ABSTRACT: This article is an attempt to review some of the essential epidemiological factors associated with three of the most frequently encountered canine tumours and some considerations that should be employed with the diagnosis and treatment of them.

Cancer is one of the most commonly encountered diseases in veterinary medicine. Some tumours are independent of age, sex or breed – affecting the young and seemingly well animal just as readily as the old and infirm. However, owing to the higher occurrence of DNA mutation in older individuals, coupled with a weakening immune system, the senior age group represents the highest proportion of the affected population.

In this paper, we review three frequently seen canine tumour types.

Mammary tumours

Although a large number of dogs develop mammary tumours, there exists a significant gender bias. Females are overwhelmingly more commonly affected with mammary tumours than males.

The reason for the skewed occurrence is down to hormones. Mammary glands have receptors on their cells for both oestrogen and progesterone. When the cell undergoes carcinogenic transformation, these hormones can act as promoters of the neoplastic process. Generally there is a long latency of tumour progression, and as a result the average age of onset is 10 to 11 years in the un-spayed bitch.

Encouragingly, the tumour type is often of epithelial origin and benign in nature – adenomas or fibroadenomas (Table 1). They can be solitary or appear in a multiple, mixed form, occupying several mammary glands synchronously.

As the mammary tissue is surgically accessible, masses are generally excised prior to histological evaluation. However, if postoperative pathology reveals a worrying tissue type or incomplete removal, then repeat surgery with wider margins – often including the draining lymph node – is indicated.

Generally, low grade carcinomas have a limited potential for metastatic spread. If tumour cells are found in the draining lymph node, there is an indication not just to remove, but to follow up the surgery with adjunctive treatment, such as chemotherapy.

This would also be the protocol in the event that the cells found in the original lesion were of the carcinoma or sarcoma group.

Interestingly, there exists a unique form of these malignant tumours which is accompanied by a severe inflammatory reaction. Painful and engorged, these tumours have poor tissue healing ability and are best treated palliatively.1

Bone tumours

Bone tumours arise from the supporting mesenchymal tissue. Any component of the bone may be involved. The nomenclature will involve the suffix ‘-oma’ or ‘-sarcoma’ depending on whether the tumour is, respectively, either benign or malignant (Table 1).

Bone cells, fibrocartilage and cartilage predominate and, as such, osteomas, fibromas and chondromas are seen in benign conditions. Whilst osteosarcomas, fibrosarcomas and chondrosarcomas occur in malignancy.

Not all bone tumours are primary neoplasms. Locally invading squamous cell carcinomas, or malignant melanomas, can spread from adjacent affected tissues. In addition, bone is a ‘hot bed’ for secondary metastasis from distant destinations, such as the previously described mammary carcinomas.

Dominating the occurrence list for dogs is the aggressively malignant osteosarcoma. This tumour has preferences. It favours middle-aged to older males rather than females, and often arises in the metaphyseal regions of the long bones. However, these areas are not exclusive or absolute.

Influencing factors that may promote carcinogenesis include the repetitive concussive forces of weight-bearing, which stress causes ‘micro-fractures’. Previous injury is implicated as another source of bone damage. There may also be famili: susceptibility traits – Rottweilers, for example, are a strongly represented breed.2   

Those patients suffering with osteosarcoma present with Lamesness. The pain becomes refractory to standard methods of analgesia. It is feasible to assume a casual link between those regions of the limb that bear the brunt of greater micromechanical stresses and those localities that most frequently display tumour – carpus, shoulder and stifle (Figure 1). 

Figure 1: Topographical illustration of the distribution of osteosarcoma in the body

In advanced disease, a hard swelling may be palpable, but it is radiography that helps to confirm suspicion. The appearance of a typical sunburst' lesion is almost pathognomic for a bone tumour (Figure 2).

Figure 2: Radiograph showing an osteosarcoma affecting the proximal tibia. Note the poorly delineated, erosive – yet simultaneously proliferative expansive – lesion and the loss of the corticomedullary boundary

As with most tumours, definitive diagnosis is achieved via fine needle aspiration (FNA) or, preferably, bone biopsy (Figure 3).

Figure 3: Bone biopsy instrument

Bone tumours can be locally invasive and, therefore, neighbouring bones are often affected. However, it is their tendency for early micro-metastatic spread that is their defining feature. This swift spread is haemodynamic rather than lymphatic. The secondaries emerge in the lungs, liver, kidneys and spleen.

Ultimately treatment is largely palliative as progression is unavoidable. In the early stages, options include amputation with adjunctive chemotherapy to challenge the almost certain metastasis. Radiotherapy may be warranted; and in the few individually appropriate cases, limb salvage surgery.

Skin tumour

Canine skin tumours are commonplace and generally benign. The occurrence of lipomas is typically biased toward older animals, whereas histiocytomas are found predominantly in the young. Lipomas and fibromas arise from mesenchymal connective tissue and basal cell tumours have an epithelial origin. Sebaceous sweat gland adenomas and follicular tricho epitheliomas are adnexal in nature (Table 1).

Although not sinister in their own right, the ability of more dangerous tumours to closely mimic these benign masses means that histological interpretation is a necessary undertaking via FNA or biopsy before any planned excision occurs.

If this information is inadequate, then follow-up surgery may be required to ensure removal of any remaining satellite cells. This is particularly true of histio¬cytomas and mast cell tumours, which can take a well-differentiated benign or poorly differentiated maligna
nt course.

The mast cell tumour is the most common malignant cutaneous canine tumour which readily metastases to local lymph nodes, the liver, spleen and bone marrow. The medical management is complicated by the inflammatory effects seen with mast cell degranulation. Histamine, amongst other cytokines, can have both local and systemic effects – involving swelling, oedema, bruising and vomiting.

Large bold excisions are required, and even then there is no certainty of attaining tumour-free surgery. For this reason, follow-up chemotherapy can both ‘mop up’ remaining cancerous cells and also address the likely metastatic spread.

Dogs are less commonly affected by soft tissue fibrosarcomas and squamous cell carcinomas.

Conclusion

These examples of commonly encountered canine tumours demonstrate the vastly differing effects and outcomes that cancer can have on the body. The neoplastic spectrum ranges from largely inconvenient but essentially non-life threatening lipomas, to fatally sinister osteosarcomas.

In many circumstances, it is acceptable to have an optimistic and 'hands off’ approach to what seem innocuous lumps and bumps, but as has been discussed, the most harmless appearing mass can be masquerading as benign when in fact if you scratch the surface it is indeed anything but.

So all irregularities are best investigated early to ascertain their true nature. 

Author

Rhian Williams BVM&S BSc MRCVS

Rhian qualified from the Royal (Dick) School of Veterinary Medicine. Edinburgh, in 1997 after gaining a Degree in Pathobiology (Hons 1st Class! from Reading University. She has remained in first-opinion practice since graduating and currently works at Vets on the Park in Cheltenham.

To cite this article use either

DOI: 10.1111/|.2045-0648.2011.00109 x or Veterinary Nursing Journal Vol 26 pp 404-406.

References

1.   MURPHY. S. 120081 Mammary tumours in dogs and cats. In Practice 30: 334-338.

2.   BLACKWOOD. L. 119991 Bone tumours in small animals. In Practice 1: 31-37.

Additional reading

MURPHY. S. (2006) Skin neoplasia in small animals Pt.1. Common canine tumours. In Practice 28: 398-402. FRANKS. L. M. and Teich. N. M. 119911 Introduction to the Cellular and Molecular Biology of Cancer. Oxford University Press.

ETTINGER, S. J. and FELDMAN. E. C. (2000) Textbook of Veterinary Internal Medicine. Harcourt Brace & Co.

• VOL 26 • November 2011 • Veterinary Nursing Journal