How would you approach an unresectable locally advanced lung cancer invading the chest wall 25 years after whole breast radiotherapy to the same side?  

The first thing to evaluate is the location of the current tumor and treatment fields for previous breast radiation. It's unlikely to get a full treatment plan, but if you can get dose, fields, and if you are lucky, some kind of field set up, you might be able to "recreate fields". If it was whole ...