Abstract
Purpose: Previously in treatment planning systems (TPS) the treatment couch was expected to be made out of air-equivalent material due to the used material (Carbon). Some studies have already shown that the treatment couch cannot be neglected during treatment planning. Nowadays the manufacturer of TPS implemented the feasibility to insert treatment couch structures. This study aimed to find the correct modeling of the treatment couch parameters in the TPS Eclipse. Method: The Varian Exact Treatment Couch consists of a carbon board (length 2.5 cm) and two moveable rails (length 8.5 cm) underneath. The treatment couch can be modeled in TPS by changing the Hounsfi eld units (HU) for each part of the treatment couch. For low and high photon energies the attenuation of the treatment couch was measured at a Clinac 2300 C/D and in the TPS the attenuation of the treatment couch model was determined for different sets of HU values. Measured and calculated attenuations were compared to each other. Results: Minimum aberration between the calculated and measured attenuation of treatment couch were found for the HU values of -750 HU for the carbon plate, -995 HU for the filling of the carbon plate and 225 HU for the rails. Additionally it was found that the attenuation is dependent on the gantry angle. Like expected the highest attenuation was found in the region of the rails underneath the treatment couch. Conclusion: For Varian Exact Treatment Couch the HU values should be adjusted to -750 HU for the carbon plate, -995 HU for the filling of the carbon plate and 225 HU for the rails. The same HU set can be used for low and high photon energies. With the correct set of HU values the treatment couch is modeled correctly in the TPS Eclipse.
Highlights
The treatment couch and some of the positioning devices of patients receiving radiotherapy treatment are made of a material which has low attenuation properties for the photon energies used in the radiotherapy
Minimum aberration between the calculated and measured attenuation of treatment couch were found for the Hounsfield units (HU) values of -750 HU for the carbon plate, -995 HU for the filling of the carbon plate and 225 HU for the rails
For Varian Exact Treatment Couch the HU values should be adjusted to -750 HU for the carbon plate, -995 HU for the filling of the carbon plate and 225 HU for the rails
Summary
The treatment couch and some of the positioning devices of patients receiving radiotherapy treatment are made of a material (carbon) which has low attenuation properties for the photon energies used in the radiotherapy. The effect of the treatment couch to the dose distribution depends on the gantry angle. The attenuation increased from 2% for beam angles as the gantry approaches the plane to 9% for posterior beam angles. This could have serious implications regarding dose to the treatment volume for treatments requiring posterior oblique angles of incidence with a possible correction factor necessary in monitor unit calculations (McCormack et al, 2005). Side bars or other design features specific of each carbon fiber tabletop disturbed the dose distribution at the isocenter at specific gantry angles (Gillis et al, 2005; Munjal et al, 2006)
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