Dosimetric evaluation of nasopharyngeal carcinomas irradiated with different IMRT techniques.

PURPOSE To evaluate the performance of volumetric arc therapy (VMAT), dynamic intensity modulated radiotherapy (IMRT) and step-and-shoot IMRT techniques in nasopharyngeal cancer (NPC) patients. METHODS IMRT plans of 48 NPC patients treated between May 2010 and December 2012, were evaluated. Twenty two patients were planned with VMAT, 18 with dynamic IMRT and 8 with step-and-shoot IMRT. Conformity index (CI) and homogeneity index (HI), the dosimetry of the planning target volumes (PTVs) and organs at risk (OARs) and the monitor units (MU) were evaluated for each IMRT modality. RESULTS The conformity indices of VMAT and dynamic IMRT were better than step-and-shoot IMRT plans (p<0.05). Step-and-shoot IMRT plans provided better homogeneity than VMAT (p=0.01). MUs for dynamic IMRT were much higher compared to VMAT (p<0.01) and step and- shoot IMRT (p<0.01). There was no significant difference between the 3 techniques in terms of PTV70 mean doses. When compared with step-and-shoot IMRT, VMAT and dynamic IMRT had a better sparing effect on optic nerves, eyes and optic chiasm (p<0.05). VMAT plans performed better sparing for brain stem than dynamic IMRT (p=0.01). There was a remarkable decrease in the maximum doses of VMAT to the eye. CONCLUSIONS VMAT outperforms dynamic IMRT by effectively reducing the MU and the dose to some OARs, with adequate PTV coverage. Also, VMAT provides better sparing of normal tissue and conformity than step-and-shoot IMRT. Differences between step-and-shoot IMRT and dynamic IMRT are thought to be due to technical differences of linear accelerator devices like fiber size, variable fiber, dose rate and gantry.