- In the primary tumor, the malignant cells arise from the intraepidermal portion of the eccrine sweat glands and may be limited to the epidermis or may extend into the dermis;

- The tumour is asymmetrical with an infiltrative growth pattern  ; 

- Infiltrating tumour shows multiple attachments to the epidermis ;

- Two types of atypical cells: eosinophilic and clear cells.

Eosinophilic cells- polyhedral or fusiform with round to oval hyperchromatic nuclei, distinct nucleoli, indistinct cell boundaries, and a variable amount of cytoplasm (small eosinophilic cells or large eosinophilic cells); 

Clear cells- large and polyhedral with round to oval nuclei, abundant clear cytoplasm, and distinct cell borders; Clear cell areas are less prominent than hidradenocarcinoma 

- Obvious nuclear atypia with frequent mitoses and focal necrosis (comedonecrosis) are characteristic features;

- Cords and nests of polygonal tumour cells penetrate to the adjacent dermis or extend into subcutaneous tissue ;

- Ductal structures are present with intracytoplasmic lumina formation ;

- Although glandular differentiation is characteristic, poorly differentiated porocarcinoma may not show obvious duct formations ;

- There is squamous differentiation ;

- Melanin pigment is demonstrated in some cases ;

- Inflammatory infiltrate around neoplastic masses is composed of lymphocytes and plasma cells, rarely mixed with neutrophils and eosinophils ;

- Tumour with only intra-epidermal component is known as in-situ porocarcinoma ;

- Intraepidermal portion of the growth is composed of large atypical cells arranged in irregular nests and islands, variable in size and in shape, and sharply demarcated from the surrounding epidermal keratinocytes ;

Benign component of poroma and hidroacanthoma simplex is present in about 10% of cases.

Neoplastic cells contain glycogen and intratubular, PAS-positive, diastase-resistant material is usually present.

Differential diagnosis:

Histopathologically, basal cell carcinoma, Paget's disease, melanoma, metastatic adenocarcinoma, trabecular carcinoma, and Merkel cell carcinoma should be included in the differential diagnosis.

Distinguishing eccrine porocarcinoma from metastatic adenocarcinoma, and especially adenocarcinoma of a breast or lung origin, can be difficult.

Trabecular carcinoma and Merkel cell carcinoma also show findings comparable with EPC.

Hyperchromatic, large, and irregular nuclei arranged in cord or trabecular patterns are common features of eccrine porocarcinoma and trabecular carcinoma; however, the latter shows nuclear molding.

On immunoperoxidase studies, EMA and CK-7 are positive, and S-100 protein and CK-20 are negative in eccrne porocarcinoma.  

Merkel cell carcinomas express neuron-specific enolase (NSE) and CK-20, and they do not express CK-7.

Immunocytochemistry:  Tumour is cytokeratin , CEA and epithelial membrane antigen positive.