Estimation of the effects of normal tissue sparing using equivalent uniform dose-based optimization

In this study, we intend to estimate the effects of normal tissue sparing between intensity modulated radiotherapy (IMRT) treatment plans generated with and without a dose volume (DV)-based physical cost function using equivalent uniform dose (EUD). Twenty prostate cancer patients were retrospectively selected for this study. For each patient, two IMRT plans were generated (i) EUD-based optimization with a DV-based physical cost function to control inhomogeneity (EUD(With DV)) and (ii) EUD-based optimization without a DV-based physical cost function to allow inhomogeneity (EUD(Without DV)). The generated plans were prescribed a dose of 72 Gy in 36 fractions to planning target volume (PTV). Mean dose, D(30%), and D(5%) were evaluated for all organ at risk (OAR). Normal tissue complication probability was also calculated for all OARs using BioSuite software. The average volume of PTV for all patients was 103.02 ± 27 cm(3). The PTV mean dose for EUD(With DV) plans was 73.67 ± 1.7 Gy, whereas for EUD(Without DV) plans was 80.42 ± 2.7 Gy. It was found that PTV volume receiving dose more than 115% of prescription dose was negligible in EUD(With DV) plans, whereas it was 28% in EUD(Without DV) plans. In almost all dosimetric parameters evaluated, dose to OARs in EUD(With DV) plans was higher than in EUD(Without DV) plans. Allowing inhomogeneous dose (EUD(Without DV)) inside the target would achieve better normal tissue sparing compared to homogenous dose distribution (EUD(With DV)). Hence, this inhomogeneous dose could be intentionally dumped on the high-risk volume to achieve high local control. Therefore, it was concluded that EUD optimized plans offer added advantage of less OAR dose as well as selectively boosting dose to gross tumor volume.