Monte Carlo simulations of the Elekta SLi Plus electron applicator system A base for a new applicator design to reduce radiation leakage

In this report the characteristics of the electron applicator system for an Elekta SLi Plus linear accelerator have been studied. The investigation will serve as a basis for a new applicator design that will give less radiation leakage. The present applicator system does not fully comply with the recommendations of the International Electrotechnical Commission (IEC) Standards recording radiation leakage. By a careful study of the different dose contribution components it was found that a simple modification of the applicator geometry could decrease the leakage dose to within the IEC recommendations. In purpose to limit the work, only the case of a 1010 cm2 applicator and a nominal energy of 20 MeV was fully investigated. The 1010 cm2 applicator is very common for patient treatments, and it is well known that the highest energy (20 MeV) gives the highest leakage dose. The Monte Carlo code BEAMnrc (2002 version) has been used to study the applicator system. Monte Carlo calculated dose distributions were found to be in good agreement with measurements verifying that the Monte Carlo method can be used for optimization of the applicator design. By using the LATCH feature of the Monte Carlo code, a history can be tracked. In that way, contribution from different system components of the total energy fluence or the total absorbed dose profile could be determined. It was found that most of the dose outside the applicator is deposited by particles that have interacted in the applicator itself. Furthermore, in this area the dose contribution from photons is almost equal to the contribution from electrons. It can also be concluded that it is the third level of the applicator that gives the highest bremsstrahlung contribution. A small modification has been done on the 1010 cm2 applicator to reduce the radiation leakage. The modification includes a change of the order of the Pb-layer and the Al-layer in the third scarper level of the applicator. An evaluation according to the IEC protocol shows that the modified applicator reduces the maximum absorbed dose outside the applicator edge from about 5% to 2% relative to the maximum dose at the central axis. Also the average absorbed dose was reduced from about 1.8% to 0.6%, which is below the IEC recommended value of 1.3%