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Beam-Attenuation in the Aktina Memorial Body Cradle: An SBRT Frame

E Hipp

E Hipp*, S Becker, K Osterman, NYU Medical Center, New York, NY

SU-E-T-427 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Stereotactic Body Radiotherapy (SBRT) frames are used to immobilize the patient during hypo-fractionated treatments. Ideally, a frame would provide 100% transmission of treatment beams. However, attenuation must be accounted for during treatment planning. This study measures the attenuation of the Aktina Memorial Body Cradle and compares the results to treatment planning calculations.

Methods: Attenuation was measured by making two sets of ion chamber readings in an homogeneous solid water phantom (CNMC Best Medical) with a ion chamber (Exradin A16)- one with the phantom in the frame and one with the phantom directly on the couch. Five coplanar 6 MV beams, 5 x 5 cm field size, (180, 230, 250, 270, 285) designed to intersect with specific cradle components were evaluated. To determine calculated transmission, a CT scan of the phantom in the cradle was acquired and two plans were then generated; one with the cradle included in the body contour and one with the cradle excluded (GE Lightspeed; Eclipse). Each plan was calculated with the pencil beam convolution algorithm (PBC) and with the anisotropic analytical algorithm (AAA).

Results: The cross-bars resulted in a measured attenuation of the beam of 4.8%, while the attenuation through the side paddles was small, at 1.4%. The beam passing through the intersection of the base frame and the crossbar fixation hardware (250°) resulted in the largest measured attenuation of 19.7%. The corresponding calculated attenuation was 21.9% and 22.5% for PBC and AAA, respectively. Inclusion of the frame in the body contour during calculation resulted in close agreement between calculations and measurements.

Conclusion: The positioning of movable SBRT frame components should be carefully considered during simulation as attenuation can vary from 1.4% for paddles to 20% for fixation hardware. Inclusion of the frame in treatment planning calculations allows flexibility in choice of treatment angles.

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