Radiotherapy is the one of the major treatment modality for thoracic esophagus cancer patients. Delivering high radiation dose to the planning target volume (PTV) while protecting the surrounding normal tissues can be achieved by volumetric modulated arc therapy (VMAT) which is the advanced radiotherapy technique. For creating VMAT dose distributions, optimization algorithms and dose calculation algorithms in commercial treatment planning systems (TPS) are used. Especially in cases of tissue heterogeneity, the final calculated dose volume histogram (DVH) differs from optimal DVH acquired via the optimization procedure. The intermediate dose calculation (IDC) module, in the Eclipse treatment planning system, is utilized on optimization of the VMAT plan to solve these differences. The aim of the present study is to investigate the impact of IDC module during the optimization of VMAT for the thoracic esophagus cancer patients. The VMAT plans were generated on Eclipse TPS v15.1 using AAA algorithm without IDC module for ten patients with thoracic esophagus cancer patients. Then, the plans were re-optimized without changing optimization criteria by using same dose calculation algorithm with IDC module. The prescribed dose to PTV was 50.4 Gy/28 fr. The homogeneity index (HI) and the conformity index (CI) of PTV, maximum dose of spinal cord, mean dose of heart, the lung volume of receiving 5 and 20 Gy, were compared between plans created with and without IDC module. The CI of PTV for VMAT plans with and without IDC module were found to be 0.822±0.030 and 0.729±0.039, respectively (p=0.005). The HI of PTV for VMAT plans with and without IDC module were found to be 0.073±0.017 and 0.126±0.022, respectively (p=0.005). The maximum dose of spinal cord (p=0.028) and the mean dose of heart (p=0.047) were found lower in VMAT plans with IDC module. However, there was no significant difference for the volume of the lung receiving 5 (p=0.236) and 20 Gy (p=0.053). In conclusion, applying IDC module on VMAT optimization increases the plan quality in thoracic esophagus cancer patients.

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