Could osteosarcoma have metastasized to my butt muscle?

I assume you: Have had a primary os, and now feel a lump in your butt. Os mostly goes to lungs. However, sarcomas can metastasize to muscles. You need to get tis looked at, not probing on line without a face to face and an exam.

Possibly.: Osteosarcoma often occurs in the bones of the pelvis. In this case, it is common to see a soft tissue mass around the involved bone. This causes hip pain certainly. The involvement of the muscle would also cause similar pain, i suppose. An MRI would tell what is going on.