Calculation and Comparison of Heart Integral Dose in The Treatment of Esophagus Cancer with Three Photon Energies & Using CT Simulation and Treatment Planning System

Introduction: Esophageal cancer is one of the most frequently occurring cancers in Iran and having a  high  incidence  rate  among  other  countries.  Radiotherapy  is  one  of  the  three  methods  (surgery,  radiotherapy and chemotherapy) for radical or palliative treatment of esophageal cancer. In this method  of treatment, the organs such as heart and spinal cord are regarded as organs at risk (OAR) which their  dose should be kept below the tolerance level.  Different techniques have been used in conventional radiotherapy, among which two parallel opposed  fields (POP) is used more often either for the whole course of the treatment or 2/3 of the fractions. In  this technique, a great portion of the spinal cord may be involved in the treatment volume.  The rate of treatment success may depend on the tolerance dose of heart and cord which act as the  limiting factor.  Materials and  Methods:  In this  study, 10 patients  with S.C.C.  of  esophagus having  indication  for  external radiotherapy were selected. The CT scan simulations were performed for all the cases and  their data were digitized to be used in computerized treatment planning system. Treatment planning for  three photon beam energies (Co-60, 6 and 10 MV) are obtained for each individual patient under the  same geometry and dose. The integral dose, absorbed dose and dose volume histogram (DVH) of the  heart and target were calculated for all the cases.  Results: The maximum point dose in the heart is 140, 125 and 115% of the reference point for Co-60,  6 and 10 MV, respectively. The integral dose for the largest A-P patient was found to be 55.33, 54.17  and 51.66 Gy.Kg for Co-60, 6 and 10 MV, respectively. In using 10 MV beam, 7% reduction in the  integral dose of the heart was obtained. Alternatively, by using 6 MV beam, a reduction of only 2%  was observed. The integral dose of the heart for the 10 MV beam had a 5% reduction in comparison to  6 MV beam.   Discussion and Conclusion:  Since lower energy beams cause a higher integral dose to the heart and  cord, the use of higher energy photon beams in the case of deep seated tumors and large size patients  are unavoidable. In spite of the advantages of cobalt machines, its energy is not suitable for deeply  suited tumors.