Diagnosis and treatment of canine dermal histiocytoma

Histiocytes are CD34+ stem cell-derived cells that differentiate into antigen-presenting cells (dendritic cell lineage) or phagocytic cells (monocyte/macrophage lineage). Disorder in these cell lines can lead to various reactive and neoplastic conditions in humans, dogs, and cats. Among them, canine cutaneous histiocytoma is a common benign skin tumor in dogs derived from Langerhans cells, an epidermal antigen-presenting cell. It accounts for 6.7% to 14% of all skin tumors in dogs. Canine dermal histiocytoma mainly presents as a red, hairless, nodular solitary lesion protruding from the skin, some with a typical “button” appearance, which may sometimes be ruptured and ulcerated. Usually, it does not cause other obvious clinical symptoms. In clinical practice, fine-needle aspiration of tumors is generally carried out for cytological examination. Microscopic examination will show flakes of light blue basophilic protein background and round cells of various sizes. The cytoplasm is mildly basophilic and nonvacuolated, and the nuclei are eccentric, oval, or reniform. Generally, canine dermal histiocytoma cells have the same appearance and few malignant cytological features. However, due to their rapid growth rate, the mitotic index may be high, and the division is apparent. It should be interpreted carefully to avoid misjudging it as a malignant tumor. Canine dermal histiocytoma can occur in dogs of any age. Still, it is more common in young dogs under three years old, and most of them will resolve spontaneously within 2-4 months without appropriate treatment or surgery. The prognosis is good. However, clinically, histiocytoma has a unique growth site and affects normal behavior and activities. Surgical resection is recommended for pet owners. Usually, surgical resection also has a good prognosis.
Prevention: There is no prevention method, and the factors affecting the regression of canine cutaneous histiocytoma are still unclear. There are two conjectures. One is that its spontaneous regression is mediated by CD8+αβT cells. Reversal is accompanied by progressive infiltration of lymphocytes, which eventually spread throughout the lesion, and these lymphocytes may be crucial to tumor cell lysis. Another hypothesis is that since histiocytoma is a tumor with a small amount of fibrovascular stroma, disruption of tumor vascularization may make tumor cell survival difficult, so induction of tumor necrosis may be a potential mechanism leading to tumor regression.
Furthermore, the natural regression of tumors in humans and animals is often attributed to a reduction in tumor cell proliferation, induction of apoptosis, or activation of the immune system. It has been found that the regression of canine histiocytoma may also be the result of an imbalance between cell proliferation and programmed cell death. Still, the current understanding of its regression remains limited.
In conclusion, canine cutaneous histiocytoma has the characteristics of self-regression. It is a benign tumor frequently occurring in young dogs under three years old and generally does not cause obvious clinical symptoms in dogs. Therefore, a cytological examination is used to confirm the tumor histiocytoma, and then the changes in the tumor can be observed. Depending on the dog’s condition, it can be decided whether surgical resection is required and whether unnecessary treatment should be avoided.