Finding of dose evaluation for organs at risk in intensity‐modulated radiation therapy for nasopharyngeal carcinoma using magnetic resonance imaging

Abstract

A critical step in predicting and avoiding radiation injury of organs at risk in radiation therapy of nasopharyngeal carcinoma is to carry out an accurate dose evaluation in planning design. In the present study, we investigated the dose evaluation feature of organs at risk on magnetic resonance imaging (MRI) images in intensity-modulated radiation therapy of nasopharyngeal carcinoma compared with computed tomography (CT) images. A total of 35 nasopharyngeal carcinoma patients were selected for this trial. CT simulation with non-contrast and contrast-enhanced scan, and MRI simulation with non-contrast and contrast-enhanced T1, T2, and diffusion weighted imaging were obtained sequentially. The organs at risk were contoured on the CT and MRI images after rigid registration, respectively. Nine-beam intensity-modulated radiation therapy plans with equal division angles were designed for every patient, and the prescription dose for the tumor target was set as 72 Gy (2.4Gy/fraction). The boundary display, volume, and dosimetric indices of each organ were compared between MRI and CT images. We found that MRI showed clearer boundary of the brainstem, spinal cord, deep lobe of the parotid gland, and the optical nerve in the canal compared with CT. MRI images increased the volume of the lens and optic nerve, while slightly reducing the volume of eye; the maximum dose of the lens, and the mean dose of the eyes and optic nerve increased to different extents, though no statistical differences were found. The left and right parotid gland volume on MRI increased by 7.07% and 8.13%, and the mean dose increased by 14.95% (4.01 Gy) and 18.76% (4.95 Gy), with a statistically significant difference (P < 0.05). The brainstem volume reduced by 9.33% (P < 0.05), and the dose of 0.1 cm 3 volume reduced by a mean 8.46% (4.32 Gy), whereas the dose of 0.1 cm 3 of the spinal cord increased by 1.5 Gy on MRI. The maximum dose region of the spinal cord was very close on CT and MRI images, and was similar to the brainstem. In conclusion, it is credible to evaluate the radiation dose of the lens, eye, brainstem, and the spinal cord by applying simulation CT; whereas MRI images are sometimes necessary to evaluate the dose of the parotid glands and the optical nerve.