Measurement of variability in patient positioning for radiation therapy with use of serial portal film measurements

This paper determines the magnitude of errors in patient positioning during external beam radiation therapy. Simulation and portal film measurement for 318 portals in patients treated with external beam radiation therapy for head and neck malignancies at the University of Pennsylvania were reviewed. Bite-block immobilization was used in most cases. The distance of anatomic landmarks from the field borders was measured on serial films. The imprecision in positioning was examined with respect to several parameters: random error, a measure of the variation between portal film measurements; systematic error, a measure of the systematic difference between the simulation film and the average of the portal film measurements; and total uncertainty, a measure of the overall variation between the simulation film and the portal films