Abstract # 2862 Evaluation of Residual Deviation in Patient Positioning and its Impact on Dose Distribution for Proton Radiotherapy

Presenter: Arjomandy, Bijan

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Orthogonal x-ray images are taken prior to treatment of proton beam delivery for positioning verification. The images are used to verify the patient positioning by comparing them to generated DRR from treatment planning. However, portal images with compensators are taken only once per week. A landmark on vertebral body was chosen to measure the positional deviation of the boney structure from the field edges on portal images. The vertebral body was chosen because it will not be affected by patient breathing during image acquisition. On each portal and DRR images, at least four measurements were made (up-down) and (left-right) from the landmark point to the field edges on straight lines. The difference in each of the corresponding measured distances from both images indicated the residual deviations from the true planned position. Eclipse Varian treatment planning was used to evaluate the change in dose distributions due to these residual deviations. The residual deviation was applied to the location of isocenter points that in effect moved the isocenter location relative to the apertures and compensators. All the evaluated plans for each patient were summed and compared to the original plan with the same number of fractions as the total number of portal images acquired during the course of treatment.