Optimizing MV CBCT Imaging Protocols Using NTCP and Secondary Cancer Risk: A Multi-Site Study in Breast, Pelvic, and Head & Neck Radiotherapy

Abstract

Purpose: To evaluate the cumulative radiobiological impact of daily Megavoltage Cone-Beam Computed Tomography (MV-CBCT) imaging dose based on Normal Tissue Complication Probability (NTCP) and Excess Absolute Risk (EAR) of secondary malignancies among radiotherapy patients treated for breast, pelvic, and head and neck cancers. This study investigated whether MV-CBCT imaging dose warrants protocol personalization according to patient age, anatomical treatment site, and organ-specific radiosensitivity. Methods: This retrospective study included cohorts of breast (n=30), pelvic (n=17), and head and neck (n=20) cancer patients undergoing radiotherapy with daily MV-CBCT. Imaging plans using two common protocols (5 MU and 10 MU per fraction) were analyzed. NTCP values were estimated using logistic and Lyman-Kutcher-Burman (LKB) models, while EAR was calculated using Schneider's Organ Equivalent Dose (OED)-based model. Statistical analysis used paired t-tests, and results were further stratified by age (under 40, 40 to 60, over 60 years). Results: In breast cancer patients, NTCP for lung increased significantly under the 10 MU protocol (p<0.001). EAR was elevated in younger breast patients (under 40 years), with some exceeding 15 cases per 10,000 person-years. In pelvic and head and neck groups, NTCP and EAR remained low (under 1 percent), with no clinically meaningful differences between protocols. Across all sites, younger age correlated with higher secondary cancer risk. Conclusion: Daily 10 MU MV-CBCT presents minimal additional risk in pelvic and head and neck radiotherapy. For breast cancer patients under 40, however, it significantly increases secondary cancer risk and lung NTCP. Personalized imaging protocols are recommended based on age, treatment site, and radiosensitivity.