Pilomatrixoma: Misdiagnosed as a round cell tumor of soft tissue on fine needle aspiration cytology.

Sir,

Pilomatrixoma is a benign tumor of the prickle cell layer of the skin and was first described by Malherbe and Chenantais in 1880. The tumor can cause diagnostic difficulty not only for the clinicians but also for the cytologists. We report one such case that was misdiagnosed as a round cell tumor of the soft tissue.

A 10-year-old girl presented with multiple neurofibroma like swelling all over the body for last two years. The nodules were superficial, firm, and 1-3 cm in size. Fine needle aspiration cytology (FNAC) was performed from one swelling in upper arm of approximately 3 × 2 cm2 in size. Stained smears were cellular, showing predominantly round to oval cells with scanty cytoplasm. The cells were arranged in sheets; clusters as well as in a discrete manner. Occasional islands of anucleate cells were also seen in the smear [Figure 1a and b]. Diagnosis of ‘round cell tumor in the soft tissue’ was suggested and excision biopsy advised. Histopathology sections showed islands of two types of epithelial cells, basaloid cells having scanty cytoplasm and pathognomonic shadow (ghost) cells which were anucleate. There were areas of cyst formation, composed of eosinophilic cornified material with foreign body giant cell reaction and calcification. The histopathology diagnosis was ‘pilomatrixoma (Malherbe's tumor)’. On review of the smear it was found that basaloid cells of pilomatrixoma were mistaken for round cells of soft tissue tumor and the pathognomonic ghost (shadow) cells had been mistaken as understained cellular clump.

Figure 1

(a and b) Photomicrograph of fine needle aspiration cytology showing clumps of anucleate cells and another photomicrograph showing cluster of round to oval cells with scaty cytoplasm in a case of pilomatrixoma (MGG, ×400)

The aspirates have been mistaken for a metastatic carcinoma by the cytologists.[12] Sivakumar[1] in his case had also considered the possibilities of mucoepidermoid carcinoma and calcifying odontogenic tumor. The diagnosis of pilomatrixoma was established only after histopathology. Others have confused the aspirates with basal cell carcinoma.[3]

Most of the lesions diagnosed histopathologically as pilomatrixoma were in the neck region and misdiagnosed in cytology.

Bhadani et al.,[4] reported five histologically proven cases of pilomatrixoma. The pre FNAC clinical diagnoses were, sebaceous cyst, tuberculous lymphadenopathy, dermatofibroma, reactive lymphadenopathy, and lipoma. Pilomatrixoma was diagnosed on FNAC in three cases on finding groups of basaloid cells, ghost epithelial cells, pink fibrillary material, and calcium deposit.

Like in our case, Thapiyal et al.,[5] misdiagnosed a histologically proven case of pilomatrixoma as a round cell tumor on cytology. The lesion had presented as rapidly growing nodular swelling on the arm.

The risk of misdiagnosing pilomatrixoma as a round cell tumor of soft tissue needs to be kept in mind, especially when the population of shadow cells is low in relation to basaloid cells.