NT-proBNP to Assess Trastuzumab-induced Cardiotoxicity

This study examines whether a blood marker called NT-proBNP (a sign of heart stress) can predict which breast cancer patients on trastuzumab (Herceptin) — a HER2-targeted drug — are at risk of heart damage, with the goal of personalizing heart monitoring during treatment. **You may be eligible if...** - You have HER2-positive breast cancer confirmed by biopsy - You are starting chemotherapy plus HER2-blocking therapy (such as trastuzumab) - Your cancer is being treated before or after surgery (not metastatic) - You are 18 or older with good overall health (ECOG 0–1) - You have adequate organ function - You are not pregnant (negative pregnancy test required) and use effective contraception if applicable **You may NOT be eligible if...** - You have a prior heart condition that would require special monitoring and may lead to stopping treatment early - You have another serious medical condition affecting the planned treatment - You are pregnant or breastfeeding - You have another active cancer requiring treatment Talk to your doctor to see if this trial is right for you.