Assessment of set-up errors in the radiotherapy of patients with head and neck cancer: standard vs. individual head support.

Background The aim of the study was to (a) compare the accuracy of two different immobilization strategies for patients with head and neck tumors, and (b) compare the set-up errors on treatment units with different portal imaging systems. Patients and methods Variations in the position of the isocenter (IC) relative to the reference point determined on the computed tomography simulator were measured in a vertical (anterior-posterior), longitudinal (superior-inferior), and lateral (medial-lateral) direction in 120 head and neck cancer patients irradiated with curative intent. Depending on the treatment unit (unit A - 2D/2D image previews; unit B- 2D image previews) and the time of irradiation, patients were divided into 6 groups of 20 patients. In patients irradiated in 2014, standard head supports were used (groups 1 and 2), whereas in those treated in 2015 and 2017 (groups 3-6) individual head supports were employed. The clinical-to-planning target volume safety margin was calculated according to the formula proposed by Van Herk. Results In total, 2,454 portal images and 3,681 set-up errors were analysed. Implementation of individual head supports in 2015 resulted in a statistically significant reduction in the average inter-fraction displacement in the vertical direction and in decreased number of IC displacements in the vertical and longitudinal direction (applies to both treatment units). The largest reduction of the safety margin was calculated in the longitudinal direction and the safety margins were larger for unit B than for unit A. Conclusions The use of individual head supports and a more advanced imaging system were found to increase set-up precision.