Practical performance for CT simulator set up and commissioning for 3d radiotherapy

Full text: We present a summary of the protocol used to commission two GE LightSpeed and One Siemens CT-Sim for 3D radiotherapy in early 2004. The protocol defined was based on AAPM TG-66 and ACPSEM 1997 position paper. Scanning Med-TEC Iso-align Laser Alignment Device for three-D isocentre alignment 1. Couch Movement Accuracy for both manual and scanning movement checked before isocentre alignment, as the reference distance is required for the external isocentre setup. 2. Gantry scan rotation circle vertical and tilted angle alignment checked by overlaying the vertical and horizontal image coordinate lines using the aligned ball-bearings in the phantom to determine the gantry cross plane image vertical accuracy and tilt angle accuracy. 3. Zero scan position determined by scanning the horizontally set Med-Align phantom, and adjusting the phantom position forward and backward to match the image scanning centre. 4. Internal lasers position determined by the position of aligned Med-Align phantom after the scanning centre determined. 5. Couch top to Gantry Perpendicular checked by matching the couch axis in the longitudinal and lateral direction to the position of the laser lines determined according to the CT scanning orientation. 6. Couch Central Axis aligned to ensure the couch travels though the internal isocentre for the full range of longitudinal of movement. 7. External isocentre and laser focus can be determined by moving the couch back a set distance (50cm or 60cm normally) from the internal isocentre. Planning image transfer and DRR image alignment 1. Scanning Centre indicated on the planning image depends on the functionality of the planning system. Some planning systems load the image with the scanning centre at zero position, others load up the image with the absolute distance. 2. Zero Slice position aligned to the centre of the phantom using the external laser. 3. Orientation and Distance were checked by scanning the Iso-align in both vertical and horizontal directions with the slice thickness less than the diameter of ball-bearing size, and the orientation and distance are measured over the slice images and DRR images. Electron Density Calibration 1. Tube Voltage kV- the electron density depends on the voltage setting (kV). Calibrations are carried out for different kV settings and separate electron density tables are generated. 2. Tube Current mA - experimental results show that change of HU's is not significant, but low mA scans produce higher Standard Deviation and noise. 3. Scan speed- made no noticeable differences if scan setup is unchanged (e.g. kV or collimation) 4. Collimation - experimental results show that the same slice thickness with different collimation, e.g. 1x5 or 5x1 in the reconstructed image, will cause electron density values to vary. It is suggested that the slice thickness in different collimation settings should be calibrated separately. 5. Sample Area - the readings taken from close to the phantom surface can be different compared with the readings taken from a deeper position in the phantom due to different scanning scatter effects. Calibration tables for the body and head-neck were performed separately. For a new CT installation the scanning isocentre is determined first before setting up the couch and external lasers. This routine assists the engineer during installation and commissioning to ensure the CT scanner meets specifications required for a therapy scanner. The electron density should be calibrated for each kV used. Collimation also affects electron density values. Separate tables for different volumes such as body and head and neck may be warranted. Copyright (2004) Australasian College of Physical Scientists and Engineers in Medicine