Small blue cell tumour NOS
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The correct diagnosis is:
- The smears are cellular and contain a mixture of cohesive to loosely cohesive basaloid cells with small but prominent nucleoli, thick sheets of anucleate squamous cells and a background of chronic inflammatory cells, multinucleate foreign body type giant cells and fibrous stroma.
- The clinical and cytologic features suggest pilomatrixoma.
- Pilomatrixoma (calcifying epithelioma of Malherbe) is a benign neoplasm of hair follicle origin.
- It usually occurs in the first 2 decades of life and most commonly in the head and neck region or upper extremities.
- Basaloid epithelial cells and anucleate squamous cells ("ghost cells") are the main specific components. With an adequate sample most cases can be diagnosed with fine needle aspiration based on the presence of these 2 features. Sampling of only one component may suggest other diagnosis.
- Skin appendage tumours such as a cylindroma contain mainly basaloid cells and may be confused for a pilomatrixoma. However these tumours typically yield cohesive, smoothly contoured clusters of basaloid cells, rather than the irregular, often jagged, cohesive to loosely cohesive monolayer sheets characteristic of a pilomatrixoma. Ghost cells and multinucleated giant cells are rarely present.
- A monomorphic population of anucleated squamous cells may suggest an epidermal inclusion cyst, trichilemmal cyst or a branchial cleft cyst. Epidermal inclusion cysts most often show well-delineated anucleated squamous cells lying singly and in clumps, as opposed to the irregular and degenerated anucleated squames of pilomatrixoma. A foreign body granulomatous reaction may be present if an epidermal cyst has ruptured. However, basaloid cells are rarely, if ever, identified in these cysts. Trichilemmal cysts usually occur on the scalp and yield groups of mature basaloid cells with more abundant and dense cytoplasm. Branchial cleft cyst typically yields cloudy fluid containing a mixture of inflammatory cells, histiocytes, nucleated and anucleated squamous cells and columnar cells.
- Fragments of proliferating basaloid cells with typical mitosis may be incorrectly interpreted as a small blue cell tumour of childhood if the other features of a pilomatrixoma are not sampled or appreciated.
- Histologically, the tumour is well circumscribed and composed of solid nests of uniform basaloid cells with prominent nucleoli. Mitosis may be frequent. Basal cells undergo abrupt squamous differentiation and degeneration, leaving anucleated (ghost) cells. A granulomatous foreign body giant cell response to keratin is common.
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Wang J, Cobb CJ, Martin SE, Venegas R, Wu N, Greaves TS. Pilomatrixoma: Clinicopathologic Study of 51 Cases With Emphasis on Cytologic Features. Diagnostic Cytopathology 2002; 27:167-172.