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The Effects of Respiratory Motion in RapidArc Spinal Stereotactic Radiosurgery

J Pursley

J Pursley*, M Manning, B Sintay, J Terrell, J Maurer, C Yount, D Wiant, Cone Health Cancer Center, Greensboro, NC

SU-E-T-677 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Spinal radiosurgery is planned to strict tolerances. Unaccounted for dosimetric changes resulting from respiratory motion might lead to delivery of a sub-optimal plan. We examine these effects for RapidArc spine radiosurgery.
Methods: Plans were created for three patients that received 4D-CT scans in a spinal radiosurgery set-up. A target covering a single vertebral body and pedicles was defined and the spinal canal was contoured for each patient. Plans that delivered 18 Gy to the targets were created on the free breathing scans (FB). The maximum inspiration and expiration 4D-CT phases were fused to the FB based on target and spinal canal position. The FB plans were copied onto these images and doses were calculated. Point doses, DVHs, and plan subtractions were used to evaluate plan differences. Motion was measured at fixed points on the 4D-CTs.
Results: The target for case 1 was defined at T4, case two at T7, and case three at T9. For all cases the target and the spinal canal moved < 1 mm, while the skin surface moved 1-2.5 mm. For the T4 plan the aortic arch moved about 6 mm. For the T7 and T9 cases the diaphragm moved 5 mm and 12.3 mm, respectively. Point dose changes in the targets between the maximum inspiration and exhalation plans were 8.1 cGy, 69.0 cGy, and 58.1 cGy for the T4, T7, and T9 plans. Point changes in the spinal canal were 13.8 cGy, 34.0 cGy, and 34.1 cGy for the T4, T7, and T9 plans. Spinal canal DVHs changes were negligible in all cases. Changes in target coverage at doses >= 18 Gy were approximately 5% for the T7 and T9 plans.
Conclusion: Respiratory motion may alter the dosimetry of spinal radiosurgery plans. The clinical impact of these changes appears to be small.

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