You need a bony pelvis MRI. I have had tumors in my "butt cheeks" <grin> to put it plainly and this is the test my orthopedic doctor ordered, from Cedar's-Sinai. This is the test that will show any tumors in this area. It needs to be done WITH CONTRAST, of course. I was having pain and could feel a lump in my "cheek" and no one thought it was anything. I, of course, insisted and, insisted again, to have surgery and it proved to be a recurrence of my uterine leiomyosarcoma. If you feel there is something wrong, ALWAYS, ALWAYS go with your feeling, I do. And, most always, I have proven to be right. You have to be your own advocate, even going against the doctors', radiologist, etc. when you feel there is something wrong. They make mistakes and it is your life on the line. I have proven mine wrong more than once and I am still here after nearly 16 years. If I can be of any help, I would be happy to be of service. Just let me know.
God Bless,
Brenda