Dose analysis of radiotherapy techniques for nasopharyngeal carcinoma

Purpose: In external beam radiotherapy of nasopharyngeal carcinoma, the treatment volume inevitably includes neighbouring normal structures and radiation complications of these organs are not uncommon, despite careful placement of radiation fields and shieldings. This study focuses on two commonly used techniques, the 'flexed-neck' technique which treats the nasopharynx and the neck lymphatics in one volume and the 'extended neck' techniques, which treats the nasopharynx and neck lymphatics in two separate volumes. A 3-D computer planning system is used to analyse dose distribution to various structures. Method: Computed tomographic images of 12 patients, each consisting of 16 to 22 slices, were loaded into a radiotherapy planning computer equipped with 3-D planning facilities. Treatment plans simulating the two radiotherapy techniques were computed. The dose distributions of the target volume and various normal structures were analysed. Results: The target volume was generally easily reached with the high-dose volume of both techniques, but the flexed neck technique provides better coverage and uniformity of dose. The pituitary, temporal lobe and the optic chiasma were found to be more heavily irradiated in both techniques, with their mean doses ranging from 65% to 94% of the prescribed dose. Doses received by most normal structures, except for the lens and eyeball, were lower in the extended neck technique. With both techniques, there were marked dose differences in the spinal cord and external auditory meatus. Conclusion: Normal structures were less affected by the extended neck technique; however, its dose coverage of the target volume might be less superior to that of the flexed neck technique.