Basaloid follicular hamartoma (BFH) is a rare malformation with characteristic histopathologic patterns. BFH consists of malformed hair follicles composed of anastomosing cords and strands of basaloid or squamoid cells1. Although BFH has been well described histologically, it occurs in the form of skin-colored papules, nodules, or plaques without specific clinical features1. Herein, we report a case of segmentally arranged hyperpigmented BFH in a 5-year-old girl.

A 5-year-old girl presented with asymptomatic, firm, hyperpigmented, grouped papules and comedones arranged segmentally on the left lateral aspect of her nose, left retroauricular area, and posterior neck (Fig. 1A, B). The lesions appeared when she was 6 months old, and the affected area had increased since then. She had no medical or family history. A skin biopsy of the retroauricular area demonstrated multiple islands composed of basaloid and squamoid cells restricted to the upper dermis, with branching cords and anastomosing strands (Fig. 2A, B). Some anastomosing strands were connected to the epidermis. No significant clefting between the tumor and stroma was observed, nor were there any atypical cellular or mitotic features. Based on clinical and histological findings, a diagnosis of BFH was made. Given the child's age and her parents' preferences, we decided to observe the lesions rather than treat them surgically or by laser.

The differential diagnosis of BFH, a benign adnexal tumor, includes infundibulocystic basal cell carcinoma (ICBCC), nevoid basal cell carcinoma syndrome (NBCCS), trichoepithelioma, and fibrofolliculoma. Of these, it is most important to rule out ICBCC, a malignant condition, to avoid unnecessary surgical excision. BFH is a superficial malformation of hair follicles and is composed of basaloid and squamoid cells, while ICBCC is composed of basaloid cells and can occasionally infiltrate into the deeper dermis, subcutaneous fat, or skeletal muscles. Additional characteristics of ICBCC include mitotic activity in the tumor cells and clefts within the stroma. Based on these histologic features, a diagnosis of BFH, rather than ICBCC, was made in this case.

NBCCS is hereditary, unlike BFH, and consists of multiple basal cell carcinoma (BCC) that manifest at an early age, along with skeletal anomalies, jaw cysts, and ectopic calcification.

Although the premalignant potential of BFH is currently undetermined, several cases of BCC within BFH have been reported2,3. Some evidence indicates that BFH is not a stable hamartomatous disease and that it might behave similarly to a sebaceous nevus, a lesion in which BCC can arise3.

Twenty-nine cases of BFH have been reported thus far. In Korea, there have been several cases of skin-colored, localized or multiple, scattered BFH4,5. Our case is unique owing to its clinical features: hyperpigmented, segmentally, and linearly arranged lesions with unilateral distribution. Although BFH does not have well-recognized clinical features, a specific histologic pattern can be found, providing a diagnostic clue. Failure to differentiate BFH from BCC may result in aggressive surgical excision of a benign adnexal tumor. However, further studies are needed to elucidate the premalignant potential of hamartomatous lesions and the possible relationship between BFH and BCC.