A hybrid phantom system for patient skin and organ dosimetry in fluoroscopically guided interventions

Abstract

Purpose: The purpose of this study was to investigate calibrations for improved estimates of skin dose and to develop software for computing absorbed organ doses for fluoroscopically guided interventions (FGIs) with the use of radiation dose structured reports (RDSR) and the UF/NCI family of hybrid computational phantoms.

Methods and materials: Institutional review board approval was obtained for this retrospective study in which ten RDSRs were selected for their high cumulative reference air kerma values. Skin doses were computed using the University of Florida's rapid in-clinic peak skin dose algorithm (or UF-RIPSA). Kerma-area product (KAP) meter calibrations and attenuation of the tabletop with pad were incorporated into the UF-RIPSA. To compute absorbed organ doses the RDSRs were coupled with software to develop Monte Carlo input decks for each irradiation event. The effects of spectrum matching were explored by modeling (a) a polychromatic x-ray energy beam made to match measured first half-value layers of aluminum, (b) an unmatched spectrum, (c) and a mono-energetic beam equivalent to the effective x-ray energy. The authors also considered the practicality of computing organ doses for each irradiation event within a RDSR.

Results: The KAP meter is highly dependent on the quality of the x-ray spectra. Monte Carlo based attenuation coefficients for configurations in which the beam is transmitted through the tabletop with pad reduced the amount by which the software overestimated skin doses. For absorbed organ dose computations, the average ratios of computed organ doses for a non-fitted to fitted spectrum and effective energy to fitted spectrum were 0.45 and 0.03, respectively. Monte Carlo simulations on average took 38 min per patient. All in-field organ tallies converged with a relative error of less than 1% and out-of-field organs tallies within 10% relative error.

Conclusions: This work details changes to the UF-RIPSA software that include an expanded library of computational phantoms, attenuation coefficients for tabletop with pad, and calibration curves for the KAP meter. For the computation of absorbed organ dose, it is possible to model each irradiation event separately on a patient-dependent model that best morphometrically matches the patient, thus providing a full report of internal organ doses for FGI patients.

Keywords: fluoroscopically guided interventions; organ absorbed dose; peak skin dose.

Figure 1

Series depicting angular definitions for C‐arm positions of fluoroscope image receptor. (a) Positioner primary angle and (b) positioner secondary angle for a patient position of recumbent, head‐first, and supine as defined by the RDSR. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

Flowchart of the process to compute organ doses from the UF‐MODS software. In the absence of patient specific information, the process is used to generate backscatter and attenuation coefficients to be used in UF‐RIPSA.

Figure 3

Frequency distributions of irradiation parameters for interventional fluoroscopic procedures in the Department of Radiology at UF Health Jacksonville using the Siemens Artis zee fluoroscopic unit. N = 23,763 irradiation events. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Measurements of KAP for the installed DIAMENTOR meter and reference chamber taken at a tube current of 200 mA and pulse width of 300 ms with 0.1 mm of Cu filtration over a period of 3 months. Radcal Month 1 and Radcal Month 3 are superimposed. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

Calibration coefficient measurements for each tube filtration available to the Siemens Artis zee system. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

Computational model of tabletop and pad used for Monte Carlo simulations. Dimensions are based on CT images obtained of the tabletop and pad used in the Siemens Artis zee suite at University of Florida College of Medicine at Jacksonville Division of Vascular and Interventional Radiology. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 7

Measurements of Al HVL for various tube voltage and filtration combinations along with a linear regression best fit line. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 8

X‐ray energy spectra for a tube voltage of 120 kVp and 0.3 mm of Cu filtration before and after matching to a measured Al HVL. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 9

Attenuation factor of the useful x‐ray beam through the tabletop and pad for the Siemens Artis zee as a function of the angle of incidence at 70 kVp. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 10

Attenuation factor of the useful x‐ray beam through the tabletop and pad for the Siemens Artis zee as a function of the angle of incidence at 0.3 mm of Cu filtration. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 11

Disparity in dose (mGy) at the reference point for each irradiation event of RDSR No. 1124. Patient underwent a bilateral uterine artery embolization. The corresponding dose at the reference point for each irradiation event is color coded to illustrate how that irradiation event compares to other events within the procedure. The horizontal lines reflect the 90, 85, 75, 50, 45, and 25th percentile cutoffs. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 12

Fraction of total organ dose when considering only irradiation events that register a cumulative reference air kerma in the 90, 85, 75, 50, 45, and 25th percentile and above. Plot corresponds to RDSR No. 1124 with 117 irradiation events. Procedure was described as a bilateral uterine artery embolization. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 13

Fraction of total organ dose when considering only irradiation events that register a cumulative reference air kerma in the 90, 85, 75, 50, 45, and 25th percentile and above. Plot corresponds to RDSR No. 1325 with 299 irradiation events. Procedure was described as an abdominal angiography; angioplasty of the superior mesenteric artery; and stenting of celiac artery origin and bilateral renal arteries. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 14

Skin dose maps for a select female and male patient corresponding to (a) RDSR 1124 and (b) 1193, respectively, from Table 5. Peak skin dose for RDSR 1124 reached 5,652 mGy and patient was matched by height and weight to the female 160 cm and 70 kg UF/NCI hybrid computational phantom. Peak skin dose for RDSR 1193 reached 5,025 mGy and patient was matched to the male 170 cm and 75 kg phantom.

Figure 15

Dose‐area histograms (DAHs) for 10 select high‐dose cases normalized to peak skin dose. Ordinate indicates what fraction of peak skin dose is delivered to an area of exposed patient skin given on the abscissa. [Color figure can be viewed at wileyonlinelibrary.com]