Purpose: Study purposes were to develop and integrate a colorectal model that incorporates anatomical variations of pediatric patients into an age-scalable computational phantom, and to validate the model for pediatric radiation therapy (RT) dose reconstructions.
Methods: Colorectal contours were manually derived by two physicians from whole-body non-contrast CT scans of 114 pediatric patients (age range: 2.1-21.6years, 74males, 40females). One contour was used for an anatomical template, 103 for training and 10 for testing. Training contours were used to create a colorectal principal component analysis (PCA)-based statistical shape model (SSM) to extract the population’s dominant deformations. The SSM was integrated into our in-house age-scalable phantom. Geometric accuracy was assessed between patient-specific and SSM reconstructed contours. Two alternative colorectal shapes (hereafter alternative #1, #2) were generated using the first 17 dominant SSM modes. Dosimetric accuracy was assessed by comparing colorectal doses from 10 test patients’ CT-based RT plans (ground-truth) with reconstructed doses for the mean, alternative #1 and #2 colorectal models in age-matched phantoms. Lastly, a proof-of-concept study was conducted to demonstrate that our colorectal models can be integrated into any computational phantom.
Results: Using all 103 PCA modes, mean(min-max) Dice similarity coefficient, distance-to-agreement and Hausdorff distance between patient-specific and reconstructed contours were 0.89(0.85-0.91), 2.1mm(1.7-3.0), and 8.6mm(5.7-14.3), respectively. Average absolute difference between ground-truth and reconstructed mean and maximum colorectal doses (normalized to 20Gy prescription dose) for the mean colorectal model (alternative #1, alternative #2) were 6.33%(8.08%, 6.13%) and 4.38%(4.28%, 4.65%), respectively. Similar agreement was observed when colorectal models were integrated into a collaborator’s 5-year-old phantom; 6.28%(11.73%, 0.34%) and 0.47%(0.19%, 0.90%), respectively.
Conclusion: We developed and validated a population-based colorectal SSM and demonstrated its use for pediatric RT dose reconstruction in two phantoms. We will use this SSM to reconstruct pre-CT era colorectal doses for irradiated individuals in the Childhood Cancer Survivor Study.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the National Cancer Institute (CA55727, G.T. Armstrong, Principal Investigator).