High Fidelity, CT on Rails-based Characterization of Total Delivered Dose Variation for Conformal Head and Neck Treatment: With evaluation of adaptive replanning time-point implications

Purpose This study aims to characterize dose variations from the original plan for a cohort of head-and-neck cancer (HNC) patients using high-quality computed tomography on rails (CTOR) datasets and evaluate a predictive model for identifying patients needing re-planning. Material and methods 74 HNC patients treated on our CTOR equipped machine were evaluated in this retrospective study. Patients were treated at our facility using in-room, CTOR Image Guidance — acquiring CTOR kV fan beam CT (FBCT) images on a weekly to near-daily basis. For each patient, a particular day’s delivered treatment dose was calculated by applying the approved, planned beam set to the post image-guided alignment CT image-of-the-day. Total accumulated delivered dose distributions were calculated and compared to the planned dose distribution and differences were characterized by comparison of dose and biological response statistics. Results The majority of patients in the study saw excellent agreement between planned and delivered dose distribution in targets — the mean deviations of D95 and D98 of the planning target volumes (PTVs) of the cohort are −0.7% and −1.3%, respectively. In critical organs, we saw a +6.5% mean deviation of mean dose in parotid glands, −2.3% mean deviation of maximum dose in brainstem, and +0.7% mean deviation of maximum dose in spinal cord. 10 of 74 patients experienced nontrivial variation of delivered parotid dose which resulted in a normal tissue complication probability (NTCP) increase compared to the anticipated NTCP in the original plan, ranging from 11% to 44%. Conclusion We determined that a mid-course evaluation of dose deviation was not effective in predicting the need of re-planning for our patient cohorts. The observed non-trivial dose difference to parotid gland delivered dose suggest that even when rigorous, high quality image guidance is performed, clinically concerning variations to predicted dose delivery can still occur.