Radiobiological Analysis of Coregistered MRI and CT Imaging in Enhancing Treatment Accuracy of Helical Tomotherapy for Rectal Cancer

Abstract

Background: Owing to the limitations of computed tomography (CT) imaging, magnetic resonance imaging (MRI) is used as a complementary tool for radiotherapy treatment planning. This study aimed to investigate the effect of combining MRI and CT on target volume determination, dose distribution, and radiobiological evaluation of treatment plans in helical tomotherapy (HT) for rectal cancer. Methods: MRI and CT images were collected from 10 patients with rectal cancer. The images were fused using a Deformable Image Registration algorithm, and tumor volumes, including GTV, CTV, and PTV, were delineated. Subsequently, the mean dose, tumor control probability (TCP), and normal tissue complication probability (NTCP) were calculated and compared between the two approaches. Findings: The mean GTV, CTV, and PTV volumes in the CT-based target delineation method were significantly larger than those in the MRI/CT-based method (P = 0.034, P = 0.034, and P = 0.024, respectively). However, the significant reduction in the PTV did not result in a statistically significant increase in the TCP (P = 0.085). With decreasing target volumes, the mean doses received by the bladder, small bowel, and femoral heads were significantly reduced (P = 0.006, P = 0.027, P = 0.002, and P = 0.045, respectively). Conclusion: The integration of MRI with CT in helical tomotherapy for rectal cancer enables more accurate target volume delineation and improves radiobiological parameters, including reduced dose to organs at risk, without compromising treatment efficacy. This approach can significantly enhance the safety and quality of the radiotherapy plans. © 2026, Isfahan University of Medical Sciences(IUMS). All rights reserved.