Is an indeterminate intramuscular lesion on an mri always mean cancer?

No: It means that there is uncertainty about what it is. Make sure the scan was read by a radiologist with fellowship training in musculoskeletal imaging and, if not, ask that it be reread by one. Most radiology groups have access to this expertise. Also, seek an opinion by an orthopedic surgeon experienced with muscle tumors. After all that, biopsy is sometimes required if questions remain.

Not necessarily: That is why it is called an indeterminate lesion. Best is to have your physician discuss with the radiologist who read it for their best opinion and then see an orthopedist who specializes in tumors. If appropriate, a biopsy is sometimes needed to determine the exact cause. Good luck!