The Effect of Beams' Orientations on the Intensity-Modulated Radiation Therapy Plan Quality

Purpose: The work aims at studying forty Intensity-modulated radiation therapy (IMRT) plans designed using KonRad system and evaluating each of them under different conditions to study the effect of beams' orientations on the plan quality. Methods: Clinical step-and-shoot IMRT treatment plans were designed for twenty patients, suffering from different types of non-CNS solid tumors, to be delivery on a Siemens Oncor accelerator with multi-leaf collimators MLCs (82 leaf). To ensure that the similarity or difference between the plans was due to effect of beams' orientations, the same optimization constraints were applied for each plan and all other parameters were kept constant. Results: The analysis of performance was based on isodose distributions, Dose Volume Histograms (DVHs) for Planning Target Volume (PTV), the relevant Organs at Risk (OARs) as well as several physical indices like mean dose (Dmean), maximum dose (Dmax), 95% dose (D95), integral dose, volume of tumor receiving 2 Gy and 5 Gy, total number of segments and monitor units (MUs). Homogeneity index and conformation number were two other evaluation parameters that were considered in this study. Conclusion: All coplanar CP and non-coplanar NC techniques result in clinically acceptable plans, with comparable target doses and dose to critical organs within prescribed dose constraints. For different organs at risk, the coplanar IMRT, the greatest advantage of non-coplanar IMRT was the marked reduction of general trend was that non-coplanar plans achieved the lowest values, while coplanar plans showed the highest. Significant differences (P<0.05) exist only between the homogeneity index HI (1.092 and 1.088) and Monitor Unit MU (357.79 and 341.80) of CP and NC plans respectively. Compared with the integral dose (p=0.007).