Analysis of dosimetric results of postoperative intensity modulated radiation therapy using single-energy or mixed-energy photons in invasive thymoma patients

Objective: To compare the dosimetric results of postoperative intensity modulated radiation therapy (IMRT) using single-energy or mixed-energy photons in invasive thymoma patients. Methods: Simulation CT images were acquired and clinical target volume (CTV), planning target volume (PTV) and organs at risk (OARs) were defined. Three sets of fixed-field IMRT planning were generated using 6 MV, 10 MV and mixed 6/10 MV photons for each case. Monitor Units (MUs) for each plan were recorded after optimization, and parameters of PTV such as conformity index (CI), homogeneity index (HI) and dose to OARs were evaluated on dose-volume histograms. Results: Near-Maximal dose (D_2%) received by PTV was better in mixed-energy IMRT as compared with 6 MV(t = 3.107, P < 0.05). HI was better in mixed-energy than in 6 MV(t = 2.924, P < 0.05). There were statistically significant differences in CI among three IMRT plans. MU was higher in 6 MV than in both 10 MV and mixed-energy IMRT. The percentages of lung volumes receiving 5 Gy(V₅),10 Gy(V₁₀), 20 Gy(V₂₀), 30 Gy(V₃₀) and the mean lung dose (D_mean)were also significantly different in most plans. V₃₀ and V₄₀ of the heart were comparable between 6 MV and mixed energy plans but better than in 10 MV plan. Conclusions: If the reasonable choice of beam angles and number, and capability of energy selection according to beam directions, with combined advantages of low and high energy photons, mixed IMRT plans can improve the quality of IMRT plans in general and has clinical potential for postoperative radiotherapy of invasive thymomas. (authors)