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EQUINE VETERINARY EDUCATION / AE / MARCH 2015


Clinical Commentary The likelihood of metastases in equidae


M. A. Smith Image Equine, Newmarket, UK. Corresponding author email: info@image-equine.co.uk, meredith.smith@aht.org.uk


The case report by Nelson et al. (2015) of multiple skeletal metastases from a penile squamous cell carcinoma in a horse describes diagnosis confirmed at post mortem examination of metastases located in 2 remote sites within the skeleton, one being within the proximal femur and the second being within the fifth cervical vertebral body following a history 3 years previously of a primary squamous cell carcinoma of the penis. Squamous cell carcinoma (SCC) is the most common


tumour of the external genitalia in equines with incidences of 49–82.5% being quoted in the literature (Brinsko 1998; van den Top et al. 2008; van den Top et al. 2014) and is also the most common tumour of the eye and adnexa (Lavach and Severin 1977). Of 49 horses with SCC of the eye, adnexa or orbit, the tumour was locally invasive or metastatic in 5 (10.2%) (Lavach and Severin 1977) and of 82 horses with penile SCC, 7 (8.5%) had regional lymph node metastasis (van den Top et al. 2008), whilst in a larger series of 114 horses, 28 had regional lymphadenopathy, 9 (32%) of which were confirmed metastases on histopathology. Diagnosis of neoplasia is increasingly common in our ageing equine population but there is a paucity of reports in the literature citing percentage likelihood of occurrence of metastasis of other equine tumours. Given our knowledge of the behaviour of SCC in the horse, we can potentially use these cases as an initial model for best practice when considering investigation and treatment of other tumours in equines. In the case report by Nelson et al. (2015), advanced


diagnostic imaging was extremely helpful in identifying the location of the first identified remote lesion, with nuclear scintigraphy being the most appropriate modality to screen for potential causes of the unresolved hindlimb lameness. The authors comment that a full body bone scan would likely have identified the second remote lesion within the fifth cervical vertebra ahead of the development of clinical signs of neck pain and lethargy. A scintigraphic study of the thoracic spine, pelvis and hindlimbs only, was performed in this case because the horse presented with clear hindlimb lameness. There is a significant difference in cost as well as in radiation exposure to personnel between performing the shorter scintigraphic study of the back, pelvis and hindlimbs vs. performing a full body bone scan, and performance of whole body scintigraphic studies routinely is therefore not justifiable in equine veterinary medicine. However, full body scintigraphic screening may be more justifiable during examination of cases in which there is a known history of or gross evidence of neoplasia and particularly in cases where potential for future metastases had previously been identified at the time of an initial debulking surgery. When considering the imaging appearance of pathology,


disease processes alter tissue absorption properties and, in general terms, changes seen on conventional radiographic or CT images are either additive or destructive. Additive


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pathologies cause increased density (opacity) and most nonmalignant pathologies fall into this category, such as oedema, effusion and abscess formation. Malignant diseases tend to cause both additive and destructive changes, for example osteolytic processes appear destructive whilst large fluid-filled masses appear additive. Necrotic areas within tumours generally appear destructive on images from all modalities. The nature of the femoral lesion was not considered clear on radiographs taken at the time; however, had a destructive pathology been visible on radiographs taken following scintigraphy this appearance could have provided sufficient guidance to recommend euthanasia. Ultrasonography did identify osseous pathology overlying the lateral aspect of the proximal femur and a decision was made not to risk performing a bone biopsy of this region, given the significantly abnormal scintigraphic appearance of the proximal femur and likely compromise of this bone. Needle aspiration cytology was performed in a humeral lesion detailed in a case report of occult metastatic intestinal adenocarcinoma and proved informative and this could have been a safer alternative option to consider, using ultrasound guidance, for further antemortem diagnosis in the current case (Jann et al. 2009). Based on the extent of the gross lesions identified at post mortem one month later, it is quite likely that the lesion within the cervical vertebra would have been radiographically apparent as a destructive lesion antemortem. Post mortem radiography of the neck may have provided additional information on the appearance of this lesion for future cross-reference. Oncologists define the process of metastasis as the spread


of cancer beyond the primary site. Metastases can be local or remote. One feature that is interesting about this case is the time frame of approximately 3 years which elapsed between surgical excision of the primary tumour and the development of clinical signs related to the metastases. Squamous cell carcinomas are often relatively rapidly growing clinically and this long time frame seems surprising. It is interesting that in spite of the time frame and the severity of the gross lesions found, the appearance at post mortem examination of the chain of lymph nodes from the local inguinal to the more regional medial iliac nodes was reported as grossly normal. The authors do not comment on the appearance of the lungs, heart or liver at post mortem examination and an assumption is made that these more common sites for metastases following SCC were grossly normal. The authors rightly note that it would have been informative to perform histopathology on regional and remote lymph nodes during post mortem examination regardless, as a significant percentage of grossly unaffected lymph nodes are in fact histologically abnormal (van den Top et al. 2011). Human patients diagnosed with prostate cancer with pelvic lymph node metastases are more likely to develop distant metastases than those with lymph nodes negative for


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