Cone beam CT evaluation of patient set-up accuracy as a QA tool

To quantify by means of cone beam CT the random and systematic uncertainty involved in radiotherapy, and to determine if this information can be used for e.g. technical quality assurance, evaluation of patient immobilization and determination of margins for the treatment planning.

Eighty four cancer patients have been cone beam CT scanned at treatment sessions 1, 2, 3, 10 and 20. Translational and rotational errors are analyzed.

For the first three treatment sessions the mean translational error in the AP direction is 1 mm; this indicates a small error in the calibration of coronal isocentric laser. The observed SD of the systematic error in each direction is 1 mm if a correction is made after the third fraction with an action limit of 4 mm. The SD of the random errors of the patient group is approximately 1 mm in each direction. The rotational errors have a vanishing mean and a systematic error of 0.5-1.2 degrees and a random error of 0.4-0.7 degrees. The uncertainties from the first three treatment sessions (disregarding rotations) lead to a margin of 4 mm from ITV to PTV for Head-and-Neck patients (all directions) and Thorax patients (AP and lateral directions). In the CC direction, the margin has to be 5 mm for the Thorax patients. The total uncertainty on the patient position grows during the treatment course, especially in the CC direction for patients receiving thoracical irradiation. This may stem from problems in the immobilization of these patients. Consequently, it may be necessary to increase the margins in the CC direction. Once the CBCT scans have been made, the information is available for off-line analysis without any extra workload. Thus, the CBCT data can supplement scheduled QA checks.