There are several variants of xanthoma, as you can see listed here (Rapini).
The dermis is infiltrated by nearly confluent aggregates of foamy cells.
Diffuse infiltrate of xanthoma cells (macrophages stuffed with lipid) are pale staining.
The xanthoma cells have abundant clear foamy cytoplasm and inconspicuous nucleoli.
The second case consists of aggregates of lipid-laden macrophages overtake the entire dermis.
The macrophages have bland round nuclei in this example; scattered lymphocytes are probably also present. One might even think briefly of renal cell carcinoma when looking at these cells.
Xanthomas are common yellow papules or plaques often seen in the setting of hyperlipidemia. There are several variants including tuberous xanthoma (Rapini).
Note that some xanthomas (solitary papular xanthoma and verruciform xanthoma which arises predominantly in the mucosa) are not associated with hyperlipidemia. Plane xanthoma may also be associated with systemic disease such as a plasma cell neoplasm, leukemia, lymphoma and cryoglobulinemia (Busam).
Histologically, xanthomas are composed of lipid-laden macrophages in the dermis, which stain positive for oil-red-O. Occasionally, one may see Touton giant cells. An inflammatory infiltrate may be seen in early lesions and eruptive xanthomas, while fibrosis and cholesterol clefts may be present in older lesions (Rapini).
Most xanthomas are benign lesions, however, their appearance should prompt an evaluation for underlying lipid disorders. The appearance of a plane xanthoma should prompt an evaluation for a possible lymphoproliferative disorder (Busam).
Busam KJ. Dermatopathology: Foundations in Diagnostic Pathology 1st Ed. Philadelphia, PA: Elsevier; 2010: 627-8.
Rapini RP. Practical Dermatopathology. Philadelphia, PA: Elsevier; 2005: 109-110.