This is based on the frequent risk of microscopic (and undetectable) cancer cells remaining in the breast. For smaller breast cancers, the combined treatment with surgery (lumpectomy or segmental resection) and subsequent radiotherapy has been shown to be equivalent to the modified radical mastectomy .
Another use of radiation therapy as primary treatment is in postmastectomy radiation to the chest wall. This may be done if the tumor was found at surgery or after tissue examination to invade the skin or chest wall muscles, if it was very large or if many lymph nodes were involved. The assumption is that there may be hidden tumor cells in the chest wall or armpit (axilla) after surgery that can be eradicated effectively by radiation. In addition to the chest wall, the area treated (the radiation field or port) sometimes includes the lymph nodes in the armpit or over the collarbone (supraclavicular nodes).
In patients who later develop chest wall recurrence , radiation therapy is often used (if the area was not previously treated with radiation), usually with a wide margin to kill the presumed microscopic tumor implants that could also be nearby. This therapy to localized recurrences may achieve permanent control over the cancer in up to half of patients.
Radiation is given daily, usually five days a week, over about six weeks. The usual technique involves external beam radiation to the entire involved breast, sometimes with an additional booster dose to the tumor area. This boost may be given by an external beam or with radiation seeds surgically implanted directly into the tumor area (brachytherapy). There is no increased risk of a secondary malignancy or a breast cancer on the opposite side as a result of radiotherapy.
