9 March 2018 Calculation of forward scatter dose distribution at the skin entrance from the patient table for fluoroscopically guided interventions using a pencil beam convolution kernel
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Abstract
The forward-scatter dose distribution generated by the patient table during fluoroscopic interventions and its contribution to the skin dose is studied. The forward-scatter dose distribution to skin generated by a water table-equivalent phantom and the patient table are calculated using EGSnrc Monte-Carlo and Gafchromic film as a function of x-ray field size and beam penetrability. Forward scatter point spread function’s (PSFn) were generated with EGSnrc from a 1×1 mm simulated primary pencil beam incident on the water model and patient table. The forward-scatter point spread function normalized to the primary is convolved over the primary-dose distribution to generate scatter-dose distributions. The utility of PSFn to calculate the entrance skin dose distribution using DTS (dose tracking system) software is investigated. The forward-scatter distribution calculations were performed for 2.32 mm, 3.10 mm, 3.84 mm and 4.24 mm Al HVL x-ray beams for 5×5 cm, 9×9 cm, 13.5×13.5 cm sized x-ray fields for water and 3.1 mm Al HVL x-ray beam for 16.5×16.5 cm field for the patient table. The skin dose is determined with DTS by convolution of the scatter dose PSFn’s and with Gafchromic film under PMMA “patient-simulating” blocks for uniform and for shaped x-ray fields. The normalized forward-scatter distribution determined using the convolution method for water table-equivalent phantom agreed with that calculated for the full field using EGSnrc within ±6%. The normalized forwardscatter dose distribution calculated for the patient table for a 16.5×16.5 cm FOV, agreed with that determined using film within ±2.4%. For the homogenous PMMA phantom, the skin dose using DTS was calculated within ±2 % of that measured with the film for both uniform and non-uniform x-ray fields. The convolution method provides improved accuracy over using a single forward-scatter value over the entire field and is a faster alternative to performing full-field Monte-Carlo calculations.
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Sarath Vijayan, Sarath Vijayan, Zhenyu Xiong, Zhenyu Xiong, Chao Guo, Chao Guo, Jonathan Troville, Jonathan Troville, Naveed Islam, Naveed Islam, Stephen Rudin, Stephen Rudin, Daniel R. Bednarek, Daniel R. Bednarek, } "Calculation of forward scatter dose distribution at the skin entrance from the patient table for fluoroscopically guided interventions using a pencil beam convolution kernel", Proc. SPIE 10573, Medical Imaging 2018: Physics of Medical Imaging, 1057363 (9 March 2018); doi: 10.1117/12.2294920; https://doi.org/10.1117/12.2294920
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