Pigmented PRRX1::NCOA1-rearranged fibroblastic tumor: A rare morphologic variant of an emerging mesenchymal tumor.

PRRX::NCOAx-rearranged fibroblastic tumor is a recently described, morphologically distinctive subcutaneous fibroblastic tumor with benign behavior. To date, 12 cases have been reported. Here, we report a new case of PRRX::NCOAx-rearranged fibroblastic tumor showing a prominent pigmented component. The lesion occurred on the shoulder of a 23-year-old male. It was an at least 2.5 cm subcutaneous tumor with a multinodular and plexiform appearance. Morphologically, the tumor was characterized by a variably cellular proliferation of uniform oval to spindle cells arranged in fascicles and cords within a myxocollagenous stroma. Irregular, elongated, dilated vessels were prominent at the periphery of tumor nodules. In addition, nests and clusters of pigment-laden epithelioid and dendritic cells were present. Immunohistochemically, the non-pigmented tumor cells showed patchy positivity for factor XIIIa and focal positivity for S100 protein. The pigmented cells were positive for S100 protein, SOX10, MITF, and a pan-melanocytic cocktail (Melan-A, HMB-45, and tyrosinase). Next-generation RNA sequencing identified an in-frame PRRX1::NCOA1 fusion. In summary, this case highlights a rare pigmented variant of PRRX::NCOAx-rearranged fibroblastic tumor, expanding the morphologic spectrum of this newly described mesenchymal tumor.

A case report of recurrent primary posterior mediastinal perivascular epithelioid cell tumour compressing the right inferior pulmonary vein, atria, and inferior vena cava.

Background: Perivascular epithelioid cell tumours (PEComas) are rare soft tissue neoplasms that commonly occur in the uterus, skin, and liver and less commonly in the retroperitoneum, colon, and mediastinum. Case summary: A 36-year-old male patient with a history of mediastinal PEComa status post resection, essential hypertension, and atrial fibrillation status post appendage ligation currently not on anticoagulation presented with a 1-week history of fevers, chills, productive cough, chest pain, dyspnoea on exertion, loss of appetite, and general weakness. Vital signs, physical exam, laboratory data, electrocardiogram, and chest radiograph were grossly unremarkable. A multimodality imaging approach utilizing transthoracic echocardiogram, transoesophageal echocardiogram (TEE), cardiac magnetic resonance imaging (cMRI), and computed tomography angiography of the chest, abdomen, and pelvis revealed a local 40 mm × 53 mm globular bilobed vascularized scar-free posterior mediastinal mass arising from the roof of the left and right atria and extending superiorly to the main pulmonary artery and inferiorly to the inferior vena cava. Based on the mass' size and proximity to vital structures and tumour recurrence, the case was presented during tumour board rounds, and the outcome was to surgically resect the mass and then have the patient follow up with medical oncology and radiation oncology for possible chemotherapy and radiation, respectively. Discussion: Perivascular epithelioid cell tumours are rare, and mediastinal PEComas are even rarer, warranting a multimodality imaging approach involving TEE and cMRI and a multidisciplinary approach involving anaesthesiologists, cardiologists, cardiothoracic surgeons, medical oncologists, pathologists, radiologists, and radiation oncologists.