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Solitary fibrous tumor (SFT) is a rare mesenchymal neoplasm known to occur at various soft tissue and visceral locations. Kidney is a rarely reported site for these tumors. Most of the SFTs described in the kidney exhibit a classical CD34-positive patternless spindle cell histology. Focal round cell morphology is seldom reported. Herein, we describe a 48-year-old male patient with renal SFT. This tumor had pure round cell morphology with a CD34−/STAT6+ immunophenotype. Fluorescent in situ hybridization and a multiplexed sequencing assay performed on an Illumina® HiSeq 4000 platform revealed NAB2 and STAT6 gene rearrangement. Renal tumors with round cell morphology are diagnostically challenging and SFT is not often considered in the differential diagnosis of a round cell tumor of the kidney. Moreover, a CD34-negative profile can be rather confounding while diagnosing such lesions. In such scenarios, a strong nuclear STAT6 immunostaining is extremely helpful in clinching the diagnosis. SFT should always be considered in the differential diagnosis of round cell tumors of the kidney due to significant diagnostic and therapeutic implications. [ABSTRACT FROM AUTHOR]

Objectives: Solitary fibrous tumor (SFT) is a mesenchymal neoplasm that can arise at various anatomic locations. It is characterized by inv12(q13q13)-derived NAB2::STAT6 fusion, resulting in the nuclear expression of STAT6. Primary SFT of the adrenal gland is rare. We launched a multi-institutional collaboration to comprehend the overarching demographics, clinical and follow-up, macroscopic, microscopic, IHC, and FISH features of 9 patients with SFT of the adrenal gland.Methods: We added a series of 9 patients to the collection of adrenal SFTs where the clinicopathologic parameters, including clinical presentation, imaging, histopathology, IHC, molecular profiles, and management and follow-up data, were analyzed comprehensively. A modified 4-variable risk stratification model, including age, tumor size, and necrosis, was applied.Results: Our series consisted of 6 male and 3 female patients, ranging in age from 19 to 64 years (mean, 49.3 years). Abdominal pain (4) and fever with abdominal pain (1) were the presenting symptoms in 5 patients. In the remaining 4 patients, the tumors were detected by abdominal imaging for hypertension and diabetes. The size of the tumor ranged from 2 cm to 10.5 cm in maximum dimension. All tumors exhibited the morphology of a spindle cell SFT with a patternless architecture; 3 had a focal storiform arrangement. STAT6 positivity was observed in all tumors, and 7 were positive for CD34. Surgical resection was the primary modality of treatment. No adjuvant therapy was administered. Follow-up ranging from 7 months to 23 months was available for 7 patients. All were alive without disease recurrence or metastasis. Risk stratification placed 8 (88.9%) patients into a low-risk category and 1 into an intermediate-risk category.Conclusions: This series is the largest of adrenal SFTs to date. These tumors of the adrenal gland are predominantly spindle cell neoplasms with indolent behavior, with a wide age distribution and a slight male preponderance. Combining our cohort with the previously published cases, the majority of tumors fall into the low-risk category for the propensity to develop metastases. Owing to the rarity and age distribution associated with these tumors, the differential diagnosis is wide and requires a systematic approach for ruling out key differential diagnoses aided by STAT6 IHC. [ABSTRACT FROM AUTHOR]