SU-E-J-183: Quantification of Motion During Hypo-Fractionated XXXX Radiation Therapy.

@entity1 @entity263 @entity1494
PURPOSE: To quantify @entity1 motion during hypo-fractionated @entity263 treatment as tracked by Calypso    4D localization system. METHODS: 50 @entity263 @entity1 with implanted Calypso beacons underwent hypofractionated IMRT treatment. Typical fraction size was 5 with doses of 5-8 Gy/fraction. 213 traces from the 50 @entity1 were analyzed to quantify the probability of motion vs time starting from beam-on. Couch corrections applied by therapists were undone to obtain the natural course of @entity1 motion. The Calypso data was used to identify vector displacements greater than 2 mm from the starting position. The direction of this vector was classified into one of the 26 directions (combinations of L/R, A/P, S/I). The probability of motion >2mm was estimated by computing the fraction of traces that exceed the 2mm threshold at each time point. The violating motion points were also binned by direction in order to identify specific directions that were more prone to movement. RESULTS: The overall probability of motion greater than 2 mm at 5 and 10 minutes from beam-on were 27 % and 50% respectively. The primary directions in which motion occurred were Posterior-Inferior (PI) and Inferior (I) with a probability of 8.5% and 4% at 5 minutes and 10% for both at 10 minutes. Motion was classified into the following bins: 0-2, 2-3, 3-4, 4-5, 5-6, 6-7, 7-8 and greater than 8 mm. It is observed that motion < 2mm decreases from the first 5 minutes to the next while the higher magnitude components increase with time. CONCLUSION: The probability of prostate motion increases with time. The trend seen in the PI and I directions can be attributed to physiological factors like @entity1494 . This probability can be factored in for scheduling intrafraction imaging and used to compare dosimetric impact of VMAT vs. IMRT plans. This work is supported in part by Varian Medical Systems.
