Abstract
Because the head and neck region is irregular and complicated in shape, dose distribution tends to be inhomogeneous with simple radiation technique. Because of the vicinity of critical structures, constraints that protect critical normal structures may cause insufficient dose distribution in the target volume. We report our easy methods to improve dose distribution in radiotherapy for head and neck cancer.
Large lateral parallel opposed fields are commonly used to treat the primary lesion and cervical lymph nodes. There is considerable dose inhomogeneity in cranial-caudal and anterior-posterior directions with this portal arrangement. The dose difference between 95% and 5% volume target doses was an average of 22% of the prescribed dose, which corresponded to 10.8Gy in absolute dose. We use a three-field technique, adding an anterior-posterior field to lateral fields with a wedge filter, to reduce dose inhomogeneity, which could be decreased to 11.6% and 5.5Gy. The anterior field was shaped to shield the parotid glands as much as possible, and the dose to the bilateral parotid glands could also be reduced.
A three-dimensional treatment planning system, and a C-arm LINAC system with a multileaf collimator, facilitate the use of non-conventional noncoplanar beam orientations and conformal beam shaping while shielding critical structures based on the beam's eye view display. Three-dimensional radiation therapy with this system enables the delivery of a highly homogeneous irradiation dose to the target volume with respect to the maximal tolerated dose to critical structures such as optic pathways.
