Author: Peter Balter, Kristy K. Brock, Clifton David Fuller, Tze Yee Lim, James Long, Brigid A. McDonald, Androniki Mitrou, Andrew J. Schaefer, Stina Svensson 👨🔬
Affiliation: The University of Texas MD Anderson Cancer Center, RaySearch Laboratories, Rice University, Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, The University of Texas MD Anderson Cancer Center UTHealth Houston Graduate School of Biomedical Sciences 🌍
Purpose: CBCT-based accumulated dose can provide up-to-date dose information on responding anatomy. To optimize resources, it is necessary to determine how many scans provide adequate information for CBCT-based dose accumulation. This work aims to establish dose-uncertainty bounds in CBCT-based accumulated dose when only one exam is used per week.
Methods: Daily CBCT exams from fourteen oropharyngeal cancer patients were corrected for artifacts and expanded to full field of view (corrCBCT). Deformable dose accumulation was performed using all daily corrCBCTs (Daily). The average dose to the ipsilateral parotid on each daily corrCBCT was used as a surrogate to analyze deviations of accumulated dose from planned dose (Planned). One exam per week was selected to create three schema of accumulated dose to be compared to Daily and Planned: maximum possible over-dose (Dacc_over), maximum possible under-dose (Dacc_under), and minimum accumulated dose deviation from Planned (Dacc_min).
Results: For 75% of CBCTs (343/460), the average daily dose to the ipsilateral parotid was larger than Planned. Dacc_min was larger than Planned for 11/14 patients (average±SD=0.62±0.73 Gy) and smaller than Daily for 10/14 patients (average±SD=-0.1±0.64). Dacc_over and Dacc_under identified the range of uncertainty possible when undersampling accumulated dose and ranged from -3.1Gy to 5.3Gy (average±SD=-0.84±2.06Gy).When Daily was more than 1Gy higher than Planned (5/14 patients, range 1.01-2.69Gy), in 3 patients weekly dose accumulation would overestimate the dose by more than half the difference between Planned and Daily. In a single patient, with only a 0.2Gy difference between Daily and Planned, weekly dose accumulation would underestimate the dose by 5.1Gy.
Conclusion: When dose accumulation is necessary to track response to treatment, weekly dose accumulation can skew the resulting dose metric and defeat the purpose of estimating delivered dose. For patients where dose accumulation has been deemed necessary, daily dose accumulation is necessary to ensure accurate results.