Although communication is well-appreciated as a potential source of error, little is known about the specific nature and causes of communication errors particularly in the radiation oncology (RO) context. This study used an institutional incident learning system (ILS) to develop a RO-specific classification schema for communication errors with the goal of informing interventions to reduce errors in treatment. Departmental ILS events from January 2012 to January 2017 were analyzed and reports specific to communication were identified. Events were classified by communication type (written, verbal, other), and written incidents were further sub-classified (electronic medical record (EMR), email, text paging). The individuals responsible for sending and receiving the communication were identified. The nature of the communication failure was categorized as: miscommunication, communication absent (not sent), communication not received, or poor communication without incident. Events were classified by point of error origination and detection based on a previously established workflow. A total of 4,617 safety-related events were captured over the 5-year period with 1002 (22%) directly involving communication. Of these, 400 events were selected at random for analysis; 20% of these were potentially severe events. Individuals responsible for sending and detecting communication errors are shown in Table 1. Written communication errors comprise 62% of communication events, whereas verbal errors were 32%. Of the written errors, 87% involved the use of the EMR for communication. Absence of communication and a miscommunication were important factors in 50% and 21% of events respectively. Most communication events (54%) originated during patient assessment or treatment planning phases. To our knowledge this is the first study to examine and classify communication errors in RO. Communication is a common contributing factor to safety events, and our results show that written communication errors are most prevalent, most often driven by poor communication through the EMR. The RO physician was the most common source of error. This work lays a foundation for further efforts to improve communication and reduce errors in patient care and treatment.Abstract 3300; Table 1VariableN (%)Person sendingRO physician154 (39%)Therapist102 (26%)Dosimetrist43 (11%)Person receivingTherapist189 (47%)RO attending157 (39%)Dosimetrist65 (16%)Patient36 (9%) Open table in a new tab