Pilomatricoma with Florid Osseous Metaplasia: A Cytologist’s Perspective

Dear Editor, Pilomatricoma is a benign skin adnexal tumor with differentiation toward the hair follicle cortex. Head and neck are usually the most common site to be involved, followed by upper extremities and trunk.[1,2] It commonly affects children and young adolescents; however, a small peak in the elderly has also been reported. Clinically, it presents as a solitary, firm, cutaneous nodule with normal overlying skin, though multiple pilomatricomas have also been reported.[2] Pilomatricomas with osseous metaplasia, also known as ossifying pilomatricomas, are rare and only a handful of cases have been reported.[1,3] Clinically, they are harder and have a longer history.[3] We present an unusual case of pilomatricoma with florid osseous metaplasia in a 32-year-old woman who presented with a nodule over her right leg for the last 16 years. The nodule was 1 × 1 cm, subcutaneous, mobile, firm-hard, painless, and non-progressive in size [Figure 1a]. Fine needle aspiration yielded blood mixed aspirate. Giemsa stained smears were examined and showed osteoclastic giant cells, occasional basaloid cells, and focal areas of calcification along with osteoid. No atypia/mitosis or necrosis was noted [Figure 1b-d]. The patient underwent excision of the lump and it was sent for histopathological examination. Grossly, it was a 2 × 2 × 1 cm grey-white mass, hard in consistency. H and E sections examined after mild decalcification revealed a tumor comprised of ghost cells, areas of calcification, basaloid cells, and osteoclastic giant cells [Figure 2]. Also seen were areas of mature lamellar bony trabeculae. However, no intervening hematopoietic elements were noted. A final diagnosis of pilomatricoma with florid osseous metaplasia was rendered.Figure 1: (a) Clinical photograph showing 1 × 1 cm subcutaneous swelling. (b) FNA smears showing a basaloid cell cluster and anucleate squames in the background. (Giemsa: 100×). (c) FNA smears showing osteoclastic giant cells and calcification (Giemsa: 200×). (d) FNA smears showing osteoclastic giant cells with eosinophilic osteoid (Giemsa: 400×)Figure 2: Section showing ghost cell areas with florid osseous metaplasia and mature bony trabeculae. (Hematoxylin and Eosin; 100×) Inset shows high power view of osteoclastic giant cells (Hematoxylin and Eosin; 400×)Pilomatricoma traditionally represents a disorder of hair matrix differentiation with limited cytologic differentiation of pilar keratinocytes and a lack of mature hair development. However, it is now considered a complex panfollicular neoplasm and can exhibit signs of differentiation of the entire follicle. The histopathological appearance of the lesion varies with the duration of the lesion, starting as infundibular cyst and ending as hard, calcified nodule.[1] Abundance of basaloid cells in early lesions might lead to misdiagnosis as small round blue cell tumors, as reported by Singh et al.[4] Cytology smears of pilomatricoma in late stage are paucicellular and show fewer basaloid cells, calcification, eosinophilic ghost cells, and foreign body giant cells. The present case also showed osteoclastic giant cells with eosinophilic osteoid formation. Osseous metaplasia in pilomatricoma is seen in old pilomatricomas like in the present case and is postulated to be a sequelae of prolonged fibroblastic response.[1,5] To the best of our knowledge, fewer than 15 cases of ossifying metaplasia have been reported so far.[1] Most of the cases were reported in the head and neck region. Only one case involving the lower extremity has been reported.[3] Ossification in pilomatricoma is usually seen next to the ghost cell areas with an associated paucity of basaloid cells. In the present case, owing to the long standing history of the lesion, very occasional basaloid cells were found. It is hypothesized that bone morphogenic protein (BMP-2) mediates fibroblast metaplasia into osteoblasts in late-stage pilomatricomas.[5] Oncostatin M, produced by activated macrophages, may also play a role in mediating the differentiation of precursor cells into osteoblasts. Deposition of calcium phosphate in the ghost cell nests is postulated to be triggered by macrophage-derived osteopontin.[1,5] Extramedullary hematopoiesis has also been reported to be associated with ossifying pilomatricomas.[6] However, in the present case, no hematopoietic elements could be found in the intertrabecular areas. Skin adnexal tumors with aberrant ossification include trichilemmal cyst, trichoepithelioma and chondroid syringoma[1] [Table 1]. Pilomatricoma is treated by surgical resection, and the rate of recurrence is very low. The patient is doing well and is free of recurrence, at 6 months of follow-up. To conclude, this is a rare case describing cytological findings of pilomatricoma with ossification confirmed on histopathology.Table 1: Differential diagnoses of pilomatricoma with osseous metaplasia on fine needle aspiration cytologyDeclaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. 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