(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: There are several studies suggesting correlation between IGRT setup errors and BMI. However, abdominal fat content has visceral and subcutaneous components, which may affect setup error differently. This study aims to develop a workflow that could utilize a quickly calculated metric of abdominal fat content to stratify these patients.
Methods: IGRT shift data was retrospectively tabulated from daily fan-beam CT-on-rails pre-treatment alignment for 8 patients treated with RT to the liver. Systematic and random errors in daily setup were characterized via averages and standard deviations of shift data. Visceral and subcutaneous fat content was defined by using a semi-automated region-growing algorithm to contour adipose on CT-simulation scans. All contours were created for a single slice near L4 on which the Visceral Fat Area (VFA) and Subcutaneous Fat Area (SFA) were calculated.
Results: VFA ranged from 26.84-359.58cm², and SFA from 71.72-357.5cm². SD of random error(s) for low vs high VFA was 0.20cm vs 0.28cm(lateral), 0.22cm vs 0.50cm(longitudinal), and 0.26cm vs 0.31cm(vertical). Additionally, s for low vs high SFA patients was 0.19cm vs 0.29cm(lateral), 0.28cm vs 0.47cm(longitudinal), and 0.25cm vs 0.31cm(vertical). The percentage of shifts greater than 7mm for low vs high VSA was 0% vs 0%(lateral), 2% vs 30%(longitudinal) and 4% vs 22%(vertical). The percentage of shifts greater than 7mm for low vs high SFA patients was 0% vs 0%(lateral), 15% vs 16% (longitudinal), and 3% vs 23%(vertical).
Conclusion: Preliminary data suggests daily setup error may be dependent on both VFA and SFA, and patients with high VFA may be more likely to need large shifts. Risk of marginal miss may increase for patients with increased visceral fat when imaged with kV-kV or cone-beam CT, which provide less soft-tissue discernment than fan-beam CT-on-rails, when 7mm or smaller PTV expansions are used. More data is needed to confirm these results.
Keywords
Not Applicable / None Entered.
Taxonomy
TH- External Beam- Photons: General (most aspects)
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