Exponential wedge for head and neck cancer treatment

Purpose/Objective: Often cancers of the nasal cavity and maxillary sinuses are treated with a three field wedge technique involving an anterior open field and lateral fields, using 45 deg. wedges. To improve the target volume isodose uniformity, the posterior volume may need to be supplemented with half beam blocked 60 deg. wedged lateral fields. The purpose of this work was to develop a single wedge that can bring the target dose within the 95% or better isodose line and at the same time simplify the technique by reducing set-up and treatment time. Material and Methods: The wedge was designed for a Clinac 4/80 Linac and made of lipowitz metal (cerrobend). A calculation scheme was developed using the Tissue-Air-Ratio (TAR) method by separating primary and secondary dose components in a 2D domain, and compensating for the fall off in dose from the AP field. The design was performed for an average head contour. The wedge profiles were measured and installed in a treatment planning system. Treatment plans were generated for different cases to observe the dose distribution. Results: The computer plans generated with this new wedge profile show a significant improvement of the dose distributions than those generated with standard 45 deg. or 60 deg. wedge profiles. With the help of the exponential wedge, it is possible to cover the target volume with the 98% isodose line for the contour for which it was made, and with 95% or better isodose line with other similar contour. Conclusion: An exponential wedge made of cerrobend can be used by a three field technique to improve the dose homogeneity for treatment of nasal cavity or maxillary cancer instead of using a three or five field technique with standard wedges. The treatment does not need any additional lateral fields to supplement the dose distribution posteriorly. This technique not only helps to save treatment set-up time but also to reduce the number of field shaping blocks. Computer planning time is also reduced