Investigating The Role of Helium Bag On Electron Contamination Removal Using Beam NRC

  Introduction: Several studies have discussed to the electron contamination caused by the air column between the head of linac and patient for photon beams. Since it is not possible to measure the electron contamination with practical dosimetry, the Monte Carlo technique is an appropriate and accurate choice to calculate particle flounce in radiation therapy. The purpose of this study was evaluated the effect of removing air between linac head and patient with Helium bag on electron contamination using BEAMnrc method. Materials and Methods: The 6-MV photon beam of Varian Clinic 2100C/D was simulated using BEAMnrc code. To evaluate the role of helium bag on electron contamination removal, the photon and electron spectra were scored for 10×10 field at a source to surface distance (SSD) of 100, 120, 140, and 160 cm in the presence of the air column or helium bag. The percentage depth dose and profile calculated at depth of 10cm using DOSXYZ code. Results: This study showed that as SSD increases, the amount of electron contamination decreases. Consequently, an increase in the height of the air column acts like a filter which it probably scatters the low-energy electrons. Replacing the helium bag in the radiation pathway led to a decrease of about 10% of electron contamination for all SSDs. Conclusion: The results of the study showed that in 6 MV energy, the air column has a significant contribution to produce the electron contaminant. By replacing helium bag, the skin dose of the patient may reduce over 10%.