The Dosimetric Effects of Different Beam Energy on Physical Dose Distributions in IMRT Based on Analysis of Physical Indices

Abstract

This work aimed at\revaluating the effect of 6- and 10-MV photon energies on intensity-modulated radiation therapy (IMRT) treatment plan outcome in different\rselected diagnostic cases. For such purpose, 19 patients,\rwith different types of\rnon CNS solid tumers, were selected. Clinical step-and-shoot IMRT treatment\rplans were designed for delivery on a Siemens\rOncor accelerator with 82 leafs; multi-leaf collimators (MLCs). To ensure that\rthe similarity or difference among the plans is due to energy alone, the\rsame optimization constraints were applied for both energy plans. All the parameters like beam angles,\rnumber of beams, were kept constant to achieve the same clinical objectives.\rThe Comparative evaluation was based on dose-volumetric analysis of both energy\rIMRT plans. Both qualitative and quantitative methods were used. Several\rphysical indices for Planning Target Volume (PTV), the relevant Organs at Risk\r(OARs) as mean dose (Dmean), maximum dose (Dmax), 95% dose (D95), integral\rdose, total number of segments, and the number of MU were applied. Homogeneity\rindex and conformation number were two other evaluation parameters that were\rconsidered in this study. Collectively, the use of 6 MV photons was\rdosimetrically comparable with 10 MV photons in terms of target coverage,\rhomogeneity, conformity, and OAR savings. While 10-MV plans showed a\rsignificant reduction in the number of MUs that varied between 4.2% and 16.6%\r(P-value = 0.0001) for the different cases compared to 6-MV. The percentage volumes\rof each patient receiving 2 Gy and 5 Gy\rwere compared for the two energies. The general trend was that 6-MV plans had\rthe highest percentage volume, (P-value = 0.0001, P-value = 0.006)\rrespectively. 10-MV beams actually decreased the integral dose (from average 183.27 ± 152.38 Gy-Kg to 178.08 ± 147.71 Gy-Kg, P-value = 0.004)\rcompared with 6-MV. In general, comparison of the above parameters showed\rstatistically significant differences between 6-MV and 10-MV groups. Based on\rthe present results, the 10-MV is the optimal energy for IMRT, regardless of\rthe concerns about a potential risk of radiation-induced malignancies. It is\rrecommended that the choice to treat at 10 MV be taken as a risk vs. benefit as the clinical significance remains to\rbe determined on case by case basis.