Evaluation the Effect of Photon Beam Energies on Organ at Risk Doses in Three- Dimensional Conformal Radiation Therapy

As regards, the selection of appropriate energy in radiotherapy tumors that are placed in non-homogen areas is important, so the aim of the present study is evaluating the effect of 6 and 15MV photon beam energies on dose distribution in 3D-CRT for lower esophageal and rectal cancers. 12 patients with lower esophageal cancer and 12 patients with rectal cancer respectively, with the prescription dose of 7000 cGy and 5040 cGy were studied. For treatment of esophagus the three-field technique and for treatment of rectum the four-field technique was used. In the thoracic area, the spinal cord and in the pelvis, the bladder and head of femurs were considered as OARs. For comparing the results, all parameters used for treatment planning except the photon beam energy were kept constant. After performing the treatment planning with two different energies in each region, the received dose rate of spinal cord, bladder and head of femurs as OARs and dose distribution in PTVs were studied. The results of this study showed that the difference between doses received by OARs in 6MV and 15MV therapeutic plans, resulted 5.2, 5.06 and -9.14%, respectively. Also the PTVs received dose difference of esophagus and rectum in the performed therapeutic plans with energy of 6 MV than energy of 15MV is 1.52 and -0.63%, respectively. We found that using of low energy photons in lower esophageal treatment and high energy photons in the rectal treatment provides the better dose coverage. Finally, with such as therapeutic plans the cumulative dose of organs at risk will be reduced.