Monte Carlo-based treatment planning for a spoiler system with experimental validation using plane-parallel ionization chambers

Evaluation of Therapeutic Properties of a Low Energy Electron Beam Plus Spoiler for Local Treatment of Mycosis Fungoides: A Monte Carlo Study

Background:

When using low-energy electron beams for the treatment of skin lesions, such as Mycosis Fungoides (MF), a beam spoiler is used to decrease electron therapeutic depth (R₉₀) while increasing the surface dose.

Objective:

The aim of this study was to evaluate the characteristics of a 5 MeV electron beam when using a spoiler for the local treatment of MF skin lesions by Monte Carlo (MC) simulation.

Material and Methods:

In this experimental study, a Siemens Primus treatment head and an acrylic spoiler, positioned at the end of applicator, were simulated using BEAMnrc, an EGSnrc user code. The modelled beam was validated by measurement using MP3-M water tank, Roos parallel plate chamber and Semi flex Chamber-31013 (all from PTW, Freiburg, Germany). For different spoiler thicknesses, dose distributions in water were calculated for 2 field sizes and were compared to those for the corresponding open fields.

Results:

For a 1.3 cm spoiler, therapeutic range 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. Maximum increase in penumbra width was 2.8 and 3.8 cm for 10 × 10 cm² and 20 × 20 cm² field sizes, respectively. Maximum increase in bremsstrahlung contamination was %2 in both field sizes.

Conclusion:

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. The results of this research can be applied to optimize the radiation treatment of MF patients in our hospital.

Use of an electron spoiler for radiation treatment of surface skin diseases

OBJECTIVES

Low-energy electron beams are characterised by low surface doses with a pronounced dose build-up and penetration of several centimetres, but often a higher surface dose and a lower penetration range is desired. The purpose of this study was to investigate the use of an electron spoiler to modify these beams for treating surface skin diseases and evaluate the feasibility of this method.

MATERIALS AND METHODS

An aluminium foil 4-mm thick covering the end of the electron applicator was used as a spoiler for a 6 MeV electron beam. The dosimetric characteristics of this beam were measured, and Monte Carlo simulations were performed.

RESULTS

The spoiler reduced the practical range and increased surface and build-up doses, but it also significantly broadened the penumbra and increased peripheral doses. Nevertheless, the beam was clinically acceptable when skin collimation with lead was employed. Monte Carlo simulations agreed well with all the experimental measurements.

CONCLUSIONS

The feasibility of using a low-energy electron beam with a spoiler for treating surface skin diseases was demonstrated. The method is hygienic and avoids some of the disadvantages associated with the bolus technique, but it is valid only for flat surfaces and perpendicular incidence. As a consequence, it can be an alternative to bolus and other reported methods in certain cases, especially when a particular sterility assurance level is required.