[Comparison of the irradiation doses for IMRT and conventional radiotherapy in nasopharyngeal carcinoma patients].

OBJECTIVE: To compare the dose difference in irradiation dose between nasopharyngeal cancer patients receiving conventional radiotherapy (CR) and intensity-modulated radiotherapy (IMRT). METHODS: The various IMRT and the CR plans for patients with nasopharyngeal carcinoma were created by a three-dimensional treatment planning system (3D-TPS). The dose-volume histograms (DVH) of normal tissues and the total number of irradiation monitor units (MU) were calculated by 3D-TPS. The treatment was delivered with 6 MV photons using a "step-and-shoot" technique on a linear accelerator. The X-ray leakage and scattered dose (LSD) at the dmax (maximal dose depth) in water were measured by dosimeter and ion chamber. The dose delivered to normal tissue adjacent to the treatment site at a certain distance was estimated by using LSD and the total MU for different radiotherapy methods. RESULTS: To IMRT patients, the volumes of normal tissues receiving the dose less than 28 Gy and higher than 35 Gy were 1.43-1.81 and 0.70-0.30 times those of CR patients respectively. The LSD irradiated that to the IMRT patients was about two times of the CR patients. CONCLUSION: The dose received by normal tissues in IMRT is two times that in CR.