1082 On line electronic portal imaging in lung irradiation: Usefulness in correction and definition of anatomical landmarks

Abstract

We studied the usefulness and feasibility of on line electronic portal imaging (EPI) with intra fractional correction of table position in the irradiation of lung cancer. Antero-posterior (AP) and oblique anterior (OA) fields of 160 sessions from 8 patients were evaluated before and after correction. For the AP fields the errors in positioning (longitudinal, lateral and rotational) were measured using the in-house developed OPIDUM system, using lung tops and carina as anatomical landmarks. After correction of the table position the rest of the fraction dose was administered and residual errors were measured using the same technique. For OA fields only the set up error was measured. In AP irradiation the positioning error before correction is more important than the movements due to breathing. Overall systematic and random error were respective −0.8 (−5.36 to 4.53)mm and 4.5 (1.1 to 3.7)mm for longitudinal position and −3.3 (15.8 to 4.5)mm and 7.0 (0.8 to 3.2)mm for lateral position. After correction we found 0.6 and 2.4mm for longitudinal and −0.02 and 1.6mm for lateral position as residual systematic and random errors. Rotational errors were comparable before and after correction, ranging from −1.4 to 0.5° (syst.) and 0.9 to 1.9° (random) before and −1.6 to 1.2° (syst.) and 0.5 to 2.20 (random) after correction. For OA fields there was a tendency to larger systematic errors in longitudinal and lateral position, the random errors were comparable. For rotational errors, both types of errors were increased compared to AP fields. As a result of this study the patient positioning technique was changed to reduce the variation of the position of the head support in respect to the patients body, which may have a large influence on positioning errors.