EQUINE VETERINARY EDUCATION / AE / FEBRUARY 2018


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A large, left lateralised, multilobular soft tissue mass with multiple, small, well defined, ovoid, central, hypoattenuating regions and variable contrast enhancement was seen within the region of the left guttural pouch. The mass compressed the left guttural pouch and medial aspect of the right guttural pouch and caused marked ventral displacement of the nasopharyngeal roof, resulting in significant narrowing of the nasopharynx consistent with that seen endoscopically. The left stylohyoid bone which was enveloped by the mass was mildly thickened and had well defined, smoothly irregular, periosteal new bone formation along its length. The administration of contrast medium clearly demonstrated the borders of the mass, which was surrounded by noncontrast enhancing, soft tissue attenuating material. The biopsy site was identified caudal to the mass lesion and gas was seen tracking from the incision into the peripheral soft tissue attenuating material; however, it did not breach the border of the mass. Following visualisation of the full extent of the mass on contrast CT no repeat attempts at surgical biopsy


were made. Differential diagnoses based on the imaging findings included soft tissue neoplasia, such as haemangiosarcoma, squamous cell carcinoma or other neoplastic lesion and granuloma formation originating from pharyngeal or lymphoid tissue. Abscessation of the left retropharyngeal lymph nodes was considered unlikely due to the pattern of contrast enhancement.


Outcome


Owing to the extensive nature of the mass and the difficulty of surgical access, combined with worsening stertor, the owner elected for euthanasia and the horse was submitted for post-mortem examination.


Post-mortem gross and histological examinations


Fig 1: a) Latero-lateral radiograph of the caudal aspect of the head, showing a large, well defined, ovoid soft tissue mass within the region of the guttural pouches. There is a narrow gas lucency dorsal to the mass (white arrow), which is likely to be air within the guttural pouches. The ventral margin of the mass is causing marked compression of the nasopharynx (between black arrowheads). b) Ultrasonographic image obtained with the probe oriented vertically over the left parotid region caudal to the mandibular ramus. The mass is of soft tissue echogenicity, containing multiple, small, ovoid, hypoechoic regions (white arrows).


mass and histopathology of cryostat and formalin fixed sections revealed fibrinous and neutrophilic inflammation with granulation tissue, fibrinoid vasculitis and thrombosis. However, it was deemed possible that these reactive and inflammatory microscopic changes may have been present towards the periphery of the mass and may not have been representative of the mass as a whole.


Advanced diagnostic imaging


Computed tomographic (CT) examination of the head (Fig 2a–d) was performed under standing sedation (as detailed above) including contrast CT using iopamidol (Niopam)4 0.2 ml/kg bwt, administered via a jugular catheter.


Grossly, a 14 9 13.5 9 9.5 cm, bilobed, heterogeneous mass (Fig 3) was present on the caudal wall of the left guttural pouch. The lateral portion of the mass that surrounded the


left stylohyoid bone was mottled pink to yellow in colour, firm and fibrous. In contrast, the medial portion was variegated dark red to yellow in colour and contained multiple, variably sized cysts. Histology of the mass (Fig 4) revealed an encapsulated, well demarcated, multilobulated, densely cellular, proliferation of cells. This was composed of broad interlacing fascicles of medium sized, spindle-shaped cells within a very fine vascular stroma. The cells had indistinct cytoplasmic borders, moderate amounts of vacuolated or fibrillar eosinophilic cytoplasm and a blunt ended oval nucleus with finely granular chromatin and a single eosinophilic nucleolus. They exhibited strongly positive immunohistochemical labelling for vimentin, desmin and alpha-smooth muscle actin with negative labelling for cytokeratin (Fig 5a–d). There was mild anisocytosis and anisokaryosis, with very rare karyomegaly, multinucleated cells and less than one mitotic figure in 10 high power fields (9400). Within the mass there were also large areas of coagulative necrosis and haemorrhage. Local capsular invasion by neoplastic cells was visible following alpha smooth muscle actin immunohistochemistry. There was no visible evidence of vascular or lymphatic invasion.


© 2016 EVJ Ltd


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