Perivascular Epithelioid Cell (PEC) tumors, also PEComas, are rare mesenchymal tumors defined by the presence of perivascular epithelioid cells that express muscle and melanocytic markers and have no known normal tissue counterparts [1]. Angiomyolipoma (AML), clear cell "sugar" tumor (CCST) of the lung, lymphangioleiomyomatosis (LAM), clear cell melanocytic tumor of the falciform ligament/ligamentum teres, and rare clear cell tumors of other anatomical sites, such as the liver, uterus, vulva, rectum, urinary bladder, abdominal wall and pancreas are tumors which have both muscle and melanocytic markers which fall in the category of PEComa [2][3][4]. These tumors are usually associated with tuberous sclerosis complex with female predominance, especially AML and LAM [5]. Pigmented PEComas are rare and difficult to differentiate from metastatic melanoma. The pigmented PEComa, unlike the typical PEComa, occurs in females without tuberous sclerosis [5]. So far only 10 reported cases of pigmented PEComas of the kidney and one in liver have been reported [6] [7]. The immunoprofile of the tumor with strong human melanocytic black-45 (HMB-45) expression and the absence of other melanocytic markers like S-100 and SOX-10 suggest the diagnosis of PEComa [7]. To the best our knowledge, this is the first reported PEComa of the lung with pigment production.

A 34-year-old female with a medical history of hypothyroidism presented with a lung mass on chest X-ray performed to evaluate chronic right upper back pain present for last six months. Physical examination and laboratory results were unremarkable. A previous chest X-ray done two years ago on a routine visit to her primary care doctor has shown a smaller lesion but was not followed-up until she became symptomatic. Computed tomography scan of chest confirmed a multi-lobular 3.4x2.2 cm soft tissue mass in the right lower lobe (Figure 1). Bronchoscopy with biopsy was negative for malignant cells and microorganisms. Computed tomography guided biopsy revealed atypical spindle cells strongly positive for HMB-45 and vimentin expression, negative for S100, SOX-10, cytokeratin, CD20, LCA, CD15, desmin and SMA. PET scan confirmed uptake of FDG in the right lung mass with no other pulmonary, mediastinal, cervical, intra or retro abdominal foci of increased uptake. Lymph nodes were unremarkable in all regions. She had wedge resection of the right upper lobe mass through video assisted thoracoscopic surgery (VATS). Grossly it was 3.5x2.5x 2.0 cm bulging dome shaped mass. There were no postoperative complications and the patient was discharged seven days after the surgery. A thorough gynecological examination did not reveal any pathology.

Pathology:
Macroscopic findings after VATS showed right upper lobe of the lung measuring 9.0x8.5x 6.0 cm and weighing 64 grams. Along the hilum was a bulging dome shaped mass measuring 3.5x2.5x2.0 cm. On section, the mass consisted of friable soft gray-white tissue. The pleural surface was smooth and showed moderate congestion.

Light microscopic examination revealed a relatively well defined sub-pleural densely cellular mass composed of sheets and short fascicles of tumor cells with scattered lymphoid aggregates. The predominant tumor cell population was ovoid to spindle, with a moderate amount of pale eosinophilic cytoplasm, vesicular chromatin and tumor giant cells, as well as cells containing fine brown intracytoplasmic pigment (Figure 2). It had conspicuous intratumoral lymphoid infiltrate with numerous eosinophils. Mitotic figures were scattered throughout the specimen with an average of 2 mitoses per 2 square mm. Focal necrosis was seen.

Immunohistochemical stains were positive for HMB 45 (Figure 3), focally for Melan-A (Figure 4), vimentin (Figure 5) and negative for S-100 (Figure 6) and SOX-10. Ki-67 immunostain was positive in 5% of the tumor cells. Remaining immunohistochemical stains are shown below in the Table 1. Table 2 shows immunostains which help in differentiating between Pigmented PEComas and malignant melanoma.

Typically PEComas of the lung are characterized by the presence of epithelioid or spindle cells with clear or granular eosinophilic cytoplasm. These tumors are also known as clear cell 'sugar' tumor of the lung (CCTL) as the PAS stain shows glycogen granules in the cytoplasm of these clear cells [8] [9].

Typically, CCTL is a rare benign pulmonary neoplasm which can occur in any age group (8–73 years) with slight female preponderance [9]. Most lesions are solitary, asymptomatic, located within the peripheral lung and pose a significant diagnostic problem. Only few develop symptoms like breathlessness, back pain, chest pain, fever and hemoptysis [9].

Macroscopically, CCTL appear as well circumscribed, peripheral nodules usually less than 3 cm in diameter and cut surface is without evidence of hemorrhage, necrosis, cavitation or calcification. Histologically, they consist of mitotically inactive round or oval cells with clear or granular eosinophilic cytoplasm and distinct cell borders, with characteristic intervening thin-walled sinusoidal vessels [9].

Though most of these tumors are benign, malignant behavior is occasionally reported. The degree of malignancy is highly variable in PEComas. The size of the tumor and the mitotic rate seem to be the most reliable prognostic factors associated with recurrence after surgical resection [10]. Our patient had tumor <5 cm, mitotic rate of 2 mitoses per 50 hpf and absence of necrosis which makes the PEComa benign as per the modified Folpe's criteria [10]. Repeat CT scan of chest done six months later showed no evidence of recurrence of the tumor.

In summary, our patient had atypical spindle cell neoplasm with focal pigment production positive for HMB-45 and negative for S-100 and SOX-10 immunostains. The differential diagnosis included metastatic melanoma and PEComa. While it is unusual for melanoma to be negative for both S-100 and SOX-10, it remained a diagnostic consideration as the tumor had melanin pigment and HMB-45 staining. However, the presence of atypical spindle shaped cells with an immunostain positive for vimentin and HMB-45 made PEComa more likely. Our case is distinguished by the presence of the melanin pigment which is unusual in PEComa. The final morphologic and immunohistochemical findings are most consistent with a tumor with perivascular epithelioid cell differentiation (PEComa).

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