THE EFFECT OF THE NASOPHARYNGEAL AIR CAVITY ON X-RAY INTERFACE DOSES

We investigated the impact of air cavities in head and neck cancer pat ients treated by photon beams based on clinical set-ups. The phantom f or investigation was constructed with a cubic air cavity of 4 x 4 x 4 cm(3) located at the centre of a 30 x 30 x 16 cm(3) solid water slab. The cavity cube was used to resemble an extreme case for the nasal cav ity. Apart from measuring the dose profiles and central axis percentag e depth dose distribution, the dose values in 0.25 x 0.25 x 0.25 cm(3) voxels at regions around the air cavity were obtained by Monte Carlo simulations. A mean dose value was taken over the voxels of interest a t each depth for evaluation. Single-field results were added to study parallel opposed field effects. For 10 x 10 cm(2) parallel opposed fie lds at 4, 6 and 8 MV, the mean dose at regions near the lateral interf aces of the cavity cube were decreased by 1 to 2% due to the lack of l ateral scatter, while the mean dose near the proximal and distal inter faces was increased by 2 to 4% due to the greater transmission through air. Secondary build-up effects at points immediately beyond the air cavity cube are negligible using field sizes greater than 4 x 4 cm(2). For most head and neck treatment, the field sizes are usually 6 x 6 c m(2) or greater, and most cavity volumes are smaller than our chosen d imensions. Therefore, the influence of closed air cavities on photon i nterface doses is not significant in clinical treatment set-ups.