SU‐E‐J‐139: Feasibility of Using EPID for Real‐Time Target Localization during Treatment

Purpose: This study aims to investigate the feasibility of using the images of the treatment fields acquired by an electronic portal imaging device (EPID) for real‐time target localization. Methods: Forty one patients treated with IMRT and RapidArc were recruited in this study including 37 prostate patients and 4 lung patients. These patients were grouped as: prostate IMRT with lymph node (n=14), prostate IMRT without lymph node (n=17), prostate RapidArc (n=6), and lung IMRT (n=4). For each patient, two to four fiducial markers were implanted inside the tumor. The DRR, which projects the patient anatomy and the fiducial marker at the EPID location, was reconstructed for each field. The MLC aperture of each control point was overlay on its corresponding DRR to evaluate the fractional time when the fiducial marker was seen on the EPID image. The probability of seeing at least one, two, three, and four fiducial markers during the treatment was recorded. Results: For the prostate IMRT patients without lymph nodes included in the target volume, the average probability of seeing at least one, two, three, and four fiducial markers during the treatment was 50% (35%–59%), 39% (23%–51%), 24% (7%–38%), and 12% (4%–29%), respectively. For the prostate IMRT patients with lymph nodes, the probability was 41% (24%–51%), 29% (12%–42%), 15% (3%–24%), and 7% (4%–15%), respectively. For prostate RapidArc treatments using two arcs, the average probability of seeing at least one fiducial marker was 81% (58%–90%) for the full arc and 74% (53%–94%) for the partial arc. For the lung IMRT treatment, the average probability of seeing at least one fiducial marker was 34% (20%–52%). Conclusions: The continuous image acquisition from the EPID during the treatment provides sufficient target movement information for real‐time target localization and intrafractional target motion correction for advanced radiotherapy treatments.