RADIOTHERAPY PORTAL VERIFICATION - AN OBSERVER STUDY

In many radiotherapy facilities radiotherapy portal verification is cu rrently a subjective process based on the visual comparison of a treat ment or portal image with a prescription or simulation image. The reli ability of this process is unknown. We describe here a study in which 16 observers (oncologists, physicists and therapists) independently ev aluated the geometric accuracy of 530 treatment fields on 45 patients. The treatment images were acquired by the BEAMVIEW(TM) on-line portal imaging system (Siemens Medical Laboratories, Concord, CA, USA). Illu strative examples of the large variation in observers' assessments of the same field are given. The kappa statistic is used to evaluate the degree of agreement between observers and between on-line (at the trea tment unit) and off-line (in a quiet viewing room) assessments. The be st interobserver agreement was between the four oncologists contributi ng to the study although this level of agreement was rated only as ''f air''. Comparison of on-line and off-line decisions made by therapists exhibited ''poor'' agreement. This study has provided statistical con firmation of the suspicions of many workers in the field of radiothera py portal verification, viz that the subjective evaluation of field ac curacy is unreliable. However, the degree of unreliability is surprisi ngly large. The inconsistencies between observers documented in this s tudy need to be clearly acknowledged in the development of protocols f or the clinical use of on-line portal imaging systems. Acceptable reli ability in radiotherapy portal verification will only be achieved when subjective decision making is eliminated.