Description and calibration of a CT simulator for use in radiotherapy

The characteristics and performance of a simulator-CT are studied. Image quality obtained with this system allows to use it for precise dosimetric calculation. Its geometric characteristics produce a tunnel diameter which is bigger than those of diagnostic CT. This allows to obtain images with the patient set in treatment position (Specially when inclined plane and other accessories are used), ensuring a correct treatment simulation. A Mecasero simulator with a Kermath tomographic system attached is studied. Image quality is evaluated from slices taken from an AAPM phantom. Obtained images are processed with ATW. a 16 bit image processing software developed in cooperation with the Optics Dept. University of Valencia. The obtained contrast allows to assure the correct localisation of internal structures such as lungs, aerial pathways, spinal chord or bones. Its spatial resolution allows to extract external contours accurately. Noise level and its origin is also discussed. The work shows that images obtained with a simulator CT properly calibrated have enough quality to be used for dosimetric calculations with reliability. Image quality loss is compensated by the geometric properties of the system, which allows taking images with the patient set in any treatment position. (Author) 9 refs