Answer: Spindle cell lipoma

Histology: Grossly the lesion was seen as a solitary circumscribed nodule involving the parotid parenchyma. Histologically, the lesion is comprised of a proliferation of small and uniform spindle cells with no significant pleomorphism. The spindle cells are set in a collagenized background. The collagen fibers are arranged in rope-like bundles. Other stromal components such as the blood vessels and mature adipose tissue are not conspicuous. An immunohistochemical panel was performed. The spindle cells were immunoreactive for CD34; and they were not immunoreactive for S100.

Discussion: Spindle cell lipoma is an unusual histologic subtype of lipoma. It is characterized by the presence of collagen-forming spindle cells at the expense of mature fat. The proportion of spindle cells and mature fat cells varies from tumor to tumor. When the histologic picture is dominated by the spindle cell component, recognition as a benign lipomatous tumor becomes challenging. In these instances, spindle cell lipoma is easily confused with other spindle cell lesions such as neurofibroma. Unlike neurofibromas, spindle cell lipomas are S100 negative. Strong staining for CD34 is well recognized in spindle cell lipomas; thus CD34 positivity should not be used as a sole criterion for establishing a diagnosis of solitary fibrous tumor (SFT). Given the morphologic and immunohistochemical overlap between spindle cell lipoma and SFT, some regard spindle cell lipoma as a member of the SFT family.

Although spindle cell lipomas typically arise in the region of the head and neck, they generally target the posterior neck. Involvement of the parotid gland is rare but not unreported. These are benign tumors that do not undergo malignant transformation and generally do not recur following conservative excision.