Background: Nearly 30,000 Mexican women develop breast cancer annually. These patients frequently present multiple unmet supportive care needs. In high-income settings, incorporating electronic patient-reported outcomes (ePROs) into cancer care has demonstrated potential for increasing patient-centred care and reducing unmet needs. No such ePRO interventions have been implemented in Mexico. The objectives of this study are (I) to design a two-component intervention for monitoring ePROs among breast cancer patients using a responsive digital application and proactive follow-up by nurses, (II) to perform intervention pilot testing of the study materials, and (III) to conduct the clinical trial to assess usability and effectiveness of the intervention. Methods. The investigators designed a two-component intervention for women receiving breast cancer treatment: a responsive web application for monitoring ePROs and clinical algorithms guiding proactive follow-up by nurses. The investigators will conduct a pilot test of the intervention with 50 breast cancer patients for six weeks to assess the feasibility and inform intervention adaptations. After that, the investigators will conduct a parallel arm randomized controlled trial assigning 205 patients each to intervention and control in one of Mexico's largest public oncology hospitals. The intervention will be provided for six months, with additional three months of post-intervention observation. The control group will receive usual healthcare and a list of information sources on relevant breast cancer topics. Women diagnosed with stages I, II, or III breast cancer who initiate chemo and/or radiotherapy will be invited to participate. The study outcomes will include supportive care needs, quality of life, use of emergency services and unscheduled hospitalizations, the usability of the ePRO App, and adherence to the intervention. Information on the outcomes will be obtained through web-based self-administered questionnaires collected at baseline, 1, 3, 6, and 9 months.