Objectives: The treatment outcome in patients can be improved with a fast and accurate treatment planning system (TPS) algorithm. The aim of this study was to design a novel head and neck phantom and to use it to test whether the accuracy of the irregular field algorithm of the Precise Plan 2.16 (Elekta Instrument AB, Stockholm, Sweden) TPS was within ±5% of the International Commission on Radiation Units and Measurements (ICRU) limit for homogenous and inhomogeneous media by rotating the Elekta Precise linear accelerator gantry angle using 2 fields.
Methods: A locally designed acrylic phantom was constructed in the shape of a block with 5 inserts. Acquisition of images was performed using a HiSpeed NX/i computed tomography scanner (GE Healthcare, Inc. Chicago, IL, USA); the Precise Plan 2.16 TPS was used to determine the beam application setup parameters and an Elekta Precise linear accelerator was used for radiation dose delivery. A pre-calibrated NE 2570/1 Farmer-type ion chamber with an electrometer was used to measure the dose. The mimicked organs were the brain, temporal bone, trachea, and skull.
Results: The maximum percentage deviation for 10×10 cm and 5×5 cm inhomogeneous inserts was 1.62 and 4.6, respectively, at a gantry angle of 180°, and that of the 10×10 cm homogeneous insert was 3.41 at a gantry angle of 270°. The percentage deviation for only the bone insert (homogeneous) and for all inserts (inhomogeneous) using parallel opposed beams was 2.89 and 2.07, respectively. Also, the percentage deviation between the locally designed head and neck phantom and the solid water phantom of the linear accelerator was 0.3%.
Conclusion: The validation result of our novel phantom in comparison with the solid water phantom was good. The maximum percentage deviations were below the ICRU limit of ±5%, irrespective of gantry angles and field sizes.