QUANTITATIVE VS SUBJECTIVE PORTAL VERIFICATION USING DIGITAL PORTAL IMAGES

Purpose: Off-line, computer-aided prescription (simulator) and treatme nt (portal) image registration using chamfer matching has been impleme nted on PC based viewing station. The purposes of this study were (a) to evaluate the performance of interactive anatomy and field edge extr action and subsequent registration, and (b) to compare observer's perc eptions of field accuracy with measured discrepancies following anatom ical registration. Methods and Materials: Prescription-treatment image pairs for 48 different patients were examined in this study. Digital prescription images were produced with the aid of a television camera and a digital frame grabber, while the treatment images were obtained directly from an on-line portal imaging system. To facilitate percepti on of low contrast anatomical detail, on-line portal images were enhan ced with selective adaptive histogram equalization prior to extraction of anatomical edges. Following interactive extraction of anatomical a nd held border information by an experienced observer, the identified anatomy was registered using chamfer matching. The degree of conformit y between the prescription and treatment fields was quantified using s everal parameters, which included relative prescription field coverage and overcoverage, as well as the translational and rotational displac ements as measured by chamfer matching applied to the boundaries of th e two fields. These quantitative measures were compared with subjectiv e evaluations made by four radiation oncologists. Results: All the ima ges in this series that included a range of the most commonly seen tre atment sites were registered and the conformity parameters were found. The mean treatment/prescription field coverage and overcoverage were approximately 95 and 7%, respectively before registration. The mean tr anslational displacement in the transverse and cranio-caudal direction s were 2.9 and 3.4 mm, respectively. The mean rotational displacement was approximately 2 degrees. For all four oncologists, the portals cla ssified as unacceptable, in terms of the field placement, exhibited si gnificantly higher (p < 0.03) translational errors in the transverse d irection, The field coverages were significantly lower (p < 0.05) and the translational errors in the craniocaudal direction were significan tly higher (p < 0.05) for the portals rated as unacceptable by two of the oncologists. Conclusions: From the parameters that were used to qu antify the degree of conformity between the prescription and treatment fields, the translational error in the transverse direction correlate d best with the oncologists' assessments on the field placement. Field coverage and translational error in the cranio-caudal direction corre lated well with assessments of only two out of the four participating oncologists. This can be explained by the fact that for the majority o f treatment sites included in the study the positioning of field borde rs was more critical for the transverse direction. A conclusion for th e design of future quantitative acid automated on-line portal verifica tion systems is that they will have to model different perceived signi ficances of different types of localization errors intrinsic to oncolo gist evaluation of portal images.