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)