Phantom dosimetry for conformal stereotactic radiotherapy with a Head and Neck Localizer frame

Linear accelerator based stereotactic radiotherapy (SRT) with the Gill-Thom as-Cosman (GTC) re-locatable frame has been in use for several years. The u se of the frame is limited to treating lesions above the hard palate. For t reating tumours in the head and neck region. the Head and Neck Localizer (H NL) frame has been designed by Radionics Inc. for use with their XPlan trea tment planning software. In this study we have used a spherical acrylic phantom commercially known a s the 'Lucy' phantom (Sandstrom Sandstrom Trade and Technology Inc.) to per form thermoluminiscent as well as film dosimetry for the HNL frame. A radio -opaque marker was placed in the phantom and a film test carried out to ver ify the accuracy in isocentre positioning. The results of the dosimetry with TLD were within 2% for points near the is ocentre and 5% (or 2 mm in steep gradients) in the planning target volume ( PTV). In regions of low dose. larger percentage differences in local dose w ere observed, but all differences were within 5% of isocentre dose. The fil m dosimetry provided dose distributions that matched well software.