Would a breast ultrasound detect a tumor that was deep in chest?

Yes it will: But there is no solution for the excessive anxiety like you seem to have.

The reported: sensitivities for ultrasound in detecting cancer vary, generally 60-80%, so ultrasound can detect breast cancers, but like any test, it is not 100%. A complete workup might include mammography, physical exam, and in some cases MRI. Talk to your doctor about your concerns. If you are talking about a mass that is actually within the chest cavity, not the breast, breast US won't help.