Analysis of the setup deviations generated in the 6D ExacTrac X-ray system for patients in hypofractionated intracranial radiosurgery treatments

The purpose of this study was to evaluate the use of the pre-treatment daily imaging system with the Brainlab ExacTrac X-Ray 6D for patients in hypofractionated intracranial treatments and radiosurgery procedures in order to quantify setups errors and review margins used by the institution in the current planning protocol. The deviations generated in the translational and rotational coordinates were given in terms of the mean values and their respective standard deviations. The random and systematic errors were determined through the Stroom and Wang relation and 36 patients treated with intracranial hypofractionated radiosurgery from August 2015 to October 2016 were evaluated. The prescribed dose was 25-30 Gy in 5 fractions for 30 patients and 12-18 Gy in a single fraction for 6 patients. A 1.3 mm margin was found for the planning target volume (PTV) from the deviations analyzed, which is consistent with that adopted by the institution. Correlation between the displacement vector, volume of the PTV and its respective loss of coverage was evaluated. Smaller volumes of PTV tend to suffer greater coverage losses when the isocenters were shifted. From the results, we suggest that for margins greater than 1 mm are required with PTV volume smaller than 2.6 cm³, in order to guarantee the expected coverage of the target. (author)