Becker Stewart J, Jozsef Gabor, DeWyngaert J Keith
Department of Radiation Oncology, New York University School of Medicine, New York, NY, USA.
Med Dosim. 2009 Summer;34(2):154-7. doi: 10.1016/j.meddos.2008.08.001. Epub 2008 Sep 4.
The purpose of this study was to measure the transmission of the Elekta Stereotactic Body Frame (ESBF) and treatment table, to calculate the transmission of the frame in the Eclipse Treatment Planning System (TPS) using analytical anisotropic algorithm (AAA), and to demonstrate a simple method of accounting for this transmission in treatment planning. A solid water body phantom was imaged inside the ESBF and planned with multiple 3D-CRT fields using AAA using both 6-MV and 16-MV energies. In the first set of plans, the frame and table were included in the "Body" contour and, therefore, used in the dose calculations. In the second set of plans, the frame and the table were not included in the "Body" contour and, therefore, were not incorporated in the calculations. The latter simulated a setup in which there was no frame or table. Eclipse TPS will only incorporate data from the CT set in calculations, if it is included in the "Body" contour. The plans were treated under two conditions: one with the phantom in the ESBF and one without the frame on a specially designed table. This table allows all the beams to enter the phantom without passing through any attenuating material (i.e., table or frame). Transmission of the frame and table was determined by the ratio of the measurements with the frame and table to the measurements without them. To validate the accuracy of the calculation model, plans with homogeneous phantom and a heterogeneous plan were compared with the measurements. The transmission of the frame varies from 89-94% depending on the angle of the beams and whether they also intercept the table. The AAA algorithm calculated the transmission of the frame and table to within 2% of the measurements for all gantry angles. Validation results showed that AAA can calculate the dose to the target to within 2% of the measured value. The attenuation caused by the ESBF must be accounted for in the planning process. For Eclipse, the frame should be contoured and included in all calculations. This can be done easily and accurately.
