Subjectivity in interpretation of portal films

We have measured the variability in identifying geometric errors and the variability in clinical decision making when using portal films.

Eight observers (four radiation oncologists and four radiation therapists) viewed 40 film pairs from 40 different patients. All films, which were acquired using conventional simulator and portal film cassettes, were selected retrospectively from a large clinical database. The observers compared the simulator and portal films under standard conditions, identified the field placement errors, and decided whether adjustments in treatment were required. In addition, all films were digitized and the field placement errors were measured objectively using image registration software.

There was much variability in identifying field placement errors and even more variability in the number of recommended adjustments. The field placement errors identified by the different observers differed by up to 50 mm for the same film pair. The number of adjustments of treatment or block position recommended by the observers also varied between 8 and 25 for the same set of films. The average field placement error, before correction, for AP lung, AP pelvis, and lateral pelvis films was 5.7 mm, 6.3 mm, and 8.9 mm, while the average error after correction (i.e., correcting all errors identified by the observers) was 5.5 mm, 4.9 mm, and 5.7 mm, respectively. Thus, for lateral pelvis films, where the initial errors were larger, the observers were able to make an improvement in patient setup.

The results suggest that human observers have difficulty identifying field placement errors accurately when the errors are around 5 mm or smaller. Although there is some evidence that experience influenced the performance of the observers, the effect of experience is not large. In routine clinical environments, the use of visual inspection will detect large field placement errors. However, tools other than visual inspection will be required if field placement errors 5 mm or smaller are to be identified accurately.