Hi Laure, sorry you're having concerns, that is one of the things I hate so much about the disease, is the nagging concerns that follow with every ache and pain.
My initial dx was in 1999 and a few years ago, I too started having pain in my sternum. Mine was evident when I moved or stretched in certain ways, not so much to the touch or just at rest. I had a bone scan which showed increased uptake there but they did not think it was cancer, but sent me for a CT next. CT confirmed no metastatic process, but instead, I had osteoarthritis with subchondral cysts on my sternal-manubrial joint. WHo knew? They think mine was just another side effect of radiation, that damaged the tissue between the sternum and manubrial joints (never knew we had a joint there!). I get flare ups now and then when it reminds me it's there, but nothing that requires any pain meds. I felt pretty lucky to get a dx of arthritis!
Hopefully yours is something as silly as what I got....hang in there!
Lori