Jonušaitė, Monika

Image-guided radiotherapy (IGRT) has enhanced the precision of cancer treatment by integrating imaging modalities such as computed tomography (CT), magnetic resonance imaging (MRI) and cone-beam computed tomography (CBCT) into daily radiotherapy workflows. In head and neck cancer, where anatomical changes are common, accurate image registration between planning and treatment scans is essential to ensure dose accuracy. However, geometric distortions in CBCT (such as translation, rotation, and scaling resulting from patient positioning variations observed in daily CBCT images) can affect tumour targeting and dose delivery. This pilot study assesses a MATLAB-based image correction algorithm that uses rigid bony landmarks and point cloud registration together with spatial transformation to align CBCT with planning CT. Two head and neck cancer patients were retrospectively analysed, selected for their contrasting anatomical responses: one with substantial tumour regression and one with minimal change. Imaging was performed on the Halcyon V3.1 linear accelerator (Varian Medical Systems), with 25 daily CBCT scans per patient (85–96 slices per scan), resulting in 50 datasets for analysis. Spatial deviations were measured along the X, Y, and Z axes, and dose recalculations were performed for each treatment fraction. The correction method significantly improved spatial congruence and reduced geometric discrepancies caused by voxel spacing and acquisition parameters. Uncorrected scans showed dose deviations of up to ± 12% in organs at risk, notably the spinal cord and parotid glands. These findings demonstrate the feasibility and dosimetric relevance of automated CBCT correction in daily head and neck radiotherapy. Although limited in sample size, the study provides a detailed technical and dosimetric analysis of spatial distortions and supports future validation in larger patient cohorts.

Abstract: Precise positioning during the radiotherapy procedure of the head and neck patients is extremely important due to the highly sensitive organs at risk surrounding the irradiated target. Set-up errors can occur due to various reasons, such as body contour changes of the patient, which may happen on behalf of weight loss or shrinkage of the tumour, or due to the inappropriate use of immobilisation tools, etc. Image-guided radiotherapy enables corrections and monitoring of these positioning deviations. However, along with known benefits, performing kV-CBCT verifications before each treatment inevitably leads to additional exposure doses. It is known that daily imaging of kV-CBCT for the patient could increase the daily exposure dose by ~1.4 mGy. Investigation was done using linear accelerator Halcyon V3.1 (Varian Medical Systems, Palo Alto, CA, USA), which ensures 100 % Image-guided radiotherapy. All the daily data were collected from medical treatment planning system EclipseTM (Offline Review tool) and were processed with statistical computing software “R”. Analysis of the results showed that despite the additional irradiation doses for the patient, the benefits of daily verifications are undeniable, because patient positioning deviations do not have a clear variation tendency and change randomly during each fraction.