Evaluation of skin dose calculation factors in interventional fluoroscopy

Abstract

PurposeThe purpose of this study was to measure fluoroscopic dose calculation factors for modern fluoroscopy‐guided interventional (FGI) systems, and to fit to analytical functions for peak skin dose (PSD) calculation.MethodsTable transmission factor (TTF), backscatter factor (BSF), and a newly termed kerma correction factor (KCF) were measured for two interventional fluoroscopy systems. For each setup, air kerma rates were measured using a small ionization chamber in fluoroscopic service mode while selecting kVp, copper (Cu) filter thickness, incident angle, and x‐ray field size at the assumed patient skin locations. Angle dependency on KCF was measured on the GE system at isocenter for angles of 0, 15, 30, and 40 degrees, using a range of kVp, Cu filters, and one field size. An analytical equation was created to fit the data to facilitate PSD calculation.ResultsFor the GE system, oblique incidence measurements show KCF decreased by about 2%, 8%, and 13% for incident angles of 15, 30, and 40°, respectively, relative to KCF at 0 degree. The GE and Siemens systems' KCFs ranged from 0.89 to 1.45, and 0.64 to 1.44, respectively. The KCFs increased with a power of field size, and generally increased with kVp and Cu filter. The average percentage difference between TTF × BSF × f and KCF was 16% at normal incidence. The KCF data were successfully fitted to function of angle, field size, kVp, and Cu filter thickness using seven parameters, with an average R‐squared value of 0.98 and maximum percentage difference of 6.0%.ConclusionsThis study evaluated scatter factors for two fluoroscopy systems, and dependencies on angle, kVp, Cu filter, and field size, with emphasis on under table beam orientations. Analytical fitting of the data with exposure parameters may facilitate PSD calculations, and more accurately determine the potential for radiation‐induced skin injury.

The American College of Radiology Fluoroscopy Dose Index Registry Pilot: Technical Considerations and Dosimetric Performance of the Interventional Fluoroscopes

“…These values are in general agreement with those from a previous study (10) that reported a factor of 0.70 for a single Innova system (GE Healthcare, Chicago, Illinois) at 100 kV with no additional filtration but using an X-ray field size of 13.5 cm  13.5 cm. In the same study (10), values of 0.61 at 90 kV and 0.65 at 120 kV with no additional filtration and an X-ray field size of 10 cm  10 cm were reported for a single Siemens fluoroscope (Siemens, Munich, Germany). The transmission values from this study (10) and the present study were substantially lower than those reported in another study (11) that reported a transmission factor of 0.79 at 100 kV without additional filtration; however, this result did not include attenuation of the table pad.…”

“…In the same study (10), values of 0.61 at 90 kV and 0.65 at 120 kV with no additional filtration and an X-ray field size of 10 cm  10 cm were reported for a single Siemens fluoroscope (Siemens, Munich, Germany). The transmission values from this study (10) and the present study were substantially lower than those reported in another study (11) that reported a transmission factor of 0.79 at 100 kV without additional filtration; however, this result did not include attenuation of the table pad. In the present study, the beam quality was fixed at 100 kV without any added Cu filtration.…”

Estimation of patient skin dose in fluoroscopy: summary of a joint report by AAPM TG357 and EFOMP

“…Since FGI device AEC settings and anatomical protocols use a variety of kV and spectral filtration, a matrix of measurements of tabletop and pad attenuation and forward scatter factors is needed for skin dose estimation, which can be paired with RDSR information on kV and spectral filtration. 51,52 The transmission of the beam through the tabletop and pad is dependent on the angle of incidence and non-normal incidence requires further correction. 53 The path length of the central ray through a horizontal tabletop and pad is increased by the secant of the CRA/CAU (Cranial and Caudal, respectively) or RAO/LAO (Right Anterior Oblique and Left Anterior Oblique, respectively) angulation (described in Section 3.B), and the entrance skin dose is decreased accordingly.…”

“…This attenuation formalism and the approximation of forward scatter being proportional to the transmitted primary X-ray beam fluence have been discussed in multiple studies. 41,51,53 Figure 8 shows an example of measured tabletop and pad attenuation and transmission, 51 which may be used for PSD estimation, as discussed by DeLorezo et al 51 the total correction required due to attenuation, back-and forward scatter might improve the accuracy of these corrections. 51…”

“…41,51,53 Figure 8 shows an example of measured tabletop and pad attenuation and transmission, 51 which may be used for PSD estimation, as discussed by DeLorezo et al 51 the total correction required due to attenuation, back-and forward scatter might improve the accuracy of these corrections. 51…”