Verification of Experimental Virtual Electron Source Position by Using Monte Carlo
Background: In the treatment with the electron beams to get the appropriate dose distribution in the tumor region, the distance between virtual electron source position and patient’s body should be correctly identified. This study is an attempt to find the effective SSD (Source-surface distance)by using the Monte Carlo N-particle (MCNP v1. 51) code to substitute the measurements in the clinic. Materials and Methods: The MC (Monte Carlo) simulation of Oncor Linac (Siemens Co. Germany) was done based on manufacturer data in 9 MeV electron beam energy. In order to obtain the values of effective SSD using the inverse slope (IS) technique, point doses are calculated at dmax (the depth of maximum dose on the central axis), inside of the water phantom model, with the phantom simulated first at the standard SSD (5 cm air gap) and then with an increasing air gap of 3, 6, 9 and 12 cm between the distal end of applicator and the phantom surface. Results: The measured and MC-calculated effective SSDs using IS method are presented and compared with those reported from other works. The effective SSDs were found to be 95.68 cm and 96.66 cm from the MC tallies of *F8 and *F4, respectively. In addition, the effective SSDs were found to be 95.59 cm from ion chamber measurement. Conclusion: Considering the results from three methods of calculation, and comparing the results with experimental methods, it was shown that Monte Carlo simulation is a useful method in determination of electron virtual source position.
Habibeh Ghasemi, Zohreh Azma, Ali Jabbary Arfaee and Mahdi Sadeghi