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A Sensitivity Study of Tissue Characterization for Brachytherapy Monte Carlo Dose Calculation

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S Bedwani

S Bedwani1*, J Carrier2, H Bouchard2, (1) Universite de Montreal, Montreal, Quebec, (2) Centre universitaire de l'Universite de Montreal, Montreal, Quebec

SU-E-T-501 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To establish the reliability of electron density (ED) mapping and tissue segmentation techniques using CT images for brachytherapy, considering stochastic and systematic Hounsfield unit (HU) variations.

Methods: Most common artifacts are simulated within a Monte Carlo theoretical framework. A set of CT data is first generated with the EGSnrc suite followed by their reconstruction performed with an iterative algorithm. A statistical analysis of HU values from reconstructed images is performed to determine uncertainties and systematic effects introduced by the iterative algorithm and the presence of artifacts. Maps of ED and tissue indexes are retrieved from an HU-ED curve calibrated at 120 kVp using experimental measurements and ICRU data. The mean energy absorption coefficient for each tissue is computed from ICRU data for an Iridium-192 source in order to evaluate the impact on dose calculations. A probabilistic approach is used to compute the uncertainty on absorbed dose generating random distributions of HU and determining the probabilistic effects on the extracted ED and absorption coefficients.

Results: For an uncertainty of +/-20 HU, absorption coefficient uncertainties raise up to 3% when HU values are near the fat-muscle intersection and up to 9% near the muscle-spongiosa intersection. Uncertainties on ED are found to be less than 1% for HU above 0 and up to 3% for fat. A systematic effect of 50 HU caused by typical artifacts yields absorption coefficient errors up to 30% for HU values near the muscle-spongiosa region, and errors in ED up to 8% for fat.

Conclusion: Results show that the main source of dose calculation uncertainty is caused by the sensitivity of the tissue segmentation technique. This study suggests that improvements in such techniques are yet to be achieved.

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