BIOLOGIC BEHAVIOR

Melanoma

The most commonly reported location for oral melanoma is the gingiva or the buccal mucosa (Figure 1), but these tumors may also involve the labial mucosa, palate, and dorsal surface of the tongue.^8,21 Melanomas are locally aggressive and highly metastatic, with metastasis to distant locations, including regional lymph nodes (59% to 80%) and lungs (51% to 66%), occurring early in the course of the disease.^{3,7,9,11,13,16,19,22}

1. A gingival (buccal mucosa) melanoma involving a dog's caudal mandible and temporomandibular joint region.

Some lingual melanomas are aggressive; in one retrospective study, five of 63 (8%) dogs with oral melanoma had lingual tumors, and location did not impact survival.²³ In two additional studies that included lingual melanoma, four of seven (57%) and five of eight (63%) dogs died of their disease.^12,24 Interestingly, a grading system correctly predicted a poor outcome in five of five dogs with high-grade tumors and positive outcomes in two of three (67%) dogs with low-grade tumors.²⁴ Another study reviewed 1,196 lingual biopsy reports, and roughly half of the specimens were neoplastic. Melanomas accounted for 23% of the tumors found, with 18 of 148 (12%) showing rare to no criteria of malignancy, but this study did not follow up on case outcomes.⁸

Histologic grade did correlate with outcome in a recent study describing histologically well-differentiated melanocytic neoplasms from the lips or oral cavity (no lingual tumors were noted).²⁵ Sixty-one of 64 (95%) dogs were alive at the end of this study or had died of unrelated causes, with a median follow-up time of 34 months. Thus, while most oral melanomas are extremely aggressive, histologic grading may identify a small subset, possibly including lingual melanomas, that is less metastatic.

Squamous cell carcinoma

Oral squamous cell carcinoma occurs most commonly in the gingiva, particularly rostral to the canine teeth.³ Squamous cell carcinoma is locally aggressive; however, unlike oral melanoma, metastasis tends to occur infrequently (10% to 20% of cases) and late in the disease course.^3,26 The exception is tonsillar squamous cell carcinoma, which metastasizes early, especially to regional lymph nodes.³ Most dogs with tonsillar squamous cell carcinoma have regional nodal metastasis at diagnosis.^27,28 In one study, 35 of 48 (73%) dogs with the tonsillar form had regional lymph node involvement noted at necropsy, and 20 of 48 (42%) of the dogs developed widespread distant metastasis, including the lungs, thyroid gland, spleen, liver, mediastinal lymph nodes, kidney, and skeleton.^3,27

Lingual squamous cell carcinomas (Figure 2) have a biologic behavior intermediate between that of gingival and tonsillar tumors, with four of 31 (13%) dogs in two studies showing regional nodal metastasis at diagnosis, and another 29% developing metastasis later, mostly to regional lymph nodes.^12,29

2. Lingual squamous cell carcinoma that has eroded completely through the midbody of the tongue on the left side.

Fibrosarcoma

Fibrosarcomas are most commonly seen in the maxilla (Figure 3) but can also affect the mandible.¹⁵ These tumors are locally invasive with a low metastatic rate. An aggressive variant of fibrosarcoma has a benign appearance histologically yet an extremely aggressive local behavior. This disease is referred to as histologically low-grade but biologically high-grade fibrosarcoma.^15,30 A histologic diagnosis of nodular fasciitis, fibroma, granulation tissue, or chronic inflammation in a rapidly growing, locally invasive and destructive oral lesion is supportive for the low-high fibrosarcoma type.¹⁵ Both variants of fibrosarcoma have a metastatic rate of around 10% to 20%.^3,15

3. A dog with an expansile, proliferative, and ulcerated fibrosarcoma of the rostral maxilla.