Rapidly Enlarging Malignant Proliferating Trichilemmal Tumor

This observation describes the presentation, histopathologic diagnosis, and treatment of a malignant proliferating trichilemmal tumor.

A 38-year old female presented with a several-year history of an enlarging, firm, and occasionally painful lesion on her left scalp (Figure A). The patient denied any bleeding or drainage from the nodule. There was no personal or family history of skin cancer. On physical exam, a 3 cm x 3 cm x 1.5 cm, well-demarcated, brown-colored multinodular, dermal lesion was noted on the patient’s left scalp. It was not ulcerated or fixed to underlying bone. A shave biopsy was performed, and histopathologic examination of the specimen revealed abrupt trichilemmal keratinization without a granular cell layer, cysts with homogenous keratin, and multiple, broad, anastomosing bands of nodules and lobules. Furthermore, pleomorphism with nuclear atypia, increased abnormal mitotic figures, hyper- chromatism, and necrotic cells led to the diagnosis of malignant proliferating trichilemmal cyst (Figure B). The patient elected for complete surgical excision given the histopathologic diagnosis. The patient continues to be closely monitored for recurrence and/or metastasis.

Wilson-Jones first described these tumors in 1966 as proliferating epidermoid cysts.¹ Pilar cysts originate from the isthmus of the outer follicular hair root sheath, giving rise to the term trichilemmal cyst or tumor.^2,3 Several years later, Saida et al introduced the classification of the malignant proliferating trichilemmal cyst to describe a trichilemmal tumor that was infiltrative and metastatic with histopathologic features of atypia and increased mitotic activity.^4,5 While benign pilar cysts are fairly common, affecting roughly 5-10% of the population, transformation into the malignant counterpart or de novo malignancies are extremely rare. Classically, these tumors are found on the scalp in females during their 8th to 9th decade of life.^3,6

Ye et al examined 76 proliferating trichilemmal tumors (PTT) in order to outline histopathologic features that could predict clinical behavior and prognosis.⁷ Three distinct PTT variants were described based on malignant features such as nuclear atypical, mitotic activity, and presence of necrosis. Group I lesions are considered benign with recurrence not observed. Histopathologic examination reveals trichilemmal keratinization, focal nuclear atypia, mononuclear infiltrates, and absence of mitotic figures, perineural invasion, or vascular invasion. Group II lesions are low-grade malignant tumors with an increased risk of local recurrence. Histologically, these tumors are noted to have abrupt keratinization, areas of single-cell necrosis, and desmoplastic stroma. Cytologic atypia is absent. Group III lesions are high-grade malignant tumors with a high risk of recurrence, lymph node involvement, and/ or distant metastasis. Group III tumors infiltrate into the dermis and have abrupt keratinization with regions of multifocal necrosis. Atypical mitoses and a desmoplastic stroma are also present. Because histological examination of our patient’s lesion demonstrated abnormal mitotic figures, cytologic atypia, and necrosis it was considered a Group 3 (high-grade) tumor based on the criteria proposed by Ye et al.

Distinguishing the variant of PTT using histologic examination is important, as no specific clinical criteria currently exist to differentiate benign and malignant tumors. Clinical signs that suggest a poor prognosis include rapidly enlarging tumors, tumors greater than 5 cm in diameter, location other than the scalp, and areas of necrosis on physical examination.⁸Histological features that favor malignant proliferating trichilemmal tumor (MPPT) include trichilemmal keratinization, lobular growth, and clear demarcation from surrounding structures.^8,10 Studies have suggested that immunohistochemical markers such as CD34, Ki-67, and P53 can be used to diagnose MPTT versus squamous cell carcinoma (SCC) and to determine the malignant potential of the lesion.^3,8,11 Distinguishing MPTTs from SCC is particularly important as the risk of recurrence and the risk of metastasis are very different between the two. In two case series, the reported rate of metastasis for Group III MPTTs, like that seen in our patient, was approximately 25%.⁴ Alternatively, in cutaneous squamous cell carcinoma the risk of metastasis is much lower at approximately 1.9-2.6%.¹² More research is required before these immunohistochemical markers are used in clinical practice. Wide surgical excision with 1 cm margins is the treatment of choice. Patients should be followed closely to monitor for recurrence and/or metastasis. Case reports utilizing adjuvant chemotherapy/radiation have been published in patients that presented with metastasis and aggressive malignant tumors.^13,14 These adjuvant therapies should be used on a case-by-case basis as no clear clinical or pathological guidelines have identified when these treatments are indicated. Mohs micrographic surgery has also been reported as a successful treatment modality in at least one case of a MPTT with local imagining recommended to investigate for regional metastases.¹⁵

Although malignant proliferating trichilemmal tumors most commonly occur in the 8th to 9th decade of life, we present a case of a high-grade tumor occurring in a 38-year-old woman. Given the risk of recurrence and metastasis in such lesions, appropriate identification of this rare neoplasm in younger patients will hasten treatment and improve prognosis.

References

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