It could be many things. The simplest and best possible thing the chest pain could be is a vertabrae disc out of place in the the spine. Usually when one of the thoracic vertabrae is out it causes impingement on the nerves going to the chest area. If this is the case a chiropractor can easily take care of that. If it something having to do with the cancer that may not be great news. X-rays should show the results.