Oral Proliferative Lesions in Dogs And Cats

Leen Verhaert Belgium

1. INTRODUCTION

Oral proliferative lesions are relatively common in small animals, but fortunately, a lot of these lesions are benign. The oropharynx is the fourth most common site of malignant neoplasia in dogs and cats.(1) Oral malignancies are estimated to represent 5.4% of all malignant tumours in dogs.(2) Prognosis of oral malignancies is often poor, partly because they are only diagnosed at an advanced stage of the disease. A variety of conditions, including infectious conditions, can present as a proliferative lesion or a local swelling in the mouth. Also, a non-healing ulcer that looks like an infectious condition may well be a malignancy. The exact nature of any lesion can only be determined by histopathological examination. The taking of a biopsy is indicated for all proliferative or other suspicious lesions such as non-healing ulcers.

2. EXAMINATION

Any examination should always start with obtaining a full history and doing a general physical examination. Clinical signs observed by the owner, duration and progression of the lesion, and former treatments and their results should all be noted.

The first step in clinical staging is careful inspection and palpation of the mass:

 Size and site.

 Presence of ulceration and/or necrosis.

 Fixation to the underlying tissues.

 Any abnormal mobility of teeth.

Regional lymph nodes should be palpated:

 Size, shape, consistency.

 Fixation to the surrounding tissues.

General physical examination may give indications of distant metastasis. Thoracic radiographs are indicated in all cases of suspected malignancy. Even when these prove to be clear and there are no signs yet of metastasis, we need to keep in mind that solitary pulmonary nodules will only be detected by conventional radiographic techniques once they reach 4�5 mm in diameter.(3)

Radiographic examination of the affected jaw is mandatory. The best detail can be obtained with non-screen dental film and an intra-oral radiographic technique in most cases. Bone infiltration may be evidenced by varying degrees of resorption and/or new bone formation. Resorption of bone will only be visible with standard technique when more than 30�50% of the bone has been resorbed.(4) In some malignancies, there may also be evidence of resorption of tooth roots. CT-scan is much more sensitive for picking up resorption of bone. Especially in maxillary lesions, the use of CT-scan can give essential extra information about the extent of the mass.

�As indicated above, the precise nature of the lesion can only be determined by histopathological examination. A representative biopsy (incisional or excisional) should be taken. A fine needle aspirate is usually of limited value in diagnosing oral masses. An atraumatically-taken biopsy, which falls within the boundaries of the lesion to be excised, has not been found to enhance the occurrence of metastasis. Care should be taken to avoid severely inflamed or necrotic parts of the lesion since these will obscure the histopathological diagnosis.

Clinical findings and results of the histopathological examination should match: a lesion that looks very aggressive probably is, even if the histopathological result tells otherwise. When they don�t match, the findings should be discussed with the pathologist and in some cases additional biopsies will be indicated.

3. CLINICAL STAGING OF ORAL TUMOURS

Oral tumours can be staged using the TNM-classification (primary tumour, regional lymph node, distant metastasis), which has been introduced by the WHO. Using this system, patients can be classified into one of four clinical stages. It has been demonstrated that prognosis worsens as the tumour stage increases from I to IV.(5) Furthermore, this system forces the clinician to evaluate the patient in a methodical and comprehensive way. Clinical staging should be combined with the taking of biopsies to determine the histopathological nature of the lesion.

4. EPULIDES

The term �Epulis� is a clinically descriptive term referring to a localized growth on the gingival.(6) Most epulides are benign non-neoplastic lesions or odontogenic tumours, but any malignant tumour can present as an epulis. Therefore, an excised epulis should always be histopathologically examined to determine the real nature of the lesion. In a retrospective study of 154 epulides at the University of Pretoria, it was found that 43.5% were focal fibrous hyperplasia, 16.9% were peripheral odontogenic fibroma, and 17.5% were canine acantomathous or peripheral ameloblastoma. In addition, a number of other tumours (fibrosarcoma, squamous cell carcinoma, malignant melanoma, etc.) were presenting as an epulis.(7)

5. ODONTOGENIC TUMOURS

Odontogenic tumours constitute a diverse group of neoplastic lesions originating from odontogenic tissues. These lesions can be classified based on their origin from the various germ layers (ectodermal, mesodermal, mixed) or as inductive or non-inductive (based on whether there is an interaction between epithelial and mesenchymal tissues similar to that seen during odontogenesis).(6,8) Common odontogenic neoplasms in the dog are peripheral odontogenic fibroma, peripheral ameloblastoma, and central ameloblastoma. These are non-inductive. Odontoma is less common and is an example of an inductive odontogenic tumour.

6. MALIGNANT MELANOMA

Malignant melanoma is the most common oral malignancy in the dog and accounts for around one third of all oral malignancies in this species.(2) There is a strong predilection for the male. Malignant melanoma typically occurs in older dogs (mean age 11 years). Cocker Spaniels, German Shepherds and dogs with heavily pigmented oral mucosa may be predisposed.(2,9) Malignant melanoma is uncommon in the cat. Malignant melanoma may be pigmented or unpigmented (amelanotic). Amelanotic melanomas may be a diagnostic problem, both clinically and histopathologically.

Malignant melanoma occurs most often on the gingiva, followed by the buccal or labial mucosa, palate, and dorsal surface of the tongue.(2) In gingival lesions, dental disruption is common and bone involvement is often seen. Metastasis to the regional lymph nodes, lungs, and other organs takes place at an early stage. Prognosis is extremely poor. Treatment of choice is wide surgical excision. Surgical excision of very small and early lesions may very occasionally be successful, but for larger lesions surgery is no more than palliative.(1) Radiation therapy may be used for local control of malignant melanoma.(10)

7. SQUAMOUS CELL CARCINOMA

SCC is the second most common malignant oral tumour in the dog.(2,9) In cats, this is the most common oral malignancy.(11)

7.1. Canine SCC

The most common site for canine SCC is the gingiva. SCC most often occurs in older dogs (average 8�9.7 years) with no sex predilection.(2) Papillary SCC has been described in very young dogs.(12,13) The primary mass is often ulcerated. SCC can present as a chronic non-healing ulcer, without proliferation.(14) Dental disruption is common; bone invasion is found in the majority of lesions.(2,9) SCC does metastatise to the regional lymph nodes and lungs, but only late in the disease process. Wide local excision is therefore often curative. SCC is sensitive to radiation, but bone involvement decreases the response to radiotherapy.(1,9) SCC of the tongue and tonsils is less common, but also much more aggressive than the gingival form (15,16). In the tonsillar form, there is a predilection for males. Tonsillar lesions are usually unilateral. Metastasis to the regional lymph nodes develops early in the disease; prognosis is poor.(16)

7.2. Feline SCC

SCC is the most common oral malignancy in cats. Oral SCC most often occurs in older cats, with no known breed or sex association.(11) It is most often located in the premolar/molar region of the maxilla, premolar region of the mandible and the tongue (1). In the tongue, the lesion may present as a non-healing ulcerative lesion in the frenulum, very comparable to what we see with foreign bodies trapped under the tongue. Often the tumour is not clearly visible, but it can be palpated as a firm mass in the ventral body caudal to the frenulum. Maxillary and tongue SCC have a poor prognosis, responding only rarely to any kind of therapy.(1) Mandibular SCC in the cat has a fair prognosis (cure possible with very early lesions and wide surgical excision).

8. FIBROSARCOMA

Fibrosarcoma is most often seen in large breed dogs, with a mean age relatively young (4�5 years). In smaller animals, fibrosarcoma most often presents at an older age (> 8 years).(1) Fibrosarcoma is seen most often in the maxilla. It may occur as a protruberant mass at the dental margins and palate. Fibrosarcoma may also originate from the nasal cartilages, the lateral surface of the maxilla or the palate, as a smooth mass with an intact epithelial covering. Fibrosarcomas are locally invasive and have a high recurrence rate after surgical excision, and a metastatic potential between that of malignant melanoma and SCC.(2,9) Prognosis is variable, to a large extent depending on both location and extent of the tumour at time of diagnosis.

9. OTHER TUMOURS

Many other malignancies occur in and around the oral cavity (e.g., osteosarcoma, hemangiosarcoma, neurofibroma, plasmocytoma, etc.). When treating these, one should always use common sense, and use literature reports on biological behaviour in humans or biological behaviour of this tumour type at other sites in the body as guidelines for treatment (e.g., margins for excision) and prognosis. More information on behaviour of the less common tumours is needed; at present, there are only anecdotal reports of treatment of other tumour types. The main issue is that whatever is cut out should be histopathologically examined by interested and experienced pathologists. Long-term follow-up needs to be done and should be reported.

10. NON-NEOPLASTIC LESIONS

Focal fibrous hyperplasia and generalised gingival hyperplasia presents as an epulis. When excising lesions that seem benign, histopathological exam always needs to be done, since clinically these lesions may look very much the same as malignant lesions.

In cats, lesions from the eosinophilic granuloma complex do present as proliferative lesions and should be differentiated from neoplastic and benign hyperplastic lesions. Eosinophilic or indolent ulcer, eosinophilic plaque, and collagenolytic granuloma are all presumed to be reaction patterns for underlying disease, though identification of this underlying disease may be difficult.(17) Eosinophilic ulcer is most commonly seen on the upper lip as a central pinkish yellow area with a raised edge. Collagenolytic granuloma (eosinophilic granuloma, linear granuloma) can present in the oral cavity and pharynx as raised, linear, pinkish yellow plaques, which may be pruritic. Treatment options includes identification of the underlying disease, glucocorticoids, antibacterial therapy and surgical excision.(17)

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16. Vos J15.���� H, van der Gaag I, van Sluys J. Oropharyngeale tumoren bij hond en kat: een overzicht. Tijdschr.Diergeneesk. 112(5): 251-263, 1987

17. Day MJ, Shaw SE. Immune-mediated skin disease. In: Clinical Immunology of the Dog and Cat. Manson Publishing, 1999. p.103-105