Shielding effect of a lead apron on the peripheral radiation dose outside the applicator of electron beams from an Elekta linear accelerator.

PURPOSE
To evaluate the shielding effect of lead aprons (LAs) on peripheral radiation doses outside the applicator of electron beams from a linear accelerator.


METHODS
Out-of-field radiation doses of 4-, 6-, 8-, 10-, 12-, and 15-MeV electron beams from an Elekta Synergy linear accelerator (linac) were measured by thermoluminescence dosimeters (TLD) at different depths (0, 0.5, 1.0, and 2.0 cm) and distances from the applicator edge (0-58 cm) in a water-equivalent slab phantom with a different number of layers of LA shielding (0-5 layers). Measurements were performed by 6 × 6, 10 × 10, 14 × 14, and 20 × 20-cm2 applicators at a gantry and collimator angle of 0°. The out-of-field radiation dose profiles were normalized to the maximum dose of every energy and measuring depth.


RESULTS
The out-of-field radiation doses (beyond 3 cm away from the field edge) decreased with an increase in the number of LA layers and distance away from the central beam axis (CAX). After shielding with the LA, the out-of-field doses decreased by up to approximately 99% compared with the no shielding group. For 4-MeV electron beams, there was a peak at 24.5 cm from the CAX, which weakened with an increasing number of LA layers.


CONCLUSION
The shielding effect of the LA varied for a different number of LA layers as well as different depths and distances away from the CAX. Four LA layers were sufficient for shielding out-of-field doses of 4-15-MeV electron beams.

levels (range 4-20 MeV) mainly depends on the tumor invasion depth. The dimension of the treatment field for electron beams is defined by an applicator, and radiation doses outside the treatment field are associated with scattered radiation and leakage radiation from the electron applicators and treatment gantry. An ideal electron applicator should remove all electrons outside the treatment field. In terms of electron therapy, out-of-field radiation is generally negligible unless it is very close to organs at risk (OARs), such as the lens.
However, current treatment planning systems (TPSs) are not designed for the calculation of out-of-field radiation doses due to a poor calculation accuracy.
Out-of-field radiation is detrimental to patients, in particular children, pregnant patients, and patients with implanted electronic devices. 1 Furthermore, in the post-treatment period, late effects, and secondary cancers induced by radiation may manifest in survivors. [2][3][4] Thus, out-of-field radiation should not be ignored. Recently, Cardenas et al. reported that an appropriate shielding strategy was very important for patients and that effective body protection measures were clinically significant. 5 However, the shielding effect of a lead apron (LA) on patient protection cannot be fully assessed using modern TPSs. In this paper, quantifying the shielding effect of LAs on out-of-field dose distributions was performed for electron beams from a linear accelerator, as this has not been reported in the literature. A satisfactory shielding effect with LAs was observed.

| MATERIALS AND METHODS
The Elekta Synergy linear accelerator (Crawley, UK) used in this study was fitted with a multileaf collimator with 60 leaves that automatically opened to a specified size whenever the electron energy was selected and the electron applicator was attached to the accessory mount within the gantry head. There were six electron beam energy levels associated with the accelerator: 4, 6, 8, 10, 12, and 15 MeV, and the irradiation fields were defined by 6 × 6, 10 × 10, 14 × 14, and 20 × 20-cm 2 applicators. The distance between the source within the gantry head and the end of the applicator was 95 cm. The linac was calibrated to deliver 1 cGy/MU beam under reference conditions (field size = 10 × 10 cm 2 , source-to-sur- LTD) that were 4.5 mm in diameter and 0.8 mm in thickness were used, whose dispersity, linearity range, and dose range were 1%, 10 −7 -10 Gy, and 10 μGy-10 Gy (data provided by production company), respectively, and TLDs were calibrated before purchase, whose reading is consistent for all energies. The LiF: Mg,Cu,P TLDs were annealed before each irradiation at 240°C for 10 min with air circulation whose temperature was controlled within ±2°C. Then, the TLDs were removed and rapidly cooled with an air fan. Readings were performed with a GRYT series TLD RGD-E reader. The readout program was as follows: heating rate of 20°C.s −1 , preheating at 140°C for 20 s, readout between 140°C and 240°C, and hold 240°C for 20 s. Doses outside of the 10 × 10-cm 2 applicator field were measured for electron beams at 4, 6, 8, 10, 12, and 15 MeV at least three times, and 500 MU were delivered for each measurement.
The gantry and collimator were set to 0°for all measurements. The TLDs were positioned on the surface of a water-equivalent slab phantom (SP34, IBA Dosimetry, Schwarzenbruck, Germany.) with the center axis parallel to the line from the gun to the target position, with the gantry at SSD = 100 cm and at 0, 0.5, 1.0, and 2.0cm depths of the phantom and distances from the field edge (0-F I G . 1. Schematic diagram of arrangement of the thermoluminescence dosimeters in the solid water phantom to measure peripheral dose outside the applicator. 58 cm), respectively (see Fig. 1). All measurements were performed at SSD = 100 cm; thus, there was a 5-cm gap between the applicator end and solid water phantom surface with the gantry at 0°. A different number of layers of an LA (0.5 mm for each layer) were applied to cover the surface of the TLDs and phantom. For electron beams with different energy levels and measuring depths, the out-of-field TLD readings were normalized to the dose (Dref) measured at the following conditions: 10 × 10 cm 2 applicator, 6 MeV,   3.C | Shielding effect at 1.0-cm depth

| DISCUSSION
A megavoltage electron beam is an important treatment strategy in modern radiotherapy that is preferred due to its dosimetric characteristics and low penetration depth. 6 The main source of an out-of-field dose contains photon and electron components. Bremsstrahlung photons may be produced when high-energy electron beams, such as 15-MeV beams, collide with the applicator building. The other component is electrons, which can occur by (a) scattering out, (b) penetrating through the applicator collimator building, or (c) leaking directly into the air from scattering foil without interacting with the applicator. [7][8][9][10][11] For high-energy electron beams, the out-of-field electron component is usually produced primarily via (2) and (3). For lower-energy electron beams, scatter radiation occurs. 12,13 For 4-MeV electron beams, the out-of-field dose peak may be caused by scattering electrons originating from the rounded surface of the MLC. 14 According to our results, the peripheral dose peak gradually weakened with an increasing number of LA layers, which was likely caused by the low penetration depth of the electron beams.
The peripheral doses were considered harmful for patients and had to be minimized, especially when they were high. Out-of- ing distance from the CAX, the out-of-field doses gradually reduced, and the effect of LA shielding also declined. It was observed that out-of-field doses at large distances from the edge of the treatment field were mainly attributed to leakage radiation. 15 The biological effect of radiation mainly consists of a deterministic effect and nondeterministic (stochastic) effect. The deterministic effect has a threshold dose, below which the effects are unobservable. However, there is no threshold dose for the nondeterministic effect. Nevertheless, the possibility of biological effects, such as cataracts, aplastic anemia, and secondary cancer, 16 has a positive correlation with the amount of absorbed dose. Although doses outside the treatment field are small, they may have a significant radiobiological effect on OARs.
Therefore, it is of great importance to protect patients from unnecessary radiation. This study suggests that out-of-field doses (beyond 3 cm from the applicator edge) from electron beams can be effectively shielded by adding several layers of an LA with 0.5 mm thickness on the surface of a water-equivalent slab phantom. But LA should selectively be implemented within a distance of 3 cm from the applicator field because of the buildup effect.
Shielding out-of-field doses thus provides significant overall protection for out-of-target volume beyond 3 cm from the applicator edge.

| CONCLUSION
In this study, we successfully evaluated the shielding effect of an LA on out-of-field radiation doses for an Elekta Synergy linac using 6 × 6, 10 × 10, 14 × 14, and 20 × 20-cm 2 applicators. We determined that the shielding effect of an LA depends on the number of layers as well as the depth and distance away from the CAX. In this study, the maximum radiation dose reduction was almost 100% after LA shielding. In addition, the results demonstrated that four LA layers were sufficient for shielding out-of-field radiation (beyond 3 cm from the applicator edge) from 4 to 15 to MeV electron beams. Measuring the shielding effect of an LA can benefit radiation oncologists to protect patients from unnecessary radiation exposure.

ACKNOWLEDG MENTS
The authors gratefully acknowledge Beijing Guangrun Yitong Radiation Monitoring Equipment Co. LTD for thermoluminescence dosimeters readout support.

CONFLI CT OF INTEREST
No conflict of interest.