EQUINE VETERINARY EDUCATION / AE / FEBRUARY 2018


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directly visualised. The CT appearances of an invasive temporomandibular joint, guttural pouch and calvarial tumour (Perrier et al. 2010) and sinonasal tumours (Cissell et al. 2012) have been described previously; however, this appears to be the first report of a primary guttural pouch neoplasm identified using antemortem standard and contrast CT. In this case, contrast CT provided superior definition of the margins of the mass when compared with endoscopy, radiography and ultrasonography integral in the decision to subject the horse to euthanasia due to the extent of the mass and its surgically inaccessible location. Equine tumours are uncommon, with an overall incidence


Fig 3: Guttural pouch mass, dorsal view, showing a smooth surfaced, bilobed appearance with firm fibrous lateral portion (lateral is to the right) and cystic medial portion.


Fig 4: Histological section from the guttural pouch mass showing interlacing fascicles of neoplastic spindle-shaped cells, HE 3100, scale bar = 200 lm.


usefulness in a clinical case. Contrast administration allowed visualisation of the cystic nature of the mass and differentiation of the true margins of the mass from the surrounding soft tissues. The ability to visualise the true margins of the mass and the biopsy site provided an explanation for the nonrepresentative biopsy. Although there was a mild imaging artefact from the surgical site this did not impact significantly on the diagnostic usefulness of this technique. Surgical artefact can of course be avoided by undertaking advanced diagnostic imaging prior to biopsy; however, in this case, the impetus to progress to more advanced imaging was in part, prompted by the nonrepresentative biopsy result. Although ultrasound and/or endoscopy are also useful adjuncts to the attainment of targeted surgical biopsies, their usefulness was precluded in this case by limited access restricting the tissue available for biopsy to the periphery of the ventral aspect of the mass which could already be


of 2–3% in the general population (Hance and Bertone 1993). Neoplasia of the guttural pouch is particularly rare and in the few reported cases, affected horses tended to be older than 10 years (Baptiste et al. 1996; Scarratt and Crisman 1998). Possibly due to their rarity, no breed or sex predilection has been identified. Tumours reported include: haemangioma (Greene and O’Connor 1986), haemangiosarcoma (Baptiste et al. 1996), melanoma (Hance and Bertone 1993; Baptiste et al. 1996; Fintl and Dixon 2001; Metcalfe et al. 2013), fibroma (Merriam 1972) and squamous cell carcinoma (Trigo and Nickels 1981; Perrier et al. 2010). The majority of these reports described malignant neoplasms, which were often a result of metastasis, especially those involving melanoma (Baptiste et al. 1996; Scarratt and Crisman 1998; Metcalfe et al. 2013). The presence of invasive metastatic disease, restricted surgical access, multiple adjacent major neurovascular structures and limited medical options warrants a grave prognosis and most horses are eventually subjected to euthanasia (Hance and Bertone 1993) as occurred here. Given the rarity of these tumours, the usefulness of alternative treatments such as radiotherapy is unknown. It may be speculated that they would be of most use early in the course of disease when tumours are small and invasion is minimal but complete excision is still prevented by limited surgical access and the multiple nerves and vasculature in this area. However, as in this particular case, significant clinical signs may not be detectable until late in the course of disease when the mass is already well developed. There are several reports of leiomyosarcoma in the horse within the digestive (Livesey et al. 1986; Clem et al. 1987; Mair et al. 1990; Boy et al. 1992; Laugier et al. 2004) and urogenital tracts (Lofstedt et al. 1987; Allison and Moeller 1999; Hurcombe et al. 2008), most likely because these organ systems contain a significant smooth muscle component. Other reported leiomyosarcoma locations include pulmonary (Rossdale et al. 2004; Davis and Rush 2013), sinonasal (Veraa et al. 2009) and multicentric sites (MacGillivray et al. 2003). Leiomyosarcomas are frequently invasive and may undergo visceral metastasis (Cooper and Valentine 2002) warranting a poor prognosis, although no preferential sites of metastasis have been identified. Histologically, they may be well differentiated and relatively well demarcated, retaining many features of normal smooth muscle and resembling benign leiomyoma, or less so, with marked cellular pleomorphism, including multinucleate forms and smaller, less well organised, fascicles. As in this case, areas of tumour necrosis are common (in contrast to leiomyoma), resulting in a cystic appearance often accompanied by haemorrhage, oedema and inflammation (Cooper and Valentine 2002). Despite a relatively low mitotic index, the presence of marked tumour necrosis and multinucleate forms combined with the locally invasive behaviour (visible after


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