Pilomatrixoma misdiagnosed as round cell tumor on fine-needle aspiration cytology

Abstract

Sir, Pilomatrixoma or calcifying epithelioma of Malherbe is a benign appendageal tumor with differentiation toward hair cortex cells and accounts for almost 20% of pilar tumors. It usually occurs as a solitary lesion on the face or upper extremities and is composed of 2 types of cells: basaloid cells and shadow cells, present in irregularly shaped islands and separated by a cellular stroma.[1] There is a tendency for basaloid cells to be overrepresented in fine-needle aspiration (FNA) smears, probably because they are more easily detached by the needle. This often leads to an erroneous cytological diagnosis.[2] We report a case of pilomatrixoma, which was misdiagnosed on fine-needle aspiration cytology (FNAC) as round cell tumor. The patient was a 16-year-old male, who presented with a rapidly enlarging, painless 1.5 Χ 1 cm nodule in the right posterior cervical region for 2 months. A clinical impression of the posterior cervical lymphadenopathy was made and an FNA was requested. FNAC smears were cellular, showing cells with round to ovoid nuclei, 1-3 prominent nucleoli and scanty cytoplasm, dispersed singly as well as in clusters [Figure 1]. Rosette-like appearance was seen in an occasional cluster. Nuclear molding was also noted in some of them. Few foreign-body type giant cells were also encountered. A diagnosis of round cell tumor was made and excision biopsy was advised.Figure 1: Photomicrograph showing clusters of round, basaloid cells, some exhibiting nuclear molding, in the fine-needle aspiration smear.Histopathologic examination of the excised specimen showed the typical histology of pilomatrixoma [Figure 2].Figure 2: High-power view showing basaloid cells and ghost cells (HandE, ×400). The inset shows foreign body giant cell reaction (HandE, ×400).Pilomatrixoma may arise in persons of any age, but 60% occur in the first 2 decades of life.[1] It has been variably misinterpreted as round cell tumor,[3] squamous cell carcinoma,[4] or even as metastatic carcinoma.[4] The cytologic diagnosis is based on the combination of basaloid cells, ghost cells, and foreign-body giant cells against an inflammatory background. Basaloid cells can be seen as single cells, in clusters, sheets, or as bare nuclei in the background. Ghost cells are usually seen in clusters. The above-mentioned diagnostic triad may not necessarily be present in all cases, especially when the aspirate is from the periphery or from an early lesion. Smears that show mainly basaloid cells can be easily misinterpreted as small round cell tumor because of their high nucleus:cytoplasmic ratio, slight nuclear hyperchromasia, and presence of nucleoli. On the other end of the spectrum, when ghost cells and foreign body giant cells predominate, the cytologic features can even mimic epidermal inclusion cyst or giant cell lesions.[4] Rapidly growing, early lesions of pilomatrixoma, predominantly composed of basaloid cells may lead to a misdiagnosis as a metastatic deposit.[4] This possibility was also considered by us, but the age of the patient and lack of significant nuclear pleomorphism made us favor a diagnosis of round cell tumor. We were unable to find ghost cells even after careful retrospective analysis of the slides. However, lack of shadow cells does not preclude correct diagnosis in the presence of the other relevant cytologic features.[5] Appreciation of the 2 different cell types, along with correlation with the clinical presentation should help the cytopathologist to arrive at a correct diagnosis.