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In this study, we compared the motion accuracy of a six degrees of freedom (6D) couch for precision radiotherapy with or without weights attached to the couch. Two digital cameras were focused on the iso-center of a linear accelerator. Images of a needle which had been fixed to the 6D couch were obtained using the cameras when the couch moved in translation and rotation around each axis. The three-dimensional (3D) coordinates of the needle were calculated from coordinate values in the images. A coordinate error of the needle position relative to the theoretical position was calculated. The errors were obtained with or without a 60 kg weight attached to the 6D couch, and these errors were compared with each other. The mean distance of the 3D error vectors for the weighted test was 0.21 ± 0.11 mm, and >0.16 ± 0.09 mm for the non-weighted test (p < 0.05). However, the difference of two values was 0.06 mm which is smaller than the minimum distance the 6D couch system can correctly move. The variance of 0.16 mm for the Y coordinate errors for the weighted test only was larger than that for the non-weighted test, which was 0.06 mm (p < 0.05). We found that a total weight of 60 kg did not affect the accuracy of the 6D couch clinically. However, the variance of the Y coordinate errors was increased. This might suggest that the addition of this weight increased the uncertainty of the motion of the 6D couch.

Aim To use cone-beam computed tomography (CBCT) images for treatment planning, the Hounsfield unit (HU)-electron density (ED) calibration table for CBCT should be stable. The purpose of this study was to verify the stability of the HU values for the CBCT system over 1 year and to evaluate the effects of variation in HU-ED calibration curves on dose calculation. Materials and Methods A tissue characterisation phantom was scanned with the field of view (FOV) of size S (FOV-S) and FOV of size M (FOV-M) using the CBCT system once a month for 1 year. A single field treatment plan was constructed on digital phantom images to validate the dose distribution using mean HU-ED calibration curves and possible variations. Results HU values for each material rod over the observation period varied with trend. The HU value of the cortical bone rod decreased by about 100 HU for the FOV-S and by about 300 HU for the FOV-M. Possible variation in the HU-ED calibration curves produced a ≤17·9% dose difference in the dose maximum in the treatment plan. Conclusions The CBCT system should be calibrated periodically for consistent dose calculation. © Cambridge University Press 2015. Aim To use cone-beam computed tomography (CBCT) images for treatment planning, the Hounsfield unit (HU)-electron density (ED) calibration table for CBCT should be stable. The purpose of this study was to verify the stability of the HU values for the CBCT system over 1 year and to evaluate the effects of variation in HU-ED calibration curves on dose calculation. Materials and Methods A tissue characterisation phantom was scanned with the field of view (FOV) of size S (FOV-S) and FOV of size M (FOV-M) using the CBCT system once a month for 1 year. A single field treatment plan was constructed on digital phantom images to validate the dose distribution using mean HU-ED calibration curves and possible variations. Results HU values for each material rod over the observation period varied with trend. The HU value of t

We propose a new metric, local uncertainty (LU), for the evaluation of deformable image registration (DIR) for dose accumulation in radiotherapy. LU measures the uncertainty of placement of each voxel in an image set after a DIR. The underlying concept of LU is that the distance between a focused voxel and a surrounding voxel on an image feature such as an edge is unchanged locally when the organ that includes these voxels is deformed. A candidate for the focused voxel after DIR can be calculated from three surrounding voxels and their distances. The positions of the candidates of the focused voxel calculated from several groups of any three surrounding voxels would vary. The variation of candidate positions indicates uncertainty of the focused voxel position. Thus, the standard deviation of candidate positions is treated as an LU value. The LU can be calculated in uniform signal regions. Assessment of DIR results in such regions is important for dose accumulation. The LU calculation was applied to a pair of computed tomography (CT) head and neck examinations after DIR. These CT examinations were for the initial radiotherapy planning and re-planning for a treatment course where the tumor underwent shrinkage during treatment. We generated an LU image showing high LU values in the shrinking tumor region and low LU values in undeformable bone. We have proposed the LU as a new metric for DIR.

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