Introduction

Electron beam therapy (EBT) plays an important role in cancer treatment [ 1 ]. Due to its remarkable advantages over photon beams, i.e. high surface dose and rapid dose fall-off beyond maximum depth, electron beams are commonly used for the treatment of superficial malignancies [ 2 , 3 ]. Penetration depth of electron beams is determined by selecting appropriate energy and for each energy, therapeutic properties of electron beams are determined by measuring percentage depth dose curves (PDD) and dose profiles in a water phantom. The properties determined from PDD curves include depth of maximum dose (d_max), therapeutic depth or the depth that %90 of dose is delivered, percentage of surface dose, the depth that %50 of dose is delivered or R₅₀ and percentage of dose which is caused by X-ray contamination, all with respect to dose at d_max. However, dosimetric penumbra width is determined using dose profiles [ 4 ].

MF is one of the superficial tumors that can be treated with low-energy electron beams. This disease is the most common type of primary cutaneous lymphoma and includes about %50 of all cases of cutaneous lymphoma [ 5 ]. Total skin electron beam irradiation (TSEI) is considered as the most effective treatment for this malignancy [ 6 ]. In TSEI, electron beams with an energy range of 2-9 MeV are used. In this energy range, dose decreases rapidly in lower depths and since photon contamination is low, superficial lesions up to depth of 1cm can be effectively treated without damaging bone marrow. In TSEI, patient is positioned at distance of 3 m from the electron source and at different angles in order to cover the entire surface of the body by treatment field [ 4 ].

For treatment of scalp, perineum, feet and other parts of skin with folds, the patient is located at source-to-skin distance (SSD) of about 100 cm and is treated with local electron fields [ 7 ]. Depending on depth of superficial lesions and the lowest electron energy available, it may be necessary to use a layer of beam attenuator or spoiler to reduce electron energy. A beam spoiler is composed of a sheet of a low atomic number material, typically Lucite or acrylic, which is placed in the path of beam to reduce penetration depth of beam and increase its surface dose [ 4 , 8 ]. When the primary beam passes through the spoiler, secondary electrons are produced and these electrons change the dose distributionin depth [ 9 ]. Several studies have been reported on the application of spoiler in order to increase the surface dose in photon beam radiation therapy [ 9 - 11 ]. McKenzie et al. used a plastic spoiler in order to match two adjacent electron fields [ 11 ]. Hernández et al. studied the application of aluminum spoilers for increasing surface dose of a 6 MeV electron beam [ 12 ]. So-Yeon Park et al. compared dosimetry characteristics of a 4 MeV electron beam without spoiler to a 6 MeV electron beam with spoiler [ 13 ]. In general, it has been shown that although low-energy electron beams have appropriate depth dose distribution for the treatment of superficial lesions, their use is restricted by their low surface dose percentage [ 14 ]. Surface dose can be increased using a tissue equivalent bolus. However, using bolus on sloped surfaces and partly bloused fields can cause remarkable dose perturbations and may also produce cold and hot spots below the edge. An alternative is to use an electron beam spoiler to decrease the electron therapeutic depth while increasing the surface dose [ 12 , 15 ]. Studies have revealed that for every treatment unit and for each spoiler material, it is necessary to determine the optimum spoiler thickness by considering the energy of electron beam, depth of tumor and tumor lateral expansion. The aim of this study was to study the application of water equivalent spoilers for electron beam treatment of different stages of MF using local treatment fields. To achieve this goal, MC simulation method was used to calculate dose distributions in a water phantom resulting from different thicknesses of spoiler.

Material and Methods

In this experimental study, a Siemens Primus medical linear accelerator treatment head was simulated for a 5 MeV nominal energy electron beam. After validating the simulation results, the changes in dose distributions in water phantom in presence of spoilers were investigated.

MC modelling of electron beam

In this study, BEAMnrc code, an EGSnrc user code, was used to simulate the linear accelerator and particle transport. Structures of treatment head were modelled as component modules (CMs) using manufacture provided information. For this purpose, the structural modules SLAB, CONETAK, CONETAK, CHAMBER, JAWS, MLC and APPLICAT were used to model the exit window, primary scattering foil, secondary scattering foil, ionization chamber, secondary collimator, multi-leaf collimator and applicator, respectively (Figure 1). In this model, a circular electron beam with a radius of 0.1 cm (ISOURC=0) was used. Lower cutoff energy for charged particle and photon transporting were 0.7 MeV and 0.01 MeV, respectively. Two field sizes 10 × 10 cm² and 20 × 20 cm² were modelled. The dimensions of radiation fields at the isocenter, located at Z=100 cm from the source (Z=0), were modelled. Information of particles transition were recorded in scoring plane located at the end of last scraper of electron applicator (Z=95.141 cm). These phase space data were used as a source for dose calculation using DOSXYZnrc code.

Figure 1. Diagram of Siemens Primus electron treatment head as simulated by BEAMnrc user code

Dose calculations

For the second step using the scored phase space data, PDD along central axis and in plane profiles at R₉₅ were calculated by DOSXYZnrc for 10 × 10 cm² field size and at SSD=100 cm in a 30 × 30 × 40 cm³ water phantom. PDD and profiles were calculated using 1 cm × 1 cm × 0.2 cm and 1 cm × 1 cm × 0.5 cm voxel sizes, respectively. Calculated dose values were used to plot PDD and dose profile curves using statdose, an EGSnrc subroutine.

Dose measurements

PDD curves along central axis and in plane profiles were measured using similar beam configurations in an automatic MP3-M water tank using Roos parallel plate (acrylic coated entrance window of 1.1 mm thickness and sensitive volume of 0.35 cm³) chamber and Semi flex 31010 (volume of 0.125 cm³) chamber (all from PTW, Freiburg, Germany), respectively.

MC model validation

MC model commissioning was performed by comparing the calculated and measured PDD and profiles. The following formula was used to calculate the percent differences [ 16 ].

$\mathrm{Percentage\; difference}=\frac{\mathrm{Calculation-Measurement}}{\mathrm{Measurement}}\times 100$

The criterion for accepting a simulated beam model is less than %2 and %3 difference for PDD in the buildup and fallout regions, respectively, and for profiles, less than %1 and %2 around the central axis and edges of the field regions, respectively [ 17 ].

In order to tune the energy of developed beam model, we initially chose nominal energy of 6 MeV, as incident electron energy for the phase-space calculation. Then, this value was altered (using 0.1 MeV steps) until the calculated PDD best fit the measured. Electron beam width was tuned by calculating the beam profile at R₉₅ depth. In order to obtain agreement between calculated and measured profiles, factors such as scattering foil geometry and FWHM of electron beam intensity distribution were modified.

Dose calculations with spoiler

An acrylic spoiler, positioned at the end of electron applicator (cone), was simulated using the SLAB CM. Dose distributions in water phantom were calculated for 10 × 10 cm² and 20 × 20 cm² field sizes and 1.1, 1.3, 1.5, 1.7, 1.9 and 2.2 cm spoiler thicknesses. The xy cross-section of spoiler for each applicator was equal to field size multiplied by the ratio of source-to-end of applicator distance to SSD, i.e. 0.95 m. Surface dose, therapeutic range (R₉₀), R₅₀, output factors and penumbra widths were compared to open field without spoiler. According to the AAPM instruction, dose profile curves were obtained in R₉₅ depth [ 18 ]. R₅₀, the depth in water whose dose is equal to half of dose at d_max, serves as a beam quality specifier [ 19 ].

For each spoiler thickness, output factor was calculated as the ratio of dose at d_max in the presence of spoiler to the dose at d_max for the open beam. Penumbra widths was calculated using the beam profiles as the difference in lateral distances of two points having %80 and %20 of dose values from central axis of the beam [ 20 ].

Results

Model validation was performed by comparing the calculated dose distributions for different beam energies to measured values for the 5 MeV electron beam. The comparison for a 5.9 MeV calculated beam showed the best agreement as shown in Figures 2 and 3. Calculated PDD curves with different thicknesses of acrylic spoiler and without spoiler are displayed in Figures 4 and 5 for field sizes of 10 × 10 cm² and 20 × 20 cm², respectively. Figures 6 and 7 show the dose profile curves with different thicknesses of acrylic spoiler. Figure 8 shows the change in R₉₀ as a function of spoiler thickness for both fields. A summary of surface dose percentage, R₉₀, R₅₀, output correction factor and penumbra width created with different thicknesses of spoiler is shown in Table 1.

Figure 2. Comparison of measured 5 MeV and calculated 5.9 MeV PDD curves for 10 × 10 cm² field size and 100 SSD.

Figure 3. Comparison of measured 5 MeV and calculated 5.9 MeV dose profile curves for 10 × 10 cm² field size and SSD=100 cm.

Figure 4. PDD curves for 5 MeV electron beam for 10 × 10 cm², open field and different thicknesses of spoiler.

Figure 5. PDD curves for 5 MeV electron beam for 20 × 20 cm², open field and different thicknesses of spoiler.

Figure 6. Dose profile of 5 MeV electron beam for 10 × 10 cm², open field and with different thicknesses of spoiler.

Figure 7. Dose profile of 5 MeV electron beam for 20 × 20 cm², open field and with different thicknesses of spoiler.

Figure 8. Therapeutic range, R₉₀, as a function of spoiler thickness for two fields 10 × 10 cm² (Field 1) and 20 × 20 cm² (Field 2), 5 MeV electron beam.

Discussion

The aim of this study was to evaluate the therapeutic properties of a 5 MeV electron beam when using a spoiler for the local treatment of MF tumors by MC simulation method. Benchmarking of the developed 5.9 MeV electron beam model demonstrated a maximum difference of %2 between calculated and measured PDD curves in the build-up region. In other areas, the differences were smaller and not more than %2. To verify the simulation of the scattering foil and collimation system, measured and calculated lateral dose profiles were compared. The comparison revealed less than %1 difference in the vicinity of central axis, and less than %2 at the edges of the field. Different thicknesses of acrylic spoiler were used to reduce the treatment depth (R₉₀) of an open 5 MeV electron beam from 1.5 cm to at least 0.1 cm while boosting the surface dose to more than %90 with respect to dose at d_max. For each spoiler thickness, investigated dosimetry specifications including PDD curves, lateral dose profiles and output factors for 10 × 10 cm² and 20 × 20 cm² field sizes. Results indicated that in the presence of a 1.3 cm spoiler, R₉₀ changed from 1.5 cm (open field) to 0.5 cm and 0.4 cm for 10 × 10 cm² and 20 × 20 cm² field sizes, respectively. Figure 8 shows an exponential relationship between increase in spoiler thickness and change in R₉₀. In this research, the general trend of increases in surface dose and shift of d_max to the surface were as reported by others. Unwanted effects of spoiler reported in the literature include bremsstrahlung photon contamination, increased width of penumbra and increased out-of-field dose [ 12 ]. In this study, the increase in bremsstrahlung contamination was noticeable only for 2.2 cm spoiler, about %2 in both field sizes. Maximum increase in penumbra width was 2.8 cm and 3.8 cm for 10 × 10 cm² and 20 × 20 cm² field sizes, respectively. Although without spoiler, the penumbra difference between the two field sizes was only 1 mm; in the presence of spoilers, the difference was as large as 1 cm. Similar increase in penumbra, 1.9 to 3.6 cm, was reported by Hernández et al. for aluminum foil spoilers in 6 MeV electron beam [ 12 ]. Increase in penumbra is due to scattering of low-energy electrons when passing through spoiler which results in rise in percent surface dose as well as out of field dose [ 12 ]. When using beam spoilers, the problem of broaden penumbra can be solved by placing lead penumbra trimmers at the outer field edge. Beam output factor (OF) for different spoiler thicknesses was measured. For the 1.3 cm spoiler, OF reduction was 0.375 and 0.173 for 10 × 10 cm² and 20 × 20 cm² field sizes, respectively. In the aluminum spoiler study, much larger reduction in output was reported, i.e. OF= 0.270 for 10 × 10 cm² field size [ 12 ]. The higher reduction in output is a function of electron scattering angle which is larger for higher density materials and for smaller field sizes.

Conclusion

The application of water equivalent spoilers for treatment of Mycosis Fungoides using local electron fields was studied. In general, R₉₀ decreased exponentially with increase in spoiler thickness. The effect of field size on penumbra was much larger for spoiled beam compared to the open beam. For a larger field size, a smaller thickness of spoiler provided a higher surface dose and output factor and a lower beam penetration. The results of this research can be applied to optimize the radiation treatment of MF patients in our hospital.

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