Evaluating the effect of the vacuum bag on the dose distribution in radiation therapy

Introduction: Immobilization of patients in radiation therapy can be performed with a vacuum bag (VB). The aim of this study is to measure the effect of the VB in the surface and depth dose of patients in radiation therapy. Materials and Methods: The effect of the VB on the surface dose and depth dose is measured in clinical conditions. Various dosimeters were used in following measurements: parallel plate chamber for depth dose, farmer ionization chamber for various gantry angles, and Mapcheck2 dosimeter for various thicknesses of VB. The effect of VB cap, which may be placed in the beam path, is also measured using EDR2 film. The measurements were performed for 6 MV and 18 MV photons with an Oncor linac. Results: The increase of 30% and 25% in the surface dose with VB was observed for 6 MV and 18 MV, respectively. Though due to the use of VB, the reduction of the absorbed dose at a 5 cm depth is under 1% and can be ignored in MU calculation. For various thicknesses of VB, 8-14 cm, the attenuation of the primary beam were up to 2.5% for 6 MV and 1.2% for 18 MV photon. The presence of VB cap in the path of radiation reduced the depth dose up to 15% and 11% for 6 MV and 18 MV, respectively. Conclusion: The use of VB can increase the surface dose of the patient up to 30% and this fact should be considered in treatment planning. For some lateral fields the cap of the VB may interfere with radiation field. If the cap of VB is placed in the beam path, it can cause a cold spot in tumor. Discussion: The use of VB can increase the surface dose of the patient up to 30%. For some lateral fields, the cap of the VB might interfere with the radiation field. If the cap of VB is placed in the beam path, it can cause a cold spot in the tumor volume.