ESTRO 2022 - Abstract Book

S829

Abstract book

ESTRO 2022

Results The Pearson’s linear correlation coefficients between the gamma passing rates and the edge penalty, 1-MCS, and MU/Gy were r=-0.83, r=-0.80, and r=-0.74, respectively. All metrics demonstrated a negative correlation with the gamma passing rate, with slightly stronger correlations for the MLC complexity metrics than with MU/Gy. Furthermore, the presence of an outlier in the data for MU/Gy (Figure 1C, patient 6) indicates a potential risk of false negatives when evaluating plan quality using only this metric. Such outliers were not observed with the MLC complexity metrics (Figure 1A and 1B), indicating a potential clinical value of defining cut-off thresholds for plan quality using these metrics. Conclusion The gamma passing rates from the phantom-based measurements has superior correlation with the MLC complexity metrics edge penalty and 1-MCS compared to the more commonly used method of evaluating the MU/Gy, as a measure for plan quality. The 1-MCS and edge penalty complexity metrics could therefore potentially serve as a valuable supplement to calculation-based QA of online adaptive IMRT plans, where a phantom-based measurement is not an option. [1] Younge KC et al. Penalization of aperture complexity in inversely planned volumetric modulated arc therapy. Med Phys. 2012;39(11):7160-70. [2] McNiven AL et al. A new metric for assessing IMRT modulation complexity and plan deliverability. Med Phys. 2010;37(2):505-15.

OC-0939 Development and validation of a population-based colorectal model for radiation therapy dosimetry

C. Owens 1,2 , B. Rigaud 3 , E. Ludmir 4,5 , A. Gupta 3,2 , S. Shrestha 2,1 , A. de la Cruz Paulino 4 , C. Peterson 5,2 , S. Kry 1,2 , S. Smith 1 , K. Brock 3,1 , T. Henderson 6 , R. Howell 1,2 1 The University of Texas MD Anderson Cancer Center, Department of Radiation Physics, Houston, USA; 2 MD Anderson Cancer Center UTHealth Graduate School of Biomedical Sciences, Graduate Program in Medical Physics, Houston, USA; 3 The University of Texas MD Anderson Cancer Center, Department of Imaging Physics, Houston, USA; 4 The University of Texas MD Anderson Cancer Center, Department of Radiation Oncology, Houston, USA; 5 The University of Texas MD Anderson Cancer Center, Department of Biostatistics, Houston, USA; 6 The University of Chicago, Department of Pediatrics, Chicago, USA Purpose or Objective There are no dose-response models establishing relationships between colorectal doses or dose-volume metrics and late colorectal sequelae (such as subsequent malignant neoplasms) in childhood cancer survivors. Such models do not exist because these studies require large cohorts with decades of follow-up. Consequently, these cohorts are largely comprised of patients treated in the pre-CT era of radiation therapy (RT) where organ dose calculations were not possible. Thus, it is common practice in late effects studies to reconstruct survivors’ RT on computational phantoms to estimate organ doses. However, the Late Effects Group computational phantom, which has been used for hundreds of late effects studies over several decades, does not have a colorectal model. Here, we aimed to (1) add a colorectal model that incorporates pediatric anatomical variations and (2) validate the geometric and dosimetric accuracy of the model across the typical age range of pediatric RT patients. Materials and Methods Whole-body non-contrast CT scans of 114 pediatric patients (age range: 2.1-21.6 years, 74 males, 40 females) were retrospectively selected. Manual colorectal contours were reviewed and approved by two radiation oncologists. 1 patient was used for the anatomical template, 103 for training and 10 for testing. All contours were normalized using median colorectal length and registered to an anatomical template using the constrained symmetric thin-plate spline robust point