Mometasone Furoate in Preventing Radiation Dermatitis in Patients Undergoing Radiation Therapy to the Breast or Chest Wall for Invasive Breast Cancer or Ductal Carcinoma in Situ

DISEASE CHARACTERISTICS: * Histologically confirmed diagnosis of primary invasive breast cancer or ductal carcinoma in situ * Planning to undergo ≥ 5 weeks of continuous definitive or adjuvant external-beam radiotherapy to 1 of the following sites: * Whole breast (as part of breast-conservation therapy) * Chest wall (as part of post-mastectomy irradiation) * Treatment of regional lymph nodes (i.e., axillary, supraclavicular, or internal mammary) allowed * Must meet the following criteria for planned radiotherapy: * Planned total radiation dose ≥ 5,000 Gy and daily radiation dose between 1.75 and 2.12 Gy * No planned split-course radiotherapy * No partial breast treatment, defined as treatment of \< 75% of the breast parenchyma * Intensity-modulated radiotherapy planning and delivery, conventional radiotherapy, or 3-dimensional radiotherapy techniques allowed * Must be entered on study within 7 days prior to beginning radiotherapy * Must start study drug prior to receiving the third radiotherapy fraction * No preexisting skin breakdown within the planned radiotherapy field at the time of study entry * No bilateral breast cancer treatment * No inflammatory carcinoma of the breast * Hormone receptor status not specified PATIENT CHARACTERISTICS: * Male or female * Menopausal status not specified * Eastern Cooperative Oncology Group (ECOG) performance status 0-2 * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contr…