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Correlation Study of Mechanical Accuracy and Longitudinal Location of a Six Degrees-Of-Freedom Couch

J Li*, M Chan, Memorial Sloan Kettering Cancer Center, Basking Ridge, NJ

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: purpose of this study is to evaluate the mechanical accuracy of ProturaTM 6D couch (CIVCO, Coralville, IA, USA) along with its longitudinal positions and its dosimetry correlation using both OBI/CBCT and StereoPHAN/SRS MapCHECK.

Methods: distance between the origin of pedestal rotational axes of the Protura System and Linac iso-center was set at various values: at each position, CBCT image of the StereoPHAN/SRS MapCHECK phantom was acquired and fused with the planned CT image, calculated 6D shifts were applied to the phantom using Protura System. After phantom’s positions were adjusted, the accuracy of each shift was evaluated by the following two separate
methods: 1) a second CBCT of the phantom was acquired and its position was compared with its planned CT image, the differences were recorded as mechanical errors from the Protura System; 2) A beam dosimetry was obtained through SRS MapCHECK measurement at those shifted positions, and its differences from the planned dose distribution were considered to be dosimetry errors incurred by the Protura System. The couch longitudinal positions were set at 10 cm interval.

Results: mechanical and dosimetry errors have shown gradual increases when the couch longitudinal distances (origin of pedestal rotation and iso-center) were reduced (moving toward the Linac). Although most mechanical errors were in the range of 1-2 mm, a large error of about 4 mm was indeed observed when the couch moved closer to the far end of its limit close to the gantry. The results from dosimetry measurements were consistent with the above observations: Gamma (3%/1mm) passing rates were decreased gradually to its lowest value after a 40-cm pedestal longitudinal shift.

Conclusion: measurements showed small gradual increases in positioning errors from the Protura System when its longitudinal distances are increased. The source of these systemic deviations was not clear and further investigation is needed.

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