Prog Med Phys.  2016 Jun;27(2):79-85. 10.14316/pmp.2016.27.2.79.

The Dose Attenuation according to the Gantry Angle and the Photon Energy Using the Standard Exact Couch and the 6D Robotic Couch

• Kim TH
• Oh SA
• Yea JW
• Park JW
• Kim SK

• KMID: 2328192
• DOI: http://doi.org/10.14316/pmp.2016.27.2.79

Abstract

The objective of this study is to increase the accuracy of dose transmission in radiation therapy using two types of treatment tables, standard exact couch (Varian 21EX, Varian Medical Systems, Milpitas, CA) and 6D robotic couch (Novalis, BrainLAB A.G., Heimstetten, Germany)). We examined the dose attenuation based on the two types of treatment tables and studied the dose of attenuation using the phase (In/Out) for the standard exact couch. We measured the relative dose according to the incident angle of a penetrative photon beam under a treatment table. The incident angle of the photon beam was from 0° to 360° in the increments of 5°. The reference angle was set to 0°. Furthermore, the relative dose of the 6D robotic couch was measured using 6 MV and 15 MV, and that of the standard exact couch was measured at the sliding rail position (In-Out) using 6 MV and 10 MV. In the case of the standard exact couch, the measured relative dose was 16.53% (rails at the "In position," 175°, 6 MV), 12.42% (rails at the "In position," 175°, 10 MV), 13.13% (rails at the "Out position," 175°, 6 MV), and 9.96% (rails at the "Out position," 175°, 10 MV). In the case of the 6D robotic couch, the measured relative dose was 6.82% (130°, 6 MV) and 4.92% (130°, 15 MV). The photon energies were surveyed at the same incident angle. The dose attenuation for an energy of 10 MV was 4~5% lower than that for 6 MV. This indicated that the higher photon energy, lesser is the attenuation. The results of this study indicated that the attenuation rate for the 6D robotic couch was confirmed to be 1% larger than that for the standard exact couch at 6 MV and 180°. In the case of the standard exact couch, the dose attenuation was found to change rapidly in accordance with the phase ("In position" and "Out position") of the sliding rail.

Keyword

Treatment couch; Status; Dose; Incident angle; Attenuation rate

Figure

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  Fig. 1. The photographs of treatment couch: (a) Standard exact couch (Varian Medical Systems, Milpitas, CA), (b) 6D robotic couch (BrainLAB A.G., Heimstetten, Germany). (a-1) The side view of the standard exact couch, (a-2) The back side view of the standard exact couch, (a-3) The top view of the standard exact couch. (b-1) The side view of the 6D robotic couch, (b-2) The back side view of the 6D robotic couch, (b-3) The top view of the 6D robotic couch.

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  Fig. 2. The photographs of universal sandwich panel rail position: (a) Bottom view of rail at the “In position” and upper carbon fiber layer apart. (b) Bottom view of rail at the “Out position” and upper carbon fiber layer apart.

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  Fig. 3. Set up images for measurement of transmission and attenuation rate from a cylinder acrylic phantom.

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  Fig. 4. The schematic diagram of the dose measurement using the ion chamber and cylinder acrylic phantom. 18 cm diameter cylindrical phantom centered on couch with ion chamber at center, beams every 5o from 0o to 360o. θ means the included angle from the couch's vertical axis to the gantry's incident angle.

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  Fig. 5. Relative dose of the standard exact couch's rail at the “in position”, field size 5×5 cm.

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  Fig. 6. Relative dose of the standard exact couch's rail at the “out position”, field size 5×5 cm.

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  Fig. 7. Relative dose of the 6D robotic couch, field size 5×5 cm.

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  Fig. 8. Dependence according to photon beam's incident angle and the photon energy about the attenuation rate for the type of the couch.

Reference

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