Basal cell carcinoma (BCC) is the most common skin cancer, occurring more frequently than malignancies of any other tissue or organ with a steadily increasing incidence12. Ultraviolet radiation is considered the single most important risk factor for BCC. Additional risk factors include exposure to arsenic, coal tar derivatives, irradiation, scars, burn sites, chronic inflammation, ulcers, and immune deficiency1.

BCC arises commonly on sun-exposed sites such as the head and neck and occurs infrequently on non-sun-exposed sites such as the axilla, buttock, groin, penis, scrotum, vulva, breast, and nipple34. Among these, the occurrence of BCC in the axilla is extremely rare4, and to date, only 5 cases have been reported in Koreans567 (Table 1). BCC can be classified histologically into nodulocystic (nodular), mixed, infiltrative, superficial, micronodular, adenoid, metatypical, morpheaform and fibroepithelioma type2. The occurrence of adenoid BCC is relatively low compared to the nodulocystic BCC, which is the most common type of BCC2. We herein report a rare case of adenoid basal cell carcinoma in the axilla in a 33-year-old woman.

A 33-year-old woman presented with a 7-month history of a solitary, well-demarcated, erythematous nodule with brown to black colored crust in the left axilla. The size increased after the patient gave stimulus to the lesion by squeezing it with her hands. There was no previous personal history of skin cancer and no other significant cutaneous or medical history was elicited. Notably she had no history of trauma, chronic axillary inflammation, immune deficiency, or exposure to artificial ionizing radiation or arsenic. There was no family history of skin cancer or other skin disease. On examination, the left axillary vault revealed a solitary, asymptomatic, erythematous nodule with brown to black colored crust measuring 1.0 × 0.8 × 1.5 cm (Fig. 1). No lymphadenopathy was appreciated. Laboratory studies including a complete blood cell count, blood chemistry, VDRL, urinalysis, chest X-ray and electrocardiogram were within normal limits or negative. Histopathologic examination showed proliferation of basaloid cells that extended into the dermis with a cyst-like enlarged structure (Fig. 2A). Peripheral palisading was prominent in many of the cellular aggregates. In the center of the cyst-like enlarged structure, the basaloid cells showed slightly enlarged, hyperchromatic nuclei and scanty amphophilic cytoplasm (Fig. 2B), and atypical cells stained negative for PAS. On immunohistochemical analysis, atypical cells stained negative for CEA, EMA and vimentin.

Based on the results of laboratory and histopathological findings, this case was diagnosed as an adenoid basal cell carcinoma. A surgical excision was performed that removed the entire lesion and no recurrence was noted.

Basal cell carcinoma (BCC) is the most common skin cancer derived from basaloid epithelia located in the follicular bulges, in the anagen hair bulbs and the follicular matrix cells, and in specific basaloid cells of the interfollicular epidermis1. Recently, the occurrence of BCC has increased. This is because the elderly population has increased due to the extension of the average life span, improved standard of living and changes in environment and lifestyle have increased exposure to sunlight, industrialization has contributed to the destruction of the ozone layer, exposure to harmful substances has increased, and improved awareness of patients have lead to more frequent visits to the hospital2.

BCC most commonly occurs in sun-exposed sites such as the face and neck, where 80–90% of BCCs occur. Ten to fifteen percent of BCCs occur in non-sun-exposed sites and usually occur in the axilla, buttock, groin, penis, scrotum, vulva, breast and nipple368. Among these, the occurrence of BCC in the axilla is extremely rare4, and to date, only 5 cases have been reported in Korean patients567 (Table 1). BCC can be classified histologically into nodulocystic (35.4%), mixed (30.1%), infiltrative (9.3%), superficial (6.7%), micronodular (6.2%), adenoid (5.9%), metatypical (4.0%), morpheaform (2.1%), and fibroepithelioma types (0.3%)2. The occurrence of adenoid BCC is rare, and relatively low compared to the nodulocystic BCC, which is the most common type of BCC2. Among the 5 cases of BCC in the axilla reported in Korean patients567, there were no other such cases of adenoid BCC.

Ultraviolet radiation is considered the single most important risk factor for BCC, and arsenic, coal tar derivatives, irradiation, scars, burn sites, chronic inflammation, ulcer and immune deficiency are also associated with the occurrence of BCC1. The genodermatoses that enhance the risk of BCC include xeroderma pigmentosa, Rasmussen syndrome, Rombo syndrome, Bazex-Christol-Dupré syndrome, albinism and Darier's disease. These syndromes variably either decrease epidermal pigmentation and thus increase the risk of UV light-induced oncogenic transformation or promote genotypic instability in the dermis13. In this case, none of these risk factors or genodermatoses were found.

Although ultraviolet radiation is thought to be the primary risk factor in development of BCC, the precise relationship is not as clear as with squamous cell carcinoma (SCC). For example SCC occurs most commonly in highly sun-exposed areas on the dorsal hands and forearms, forehead, superficial pinna, and lower lip. SCC also correlates with chronic, cumulative ultraviolet radiation exposure. In contrast, BCC is less likely to arise on the dorsal hands and it occurs more commonly in sunprotected areas than SCC4.

Several theories explaining why BCC occurs at sun-protected sites have been proposed. Gibson and Ahmed9 reported 51 cases of perianal and genital BCC. They reported no association with human papillomavirus and suggested that local trauma and advancing age may contribute to the development of BCC at these sites. In this case, given the fact that the size of the lesion increased after the patient gave stimulus to it, it is thought that the development of BCC can be associated with trauma. Heckmann et al10 proposed that areas with high facial BCC frequency despite low UV exposure, such as the medial quadrant of the orbit, are characterized by a concave shape, a reduced skin tension, and the presence of marked skin folds. They suggest that the disturbed cell matrix interactions found at these sites may be a cofactor for developing BCCs. LeSueur et al.4 proposed that non-sun-exposed sites, including the axilla, might share these same characteristics.

Location, size, histologic tumor type and treatment strategies are important factors in the recurrence of BCC. Lesions that developed a long time ago, that occur in the center of the face or the ear, that exceed 2 cm in size, that have a history of therapeutic management and that are histologically infiltrative, micronodular and morpheaform have a high risk of recurrence2. As a therapeutic method, Mohs micrographic surgery is known to be most effective12511. However surgical excision is preferred because of the good prognosis of the disease and the fact that Mohs micrographic surgery is complicated and time-consuming. In this case, the lesion was removed by surgical excision, and no evidence of recurrence has been found during the follow-up visits until now.

Because the axilla is a site that is less likely to be monitored by the patient, there is the potential for delay in diagnosis and treatment, increasing the possibility of more extensive surgery and recurrence. This report highlights the importance of performing a complete cutaneous examination including sunprotected sites. This is especially important for patients with a history of skin cancer or those with other potential risk factors45.