Couch Sag Discrepancies Between Planning CT and Radixact: A Wide-area 3D Measurement Study

This study aimed to quantitatively evaluate and compare the couch motion, particularly the sag, of a treatment planning CT system and a Radixact treatment system using a wide-area three-dimensional coordinate measuring machine.

We assessed the couch movement of a treatment planning CT and a Radixact X9 system. A total load of 60 kg was applied to each couch. The three-dimensional positions of three points on each couch were measured as the couch moved in 200 mm increments up to a distance of 1,200 mm. We evaluated vertical displacement (sag), longitudinal movement accuracy, and lateral deviation using a wide-area three-dimensional coordinate measuring machine.

A significant couch sag, reaching a maximum of 4.12 mm in the Z-axis, was observed on the imaging plane of the treatment planning CT. In contrast, the Radixact system, equipped with a couch catcher, showed a maximum sag of only 0.13 mm in the treatment plane. Both couches demonstrated extremely high accuracy in longitudinal movement and straightness, with minimal lateral deviation. The Radixact's couch catcher effectively suppresses couch sag during treatment. However, the substantial discrepancy in sag between the treatment planning CT and the Radixact system poses a risk of compromising treatment accuracy by creating a gap between the planned and delivered positions.

A three-dimensional coordinate measuring machine enables highly accurate measurement of couch motion, revealing discrepancies that should be accounted for during the treatment planning process.