Popliteal soft tissue tumor associated with hamstring injury

We report herein a very rare case of semitendinosus tear that formed a tumor in the popliteal region after not having recovered as a result of only being instructed to rest. The soft tissue tumor was discovered on ultrasonography 4 months after pain and sensation of discomfort appeared in the popliteal region. We considered this symptom as dependent on the presence of the tumor and selected surgical treatment. Intraoperatively, this soft tissue tumor was connected with the pes anserinus by tendinous tissue. On pathological examination, the soft tissue tumor was diagnosed as skeletal muscle showing necrosis. From imaging, operative findings, and pathological diagnosis, this was considered to represent a rare case in which myorrhexis developed into a soft tissue tumor in the popliteal region after a semitendinosus tear remained unhealed and was neglected over the long term. Although we had trouble confirming a diagnosis and treatment procedure, we were able to acquire good results with surgical treatment. This is, to the best of our knowledge, the first report of damaged semitendinosus becoming a soft tissue tumor in the popliteal region after long-term neglect without healing.