Osimetric comparision and analysis of two kinds of intensity-modulated plan for nasopharyngeal carcinoma (NPC)

Objective: To compare IMRT with VMAT for NPC patients with regard to the sparing effect on organ at risk (OAR), dosimetric quality, treatment delivery time and monitor units(MUs). Methods: Fifteen patients with NPC were planned by IMRT and VMAT for dosimetric comparison. Results: For most parameters such as HI CI D_means and D_1% in target area, the results of VMAT plans were better than those of IMRT plans. In OAR, although for D_means of brain stem and D_1% of optic nerves and right lense, there were no significant difference between VMAT and IMRT, VMAT plans had a better sparing effect for most parameters. The delivery time per fraction for VMAT(4.1 min) was much shorter than the time for IMRT(8.8 min). Compared to IMRT (823.5 MUs), VMAT(597.7 MUs) reduced Mus by 37.8%. Conclusion: The results indicate that IMRT plan and both VMAT plan could have similar target coverage and meet clinical needs, however for most parameters in target area and OAR, the results of VMAT plans were better than these of IMRT plans, particularly on HI and Cl of target and to protect OAR such as brain-stem spine-cord and parotid, but VMAT plans were as if not enough to protect small volume OAR such as optic nerve, optic chiasma and lense. VMAT provided shorter delivery time per fraction and fewer monitor units than IMRT. (authors)