Chemotherapy is essential in cancer treatment but often causes serious toxicities such as cardiotoxicity and neutropenia. In current practice, toxicity monitoring relies mainly on population guidelines and periodic visits, so adverse events are often detected late. ChemoTwin is a clinical decision support prototype that uses a digital twin approach to help clinicians monitor chemotherapy toxicity. The system combines continuous monitoring of patient data, toxicity risk prediction, and access to clinical guidelines so that recommendations stay evidence-based. The prototype brings together more than 15 clinical parameters and estimates risks for key adverse events. Preliminary evaluation shows promising prediction performance and positive feedback from oncology practitioners, supporting the feasibility of this approach for future clinical validation.