Abstract
Tumor tracking is an option for intra‐fractional motion management in radiotherapy. The VERO gimbal tracking system creates a unique beam geometry and understanding the effect of the gimbal motion in terms of dose distribution is important to assess the dose deviation from the reference conditions. Beam profiles, output factors (OF) and percentage depth doses (PDD) were measured and evaluated to investigate this effect. In order to find regions affected by the pan‐tilt motion, synthesized 2D dose distributions were generated. An evaluation of the 2D dose distribution with the reference position was done using dose difference criteria 1%–4%. The OF and point dose at central axis were measured and compared with the reference position. Furthermore, the PDDs were measured using a special monitoring approach to filtering inaccurate points during the acquisition. Beam profiles evaluation showed that the effect of pan‐tilt at inline direction was stronger than at the crossline direction. The maximum average deviation of the full width half maximum (FWHM), flatness, symmetry, penumbra left and right were 0.39 ± 0.25 mm, 0.62 ± 0.50%, 0.76 ± 0.59%, 0.22 ± 0.16 mm, and 0.19 ± 0.15 mm respectively. The ÔF and the measured dose average deviation were <0.5%. The mechanical accuracies during the PDD measurements were 0.28 ± 0.09 mm and 0.21 ± 0.09 mm for pan and tilt and pan or tilt position. The PDD average deviations were 0.58 ± 0.26 % and 0.54 ± 0.25 % for pan‐or‐tilt and pan‐and‐tilt position respectively. All the results showed that the deviation at pan and tilt position are higher than pan or tilt. The most influences were observed for the penumbra region and the shift of radiation beam path.
Highlights
Motion management plays an important role in an advanced external beam radiotherapy treatment, especially when the target and organ at risk (OAR) are moving during the treatment delivery
In total there were 9 gimbal positions involved as shown in Fig. 1, which made a total data set of 720 profiles, 90 percentage depth doses (PDD) curves and 360 points for the output factors (OF)
The dose deviation for pan and tilt motion are higher than for gimbals moved in pan or tilt due to a longer source surface distance (SSD) and a higher pan-tit angle relative to the water surface
Summary
Motion management plays an important role in an advanced external beam radiotherapy treatment, especially when the target and organ at risk (OAR) are moving during the treatment delivery. Mitsubishi (Heavy Industry, Ltd., Japan) and Brainlab (Feldkirchen, Germany) have developed a linac system called VERO that is capable of compensating intra-fractional motion.[1] The machine is coupled with a dedicated Exactrac VERO infra-red stereo camera and a dual tube fluoroscopy tracking system. The reduced intra-fractional uncertainties will potentially shrink CTV-PTV margins.[3] In comparison to other motion mitigation strategies, the treatment time can potentially be reduced since the beam is delivered continuously while the target is in motion.[1,2,3,4,5,6,7,8,9,10]
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have