AN ANALYSIS OF INTRATREATMENT AND INTERTREATMENT DISPLACEMENTS IN PELVIC RADIOTHERAPY USING ELECTRONIC PORTAL IMAGING

Purpose: To evaluate the relative frequency and magnitude of intratrea tment and intertreatment displacements in the patient positioning for pelvic radiotherapy using electronic portal imaging. Methods and Mater ials: Five hundred ninety-four electronic portal images of seven patie nts treated with a four-field pelvic technique were evaluated. All pat ients were treated prone without an immobilization device. Two fields were treated per day, from which an average of two electronic portal i mages were obtained for each field. No treatment was interrupted or ad justed on the basis of these images. Each image was aligned to the cor responding simulation film to measure the displacements in the mediola teral, craniocaudal, and anteroposterior directions relative to the si mulated center. The intertreatment displacement was the displacement m easured from the initial image for each daily treated field. For each daily treated field the intratreatment displacement was calculated by subtracting the displacement measured on the initial image from the di splacement measured on the final image. Results: The frequency of inte rtreatment displacements exceeding 10 mm was 3%, 16%, and 23% for the mediolateral, craniocaudal, and anteroposterior translations, respecti vely. There were no intratreatment displacements exceeding 10 mm (p < 0.001). The frequency of intertreatment displacements exceeding 5 mm w as 40, 52, and 51% for the mediolateral, craniocaudal, and anteroposte rior translations, respectively; whereas, the frequency of intratreatm ent displacements exceeding 5 mm was 1, 5, and 7% for the same transla tions, respectively (p < 0.001). The standard deviation of the intertr eatment displacements was at least three times as great as the standar d deviation of the intratreatment displacements for all translations. These deviations were greater than the precision limit of the measurem ent technique, which is approximately 1 mm. Each patient had one direc tion where systematic error predominated in intertreatment positioning . Random error predominated for intratreatment positioning and for the other two directions in intertreatment positioning. Conclusions: Duri ng a course of pelvic radiotherapy, the frequency of intertreatment di splacements exceeding 5 and 10 mm is significantly greater than the fr equency of intratreatment displacements of these magnitudes. Errors in intertreatment positioning are predominantly systematic in one direct ion for each patient, whereas intratreatment error is predominantly ra ndom. Because patients do not move considerably during the daily treat ment of a pelvic field, a single electronic portal image per daily fie ld may be considered representative of the treated position.