Pathology of Perivascular epithelioid cell neoplasms (PEComa) -Infographic

Pathology of Perivascular Epithelioid Cell Neoplasms (PEComa)

"Perivascular epithelioid cell neoplasms (PEComas) are defined by the World Health Organisation Classification of Tumors of Soft Tissue and Bone as mesenchymal tumors composed of perivascular epithelioid cells with unique histological properties and immunophenotypes".

Bonetti et al. firstly proposed the descriptive name of Perivascular Epithelioid Cells (PEC) in 1992. Zamboni et al. introduced the term PEComa in 1996.

"Perivascular Epithelioid Cell Tumours (PEComas) represent a family of rare mesenchymal neoplasms, including angiomyolipoma (AML), clear cell "sugar" tumour (CCST) of the lung, lymphangioleiomyomatosis (LAM), clear cell myomelanocytic tumour (of the falciform ligament/ligament teres), and unusual clear cell tumours of the pancreas, rectum, abdominal serosa, uterus, vulva, thigh, and heart.

The tumour is more frequent in females than in males (6:1) with a median age of 45 years. Some cases are associated with the Tuberous Sclerosis Complex, especially AML, CCST, and LAM"

Renal angiomyolipoma (AML) is the most common PEComa.

Both benign and malignant tumours have been described.

Malignant tumours are diagnosed by histologically confirmed lymph node metastases and radiological evidence of pulmonary metastases.

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Histopathological features:

These tumours are closely located to vascular structures and are characterized by spindle to epithelioid cells with clear to pale eosinophilic cytoplasm arranged in fascicular or nested patterns.

Immunohistochemically, the tumour cells expressed HMB45 and smooth muscle actin and micropthalmia transcription factor.

Over 50% of them stain for melan-A (sometimes in the absence of HMB45), and around 25% are desmin positive.

A subset of cases stain for S-100 protein (may cause confusion with melanoma) and TFE3.

Differential diagnosis:

The differential diagnosis of PEComas depends on the morphology (epithelioid vs spindled) and location of the neoplasm.

PEComas must be distinguished from conventional melanoma and clear cell sarcoma.

Gastrointestinal stromal tumor (GIST) and carcinoma (especially clear cell carcinoma) may also be confused with epithelioid PEComas.

Smooth muscle neoplasm (with epithelioid and spindled morphology) is another tumour for which PEComas may be mistaken.

Folpe et al proposed the categorization of PEComas into three groups based on tumor diameter, nuclear grade and cellularity, mitotic rate, necrosis, vascular invasion and infiltrative growth.

Proposed classification of PEComas: Folpe A.L., McKenney J.K., Li Z., Smith S.J., Weiss S.W. Clear cell myomelanocytic tumor of the thigh. Am J Surg Pathol. 2002;26(6):809–812.

Benign	<5 cm in diameter Non-infiltrative Non-high nuclear grade and cellularity Mitotic rate ≤1/50 HPF No necrosis No vascular invasion
Uncertain malignant potential	One or both of the following features: Nuclear pleomorphism/multinucleated giant cells >5 cm in diameter
Malignant	Two or more of the following features: >5 cm in diameter Infiltrative High nuclear grade and cellularity Mitotic rate ≥1/50 HPF Necrosis Vascular invasion