Analysis of electron beam output factors by Monte Carlo method

Objective: To investigate the application of the Monte Carlo dose calculation of output factors for electron beams in radiotherapy. Methods: The code EGS4/MCTP was used to simulate the head of a medical linear accelerator (Varian 23EX) to calculate the output factors for 6 MeV, 9 MeV and 18 MeV electron beams. The source-to-surface distance used was 100 cm. The field sizes ranged from 2 cm × 2 cm to 25 cm × 25 cm. The calculated output factors agreed with the corresponding measured factors which were measured by the IBA Phantom system to within 2%. Then, the output factors of direct articles and indirect articles which were under different energy and various cone-insert combinations were calculated by the code EGS4/MCTP. Results: The calculated output factors agreement with the measurements is found to be mostly under the 1% level. The variation of output factors depends on the characteristics of the beams and the modifications that the various cone-insert combinations introduce to these characteristics. Conclusions: Monte Carlo dose calculations for electron beams in homogeneous water phantoms have been demonstrated to be accurate under the 1% level in comparison with measurements. The output factors were influenced by energy and cone-insert combinations in complex ways. (authors)