Histiocytoma

Histiocytoma is a term used to describe a lesion composed of histiocytes, a type of immune cell derived from monocytes, including macrophages and dendritic cells. The name can apply to several benign or reactive proliferations in humans and animals, but in veterinary medicine the best known form is canine cutaneous histiocytoma, also called the button tumor.

In dogs, cutaneous histiocytomas typically occur in young individuals as a solitary, rapidly enlarging, dome-shaped or pedunculated skin nodule, often on the limbs or face. They are usually nonulcerated and may be hairless at the site. The tumor is generally benign and has a strong tendency to regress spontaneously within weeks to a few months without treatment, though complete resolution may take longer in some cases. Metastasis is rare.

Diagnosis is based on clinical appearance and confirmation by biopsy. Histopathology reveals a well-circumscribed dermal nodule composed of histiocytic cells with variable cytoplasm, sometimes accompanied by eosinophils and occasional multinucleated giant cells. Immunohistochemistry can aid diagnosis, supporting histiocytic origin and helping to distinguish it from other dermal tumors.

Differential diagnoses for a canine skin nodule include mast cell tumors, melanocytic tumors, epitheliotropic lymphoma, and other soft tissue nodules. Treatment is often unnecessary beyond biopsy for diagnosis; if the lesion persists, causes cosmetic concern, or diagnostic uncertainty remains, surgical excision or laser therapy can be curative. Prognosis is excellent, with recurrence being uncommon after complete regression or removal.

In humans, histiocytomas are less clearly defined and may overlap with other histiocytic disorders, such as juvenile xanthogranuloma or Langerhans cell histiocytosis, depending on histology and distribution.