Firm lesion on the lateral thigh

THE DIFFERENTIAL DIAGNOSIS

Firm, hyperpigmented, macular or nodular skin lesions are prevalent and seen in a number of conditions (Table 1).

Dermatofibroma (fibrous histiocytoma) is a benign proliferation of collagen fiber and other mesenchymal cell lines, likely in response to local inflammation or trauma.¹

Dermatofibromas are more common in women and typically develop between ages 20 and 50.^1,2 They often present as smooth, slow-growing, firm, tan to reddish brown papules or nodules less than 1 cm in diameter that classically dent on compression.^1–3 They are mostly asymptomatic and may appear anywhere on the body, though 20% are on extremities.^1–3 Lesions are typically darker in color in the center and lighter toward the perimeter.^1–3

Lentigo maligna is a premalignant melanocytic nevus that may be considered melanoma in situ in its most advanced stages.⁴ It has a high risk of progression to invasive melanoma.⁴ Lesions typically present on sun-exposed skin such as the head or neck.⁴ They appear as heterogeneous asymmetric macules with irregular borders that grow centrifugally.⁴ Ultraviolet light examination with a Wood lamp can show extension of the lesion far beyond the pigmented borders.⁴

Treatment is typically by surgical excision with borders greater than 7 mm and histopathologic examination of the margins, though radiation and topical imiquimod may be used in specific circumstances.⁴

Dermatofibrosarcoma protuberans is a malignant neoplastic lesion, more common in women and darker-skinned individuals age 30 to 50.⁵ It can present as an asymptomatic, slow-growing, violaceous nodule or plaque, more often on the trunk or upper extremities.⁵ It is typically larger than a dermatofibroma, with an irregular border and deeper palpable skin invasion.³ Diagnosis is typically by excisional biopsy.⁵ Though it has a low metastatic potential, it can have a great capacity for local invasion and destruction.⁵

Treatment requires excision with exhaustive histopathologic examination of boundaries for tumor cells, either by Mohs micrographic surgery or wide local excision.⁵ Adjuvant and neoadjuvant therapies such as radiation and imatinib may also be used in cases refractory to excision or with extensive invasion.⁵

Seborrheic keratosis is a common benign skin tumor that becomes increasingly common with age, though lesions can present at any age.⁶ They can be pigmented and so may be mistaken for dermatofibroma, but they do not dimple to lateral compression.⁶ They typically present as sharply demarcated ovoid macules or papules 1 cm in diameter and with a shiny (“oily”) appearance.⁶ They classically appear raised and “stuck on” to the skin.^1,6 Obvious seborrheic keratoses may be monitored, but questionable lesions should be diagnosed with shave excision or curettage and histopathology.⁶

Epidermoid inclusion (sebaceous) cyst is a common cystic lesion that can be flat or raised, with size ranging from a few millimeters to a few centimeters.¹ They often have a dark central punctum, which occasionally drains.¹ They are benign and should be removed only if they cause symptoms such as frequent infection or for cosmetic reasons.¹

MANAGEMENT OF DERMATOFIBROMA

Most dermatofibromas do not require treatment unless they show signs of malignant progression such as a change in quality or rapid growth.^1,2 It is essential to distinguish them from the far more malignant dermatofibro-sarcoma protuberans, as well as melanoma and other malignant lesions. Irregular borders or substantial palpable depth of invasion through skin should prompt excisional biopsy for definitive diagnosis.³