Adverse event (AE) reporting is crucial in oncology for evaluating the safety and efficacy of treatments. Common Toxicity Terminology for Adverse Events (CTCAE) is the standard tool used for AE assessment, but its reliability has been questioned. Patient-reported outcomes (PROs) have gained importance in assessing the impact of cancer treatment on patients' lives. Integrating PRO data with CTCAE ratings could enhance the reliability of AE assessment. The objective of the study is to evaluate the impact of providing PRO data to physicians on the accuracy of AE information assessed by CTCAE ratings. The primary objective is superior inter-rater reliability of CTCAE ratings from physicians using PRO data compared to traditional CTCAE ratings without PRO information. This is an unblinded, randomized controlled trial with two study arms and a single assessment time point. Adult cancer patients undergoing chemotherapy or immunotherapy and experiencing any AEs are recruited. Patients complete the PRO measures using the EORTC QLQ-C30 and additional items questionnaire. Two physicians conduct independent CTCAE ratings for each patient, with one group of physicians receiving PRO data. Inter-rater reliability of CTCAE ratings is compared between the group with PRO data and the control group. In this multinational study, we will be recruiting 100 patients.