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Modeling of carbon fiber couch attenuation properties with a commercial treatment planning system

2008; Wiley; Volume: 35; Issue: 11 Linguagem: Inglês

10.1118/1.2982135

2473-4209

I Mihaylov, Peter M. Corry, Yulong Yan, Vaneerat Ratanatharathorn, Eduardo G. Moros,

Medical Imaging Techniques and Applications

The purpose of this work is to evaluate the modeling of carbon fiber couch attenuation properties with a commercial treatment planning system (TPS, Pinnacle 3 , v8.0d). A carbon fiber couch (BrainLab) was incorporated into the TPS by automatic contouring of all transverse CT slices. The couch shape and dimensions were set according to the vendor specifications. The couch composition was realized by assigning appropriate densities to the delineated contours. The couch modeling by the TPS was validated by absolute dosimetric measurements. A phantom consisting of several solid water slabs was CT scanned, the CT data set was imported into the TPS, and the carbon fiber couch was auto‐contoured. Open (unblocked) field plans for different gantry angles and field sizes were generated. The doses to a point at depth, placed at the linac isocenter, were computed. The phantom was irradiated according to the dose calculation setup and doses were measured with an ion chamber. In addition, percent depth dose (PDD) curves were computed as well as measured with radiographic film. The calculated and measured doses, transmissions, and PDDs were cross‐compared. Doses for several posterior fields (0°, 30°, 50°, 75°, 83°) were calculated for 6 and photon beams. For model validation a nominal field size of was chosen and were delivered for each portal. The largest difference between computed and measured doses for those posterior fields was within 1.7%. A comparison between computed and measured transmissions for the aforementioned fields was performed and the results were found to agree within 1.1%. The differences between computed and measured doses for different field sizes, ranging from in increments, were within 2%. Measured and computed PDD curves with and without the couch agree from the surface up to depth. The PDDs indicate a surface dose increase resulting from the carbon fiber couch field modification. The carbon fiber couch attenuation for individual posterior oblique fields (75°) can be in excess of 8% depending on the beam energy and field size. When the couch is contoured in Pinnacle 3 its attenuation properties are modeled to within 1.7% with respect to measurements. These results demonstrate that appropriate contouring together with relevant density information for the contours is sufficient for adequate modeling of carbon fiber supporting devices by modern commercial treatment planning systems.