A parametric study of occupational radiation dose in interventional radiology by Monte-Carlo simulations.

This paper presents the results of a parametric study on the occupational exposure in interventional radiology to explore the influence of various variables on the staff doses. These variables include the angiography beam settings: x-ray peak voltage (kVp), added copper filtration, field diameter, beam projection and source to detector distance. The study was performed using Monte-Carlo simulations with MCNPX for more than 5600 combinations of parameters that account for different clinical situations. Additionally, the analysis of the results was performed using both multiple and random forest regression to build a predictive model and to quantify the importance of each variable when the variables simultaneously change. Primary and secondary projections were found to have the most effect on the scatter fraction that reaches the operator followed by the effect of changing the x-ray beam quality. The effect of changing the source to image intensifier distance had the lowest effect.

Field correction factors for a PTW-31016 Pinpoint ionization chamber for both flattened and unflattened beams. Study of the main sources of uncertainties.

PURPOSE: The primary aim of this study was to determine correction factors, kQclin,Qmsrfclin,fmsr for a PTW-31016 ionization chamber on field sizes from 0.5 cm × 0.5 cm to 2 cm × 2 cm for both flattened (FF) and flattened filter-free (FFF) beams produced in a TrueBeam clinical accelerator. The secondary objective was the determination of field output factors, ΩQclin,Qmsrfclin,fmsr over this range of field sizes using both Monte Carlo (MC) simulation and measurements. METHODS: kQclin,Qmsrfclin,fmsr for the PTW-31016 chamber were calculated by MC simulation for field sizes of 0.5 cm × 0.5 cm, 1 cm × 1 cm, and 2 cm × 2 cm. MC simulations were performed with the PENELOPE code system for the 10 MV FFF Particle Space File from a TrueBeam linear accelerator (LINAC) provided by the manufacturer (Varian Medical Systems, Inc. Palo Alto, CA, USA). Simulations were repeated taking into account chamber manufacturing tolerances and accelerator jaw positioning in order to assess the uncertainty of the calculated correction factors. Output ratios were measured on square fields ranging from 0.5 cm × 0.5 cm to 10 cm × 10 cm for 6 MV and 10 MV FF and FFF beams produced by a TrueBeam using a PTW-31016 ionization chamber; a Sun Nuclear Edge detector (SunNuclear Corp., Melbourne, FL, USA) and TLD-700R (Harshaw, Thermo Scientific, Waltham, MA, USA). The validity of the proposed correction factors was verified using the calculated correction factors for the determination of ΩQclin,Qmsrfclin,fmsr using a PTW-31016 at the four TrueBeam energies and comparing the results with both TLD-700R measurements and MC simulations. Finally, the proposed correction factors were used to assess the correction factors of the SunNuclear Edge detector. RESULTS: The present work provides a set of MC calculated correction factors for a PTW-31016 chamber used on a TrueBeam FF and FFF mode. For the 0.5 cm × 0.5 cm square field size, kQclin,Qmsrfclin,fmsr is equal to 1.17 with a combined uncertainty of 2% (k = 1). A detailed analysis of the most influential parameters is presented in this work. PTW-31016 corrected measurements were used for the determination of ΩQclin,Qmsrfclin,fmsr for 6 MV and 10 MV FF and FFF and the results were in agreement with values obtained using a TLD-700R detector (differences < 3% for a 0.5 cm square field) for the four energies studied. Uncertainty in field collimation was found to be the main source of influence of ΩQclin,Qmsrfclin,fmsr and caused differences of up to 15% between calculations and measurements for the 0.5 cm × 0.5 cm field. This was also confirmed by repeating the same measurements at two different institutions. CONCLUSIONS: This study confirms the need to introduce correction factors when using a PTW-31016 chamber and the hypothesis of their low energy dependence. MC simulation has been shown to be a useful methodology to determine detector correction factors for small fields and to analyze the main sources of uncertainty. However, due to the influence of the LINAC jaw setup for field sizes below or equal to 1 cm, MC methods are not recommended in this range for field output factor calculations.