Monte Carlo Study of 3 D Stray Radiation during Interventional Procedures

In interventional medical procedures, other than the highly important issue of optimizing image quality and patient exposure using the primary beam, there remains a continuing need for the study of staff exposure from the scattered radiation. Herein, investigation is made of the 3D stray-radiation distribution, the simulation being made of a realistic interventional scenario through use of the Monte Carlo code Geant4 (version 10.3). The simulation is conducted based on the high definition reference Korean-man (HDRK-man) computational phantom and a GE Infinia 3/8” C-arm machine, focusing on the effect of variation of kVp and field of view (FoV) on the scattered particles’ spatial distribution. With direct measurement of the absorbed dose remaining challenging, not least in respect of the organs at risk, we computed the scatter fractions, defined as the ratio of the air kerma free-in-air to the entrance surface air kerma (ESAK), which are both easily quantifiable. Scatter fraction distributions were simulated for X-ray tube outputs (and half-value layers, HVL) of 60 kVp (2.3 mm Al), 80 kVp (3.2 mm Al) and 120 kVp (4.3 mm Al) and FoV of 15, 20, 25 and 30 cm. The distributions are obtained for different height levels, corresponding to the lens of the eye, and the lung and prostate, all radiosensitive organs. Investigations are made for eight likely locations around the patient. At fixed FoV results reveal an inverse relationship between ESAK and kVp, also that change in kVp from 60 to 80 has a greater effect than from 80 to 120. For change in FoV at fixed kVp, the scatter fraction remains constant. The particular staff locations are found to be optimal in seeking mitigation of dose. Moreover, the combined usage of numerical human model and Monte Carlo simulation can be considered as an added value to the radiation safety research field, especially to the interventional radiology staff and to the patient.


Introduction
Interventional radiology (IR) and cardiology (IC) are of undoubted value in the diagnosis and treatment of diseases, their efficacy being reflected in the growing availability of these procedures within many larger medical centers throughout the world. The procedures are conducted by a team of medical staff who during the intervention all remain within the specialized theatre, typically being in close proximity to the patient. Given that such live-time imaging procedures are typically of protracted duration, it is of no surprise that when compared to other frequent radiological methods, IR and IC represent two of the most significant in terms of radiation exposure to medical staff. Although perhaps obvious, it is nevertheless worth noting that with the ever-increasing demand for the conduct of such procedures there comes an associated increase in radiation exposure to staff. A priori knowledge of the occupational absorbed dose to such staff can assist in keeping exposure levels as low as reasonably achievable [1]. Of interest is that previous success in maintaining levels below the dose limit has not always prevented cataract formation, with incidences of brain cancer among radiologists also being suggested as being implicated [2] [3] [4] [5] [6]. As such, computational-exposure scenarios mimicking realistic interventional procedures are important in predicting the 3D scatter (stray) radiation arising from the patient and apparatus. Over the last decade, investigations of patient and staff exposure during IR and IC have been performed via measurement [7] [8] [9] [10] [11].
Study of interventional radiology occupational exposures has also been made using the Monte Carlo (MC) technique [12] [13] [14]. This said, the stray radiation 3D distribution has not been previously obtained via MC simulation using the high definition reference Korean-man (HDRK-man) virtual anthropomorphic phantom [15]. Herein modeling has been made of staff exposure, the patient being imaged with a GE Infinia 3/8" machine, utilizing the versatile Geant4 toolkit [16]. The aim has been to study the effect of key parameters on occupational exposure distribution within the theatre. Specifically, the influence of kVp, field of view (FoV) and staff position has been investigated, focusing on three radiosensitive organs (lens of the eye, and lung and prostate).

Materials and Methods
Description is provided herein of the adopted methodology, presenting the voxelized phantom corresponding to the patient and staff, the X-ray imaging machine and X-ray tube output. Different proposed interventional imaging procedure scenarios are also outlined. We note that there are three phantom format types (stylized, voxel and hybrid) and four morphometric categories (reference, patient-dependent, patient-sculpted and patient-specific). Here we are limited to voxel type and reference category (patient matching by age only) to model phantoms.

Patient and Operator Modeling
In modeling patient and staff, close similarity has been noted between the ICRP reference man and the voxelized phantom HDRK-man, including height and weight, a matter encouraging the use of the latter. Constructed from high-resolution photographic anatomical images, HDRK-man has a height of 171 cm and weight of 68 kg. Whereas, the ICRP models human having a height of 176 cm and weight of 73 kg. The considered phantom consists of some 30 million voxels, resolution 1.981 × 1.981 × 2.0854 mm 3 , distributed along a 3D array of dimension 247 × 141 × 850, along the x, y and z coordinate axes, respectively. The elemental composition and density of each voxel were obtained from ICRU 46 [17], defining some 40 tissues/organs.

Gamma Camera and X-Ray Spectra
The GE Infinia II 3/8" C-arm machine modeled in this study has an inner radius of 1m and is made from a sturdy, rigid foam material (density 0.7 g/cm 3

Irradiation Scenarios
The parameters studied during this work were as follows:

Results and Discussion
Results and analysis are provided herein, concerning the three dominating parameters modifying the 3D stray radiation dose distribution, kVp, FoV and staff

Effect of kVp
To Such results are in accord with literature [10] and theory, the latter depicted in Figure 5 showing polar plots of the Compton scattering cross-section as a function of energy [21], the forward direction becoming increasingly favored. Table   1 illustrates the ratios of air kerma free-in-air to entrance surface air kerma (ESAK), namely the scatter fraction, quoted in µGy/Gy•cm 2 [22], for all combi- This can be rationalized to be due to the penetration and scattering abilities characterizing the spectral X-ray beam. Given the limited number of interventional scenarios studied, including the overall shape of the phantom, the choice   of C-arm machine and buildup specifications, the findings are not to be generalized. Nevertheless, while each interventional procedure will undoubtedly produce its own scattered radiation distribution, the current study can be considered to provide a good estimation of the overall situation.

Effect of the FoV
In and 28cm circular shaped FoVs, Bethe et al. [23] found this to be an important effect on the stray radiation when changing the FoV parameter; the present work confirms their findings. Also supported is the trend towards near constancy of scatter fractions for a given value of kVp. Thus, we can generalize that the scatter fraction remains an almost constant function of the FoV for a fixed kVp parameter. This situation should be explored by extending the present study to take into account the effect of focal-to-skin and focal-to-detector distances.

Effect of Operator Location
The effect of changing staff location (as shown in Figure 1) on the scatter fraction at the level of the lens of the eye, lung and prostate is illustrated in Table 1.
Despite buildup effects as observed in the measurements, applicability of the inverse square law has been confirmed, in terms of scattered fraction values, for the coupled staff locations of (1 and 5), (2 and 6) and (3 and 7). Moreover, we observe the typical times spent by the different members of staff (IR, IC and N) during an interventional procedure [19] to be optimal. This said, the present K. S. Alzimami study has been limited to the PA projection; further simulations would help, extending findings to other irradiation conditions of AP, LAO, RAO and such like.

Conclusion
There is an increasingly pressing need to quantify medical staff during interventional procedures. The present study, concerning scattered radiation from the patient and C-arm, has examined the influence of kVp, FoV and staff location on the 3D distribution and dose. Using an anthropomorphic voxelized phantom incorporated into a Geant4-based program, evaluation has been made of ESAK and scatter fraction at three organ levels of members of staff (eye lens, lung and prostate). For a given value of the FoV, ESAK is seen to increase with a decrease in kVp, supporting existing literature. Secondly, modification of kVp from 120 to 80 has a negligible effect (average deviation of 13.4%); conversely, in changing from 80 to 60 kVp an average decrease of 35.9% is found. Finally, the typically adopted locations of the various members of staff during interventions, in accordance with their roles, are found to be an optimal choice. From such findings, we urge the implementation of appropriate in-house protocols in seeking to mitigate the potential deleterious effects of radiation on members of staff.

Conflicts of Interest
The author declares no conflicts of interest regarding the publication of this paper.