Pilomatricoma Simulating Metastatic Pancreatic Carcinoma: A Diagnostic Pitfall

Sir,

A 69 year old male presented with a nodule in the right inguinal region for 5 days. On examination, the nodule was subcutaneous in location, firm, nontender and measured 1.3 cm × 1.1 cm. He had a history of moderately differentiated adenocarcinoma pancreas.

A clinical diagnosis of cutaneous metastasis was considered, and fine-needle aspiration cytology (FNAC) was performed with a 23G needle. Air-dried smears were made and stained with May–Grünwald–Giemsa stain. On examination, smears were highly cellular with cells arranged in sheets and groups. Most of the cells had a high nuclear-cytoplasmic ratio, hyperchromatic round nuclei, and scanty cytoplasm.

Few of the groups had a papillary appearance and were admixed with eosinophilic stromal substance. Also found admixed were occasional anucleated squamous epithelial cells and multinucleated giant cells [Figure 1]. On correlating with radiological findings, the lesion was found to be positron emission tomography (PET) avid on a PET scan. There was no other lesion anywhere in the body. Collating the findings, a possibility of metastatic carcinoma was suggested and CA19.9 levels advised. The case was discussed with the clinician, and considering this lesion to be an oligometastasis, an excision was advised. Serum CA19.9 levels were not done.

Figure 1.

(a) Cluster of shadow cells with abundant cytoplasm, central unstained area, and distinct cell borders (MGG, ×400). (b) Cluster of anucleate squamous epithelial cells (MGG, ×400). (c): Basaloid cells admixed with anucleate squamous cells and multinucleated giant cells. (d) A cluster of basaloid cells with round, hyperchromatic nuclei and scant cytoplasm (MGG, ×400)

A subsequent histopathological examination revealed epidermis with a tumor in dermis composed of basaloid tumor islands with intervening stroma. These cells were admixed with sheets of shadow cells. Foci of dystrophic calcification, sheets of histiocytes, and few multinucleated giant cells were also seen [Figure 2]. A final diagnosis of pilomatricoma was given.

Figure 2.

(a) Overlying unremarkable epidermis with well-circumscribed tumor in dermis composed of epithelial islands embedded in cellular stroma (H and E, ×100). (b) Biphasic population of tumor comprising basophilic basaloid cells (left) and shadow or ghost cells (right) (H and E, ×100). (c) Basophilic cells with round and elongated nuclei (H and E, ×400). (d) Transformation of basaloid cells into shadow (ghost cells), associated with loss of nucleus (H and E, ×400)

Pilomatrixoma (PMX), also known as “pilomatricoma” or “calcifying epithelioma of Malherbe,” is an uncommon benign slow-growing neoplasm originating from pluripotential cells of the outer sheath cell of the hair follicle root.[1,2] PMX most commonly occurs in head and neck region and rarely at other sites (upper extremity, trunk, and lower extremities).[3,4,5] Although this neoplasm can present at any age, it demonstrates a bimodal peak presentation during the first and sixth decades of life.[4]

Pilomatricomas are benign adnexal tumors that are frequently misdiagnosed by the physicians. The clinical differential diagnosis for head and neck pilomatricoma includes a sebaceous cyst, ossifying hematoma, chondroma, degenerating fibroxanthoma, giant cell tumor, foreign body reaction, dermoid cyst, osteoma cutis, squamous cell carcinoma, basal cell carcinoma, amelanotic malignant melanoma, and metastatic bone formation.[6]

The tumor can show a spectrum, depending on the age of the lesion and site of aspiration. The triad of basaloid cells, squamous cells, and characteristic shadow or ghost cells allows confident diagnosis of PMX. However, the diagnostic accuracy of PMX on FNAC is low and varies from 21% to 44% in different studies.[7]

In the present case, a retrospective analysis of the cytosmears revealed few groups of ghost cells, anucleated squamous epithelial cells, and occasional multinucleated giant cells admixed with the basaloid cells, which favored the diagnosis of pilomatricoma. However, on extensive search, no foci of calcification were noted.

To conclude, the present case highlights the importance of considering pilomatricoma as a differential diagnosis of any dermal/subcutaneous nodule in any age and locations other than head and neck. It also reiterates the fact that cytopathologists who play an important role in diagnosing such lesions should be aware of the variability of the cellular composition of these types of lesions to avoid misdiagnosis.

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Conflicts of interest

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