Verification of Experimental Virtual Electron Source Position by Using Monte Carlo

Research & Reviews: Journal of Pure and Applied Physics

article

Habibeh Ghasemi¹  Zohreh Azma²  Ali Jabbary Arfaee³  Mahdi Sadeghi⁴ 

[1] Medical Radiation Engineering Department, Science and Research Branch;Medical Radiation Engineering Department, Shahid Beheshti University;Shohada Tajrish Hospital Radiation Oncology, Shahid Beheshti University;Medical Physics Department, School of Medicine, Iran University of Medical Science

关键词: Electron beam;    Effective SSD;    Virtual source position;    MCNP5. Introduction Electron beams have an important role in the treatment of superficial and shallow cancers in modern radiotherapy [1]. Because of the electron interactions with the accelerator head materials including scattering foils;    monitoring chambers;    photon jaws;    and the applicators;    they may be thought of as emanating from the virtual source that is not in the real position of the accelerator exit window. The International Commission on Radiation Units and Measurements (ICRU) Report No. 35 [2];    defines an effective extended electron source as the source which when placed in a vacuum at some distance SSD from the phantom surface (Z=0) would produce exactly the same electron fluence at Z=0 as the actual beam. The distance from this source position to the patient’s skin surface is called the effective SSD. To obtain an accurate dose calculation of electron beams;    correction for measured effective SSD must be applied. In addition;    effective SSD may be used to calculate output at extended SSD for corrections of central axis percentage depth dose (PDD) [3];    off-axis dose (OAD) values;    output factors;    and X-ray contamination [4]. Hence;    it is essential that these corrections should be examined for each treatment unit prior to referral to treat at extended SSD using electron beams. The SSDeff is dependent on the electron energy and the field size used.;   

DOI  :  10.4172/2320-2459.1000197

来源: Research & Reviews

【 摘 要 】

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.

【 授权许可】

Unknown