A Nodule on the Back.

An 89-year-old woman presented with a recurrent, gradually enlarging, painless nodule on her back. The nodule had been excised twice before at another institution most recently more than 2 years ago. The patient has severe asthma and hypertension. She sought to remove The nodule because it was "getting in the way." Physical examination revealed a tumor 8 cm in diameter with attenuated overlying skin but no ulceration, bleeding, or drainage. The clinical diagnosis was sebaceous cyst. The lesion was excised. Grossly, the excision specimen consisted of a well-demarcated, lobulated subcutaneous mass that measured 7.0 X 4.0 X 2.5 cm and was covered by unremarkable skin that measured 8.0 X 2.5 cm. Cut surface of the mass revealed a gray-tan firm tumor with multiloculated cystic spaces filled with dark yellow-tan fluid. Microscopically, the tumor was well demarcated from the surrounding tissue and showed coalescing masses of squamous epithelium with abrupt keratinization and keratin pearls. There were solid and cystic areas, some containing cholesterol clefts and blood. Foci of calcifications were present. There was minimal cellular atypia and only rare mitotic figures. The margins of excision were free of tumor (Figures 1 through 3).

[FIGURE 1-3 ILLUSTRATION OMITTED]

What is your diagnosis?

Pathologic Diagnosis: Proliferating Trichilemmal Tumor

Proliferating trichilemmal tumor is an uncommon but usually benign lesion, which may be misdiagnosed as squamous cell carcinoma. It is also known as proliferating trichilemmal cyst and pilar tumor of the scalp.[1] Ninety percent of proliferating trichilemmal tumors occur on the scalp, but they have also been found on the forehead, nose, back, chest, abdomen, buttocks, elbow, wrist, mons pubis, and vulva.[2,3] Most patients are women (84%) who range in age from 27 to 83 years. Most cases occur in the sixth and seventh decades of life.[2] A proliferating trichilemmal tumor may be present for many years. Typically, it appears as a slowly enlarging, painful subcutaneous scalp nodule. It may ulcerate, bleed, and produce a purulent discharge. Proliferating trichilemmal tumors may be mistaken for squamous cell carcinoma. Clinically, they have also been diagnosed as sebaceous, pilar, epidermoid, or trichilemmal cysts and dermatofibroma.[2] The development of proliferating trichilemmal tumors is thought to be related to trauma of trichilemmal cysts.[4] They arise from the outer sheath of the hair follicle.[3] Grossly, the tumors are sharply demarcated and lobulated, ranging from 0.4 to 10 cm in diameter. Cut surface reveals solid and cystic areas, sometimes with honeycomb configuration.[2] Proliferating trichilemmal tumors can occasionally become large, up to 25.0 cm.[5] Histologic characteristics of this lesion are sufficient for diagnosis. They include sharply circumscribed convoluted lobules of squamous epithelium, with pushing margins, extensive areas of necrosis with keratin debris, abrupt keratinization without formation of granular layer, and continuity with epidermis.[3,5,6] Small cystic areas may be present. Trichilemmal keratinization is the most characteristic feature.[2]

Differentiation of trichilemmal tumor from squamous cell carcinoma is the most significant problem in pathologic diagnosis. The presence of abrupt keratinization, minimal pleomorphism, low mitotic activity, sharp circumscription, foci indistinguishable from a trichilemmal cyst, calcification, and absence of a premalignant lesion such as actinic keratosis help to differentiate proliferating trichilemmal tumor from squamous cell carcinoma.[2,6,7] Despite the presence of cytologic atypia and the number of mitoses in some proliferating trichilemmal tumors, they behave as benign lesions.[6]

Malignant transformation occurs occasionally, which can be manifested by sudden rapid growth. Histologically, malignant proliferating trichilemmal tumors show severe nuclear atypia, marked cellular pleomorphism with atypical mitoses, dyskeratotic cells, and infiltrating margins.[3,8] Mutations of p53 gene have been reported,[9] although others failed to detect overexpression of p53 protein.[8] Lymph node and distant metastases have been reported in malignant proliferating trichilemmal tumors.[7,8] The treatment of malignant proliferating trichilemmal tumors is the same as in benign cases: surgical excision with clear margins. Both malignant and benign tumors, however, can recur even after adequate resection. Chemotherapy and x-ray therapy have been used for malignant tumors.[8]

The present case of proliferating trichilemmal tumor is typical in several respects: patient's age and sex, tumor size, duration, recurrence after excision, and gross and histologic appearance. Its location on the back is slightly unusual. Its benign histologic appearance led to a straightforward diagnosis.

References

[1.] Janitz J, Wiedersberg H. Trichilemmal pilar tumors. Cancer. 1980;45:1594-1597.

[2.] Brownstein MH, Arluk DJ. Proliferating trichilemmal cyst: a simulant of squamous cell carcinoma. Cancer. 1981;48:1207-1214.

[3.] Markal N, Kurtay A, Velidedeoglu H, Hucumenoglu S. Malignant transformation of a giant proliferating trichilemmal tumor of the scalp: patient report and literature review. Ann Plast Surg. 1998;41:314-316.

[4.] Leppard BJ, Sanderson KV. The natural history of trichilemmal cysts. Br J Dermatol. 1976;94:379-390.

[5.] Headington JT. Tumors of the hair follicle: a review. Am J Pathol. 1976;85: 480-505.

[6.] Amaral ALMP, Nascimento AG, Goellner JR. Proliferating pilar (trichilemmal) cyst: report of two cases, one with carcinomatous transformation and one with distant metastases. Arch Pathol Lab Med. 1984;108:808-810.

[7.] Park BS, Yang SG, Cho KH. Malignant proliferating trichilemmal tumor showing distant metastases. Am J Dermatopathol. 1997;19:536-539.

[8.] Weiss J, Heine M, Grimmel M, Jung EG. Malignant proliferating trichilemmal cyst. J Am Acad Dormatol. 1995;32:870-873.

[9.] Takata M, Rehman I, Rees JL. A trichilemmal carcinoma arising from a proliferating trichilemmal cyst: the loss of the wild-type p53 is a critical event in

malignant transformation. Hum Pathol. 1998;29:193-195.

Accepted for publication September 8, 1999.

From the departments of Pathology, Rhode Island Hospital and Brown University School of Medicine, Providence, RI (Dr Gray); and Departments of Pathology, Roger Williams Medical Center, Providence, RJ, and Boston University Scholl of Medicine, Boston, Mass (Dr Tibbetts).

Reprints not available from the authors.