Large Animal Diagnostic Imaging Society

17-year-old Arabian gelding

Presented for right hock swelling of 3 month duration


  • Presented for right hock swelling of 3 month duration; rapid progression in size in the last month.
  • No associated lameness. Currently being ridden lightly.
  • No history of prior injury.
  • Mild self trauma (biting at it).

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    Lateral view, right tarsus
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    Dorsoplantar view, right tarsus
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    Dorsolateral palmaromedial view, right tarsus
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    Dorsomedial palmarolateral view, right tarsus
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There is marked soft tissue swelling on the dorsomedial aspect of the tarsus extending from the distal tibia to the level of the tarsometatarsal joint, centered at the talus. Within this swelling, there is faint, diffuse stippled mineralization. The distal tibia, talus, calcaneus and small tarsal bones are normal.

An ultrasound of the right tarsus and a tibiotarsal arthrogram were performed.

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  • There is a large, mixed echogenic soft tissue mass within the subcutaneous tissues of the dorsomedial aspect of the right tarsus, measuring at least 5cm x 6.5cm. This mass contains numerous, mildly shadowing, hyperechoic foci throughout with a few small, superficial areas of echogenic fluid. The margins of this mass are indiscrete with infiltration into the surround subcutaneous tissue. The mass is lateral to the medial collateral ligaments and effaces the lateral margin of the long medial collateral ligament with moderate fiber disruption at the distal aspect. The short medial collateral ligament has normal fiber pattern. There is a mild amount anechoic fluid within the tibiotarsal joint.
  • Extra-articular, partially mineralized soft tissue mass, right tarsus. Differentials include neoplasia (sarcoma, mast cell tumor) or benign (mineralized granuloma).
    • The mass compromises the long medial collateral ligament and has poorly defined margins.

Using 50% diluted 300mg/ml Omnipaquesolution, ~10ml was injected with sterile technique into the dorsoplantar pouch of the tibotarsal joint to produce the following images.

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  • The contrast within the tibiotarsal joint does not communicate with the dorsomedial soft tissue swelling.
  • Extra-articular soft tissue mass with internal mineralization, dorsomedial right tarsus. Differentials include neoplasia (sarcoma, mast cell tumor) or benign (mineralized granuloma). No evidence of communication with the tibiotarsal joint.


  • Punch biopsy
    • Mast cell tumor


  • Surgical excision/debulk
  1. Cole R et al. Imaging Diagnosis – Equine Mast Cell Tumor. Veterinary Radiology & Ultrasound, Vol 48, No 1, 2007, pp 32-34.
  2. Johnston GCA, Zedler ST. Treatment of an invasive equine mast cell tumor in the extensor carpi radialis by extensive tenomyectomy and local corticosteroid injections.  Equine vet. Educ. (2018) Vol 31:6, pp e34-e38.