Comparison of skin dose calculated by the dose tracking system (DTS) with a beam angular correction factor and that calculated by Monte-Carlo

Abstract

Skin dose is dependent on the incident beam angle and corrections are needed for accurate estimation of the risk of deterministic effects of the skin. Angular-correction factors (ACF) were calculated and incorporated into our skin-dose-tracking system (DTS) and the results compared to Monte-Carlo simulations for a neuro-interventional procedure. To obtain the ACF’s, EGSnrc Monte-Carlo (MC) software was used to calculate the dose averaged over 0.5, 1, 2, 3, 4 and 5 mm depth into the entrance surface of a water phantom at the center of the field as a function of incident beam to skin angle from 90–10 degrees for beam field sizes from 5–15 cm and for beam energies from 60–120 kVp. These values were normalized to the incident primary dose to obtain the ACF. The angle of incidence at each mesh vertex in the beam on the surface of the DTS patient graphic was calculated as the complement of the angle between the normal vector and the vector of the intersecting ray from the tube focal spot; skin dose at that vertex was calculated using the corresponding ACF. The skin-dose values with angular correction were compared to those calculated using MC with a matching voxelized phantom. The results show the ACF decreases with decreasing incident angle and skin thickness, and increases with increasing field size and kVp. Good agreement was obtained between the skin dose calculated by the angular-corrected DTS and MC, while use of the ACF allows the real-time performance of the DTS to be maintained.

1. INTRODUCTION

Fluoroscopically guided interventional procedure can cause high dose to the skin on the entrance surface of the patient. This high dose can cause threshold-dependent injury to skin¹, such as erythema, epilation, and desquamation². Accurate estimation of the skin dose to the patient is important to evaluate the risk of deterministic cutaneous effects during fluoroscopically-guided procedures. Skin dose varies with the angle of beam incidence and, as seen in Fig. 1, the incident angle of the x-ray beam on the patient’s skin varies during these procedures. Skin dose is typically calculated assuming normal incidence of the beam including the skin-dose-tracking system (DTS) ^3–4 shown in Fig. 2. Corrections for the angle of beam incidence are needed for more accurate skin dose estimation and angular correction factors were thus determined for application in the DTS. The skin dose for this study was defined as the average over the skin thickness. Since skin thickness varies by location over the patient’s body ranging from 0.5 mm to 5 mm^5–6, angular correction factors were determined for this range of thicknesses. For comparison, skin dose maps were determined for a clinical neurovascular procedure with and without the angular correction. The mesh-based patient computational phantom in the DTS was converted to be compatible with Monte-Carlo (MC) software and the skin dose map for the same neuro-interventional procedure was calculated by MC to determine the accuracy of the map resulting from the angular-corrected DTS.

Fig. 1.

Fluoroscopic procedures utilize various angulations of the c-arm gantry that, along with the body contours, can result in the incident beam being non-normal to the skin surface. The incident angle (θ) is defined as the angle of the beam central axis with the skin surface and normal incident is defined as the angle of 90 degrees⁷.

Fig. 2.

The dose tracking system or DTS that we developed provides a real-time, color-coded mapping of the skin dose distribution during fluoroscopically-guide interventional procedures. It indicates the peak skin dose value on the entire surface as well as the peak dose within the current field of view or the FOV-PSD to make the physician aware of the patient’s potential for skin injury.

2. METHODS

To determine the angular correction factor for application in the DTS, EGSnrc Monte-Carlo software⁸ was used to calculate the dose averaged over 0.5, 1, 2, 3, 4, and 5 mm depths into the entrance surface of a water phantom which is 40 × 40 cm on the surface and 20 cm thick; as illustrated in Fig. 3a, dose was calculated at the center of the field as a function of incident beam angle from 90–10 degrees for beam field sizes from 5–15 cm and for beam energies from 60–120 kVp with 3×10¹⁰ photons incidence. An angular-correction factor (ACF) was determined by normalizing these dose values to the incident primary dose, which was calculated using NIST mass-energy absorption coefficients⁹ and by integrating over the beam-energy spectrum (Fig. 3b). To apply the ACF in the DTS, the angle of incidence at each mesh vertex in the beam on the surface of the patient graphic was calculated as the complement of the angle between the normal vector and the vector of the intersecting ray from the tube focal spot (see Fig. 4). Skin dose at each point of intersection was corrected using the ACF for the ray angle at each vertex for each procedure projection saved in a DTS log file from a clinical neurovascular interventional procedure. For the MC calculation of the skin dose map over the DTS patient graphic for the neurovascular procedure, the DTS mesh phantom was voxelized into 1 mm³ size and was converted into a MC-compatible phantom format. The patient skin-dose was calculated using MC with 10¹⁰ incident photons for each projection and the final dose map was compared to that calculated by the angular-corrected DTS. A 1 mm skin thickness was assumed in the calculations for the neurointerventional procedure results shown here.

Fig. 3.

a) Geometry for Monte Carlo (MC) calculation of angular correction factors. The incident angle (θ) is the angle of the central axis with the skin surface. Field size is defined in a plane perpendicular to the central axis at its intersection with the skin. Results are the average over a surface thickness on a water-equivalent, 20 cm thick phantom with a 40 × 40 cm entrance surface. b) Method for calculation of the incident primary dose and equation for angular correction factors.

Fig. 4.

a) Geometry of x-ray intersection with a patient graphic vertex (overtable tube is shown). The normal vector, $\stackrel{\rightharpoonup }{\mathrm{n}}$ at the vertex intersection point, P, and the coordinate of intersection point (X1, Y1, Z1) are recorded in the patient graphic file. b) The incident beam angle is calculated using the normal to each vertex and the vector from the vertex to the focal spot; the skin dose at the vertex point (P) in the DTS is calculated using the angular correction factor at this angle, θ.

3. RESULTS AND DISCUSSION

3.1. ACF vs incident beam angle

The curves in Fig. 5 show that the ACF, or the primary plus scatter to incident primary ratio, decreases with decreasing angle, especially below 50 degrees as the rays become more tangent to the surface, primarily due to greater primary path length through the skin layer.

Fig. 5.

The ACF’s, or ratios of the primary plus scatter to the incident primary ratio at the field center, are shown as a function of incident angle for a 10 × 10 cm field size averaged in 1 mm skin thickness at 60, 80, 100, and 120 kVp

3.2. ACF vs field size

The results of Fig. 6 show the ACF increases with increasing field size. This is due to the increase in backscatter at the field center as the field size increases.

Fig. 6.

The ACF’s, or ratios of the primary plus scatter to the incident primary ratio at the field center, are shown as a function of field size averaged over 3 mm skin thickness at a) 30, b) 60, and c) 90 degrees at 60, 80, 100, and 120 kVp.

3.3. ACF vs kVp

Fig. 7 shows that the ACF increases with increasing kVp for 5, 10 and 15 cm field sizes at 30, 60, and 90 degrees incident angle since the backscatter increases with beam energy. The difference in ACF between different angles of incidence is seen to increase with increasing field size again due to increasing backscatter.

Fig. 7.

The ACF’s, or ratios of the primary plus scatter to the incident primary ratio at the field center, are shown as a function of kVp averaged over 0.5 mm skin thickness for a a) 5 × 5, b) 10 × 10, and c) 15 × 15 cm field size.

3.4. ACF vs skin thickness

Fig. 8 shows that the ACF decreases with increasing skin thickness at lower kVp, small field size, and small angle of incidence. However, as the beam energy goes above 60 kVp, for field sizes larger than 5 cm and angles of incidence larger than 10 degrees, the ACF is rather invariant with skin thickness.

Fig. 8.

The ACF’s, or ratios of the primary plus scatter to the incident primary ratio at the field center, are shown as a function of skin thickness a) for a 5 × 5 cm field size at 60 kVp, b) for a 5 × 5 cm field size at 120 kVp, c) for a 20 × 20 cm field size at 60 kVp, and d) for a 20 × 20 cm field size at 120 kVp.

Fig. 9 demonstrates why the ACF is rather invariant with skin thickness. The primary and secondary components of the dose were separately determined. The primary component was determined by integrating the following equation over the beam-energy spectrum

$$\text{Averaged}\text{primary}\text{dose}=\text{Incident}\text{primary}\mathrm{dose}(\text{E})\times \frac{1-{\text{e}}^{\frac{-\mathrm{\mu }(\text{E})\ast \mathrm{t}}{\sin \mathrm{\theta }}}}{\frac{\mathrm{\mu }(\text{E})\ast \text{t}}{\sin \mathrm{\theta }}}$$ (1)

where t is the skin thickness and μ is the linear attenuation coefficient at energy E and θ is the angle of incidence, and the secondary was determined by subtracting this averaged primary dose from the total averaged primary plus scatter calculated by MC. It is seen that the average primary dose is lower for greater thicknesses, while the scatter component increases with increasing thickness so that there is some cancellation of the effect of thickness on total dose. The scatter increases with increasing beam energy. Table 1 summaries the ACF dependences.

Fig. 9.

a) The averaged primary to the incident primary ratio, and b) scatter to the incident primary ratio, averaged over various skin thickness at the field center, are shown as a function of incident angle for a 10 × 10 cm field size at 60 and 120 kVp.

Table 1.

The percent difference of the ACF between skin thicknesses of 0.5 and 5 mm at various incident angles at 60, and 120 kVp for a 5 × 5 cm, and 20 × 20 cm field size.

Angle (degrees)	10	20	30	40	50	60	70	80	90
60 kVp, 5 cm field	26.82	11.86	7.64	4.98	3.82	3.13	2.69	1.94	2.70
120 kVp, 5 cm field	6.78	2.13	0.39	0.14	0.72	1.44	1.87	2.14	1.97
60 kVp, 20 cm field	20.78	9.38	4.89	3.48	3.60	1.31	1.40	1.83	0.62
120 kVp, 20 cm field	4.07	1.18	0.68	2.88	2.98	3.25	3.61	1.73	2.77

$\mathrm{Percent}\mathrm{ACF}\mathrm{difference}=100\ast \{\left|\frac{\mathrm{ACF}(0.5\mathrm{mm})-\mathrm{ACF}(5\mathrm{mm})}{\mathrm{ACF}(0.5\mathrm{mm})}\right|\}$

3.5. The dose-area-histogram (DAH) of a selected neurovascular carotid procedure.

Fig. 11 shows the cumulative dose-area-histogram (DAH) for the angular-corrected DTS, the uncorrected DTS and the MC skin dose maps for a selected neurovascular carotid procedure whose DTS dose map is shown in Figure 10. The DAH is a plot of the area of skin with a dose exceeding the value on the x-axis. The repair of skin injury is dependent on the area of skin exposed to a dose exceeding a threshold and thus the DAH provides a good indication of the risk of serious cutaneous injury. For this example, Table 2 shows that the percent difference in skin dose-area products (SDAP) and peak skin dose (PSD) between angular-corrected DTS and MC are 10.0% and 0.4%, respectively. Without DTS angular correction, the percent difference in SDAP and PSD compared with MC is increased to 18.6% and 2.2%, respectively.

Fig. 11.

The cumulative DAH with uncorrected DTS, angular corrected DTS, and MC.

Fig. 10.

The DTS display in playback mode for the neurovascular carotid procedure selected for skin dose comparison in the head. Exposure outside the interventional region occurred during catheter guidance.

Table 2.

The skin dose-area products (SDAP) and peak skin dose (PSD) as calculated by MC compared to that calculated by the DTS with and without angular correction for a clinical carotid interventional procedure.

	MC	DTS with angular correction	DTS without angular correction
SDAP (mGy · m2)	52.8	58.1	62.6
% SDAP Diff	0	10.0	18.6
PSD (Gy)	2.78	2.79	2.85
% PSD Diff	0	0.4	2.2

% Diff = (DTS - MC) / MC × 100

4. CONCLUSIONS

Skin dose is dependent on the angle of beam incidence, decreasing with decreasing angle of incidence due to the increased path length through the skin layer and cause more attenuation of the incident primary dose, increasing with increasing field size and beam energy due to more backscatter. The skin dose is rather invariant with skin thickness due to the primary dose is lower for greater thicknesses, while the scatter component increases with increasing thickness. The angular-corrected DTS had good agreement with the MC simulation for a carotid procedure. The skin dose area products (SDAP) reductions and peak skin dose (PSD) reduction are 10.0% and 0.4% with angular correction and 18.6% and 2.2 % without angular correction compared to MC on the head of a selected carotid procedure, showing that angular correction provides improved accuracy in estimating skin dose from fluoroscopic procedures. However, the skin dose from the corrected DTS is larger than MC, and this may due to the ACF being calculated for a flat phantom at the center of the field. Typically, the dose profile drops toward the field edge and is less for a curved surface due to reduced backscatter. These factors are included in the MC dose mapping but not with the ACF used here. Although this AFC method slightly overestimates the dose compared to the MC mapping, it can be used to improve the accuracy of the DTS.