A new multipurpose quality assurance phantom for clinical tomotherapy

We present a new multipurpose phantom for routine dosimetric and image quality assurance (QA) testing of a tomotherapy unit. The phantom is designed for verification of delivered dose from the intensity modulated radiation therapy (IMRT) fan beams used to provide conformal dose distributions. The phantom is used to assess imaging characteristics and verify CT number calibration for the integrated megavoltage computed tomography (MVCT) detector mounted on the gantry across from the therapy linac. Functional requirements are outlined for the unique dosimetric and imaging needs of the QA tests of the tomotherapy system. The 36 cm long phantom is constructed mostly of Solid Water^TM and consists of 6 adjacent discs. The phantom consists of head and torso modules. The phantom is divided into imaging and dosimetric regions. For fan beam IMRT dosimetry, both a standard Farmer-style ionization chamber and a CT pencil chamber are used to measure the dose and dose rate under a variety of delivery conditions and phantom heterogeneities. The feasibility of using an existing kV CT chamber at therapy energies was investigated. The response to irradiation by small individual fields (7.6 mm) along its length is within one standard deviation of the average for the central 80% of its active volume (10 cm). For imaging QA, this phantom contains cavities to accommodate standard inserts to test spatial resolution, low contrast detectability, noise and edge contrast. Addition inserts are required for helical pitch verification and detector centering