AbstractID: 7428 Title: Prediction of isocenter displacement using depth helmet in fractionated stereotactic radiotherapy (FSRT) A mathematical algorithm that predicts isocenter displacement, in standard BRW coordinate system, from depth helmet data is developed. The goal of this study is to predict relative patient shift at each treatment based on corresponding depth helmet measurements and make necessary adjustments to obtain better isocenter accuracy. The algorithm is capable of translating depth helmet measurements to patient shift in anterior-posterior, lateral, and vertical directions. This is obtained by mainly assuming that patient movement is limited to three degrees of freedom: rotational movement about the mouthpiece in sagittal plane on the GTC frame, anterior-posterior movement, and lateral movement. We assumed that the rotational movement of the patient was the sole cause of the vertical isocenter displacement. We also assumed that the amount of rotation is small (<2°). Based on these assumptions, calculations are performed on the depth helmet data of two FSRT patients. The results are compared to the CT images taken before each treatment. As a result, the predictions were in good agreement with the displacements shown in CT images. There were, however, some cases where the algorithm had difficulty in predicting the displacements. This is due to many factors including CT image resolution, depth helmet uncertainty, patient weight status, etc. Therefore, a criterion is also developed to guide when the algorithm can be trusted or not.