Physical characteristics of electron beam from conventional and beam shaper IOERT applicator: A comparison study

Introduction: Intraoperative electron radiation therapy (IOERT) is one of the cancer treatment techniques that delivers high doses to tumor bed during surgery. IOERT can be performed by either conventional LINACs or dedicated IORT accelerators such as LIAC (Light Intraoperative Accelerator). Two types of applicators can be used with LIAC dedicated accelerator including conventional applicator and beam shaper one. Standard applicators are cylindrical ones which can produce circular fields. On the other hand, beam shaper applicator is able to produce various square and rectangular fields through dedicated shielding blades located at the distal end of applicator. The aim of this study is to compare the dosimetric parameters and physical characteristic of electron beam produced by these two different types of IOERT applicators through Monte Carlo simulation.
 
Materials and Methods: The LIAC head in conjunction with conventional cylindrical and beam shaper applicator were simulated by MCNPX Monte Carlo code. The validity of simulated models was evaluated through comparing the Monte Carlo Based data (including PDDs and TDPs) and ionometric measured ones for both conventional and beam shaper applicator at different energies. Then, the energy and angular distribution of electron beam at the distal end of mentioned applicators were calculated for different combinations of energy/field sizes and obtained physical characteristics were quantitatively compared.
 
Results: For both types of studied applicators, the mean electron energy increases with increment of electron field size at the same energy. For conventional cylindrical applicators, the variations of mean electron energy with increasing field sizes ranges from 4.48 to 4.57,
5.63 to 6.13, 6.61 to 7.44 and 7.25 to 8.35 at 6, 8 10 and 12 MeV, respectively. Also, with increasing the field size for beam shaper applicator, the mean electron energy varies from 4.47 to 4.86, 5.76 to 6.65, 6.67 to 8.10 and 7.27 to 9.06 at 6, 8 10 and 12 MeV, respectively. Increasing the electron energy at the same field size causes the increment of mean scattering angle of electron beam at the distal end of both applicators understudy. With increasing field size for conventional applicators, the mean scattering angle of electron beam varies from 5.98 to 7.24, 5.62 to 6.68, 5.75 to 6.43 and 5.98 to 6.29 at 6, 8 10 and 12s MeV, respectively. Variations of mean scattering angle with increasing field sizes for beam shaper applicator also ranges from 6.89 to 7.19, 6.23 to 6.39, 5.93 to 5.95 and 5.50 to 5.69 for 6, 8 10 and 12 MeV, respectively.
 
Conclusion: Based on the obtained results, it can be concluded that the physical characteristics of produced electrons by shaper applicator are different from those of conventional applicator. This fact can be mainly attributed to the presence of shielding blades in design of beam shaper applicator.