoneal spread and poor overall survival. Although originally considered to be pathognomonic for DSRCT, EWSR1::WT1 gene fusions have recently been detected in rare tumors lacking the characteristic morphologic and immunohistochemical features of DSRCT. Here, we report 3 additional cases of neoplasms other than conventional DSCRCT with EWSR1::WT1 gene fusions that occurred outside the female genital tract. Two occurred in the abdominopelvic cavities of a 27-year-old man and a 12-year-old girl, whereas the third arose in the axillary soft tissue of an 85-year-old man. All cases lacked prominent desmoplastic stroma and were instead solid and cystic with peripheral fibrous pseudocapsules and occasional intervening fibrous septa. Necrosis was either absent (1/3) or rare (2/3), and mitotic activity was low (<1 to 3 per 10 hpf). In immunohistochemical studies, there was expression of smooth muscle actin (3/3) and desmin (3/3), rare to focal reactivity for EMA (2/3), and variable expression of CK AE1/AE3 (1/3). Myogenin and MyoD1 were negative, and C-terminus-specific WT1 was positive in both cases tested (2/2). All 3 tumors followed a more indolent clinical course with 2 cases demonstrating no evidence of disease at 20 and 44 months after resection. The patient from case 3 died of other causes at 14 months with no evidence of recurrence. DNA methylation profiling showed that the 3 cases clustered with DSRCT; however, they demonstrated fewer copy number variations with 2 cases having a flat profile (0% copy number variation). Differential methylation analysis with hierarchical clustering further showed variation between the 3 cases and conventional DSRCT. Although further study is needed, our results, in addition to previous reports, suggest that EWSR1::WT1 gene fusions occur in rare and seemingly distinctive tumors other than conventional DSRCT with indolent behavior. Proper classification of these unusual soft tissue tumors with EWSR1::WT1 gene fusions requires direct correlation with tumor morphology and clinical behavior, which is essential to avoid overtreatment with aggressive chemotherapy.

Epithelioid mesenchymal neoplasm with FUS::CREM gene fusion in the tongue: Report of a rare and challenging diagnosis

FET (encompassing both EWSR1 and FUS) fusions with genes from the CREB family (CREB1, ATF1, and CREM) are involved in a variety of neoplasms. Recently, FET::CREB fusions were recognized in a group of malignant epithelioid neoplasm with a striking predilection to mesothelial-lined cavities and frequent cytokeratin immunoexpression. Herein, we report a rare mesenchymal neoplasm with epithelioid morphology and nonspecific immunoprofile harboring a FUS::CREM fusion arising in the oral tongue of a 53-year-old man. Histology showed a well-circumscribed tumor composed of epithelioid cells with eosinopohilic or clear cytoplasm with sparse stroma, accompanied by peripheral lymphoplasmacytic infiltrates. Immunohistochemically, an extensive panel revealed only patchy expression of synaptophysin and weak-to-moderate nuclear expression of TFE3, and negativity for other markers including cytokeratins, epithelial membrane antigen, p63/p40, vimentin, S100, smooth muscle actin, CD34, desmin, SOX10, glial fibrillary acidic protein, melan-A, HMB45, and CD68. A FUS::CREM gene fusion was detected by next generation sequencing at an outside institution, and subsequent fluorescence in situ hybridization analysis confirmed the presence of FUS gene rearrangement. The identification and analysis of additional cases should help to clarify the nosologic status and the biologic potential of this tumor.

Primary lung hyalinizing clear cell carcinoma: a diagnostic challenge in biopsy

Introduction

Hyalinizing clear cell carcinomas (HCCCs) are rare, low-grade, malignant tumors. They most commonly involve the minor salivary glands of the head and neck. HCCC that occurs in uncommon locations and examining samples from small biopsy pose a diagnostic challenge for most pathologists.

Case presentation

We herein report a primary pulmonary HCCC diagnosed by small biopsy and summarize its histologic, immunophenotypic, and molecular features along with a review of 11 previously reported cases to emphasize the potential diagnostic pitfalls.

Conclusions

Small biopsy diagnosis of primary pulmonary HCCC is challenging. A collection of mimics needed to be ruled out. Awareness of the key morphologic features of pulmonary HCCC combined with essential immunohistochemistry and molecular tests contributes to the correct diagnosis.