Development of Novel Immobilization Adapter for Head and Neck Radiotherapy with Low-attenuation Material

Purpose: The dosimetric error due to immobilization devices has been highlighted by the AAPM Task Group 176. We developed a novel low-radiation-absorbent immobilization adaptor (HMA), which can be used with a Styrofoam headrest for head and neck region in radiotherapy. The purpose of this study was to investigate the impact of the HMA on the dose distribution and compare with a commercially released plastic adapter. Methods: Computed tomography (CT) simulation and dose calculation on a treatment planning system (TPS) were performed by the use of HMA and the plastic adapter with a cylindrical phantom. Both the adapters were placed on the phantom upside and the attenuation rate was measured. Gantry angles were changed at every 1°interval from 0°to 50°for measurements. The measured dose was normalized by the value of 90°. The treatment equipment was TrueBeam (Varian medical systems); X-ray energies were set on 4, 6 and 10 MV, respectively. The measured attenuation rates were also compared with calculation results of TPS. Results: The highest differences on attenuation rate of both the adapters were observed at a gantry angle of 32.0°; the differences were 3.0% at 4 MV, 2.7% at 6 MV and 3.0% at 10 MV, respectively, and lower absorption was HMA. TPS calculation results of monitor unit for the HMA were within 1.0% in each energy. Conclusion: The HMA was able to provide absorption dose and calculation errors lower than a commercially released adapter. It can also provide more accurate dose delivery for radiotherapy in head and neck because of the low absorption characteristics.