Fibrosarcoma in a horse!

A 14 year old gelding presented for a complaint of recent lameness and swelling of the right knee or carpus. There was no history of trauma but a plumb size swelling was palpated along the outside of the knee. The gelding resented direct pressure over the knee and any manipulation of the knee. When radiographed, there were no significant abnormalities noted with regards to the bones or articular surfaces that make up the carpus. However, a moderate soft tissue swelling was noted along the dorsal (front) and lateral (outside) aspect of the carpus (Blue circles in Figures 3-4). 

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An ultrasound exam was performed on the soft tissue swelling and a hypoechoic (dark) soft tissue structure was identified (Figure 5). This structure appeared well demarcated and was consistent with a mass or tumor which was below the surface of the skin yet outside the joint capsule of the carpus.

Figure 5

When the ultrasound image is flipped into the same projection as the radiograph, the soft tissue mass aligns perfectly with the soft tissue swelling noted on the digital radiograph (Figure 6). The red line corresponds to the middle carpal joint and helps demonstrate how close the tumor is to the joint yet does NOT communicate with the joint.

Figure 6

A core biopsy was taken of the soft tissue mass through a small skin incision directly over the area in question and submitted for analysis.  The histopathology report indicated that the core biopsy was consistent with a fibrosarcoma. This type of tumor is not common in horses and although it does not commonly spread to other regions of the horse, it can be locally destructive and aggressive. As such, it was determined that the immediate course of action was intra-lesional injections of a potent chemotherapy drug. Due to the close proximity with the joint capsule, surgery was considered too risky at this point. The gelding has been treated 1x with a chemotherapy medication and a follow-up exam is expected within 2-3 weeks. To be continued...