Evaluation of the Influence of Lung Inhomogeneity on Depth Dose Distribution before and after the Lung in Electron Therapy: A Semi-Experimental Study

Because of the vast usage of high energy electron beams in irradiation of the chest wall and the possibility of exposing the whole or a part of lung in their paths and significant uncertainty in calculating the absorbed dose caused by this inhomogeneity, evaluation of the influence of lung inhomogeneity on depth dose distribution before and after the lung is necessary. Subjects and Three cork plates as lung-equivalent tissues with different thicknesses of 1, 2 and 3 centimeter were placed in a water phantom and absolute dosimetry was done by PPC40 ion chamber for beam energies of 9, 12 and 15 MeV generated by Siemens Primus Plus linear accelerator in different field sizes of 2r=5, 10×10 and 15×15 cm². In addition, some correction factors were calculated by effective SSD method for different beam energies and were exerted on primary data in this semi-experimental study. For subsequent comparisons to evaluate the CET method, dosimetry also was done at depths of 1.5, 1.8, 2.2, 2.5 and 3.5 centimeter in homogeneous water phantom. After exerting correction factors on primary data, for all thicknesses of inhomogeneous tissue, energies and fields, the absorbed dose before the inhomogeneity is lower and the absorbed dose after the inhomogeneity is higher than the absorbed dose at its equivalent depths in homogeneous water phantom. For the energy of 9 MeV in the field of 2r=5, the error percentage of dose estimation with CET method is significantly higher than other energies and fields. This percentage also increases by increasing in inhomogeneity thickness for all energies in the 2r=5 field. In comparison with depth dose in homogeneous phantom, the lung-equivalent tissue results dose reduction before and dose increase after the lung. In addition, the error percentage of CET method in smaller field and energy is significantly high and its application in similar cases is not recommended.