Transit dosimetry with an electronic portal imaging device (EPID) for 115 prostate cancer patients

Purpose: Comparison of predicted portal dose images (PDIs) with PDIs measur ed with an electronic portal imaging device (EPID) may be used to detect er rors in the dose delivery to patients. However, these comparisons cannot re veal errors in the MU calculation of a beam, since the calculated number of MU is used both for treatment (and thus affects the PDI measurement) and f or PDI prediction. In this paper a method is presented that enables "in viv o" verification of the MU calculation of the treatment beams. The method is based on comparison of the intended on-axis patient dose at 5 cm depth for each treatment beam, D-5, with D-5 as derived from the portal dose D-p mea sured with an EPID. The developed method has been evaluated clinically for a group of 115 prostate cancer patients. Methods and Materials: The patient dose D-5 was derived from the portal dos e measured with a fluoroscopic EPID using (i) the predicted beam transmissi on (i.e., the ratio of the portal dose with and without the patient in the beam) calculated with the planning CT data of the patient, and (ii) an empi rical relation between portal doses D-p and patient doses D-5. For each bea m separately, the derived patient dose D-5 was compared with the intended d ose as determined from the relative dose distribution as calculated by the treatment planning system and the prescribed isocenter dose (2 Gy). For int erpretation of observed deviating patient doses D-5, the corresponding on-a xis measured portal doses D-p were also compared with predicted portal dose s. Results: For three beams, a total of 7828 images were analyzed. The mean di fference between the predicted patient dose and the patient dose derived fr om the average measured portal dose was: 0.4 +/- 3.4% (1 SD) for the anteri or-posterior (AP) beam and -1.5 +/- 2.4% (1 SD) for the lateral beams, For 7 patients the difference between the predicted portal dose and the average measured portal dose for the AP beam and the corresponding difference in p atient dose were both greater than 5%. All these patients had relatively la rge gas pockets (3-3.5 cm in AP direction) in the rectum during acquisition of the planning CT, which were not present during (most) treatments. Conclusions: An accurate method for verification of the MU calculation of a n x-ray beam using EPID measurements has been developed. The method allows the discrimination of errors that are due to changes in patient anatomy rel ated to appearance or disappearance of gas pockets in the rectum and errors due to a deviating cGy/MU-value. (C) 1999 Elsevier Science Inc.