Quantitative evaluation of entrance skin dose from kV imaging in respiratory motion-tracking radiotherapy

Although some studies have reported the radiation doses associated with kilovoltage (kV) X-ray imaging in motion-tracking radiotherapy using Radixact Synchrony, detailed assessments of skin doses that reflect the actual imaging frequency and patient positioning remain insufficient. This study aimed to estimate the entrance skin dose (ESD) associated with frequent kV image acquisition, evaluate the radiation dose in past patients at our hospital, and identify the contribution of imaging dose relative to the prescription dose and dose constraints. For each protocol available on the Radixact Synchrony system, the half-value layer, X-ray tube voltage, and air kerma at the kV imaging isocenter were measured using a semiconductor detector (RaySafe X2, Unfors RaySafe AB, Sweden). The ESD associated with the kV image acquisition was estimated based on the number of kV image acquisitions during tracking and corresponding imaging angles from 109 patients who underwent motion-tracking radiotherapy. The total number of imaging exposures throughout the treatment period averaged 1230 per patient, with a maximum of 3750. The cumulative ESD per acquisition angle averaged 69.1 mGy, with a maximum of 367.2 mGy. Furthermore, by considering the overlap of imaging angles and the influence of transmitted X-rays, the estimated maximum skin dose was 951.9 mGy. In this case, the maximum skin dose in the treatment plan was 47.9 Gy, and so the imaging dose corresponded to approximately 2% of the prescribed skin dose. Our findings indicate that the contribution of the imaging dose to the treatment dose is sufficiently low.