Monte Carlo study of Target design

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Introduction

The linear accelerator’s electron target is designed to produce a beam of photons which will deliver a therapeutic dose at some depth within a patient. For the Elekta linac, operating at 6 MV, the target consists of a thin metallic disc, the ‘target insert’, set into a larger metallic cylinder. (The composition and geometry of this component is described in further detail in Ref. [1].)
To simplify the design of this component in the model Elekta linac, we remove the portion of the target that extends upstream, above the insert. We examine the effects of modeling the target as a single material (using first only the higher-Z target insert material and then using only the lower-Z surrounding material). We also model the target as made of two flat slabs of metal, rather than as a smaller insert in a larger cylinder. Additionally, the effects of imprecisely defining the target insert geometry in the lateral direction are examined by increasing and decreasing the radius of the target, away from its specified extent. Radii examined are approximately 1/3 and 2/3 of the specified target insert radius, with the first of these being much smaller than the upstream end of the primary collimator.
To test the effects of altering the design of the electron target in the Elekta linac, dose delivered by an accurate and detailed model (henceforward referred to as ‘detailed’) is compared to the results of using  model linacs containing modified targets.
Results
Figure 1: (a) Lateral profiles, (b) depth dose profiles and (c) energy spectra at linac exit plane, for various target models. Depth-dose profiles are normalised to the detailed-model dose at 5.5 cm depth. Lateral profiles are normalised to the detailed-model dose on the central axis (CAX) and are generated at 8.5 cm depth. The ‘population’ in the each energy spectrum is the number of photons reaching the detector with a given energy divided by the product of the energy bin size, the area of detector under examination and the number of particle histories simulated. Solid lines are detailed model linac data; filled diamonds are for target with air above $z=0$; open diamonds are for wide insert; open squares are for `2/3 insert’; open triangles are for `1/3 insert’; crosses are for target without insert; and plus signs are for target made entirely from insert material. (Some data points omitted for clarity.)

As is shown by Figure 1, removal of the portion of the target which extends upstream, above the insert, does not appreciably affect the dose delivered by the beam. Changes to the geometry of the insert itself, however, can alter the resulting profile.

The results of the replacement of the higher-Z target insert with the lower-Z surrounding metal are shown in Figure 1. Here, the use of the simpler target can be seen to result in a reduced dose across the lateral profile (Figure 1(a)) and at all depths (Figure 1(b)). Additionally, Figure 1(c) shows that the target with the insert replaced by the surrounding metal also produces an energy spectrum which is distinctly shifted in the lower-energy direction. Evidently, this change to the target materials produces a target with a reduced overall effective Z, which produces lower energy bremsstrahlung photons.

By contrast, using the higher-Z insert material to model the entire target produces a beam with an increased mean energy (as shown by Figure 1(c)). However the depleted fluence (also observable in Figure 1(c)) arising from self-absorbtion of photons by this high-Z target results in a beam which produces a greatly reduced dose in water (as shown by data in Figures 1(a) and (b)).

Increasing the width of the target insert, across the field, does not affect the profile of the beam. This observation is in keeping with the conclusion of Sheikh-Bagheri and Rogers [2], that the width of the target itself is “not important as long as the target width is much larger than the lateral spread of electrons or the radius of the upstream opening of the primary collimator”. These conditions are fulfilled by both the target and its insert in our detailed model. Reducing the width of the insert, however, can be expected to affect the resulting beam, resulting in a depletion of dose. That this is the case can be seen by further data in Figure 1, where ‘1/3 insert’ and ‘2/3 insert’ denote the results of reducing the radius of the insert to, respectively, one third and two thirds of its detailed-model value. The ‘1/3 insert’ data show an especially clear depletion of dose across the profile.

References

1. A. Mesbahi, “Development of a simple point source model for Elekta SL-25 linear accelerator using MCNP4C Monte Carlo code”, Iranian Journal of Radiation Research 4(1), 7-17 (2006)
2. D. Sheikh-Bagheri and D. W. O. Rogers, “Sensitivity of megavoltage photon beam Monte Carlo simulations to electron beam and other parameters”, Med. Phys. 29(3), 379-390 (2002)

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