Axillary basal cell carcinoma: A retrospective review of 57 cases

To the Editor: Basal cell carcinoma (BCC) predominantly arises on chronically sun-exposed skin, yet rare cases occur at sun-protected sites. Among these, the axilla merits specific attention for several reasons. First, the axilla is a rarely occluded, hair-bearing intertriginous site with a high density of apocrine glands and a distinct microbiome, features that distinguish it from other sun-protected locations, such as the anogenital region. Second, axillary lesions pose a particular diagnostic challenge because they mimic common benign axillary pathology, including cysts, skin tags, and melanocytic neoplasms. Third, recent genomic studies of BCCs arising at sun-protected sites have demonstrated recurrent PTCH1 alterations without ultraviolet mutational signatures, supporting ultraviolet-independent Hedgehog pathway activation as a driver of tumorigenesis at these locations.^1,2 Whether the axillary microenvironment contributes to this alternative mutagenic mechanism remains unknown. We therefore retrospectively reviewed biopsy-proven axillary BCCs diagnosed at our tertiary referral center over a 15-year period.

Fifty-seven axillary BCCs were identified in 56 patients (Table I). Median age was 69 years, and 57.1% were men. Twenty-three patients (41.1%) had a history of BCC, whereas only 1 patient each had immunosuppression, prior axillary radiation, or Gorlin syndrome. Tumors were evenly distributed between the right and left axillae, with a median clinical diameter of 10 mm (Table II). Most lesions demonstrated nodular or superficial histology; only 1 infiltrative BCC was identified.

Clinically, 35.1% of tumors were asymptomatic, and 22.8% were discovered incidentally during examination. BCC was included in the clinical differential diagnosis in 68.4% of cases, although pigmented lesions were frequently mistaken for melanocytic neoplasms. Curettage, excision, and Mohs micrographic surgery were the most common treatments.

To our knowledge, our series of 57 tumors represents one of the largest single-institution cohorts of axillary BCCs reported to date.^3,4 Consistent with prior reports, axillary BCCs in our cohort were predominantly nonaggressive histologic subtypes.3, 4, 5 The frequent absence of symptoms and high rate of incidental discovery underscore the importance of complete skin examinations that include intertriginous and sun-protected areas. Clinicians should maintain suspicion for BCC when evaluating persistent axillary lesions, even in patients without classic risk factors. Future molecular characterization of axillary BCCs may clarify whether the rare microenvironment of this site contributes to ultraviolet-independent pathways of tumorigenesis.