Working Diagnosis:
Tenosynovial Giant Cell Tumor, Diffuse Type of the Posterior Tibial tendon and Flexor Digitorum Longus

Treatment:
Excisional biopsy

Outcome:
Our patient had an excisional biopsy of the presumed fatty mass, and the pathology revealed a tenosynovial giant cell tumor, diffuse type. Ninety-nine percent of it was successfully removed during surgery. She was able to return to full activity post-operatively. To our knowledge, the tumor has not recurred, and she is doing well with no more left sided ankle pain.

Author's Comments:
This case highlights the importance of using different diagnostic modalities in sports medicine. On initial work up, our patient's tumor was missed. Her in-office ultrasound was clearly abnormal, consistent with possible tenosynovitis, though with some notable differences from the typical appearance of tenosynovitis. This prompted the decision to pursue additional imaging which led to a series of MRIs that identified a mass. The mass was eventually described as a synovial tendon lipomatosis, which was not the biopsy-proven diagnosis. Tenosynovial giant cell tumors are rarely seen in the ankle, and without the eventual biopsy, this would have been misdiagnosed. Despite advanced imaging, over 50% of cases of these tumors reported in the ankle in the literature are misdiagnosed prior to biopsy. Given the rarity and high recurrence (up to 55%) of these tumors in the ankle, having the correct diagnosis is critical.

Editor's Comments:
As previously mentioned, when a patient has chronic, recurrent pain without a clear diagnosis, it is important to obtain more advanced imaging. In this case, the patient's symptoms seemed to localize to the soft tissues around the ankle and foot with initial xrays that were normal. Although she had an MRI early in her course of symptoms, repeated soft tissue imaging given recurrent episodes without clear inciting factors would be an important part of her ongoing work up rather than just assuming the original diagnosis was accurate. In this case POCUS revealed findings suspicious for pathology beyond a typical tendonosis or tenosynovitis that prompted repeat MRI.
This case is interesting also due to the fact that it raises for discussion two rare conditions: Lipoma arborescens and Tenosynovial Giant Cell tumor. Initially the MRI raised the possible diagnosis of lipoma arborescens. This condition is rare in general and typically affects the synovium of joints, the most common being the knee. Involvement of a tendon sheath is extremely rare. The condition is most common in people in their 6th and 7th decades of life but can occur in the young. Lipoma arborescens is a benign condition and treated with synovectomy.
In this case the ultimate diagnosis based on pathologic evaluation was a Giant Cell Tumor of the tenosynovium. Although Giant Cell Tumors of the bone are the more common representation of this rare condition, Giant Cell Tumors affecting the soft tissues are even more rare.
For both Lipoma Arborescens and Giant Cell Tumors, malignant transformation is extremely rare and surgical excision is typically curative.

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