While this is a breast st 3/4 forum, there are still people who post outside this category so I am including some comments for people without cancer also:
if you are a diagnosed case of breast cancer or lymphoma:
The CT scan is a fairly reliable test and it is still largely the choice imaging tool for investigating metastatic disease from known cancer. If the main problem is the back pain, the problem may be limited in the spine. For evaluating the spinal cord, the CT scan may have some problems resolving such tissues especially if there is no apparent involvement of bone. The MRI is the optimal test to perform as it would adequately visualize these soft-tissues.
If you are neither a diagnosed case of breast cancer or lymphoma;
The CT scan is not a screening test for breast cancer or lymphoma.
The screening tests for breast cancer include a clinical breast examination (done by a doctor or trained medical personnel), mammography, and for some high-risk women (based on personal and family history) a breast sonogram or MRI.
There is no screening test for lymphoma. However, the CT scan may find potential lymph nodes to evaluate with a subsequent biopsy,
Why was the CT scan requested? Discuss with your doctor what was being sought specifically with the scan. The physical and neurologic examination would be important in finding the area most involved.