COMPARISON OF GEOMETRIC ERRORS IN RADIATION THERAPY DEPENDING ON THE FREQUENCY OF THE PATIENT’S POSITIONING CORRECTION

Aim: to determine additional anisotropic margins for planning target volume (PTV) in the absence of daily X-rays monitoring of the patient’s position to reduce the radiation expose on healthy tissues and increase the dose to the tumor. Object and methods: geometric inaccuracies depending on the isocenter location was calculated. Based on tumor location (radiation zone), there were 5 groups of patients devided: head (n = 67), head-and-neck (n = 27), chest (n = 58), abdominal cavity (n = 12), pelvis (n = 58). Patients were treated on a TrueBeam 2.7 accelerator with 3D conformal radiotherapy, IMRT (intensity modulated radiation therapy) and VMAT (volumetric modulated arc therapy). The Eclipse treatment planning system was used to generate patient treatment plan. Results: the patient displacement vector for each of the localizations was determined. The systematic and random patient set-up errors for all studied irradiation zones were calculated. Van Herk equation was used to calculate anisotropic margins. Conclusion: the main result of the study is that there were determined the anisotropic margins of PTV which should be used in the absence of daily X-rays monitoring of patient positioning. The largest variation was shown to be in the areas of pelvis, abdomen and chest; errors in the areas of the head, head-and-neck are much smaller. It was estimated that in the lateral direction the largest margins are needed.