A retrospective review of the histologic features and prognosis of hypercellular canine peripheral odontogenic fibromas.

Peripheral odontogenic fibromas (POFs) are benign masses of mesenchymal cells with features of periodontal ligament/gingival ligament fibroblasts and are among the most commonly diagnosed oral masses in dogs. Recently, a subset of hypercellular POFs (hPOFs) has garnered attention due to atypical histologic features giving concern for malignant potential. This retrospective study describes 54 hPOFs characterized by increased cellularity, increased pleomorphism, increased mitotic count, and/or bony remodeling in the absence of inflammation. Data collected from records included signalment, degree of excision, and location of the mass. Follow-up questionnaires were distributed to referring veterinarians to assess biologic behavior and patient outcomes. The hPOFs represented 76/6303 (1.2%) of all canine POF diagnoses in a 12.6-year time frame. Of 29 cases where follow-up data were available, 4/29 (14%) experienced local recurrence, similar to published recurrence rates of typical POFs. No evidence of malignant behavior nor metastasis was identified in any case. The median survival time (17 months) was greater than the median follow-up time for living patients (14 months), and the deaths of 14 patients were all attributed to unrelated illnesses. These results suggest that despite concerning histologic features, hPOFs are not associated with a shorter survival time, nor do they carry a greater risk of local recurrence or metastasis relative to histologically typical POFs. Our findings suggest that hPOFs can be clinically managed similar to typical POFs. Pathologists presented with POFs with hypercellularity, increased pleomorphism, increased mitotic count, and bone remodeling should be aware of hPOF as a potential diagnosis.