Effects of remedies made in patient setup process on residual setup errors and margins in head and neck cancer radiotherapy based on 2D image guidance

Abstract

AimPatient setup errors were aimed to be reduced in radiotherapy (RT) of head-and-neck (H&N) cancer. Some remedies in patient setup procedure were proposed for this purpose. BackgroundRT of H&N cancer has challenges due to patient rotation and flexible anatomy. Residual position errors occurring in treatment situation and required setup margins were estimated for relevant bony landmarks after the remedies made in setup process and compared with previous results. Materials and methodsThe formation process for thermoplastic masks was improved. Also image matching was harmonized to the vertebrae in the middle of the target and a 5mm threshold was introduced for immediate correction of systematic errors of the landmarks. After the remedies, residual position errors of bony landmarks were retrospectively determined from 748 orthogonal X-ray images of 40 H&N cancer patients. The landmarks were the vertebrae C1–2, C5–7, the occiput bone and the mandible. The errors include contributions from patient rotation, flexible anatomy and inter-observer variation in image matching. Setup margins (3D) were calculated with the Van Herk formula. ResultsSystematic residual errors of the landmarks were reduced maximally by 49.8% (p≤0.05) and the margins by 3.1mm after the remedies. With daily image guidance the setup margins of the landmarks were within 4.4mm, but larger margins of 6.4mm were required for the mandible. ConclusionsRemarkable decrease in the residual errors of the bony landmarks and setup margins were achieved through the remedies made in the setup process. The importance of quality assurance of the setup process was demonstrated.