Microscopic description:

Ovoid to fusiform spindle cells with indistinct cell borders arranged around dilated, branching, hemangiopericytoma-like vasculature.

IHC: Stained positive for STAT6, CD34, Ki67 (<1%); negative for AE1/AE3,SOX10.

Based on the histopathological features and IHC stains, the tumor shows features consistent with recurrent perineal solitary fibrous tumor.

Discussion:

Solitary fibrous tumor (SFT) is classified as a rare mesenchymal neoplasm of spindle cells, accounting for <2% of all soft tissue tumors, with low risk of metastasis and usually benign behavior.

Presentation ranges between 20-80 years old, with equal incidence between males and females, mostly commonly found in the pleura and at the abdominal level (60-70%).

Research is ongoing to differentiate between SFTs with benign behavior and those that are more aggressive, though it is suggested age, location, large tumor size, high mitosis rate (>4/10 HPF), and cell paleo-morphine may account for malignant characteristics.

SFTs are generally slow-growing, painless masses, with location-specific symptoms.

Genetically associated with the fusion of genes NAB2-STAT6, SFTs are activated by EGR-mediated transcription causing cell growth with secondary neoplastic progression.

Immunohistochemically, important positive markers are CD34, CD99, Bcl-2, and STAT-6. Consistently negative markers include EMA and S100.

Evaluating diagnostic imaging is crucial for accurate diagnosis and therapeutic approach. Common findings include well-defined masses with smooth margins, well vascularized, and may have central necrosis and very rarely calcification. The presence of fatty tissue is suggestive of lipomatous hemangiopericytoma.

Due to low incidence of SFT, the standard treatment is surgical resection, however about 20% of those patients have a subsequent local recurrence.

References:

Gonzalez JA, Sanchez E, Messa Botero O, Cervera-Bonilla S. Solitary Fibrous Tumor in the Perianal Region: Report of Two Cases With an Atypical Location. Cureus. 2021 Jan 24;13(1):e12887. doi: 10.7759/cureus.12887. PMID: 33643733; PMCID: PMC7901715.

Demicco EG, Park MS, Araujo DM, Fox PS, Bassett RL, Pollock RE, Lazar AJ, Wang WL. Solitary fibrous tumor: a clinicopathological study of 110 cases and proposed risk assessment model. Mod Pathol. 2012 Sep;25(9):1298-306. doi: 10.1038/modpathol.2012.83. Epub 2012 May 11. PMID: 22575866.

Paramythiotis D, Moysidis M, Kourtidis L, Karakatsanis A, Poulios C, Michalopoulos A. Perianal Solitary Fibrous Tumor in a Rare Anatomical Presentation: A Case Report and Literature Review. Am J Case Rep. 2021 May 19;22:e929742. doi: 10.12659/AJCR.929742. PMID: 34010267; PMCID: PMC8141335

Geramizadeh B, Marzban M, Churg A. Role of Immunohistochemistry in the Diagnosis of Solitary Fibrous Tumor, a Review. Iran J Pathol. 2016 Summer;11(3):195-203. PMID: 27799967; PMCID: PMC5079451.

CASE IMAGES