Physical validation of UF‐RIPSA: A rapid in‐clinic peak skin dose mapping algorithm for fluoroscopically guided interventions

Abstract Purpose The purpose of this study was to experimentally validate UF‐RIPSA, a rapid in‐clinic peak skin dose mapping algorithm developed at the University of Florida using optically stimulated luminescent dosimeters (OSLDs) and tissue‐equivalent phantoms. Methods The OSLDs used in this study were InLightTM Nanodot dosimeters by Landauer, Inc. The OSLDs were exposed to nine different beam qualities while either free‐in‐air or on the surface of a tissue equivalent phantom. The irradiation of the OSLDs was then modeled using Monte Carlo techniques to derive correction factors between free‐in‐air exposures and more complex irradiation geometries. A grid of OSLDs on the surface of a tissue equivalent phantom was irradiated with two fluoroscopic x ray fields generated by the Siemens Artis zee bi‐plane fluoroscopic unit. The location of each OSLD within the grid was noted and its dose reading compared with UF‐RIPSA results. Results With the use of Monte Carlo correction factors, the OSLD's response under complex irradiation geometries can be predicted from its free‐in‐air response. The predicted values had a percent error of −8.7% to +3.2% with a predicted value that was on average 5% below the measured value. Agreement within 9% was observed between the values of the OSLDs and RIPSA when irradiated directly on the phantom and within 14% when the beam first traverses the tabletop and pad. Conclusions The UF‐RIPSA only computes dose values to areas of irradiated skin determined to be directly within the x ray field since the algorithm is based upon ray tracing of the reported reference air kerma value, with subsequent corrections for air‐to‐tissue dose conversion, x ray backscatter, and table/pad attenuation. The UF‐RIPSA algorithm thus does not include the dose contribution of scatter radiation from adjacent fields. Despite this limitation, UF‐RIPSA is shown to be fairly robust when computing skin dose to patients undergoing fluoroscopically guided interventions.


| INTRODUCTION
Fluoroscopically guided interventional (FGI) procedures are frequently associated with relatively high dose rates and prolonged irradiation times. There is an expressed need for physicians and clinicians to be aware of patient dose and to minimize the risk of radiation-induced injury. 1 Real-time knowledge of peak skin dose has been shown to assist clinicians in managing and even reducing patient risks. 2 In response to these issues, the authors introduced a rapid in-clinic peak skin dose mapping algorithm developed at the University of Florida (called UF-RIPSA) as previously presented by Johnson et al. 3 and later modified by Borrego et al. 4 The skin doses reported by UF-RIPSA are evaluated based on the following expression: where D skin is the estimated dose to the skin from a single irradiation event, K a,r is the reference air kerma reported by the radiation dose structured report (RDSR) for each irradiation event, b is a calibration factor for the KAP meter, d ref is the distance to the reference point from the source location, and d skin is the distance to the skin location from the source location. Other terms include BSF, which is a correction of backscattered x rays and secondary electrons at the skin dose point, l en q skin air is the ratio of the mass energy-absorption coefficients (skin-to-air) for the relevant x ray energies considered in the dose assessment, and AF is an attenuation factor that accounts for the loss in energy deposition due to the presence of the tabletop and pad provided that the x ray beam intercepts these structures. domain. UF-RIPSA used the reported reference air kerma, K a,r , and a calibration factor, b, derived from cross-calibration of the kerma-area product meter with values with a Radcal Model 10 9 6-6 6-cm 3 ion chamber. To relate the OSLDs dose readings with UF-RIPSA, a freein-air † cross-calibration of the OSLDs with the ion chamber was computed for various x ray spectra of differing beam qualities. This study also derived Monte Carlo correction factors to adjust for any dose-response differences due to a geometry other than free-in-air.
All measurements performed are summarized in Table 1 and discussed at length below.

2.A | FLUOROSCOPIC BEAM PARAMETE RS
All exposures were performed using a Siemens Artis zee bi-plane flu-

2.B | FREE-IN -AIR MEASUREME NTS
To characterize the OSLDs, dose measurements were taken for an ion chamber free-in-air and a sample of three OSLDs free-in-air. A Radcal Model 10 9 6-6 6-cm 3 ion chamber was used to assess air kerma at the isocenter of the C-arm for each beam quality at two field sizes. The field sizes, both defined at isocenter, were       The peak tube potentials used were 50, 80, and 100 kVp. The amount of added filtration was varied from 0.2 to 0.6 mm Cu with no additional filtration other than inherent filtration. In total, nine beam qualities were used. c Exposures were made with the central ray angle of incidence set at 0 0 , 30 0 , and 60 0 .

2.E | CORRECTION FACTORS
OSLD readings can be characterized partly by correction factors evaluated for each beam quality and irradiation geometry to convert dose quantities from one geometry to another. Equation (2) The Monte Carlo correction factor given in eq. 3 then accounts for the dose-response variation between an OSLD on-phantom to that of an OSLD free-in-air. It is defined as the ratio of the Monte      Table 5, percent errors range only from 6.6% to À11.0%, with increasing negative dose errors at larger angles of incidence.
F I G . 6. The photon energies at the OSLDs for each irradiation geometry at a peak tube potential of 80 kVp and 0.2 mm of added Cu filtration. For comparison, the initial equivalent spectrum is also shown where the BSK term in the legend indicates an energy fluence spectra with the backscattering phantom present. Similarly, the terms Table or Pad indicate that the beam first traversed these structures before the energy fluence was scored. The recorded values by the OSLDs differ from the results of UF-RIPSA by less than 9%, with the OSLDs always recording a higher dose value, primarily due to the inclusion of latter x ray scatter not accounted for in the UF-RIPSA algorithm.
In Fig. 8, the beam first traverses the table and pad before normally striking the surface of the tissue-equivalent phantom containing the OSLDs. The displacement in Fig. 8 (Fig. 7) or Field C (Fig. 8).

| CONCLUSIONS
UF-RIPSA is a rapidly deployed algorithm for assessing skin doses incurred during fluoroscopically guided interventional procedures which is based upon a ray-tracing of the reported reference air kerma given by the RDSR on a per irradiation event basis, with subsequent

CONFLI CTS OF INTEREST
There are no conflicts of interest.

N O T E †
Free-in-air in this study placed the detection instrument, Radcal ion chamber or OSLD, at isocenter in the absence of a backscattering phantom with the surface of the tabletop 25 cm below.
T A B L E 5 Assessment of the angular dependence of D Predicted OSL on the backscatter phantom with the fluoroscopic beam first traversing the table and if present, the pad (via application of eq. 4). The table data are for beam qualities with 0.2 mm of added Cu filtration, a peak tube potential of 80 kVp, and a square field size of 5 9 5 cm 2 at the OSLD location.