This study will be a prospective double- blind trial. Patients with breast cancer with stages I-III undergoing to radiotherapy will be included. The participants will be allocated into an intervention group (laser therapy in 26 patients) or a control group ( in 26 patients the laser will be disabled without affecting its apparent function) , five days a week before radiotherapy . The low level laser therapy used in this trial will be Photon Lase III (DMC, approved by ANVISA for medicinal purposes). This InGaAIP laser emits a pulsed 660 nanometer beam with an average output of 80 milliwatts and the average energy density delivered to the breast will be 3 J/cm2. Skin reactions will be graded weekly by a nurse, by a radiotherapist and by an oncologist using the common toxicity criteria (CTC -developed by the National Cancer Institute ) and Acute Radiation Morbidity Scoring Criteria (developed by Radiation Therapy Oncology Group). The patients will answer the modified visual analogue scale for pain (patient self-evaluation).