Definition of action levels for a new external quality control procedure of stereotactic beams on cyberknife

Abstract

Objectives Defining actions levels to develop a protocol of external quality control for stereotactic radiation therapy treatments using thermoluminescent detectors and radiochromic films. The irradiation protocol have been defined then firstly tested at the Institut de Cancerologie de Lorraine in Nancy then in 6 Cyberknife institutions. Material and methods Measurements have been performed using thermoluminescent detectors (TLD) (TLD-100 Harshaw) and radiochromic films (Gafchromic EBT2). Detectors were inserted in a test-object, designed by Equal-Estro and two physicists participating to the study, containing 4 locations for TLD capsules and 3 for the films. The protocol consists in an irradiation to a maximal dose of 4.1 Gy on the target volumes defined by the TLD + a 6 mm margin. The forth TLD was considered as an organ at risk. A preliminary statistical study was used to estimate the uncertainty levels of measurements with each of the detectors in the Cyberknife beams. In order to determine the action levels, evaluation was performed for absolute dose using TLD measurements and using Gamma Index for relative dose distributions measured by comparison with dose calculations. For film analysis, different pairs of dose/distance to agreement criteria were studied as well as different normalization methods. Results Absolute dose measurements performed with TLD on Cyberknife demonstrated a reproducibility of 1.7% for a single beam irradiation. Regarding irradiations performed according to the protocol, absolute dose measurements using TLDs was lower than 5% for most of the tested points in the 7 institutions. Larger deviations were observed for the TLD located in the organ at risk. The deviations were identified to be caused by high dose gradients resulting in an inhomogeneous dose distribution in the TLD. Study of dose distributions performed with different Gamma-index criterii (5%/2 mm to 3%/1.5 mm) showed that the criteria 5%/2 mm was the most appropriated one in the context of an external audit because it allows to take into account the precision of dose delivery of Cyberknife beam, uncertainty on the absolute dose measurements and uncertainty in the dose distribution matching to calculate the Gamma-Index value. Conclusion The protocol defined has been successfully tested. A precision of 5% on absolute dose can be achieved. A second row of measurements will soon be performed in order to confirm this preliminary study.