Slice-based plan evaluation methods for three dimensional conformal radiotherapy treatment planning

Dose volume histograms (DVHs) play a vital role in determining the optimal plan for radiotherapy treatment delivery. The current concepts of conformality index (CI), equivalent uniform dose (EUD) derived from dose volume histogram (DVH) does not provide any spatial information. In this study, slice-based evaluation methods have been proposed for spatially analyzing the radiotherapy treatment plans. A case of prostate cancer has been selected for demonstrating the proposed tools for evaluating the dose distribution. Three dimensional conformal radiotherapy treatment planning (3D-CRT) was performed to a dose of 27 Gy/15# with three fields (6 MV anteroposterior and two 15 MV lateral fields) employing multileaf collimator after delivering 45 Gy/25#. The dose was normalized to isocenter and the treatment plan was evaluated with DVH. The dose maximum point, conformality index, planning target volume coverage index (PCI), planning target volume overdose index (POI) and equivalent uniform dose (EUD) were evaluated for every single slice along the cranio-caudal direction for all the planning target volume (PTV) contours and plotted against the slice location. The dose maximum point plotted against the slice position helps in identifying the slices where the dose maximum point is outside the target volume. The plot of conformality index gives the information about the location of those slices where excess of surrounding normal tissues is encompassed inside the prescription isodose. POI quantifies the high dose regions inside the PTV slices that receive doses above 107% of the prescription dose. Similarly, the plot of PCI and EUD with slice position gives the information about those slices where the tumor is not covered adequately. The proposed methods in this study forms as a simpler way to assess the spatial distribution of the dose inside the target volume. It could be used in combination with the current plan evaluation tools and will be very helpful in analyzing the treatment plans.