From Janet Tobiassen Crosby, DVM,
Your Guide to Veterinary Medicine.
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As readers of the Dermatology Diary learned in the entry for week 3, a histiocytoma is "a benign skin tumor, usually seen in young dogs, that often spontaneously regresses without treatment". This FAQ offers additional information on this common skin tumor seen in young dogs.

A. What does a histiocytoma look like?
The appearance of a raised, hairless and bright red "angry" looking skin lump on a young dog is characteristic of a histiocytoma. This is often alarming to dog owners; as it they often appear suddenly, almost overnight. Histiocytomas are not painful, and most dogs do not even seem to notice them, despite the fact that they lump may be ulcerated. Click here to view photos of a histiocytoma in a young dog.

Where are histiocytomas seen?
Dogs less than three years old are the most common presentation, and the head, ears and limbs are the most common locations to find histiocytomas.¹

Are histiocytomas cancerous?
Histiocytomas are classified microscopically as a round cell tumor. There are a variety of tumors included in the round cell tumor classification. While histiocytomas are benign, other tumors in this class may be more serious. It is important to have your veterinarian check out any new lumps and bumps on your pet to be sure.

How are histiocytomas diagnosed?
Definitive diagnosis is established via cytology or biopsy. Your veterinarian may want to wait and watch the lump, based on the age of your dog and the location and presentation of the lesion. Histiocytomas will spontaneously regress in 2- 3 months. If in doubt, biopsy will provide an answer as to the nature of the lesion.

Reference: ¹ Merck Veterinary Manual, 8th ed.

Related Reading:
Canine Cutaneous Histiocytoma
The Histiocytoma
Histiocytoma Information from UC Davis