In general, you’re receiving radiation to eliminate any cancer cells that may have been left behind in the breast area after surgery (mastectomy or lumpectomy, with or without lymph node removal). And because cancer cells are too small to be seen (except by microscope), there is no way of really knowing if the radiation is working. Since there is no physical evidence of how cancer cells may be responding to radiation, it is essential that you receive the entire course of treatment as originally planned.
The port films (X-rays of your treatment field, taken weekly throughout radiation) tell your doctor if radiation is reaching the areas where it can do the most good. But they cannot tell you if there were cancer cells present or how they are reacting to the radiation. After treatment, your doctors will perform regular diagnostic tests to check the treated areas.