The representitativeness of patient position during the first treatment fractions

BACKGROUND: During external radiotherapy daily or even weekly image verification of the patient position might be problematic due to the resulting workload. Therefore it has been customary to perform image verification only at the first treatment fraction. In this study it is investigated whether the patient position uncertainty at the initial three treatment fractions is representative for the uncertainty throughout the treatment course. METHODS: Seventy seven patients were treated using Elekta Synergy accelerators. The patients were immobilized during treatment by use of a customized VacFix bag and a mask of AquaPlast. Cone beam CT (CBCT) scans were performed at fractions 1, 2, and 3 and at the 10th and 20th treatment fractions. Displacements in patient position, translational and rotational, have been measured by an image registration of the CBCT and the planning CT scan. The displacements data are evaluated retrospectively and the effect of Action Level (AL) image verification protocols based on sessions 1, 2, 3 are simulated. The resulting overall patient position uncertainties of the different protocols are evaluated at the 10th and 20th fractions. RESULTS AND CONCLUSIONS: The differences between the addressed protocols are shown to be very small compared to the overall increase in patient position uncertainty during the treatment course. Thus the main problem in achieving the smallest possible uncertainty for the overall treatment is not the selection of 'the best' image verification protocol for the initial three fractions. The main challenge is that the overall patient position uncertainty increases during the treatment course. Information about the patient position during the first three fractions is therefore not representative for the overall patient position. For these types of patients and immobilization equipment it would consequently be an advantage to reduce the number of image verification sessions during the initial fractions and then compensate with additional imaging sessions during the remaining treatment course.