What is the probability of physical therapy helping a thoracic muscle strain in back?

Very good: Physical therapy is a good modality to treat the kind of muscle strain you report.

Make a correct diagn: It is important to try to make an accurate diagnosis of possible. I have seen some patients diagnosed with strains when in fact the cause was something else. If it is indeed a muscle strain, then yes, physical therapy can be helpful. Most muscle strains if not too severe will heal even without treatment because muscles have a good blood supply. If it is not getting better see doctor.