What is the value of cancer surgery in horses? Is it practicable, humane and to the betterment of the welfare of affected horses?

Neoplastic prevalence surveys in equids confirm that most primary tumors are cutaneous. These are fortunately easily seen/palpated and sampled. Non-cutaneous neoplasms are less accessible and more difficult to diagnose. Surgery is widely regarded as the mainstay of cancer therapy in horses, The options for surgical intervention include sharp surgery, laser excision, cryosurgery, RF-electro cautery, electrochemotherapy, and surgical ligation. Additionally, chemotherapeutic immunotherapy and radiotherapeutic options can be used adjunctively where relevant. It is certainly true that for almost all cancer therapy in other species including man, the use of multimodality treatment is de rigeur. In equine cancer medicine, however, this is regrettably not the case and while surgery is clearly a major player in the potential resolution of tumors, good results possibly rely more on good fortune than oncological rationale. The concept of multimodality treatment with surgery needs to be developed. However, the range of therapeutic options with evidence is very narrow and limited by circumstance, availability, and expertise.

Since most accessible tumors will be either sarcoid, melanoma, carcinoma or mast cell tumors, these are the main target for most surgical treatment.

1. Is surgery and effective way of resolving the equine sarcoid?

This is one of the most dangerous tumors in terms of the requirement to remove every single solitary cell. Leaving a single cell behind is probably going to result in a significant recurrence and usually with significant exacerbation. A rather benign-looking occult or verrucose sarcoid that is incompletely removed will almost always develop into an invasive dangerous and difficult fibroblastic complex and in any case the resulting wound will fail to heal (Figure 1).

2. Does surgery work for the removal of melanoma lesions?

Historically, the profession has carried a negative and harmful dogma of neglect that stated that disseminated malignancy was certain after minor surgical interference. In fact, over 80% of melanoma lesions will become malignant if they are left long enough, and there is absolutely no justification to leave them. The metastatic spread of equine melanoma is extremely inefficient. The blood-borne metastatic cells are delivered to the pulmonary circulation where they are filtered out in congregated masses. Thisfiltration system is extremely efficient but some cells will escape this filtration mechanism. According to Paget hypothesis, neoplastic cells in the pulmonary parenchyma are held in G0 and cannot grow into meaningful tumors; the environment is not suitable for the development of metastatic tumor in the lung tissue itself. However, any cells that escape this filtration mechanism can invade any organ or structure – some with greater tolerance and higher replicative speed and devastating consequences. The metastatic spread occurs very early – often long before tumors become apparent; malignancy starts with a single cell – it does not start in big tumors or little tumors – it just starts wherever it needs to. This therefore means that there is an obligation and a professional responsibility to remove all external melanoma lesions as early as possible; if they are malignant already, they must be removed to reduce the amount of metastatic spread and if they are not malignant yet, they are likely to become so and therefore, must be removed. There is absolutely no justification to allow any melanoma lesion on any horse at any time. Melanoma in non-grey horses is even more serious and there can be absolutely no delay here; histology is critical in these cases.

3. Can squamous cell carcinoma be resolved surgically

The challenge with squamous cell carcinoma is the definition of the outer limit of the tumor since there invade lymphatics and blood vessels and the fact that squamous cell carcinoma tumors are quite frequently destructive. This makes the reconstruction of excised tumor very challenging. Where they are in the proliferative form, they are technically much easier to remove surgically. Surgical pathology and concurrent chemotherapy are obligatory in all cases. For example, removal of a small localized penile carcinoma could reasonably be assumed to have an effective and safe surgical margin but it may not, and since carcinoma tracks along blood vessels and more particularly along lymphatics, the lymph-node interpretation proximal to the lesion itself is a very important prognostic indicator as to whether surgical intervention is likely to be successful or not. On the other hand, phallectomy is often used to deal with extensive distal penile carcinoma and a variety of different surgical methods are available to cope with this circumstance.

4. Once diagnosed, what are the surgical implications for mast cell tumor removal?

There is considerable debate about the true nature of the equine mast cell tumor (mastocytoma). It is not clear whether this is a genuine tumor or not. In any event, they do have a lot of the characteristics associated with neoplastic disease, and surgical removal of mast cell tumors is the treatment of choice. . Therefore histological confirmation. These lesions can become very big and very aggressive and others are small and easily removed; it is always better to remove a small tumor in the expectation of it getting larger and becoming impossible rather than waiting until it is impossible and then regretting having not interfered earlier. All mast cell tumor lesions should be removed surgically. Partial removal does not appear to be harmful and therefore, whilst in some circumstances, some dangers are easily envisaged, removal of the bulk of the tumor seems to result in a significant long-term benefit. Malignant mast cell tumors are extremely rare in horses and there is no evidence one way or the other as to whether dissemination can follow a surgical intervention – it just seems unlikely!

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