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A Novel Analytic Approach to Assessing Rotation-Induced Dosimetric Errors of Stereotactic Radiosurgery Cases

Q Zhang

Q Zhang*, Y Song, M Chan, C Burman, Y Yamada, Memorial Sloan-Kettering Cancer Center, New York, NY

MO-F-108-6 Monday 4:30PM - 6:00PM Room: 108

Purpose: To assess 3D rotational setup error effect on dose distribution, we derive a new set of beams such that the beams-eye-views of this new set of beams with respect to the planning CT are the same as those of the original plan for the patient, but with setup rotation errors incorporated.

Methods: To assess the rotation effects on the dose distribution of stereotactic radiosurgery (SRS) cases, a methodology was developed and two sets of table, gantry, and collimator angles were obtained in analytical forms. Those new angles emulate the rotation effects of setup errors. Eight SRS cases were computed with a series of different combinations of patient rotation errors, ranging from (-5°, -5°, -5°) to (5°, 5°, 5°) (roll, pitch and yaw) with an increment of 1°. For each set of rotational errors, its corresponding equivalent beams were computed using our analytical solution and used for dose calculation.

Results: The above new beams were implemented in a treatment planning system (TPS). Based on all the plans that accounted for different rotations, we have found that rotations have an insignificant effect on the minimum, maximum, mean doses, and V80% of the planning target volume when the rotations were relatively small. This was particularly true for the small and near-spherical targets. They, however, did change V95% significantly when the rotations approached 5°. Our theory has been validated with SRS cases and proven to be practical and viable.

Conclusions: We have derived analytical solutions to a new set of table, gantry and collimator angles for a given beam configuration as a function of patient rotation errors. Compared to the traditional method of rotational effect assessment by importing rotated CT images into TPS, our equivalent beam approach is simple and accurate. It is a promising technique for real time adaptive treatment planning.

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