Verification of Uniformity of Dosimetric Characteristics of Couch Tops in Proton Radiation Therapy

Abstract

In proton therapy the couches used for patient positioning and alignment can affect the uncertainty in the dose distribution from the applied proton beam. In order to limit the additional uncertainty of the proton range, it is necessary to know the determined water equivalent thickness (WET) of the couch top and carefully reflect it in the calculations of the dosimetry planning system. The method presented in this article is based on the difference in the measured depth of the proton beam detected by the multilayer ionization chamber with the beam passing through the couch top and without passing through it.
 A range shifter (proton beam modifier) installed in the nozzle with the known WET for evaluating couch tops before commissioning was used in this research work. A nozzle is a proton beam delivery system consisting of quadrupole magnets, ionization chambers and an X-ray tube. CT scans were also acquired to evaluate the internal homogeneity of the test couch tops.
 Several kVue couch inserts Proton one (Qfix Systems, Avondale, PA) were evaluated in this study. The results show that the inserts are largely interchangeable for this type of design and the use of the range shifter during treatment, using gantry angles with the proton beam passing through the couch top, does not affect its WET, adding a deflection of approximately 0.1 mm. It was also determined that the reproducibility of the WET determination method has a deviation less than ±0.03 mm. In conclusion, the described above method of couch top testing before commissioning in proton therapy can also be applied to other patient immobilization systems.