Exploring the Margin Recipe for Online Adaptive Radiation Therapy for Intermediate-Risk Prostate Cancer: An Intra-fractional Seminal Vesicles Motion Analysis

Publication date: Available online 24 February 2017
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Yang Sheng, Taoran Li, W. Robert Lee, Fang-Fang Yin, Q. Jackie Wu
PurposeTo provide a benchmark for seminal vesicle (SV) margin selection to account for intra-fractional motion, and to investigate the effectiveness of two motion surrogates in predicting intra-fractional SV coverage.Methods and Materials15 prostate patients were studied. Each patient has five pairs (one patient has four pairs) of pre-treatment and post-treatment cone-beam CTs (CBCTs). Each pair of CBCTs was registered based on prostate fiducial markers. All pre-treatment SVs were expanded with 1, 2, 3, 4, 5 and 8 mm isotropic margins to form a series of PTVs, and their intra-fractional coverage to the post-treatment SV determined the "ground truth" for exact coverage. Two motion surrogates, the center-of-mass (COM) and the border of contour, were evaluated using Pearson product-moment correlation coefficient and exponential fitting for predicting SV underdosage. Action threshold of each surrogate was calculated. The margin for each surrogate was calculated using a traditional margin recipe.Results95% post-treatment SV coverage was achieved in 9%, 53%, 73%, 86%, 95% and 97% fractions with 1, 2, 3, 4, 5 and 8 mm margin, respectively. 5 mm margins provided 95% intra-fractional SV coverage in over 90% fractions. The correlation between the COM and border was weak, moderate and strong in the LR, AP and SI directions, respectively. Exponential fitting gave the underdosage threshold of 4.5 and 7.0 mm for the COM and border. Van Herk's margin recipe recommended 0, 0.5 and 0.8 mm margins in the LR, AP and SI directions based on the COM, and 1.2, 3.9 and 2.5 mm based on the border.Conclusions5 mm isotropic margins for the SV is the minimum required to mitigate the intra-fractional motion. Both the COM and border are acceptable predictors for SV underdosage with 4.5 and 7.0 mm action threshold. Traditional margin based on the COM or border underestimates the margin.

Teaser

We investigated the minimal margin for seminal vesicles based on volumetric coverage to mitigate intra-fractional motion during prostate radiation therapy. The volume coverage based margin provided ground truth for analyzing the effectiveness of two alternative motion surrogates, the center-of-mass and the border of contour. The exponential fitting was performed to test the ability of predicting underdosage of SV for two surrogates and the Van Herk's margin calculated from both surrogates was provided for reference.


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