Quantification and modelling of the dosimetric impact of the treatment couch in volumetric modulated arc therapy (VMAT)

Background: As the volumetric modulated arc therapy (VMAT) becoming a main role of treatment ways, the effect of couch top becomes more significant. It is imperative to re-evaluate the couches that previously may have been considered of no importance during early treatment techniques. The impact of couch top on radiation delivery was explored and the couch model was tested with the aim of reducing the couch absorption influences.  Materials and Methods: Attenuation measurements were performed in a cylindrical phantom with an ionization chamber positioned at the isocenter. Couch model was obtained by importing its actual CT scan, and the accuracy was evaluated by comparing percentage deviation at 2 and 5 mm voxel grid size. Effects on surface dose were measured using EBT3 film with the constant SSD at different depths and beam energies at the gantry angle 180° and 0°, respectively. Results: Couch top increases surface dose from 45.9 % to 95.8 % , from 35.0 % to 87.9 % and from 29.2% to 73.9 % for 10 cm ×10 cm field at 6 ,10 and 18 MV, respectively. Due to the couch absorption the case of vertebral metastasis VMAT plan D50 of the PTV changed from 30 Gy to 29.3 Gy. Couch model with uniform electron density of 0.18g/cm3 demonstrated an excellent agreement between measured and TPS computed dose. Conclusion: The treatment couch presence between the patient and beam source significantly alters dose in the patient. Modelling the couch in the Monaco TPS can adequately predict the altered dose distribution.