Session Item

Retrospective cohort studies of radiation therapy (RT)-related late effects involve longitudinal follow-up of cancer survivors (N>10,000) from multiple institutions treated with conventional RT without CT-based treatment planning.  To estimate organ doses, patients’ historic RT records are abstracted and their treatments are reconstructed on age-specific computational phantoms. In past studies, we have reconstructed heart doses using a simple anatomy atlas-based whole heart model, i.e. stylized and without substructures. The primary objective of this study was to enhance the heart model, including substructures, such that it was more anatomically realistic in size, shape, and position using CT images and demonstrate that the refined heart model is representative across the typical age range of a pediatric cohort.

Our atlas-based heart model was refined and expanded using whole heart and substructure contours from the National Cancer Institute (NCI) computational human phantom series. Within a commercial treatment planning system, we registered the 5-year-old NCI phantom with our in-house phantom. Using a Python script, whole heart and substructure contours were extracted, deformed, and converted to a grid of evenly spaced points in our in-house master phantom. We then scaled our phantom to ages 1, 5, 10, 15 years and adult. Scaling functions in our computational phantoms take into account non-uniform growth along all three dimensions. To evaluate if our new cardiac model was representative from infant to adolescent, we calculated Dice similarity coefficient (DSC) values comparing our atlas-based and new cardiac models scaled to aforementioned ages with NCI phantoms of same age and gender.

We developed a new cardiac model with 14 substructures (including aorta, ventricles, atriums, arteries and valves) for our computational phantom. Importantly, the phantom (along with heart and substructures) can be scaled to any age at RT for individual dose reconstructions, which is essential for pediatric cohorts. The new cardiac model is anatomically similar to NCI cardiac model across all ages with an average DSC value of 0.80 ± 0.04. This is a significant improvement (Wilcoxon Signed-Rank Test, p < 0.01) over the atlas-based heart model with an average DSC value of 0.37 ± 0.05. The improvement is consistent for each age and both genders.

The new cardiac model developed here is anatomically superior to the previous model and enables dose estimation for cardiac substructures for individuals in pediatric cohorts whose historic RT was not planned with CT-images. Our model can be used for late cardiac toxicity studies to evaluate the relationship between cardiac substructure doses and late cardiac disease.