Asymptomatic Soft Tumor on the Forearm

The Diagnosis: Aneurysmal Dermatofibroma

A shave biopsy of the entire tumor was performed at the initial visit. Histologic examination with hematoxylin and eosin staining revealed a fibrohistiocytic infiltrate containing cleftlike cavernous spaces lined by epithelial cells (Figure, A). Immunohistochemical staining revealed factor XIIIa expression on fibrohistiocytic cells (Figure, B). CD34 was expressed on vascular endothelial cells, but it failed to highlight the fibrohistiocytic space (Figure, C). Overall, these findings supported the diagnosis of aneurysmal dermatofibroma. The lesion healed without complications, and the patient was counseled on the risk for recurrence. He was offered localized excision but opted for conservative management without excision and close follow-up and monitoring.

Findings from a shave biopsy of the right forearm tumor supported the diagnosis of aneurysmal dermatofibroma. A, Hematoxylin and eosin staining revealed a fibrohistiocytic infiltrate containing cleftlike cavernous spaces lined by epithelial cells (original magnification ×40). B, Immunohistochemical staining for factor XIIIa diffusely highlighted the fibrohistiocytic infiltrate (original magnification ×40). C, Immunohistochemical staining for endothelial cell marker CD34 highlighted cavernous spaces (original magnification ×100).

Dermatofibromas are common benign cutaneous nodules that often are asymptomatic and occur on the extremities. Dermatofibromas also are known as cutaneous fibrous histiocytomas and have numerous histologic variants. Aneurysmal dermatofibroma (also called aneurysmal fibrous histiocytoma) is a rare histologic variant of dermatofibroma presenting as a slow-growing exophytic tumor that can be purple, red, brown, or blue. Although classic dermatofibromas typically constitute a straightforward diagnosis, aneurysmal dermatofibromas often are more challenging to clinically differentiate from other cutaneous neoplasms. Additionally, due to the exophytic nature and larger size (0.5–4.0 cm), aneurysmal dermatofibromas do not exhibit the characteristic dimple (Fitzpatrick) sign found in many dermatofibromas. Aneurysmal dermatofibromas are 10 times more likely to recur than classic dermatofibromas.^1-4

Aneurysmal dermatofibromas can mimic other cutaneous neoplasms, some indolent and others more aggressive. Similar to aneurysmal dermatofibromas, solitary neurofibromas and nevi lipomatosus can appear as asymptomatic exophytic nodules with a similar spectrum of color and indolent clinical courses. In nevus lipomatosus, the dermis is almost entirely replaced by mature adipose tissue.⁵ Solitary neurofibromas represent a proliferation of neuromesenchymal cells with haphazardly arranged, wavy nuclei characteristic of nerve cells.⁶ Dermatofibrosarcoma protuberans can be distinguished from aneurysmal dermatofibroma by lack of factor XIIIa expression and diffuse positivity for CD34.⁷ Finally, aneurysmal dermatofibromas may resemble vascular tumors such as nodular Kaposi sarcoma. Kaposi sarcoma can be differentiated from an aneurysmal dermatofibroma by the presence of characteristic vascular wrapping, the absence of fibrohistiocytic cells, and expression of human herpesvirus 8 latent nuclear antigen-1.^1,8 Although aneurysmal dermatofibromas are of low malignant potential, they are associated with a higher rate of recurrence compared to common dermatofibromas.⁹ Definitive treatment involves complete excision with follow-up to ensure no signs of recurrence.¹⁰ Incomplete excision can increase the likelihood of recurrence, especially for larger aneurysmal dermatofibromas. Aneurysmal dermatofibromas are one of the subtypes of dermatofibromas that may extend into the subcutaneous tissue. Han et al² found that 77.8% of aneurysmal dermatofibromas extended into subcutaneous tissue. Recognizing the clinical and pathological features of this rare subtype of dermatofibroma can aid dermatologists in appropriate recognition and management.