Session Item

The increased complexity of the radiation therapy process makes essential to implement tests to commission the whole procedure, from de CT to the treatment administration in the radiation unit. In this work, we use a new modular phantom (RUBY phantom, PTW Freiburg, Germany) for the end to end testing of several VMAT irradiations. 

End to end testing was made for different Vmat treatments: A non-coplanar cranial stereotactic treatment of one small centred lesion (volume of 0.5 cc), a cranial coplanar and non-coplanar stereotactic treatment for three lesions and a prostate SBRT. The main modules of the RUBY phantom were used for cranial (head phantom) and prostate (body phantom) treatments, combined with different inserts to check point dose and dose distributions, IGRT, heterogeneity corrections and volume calculations. For point dose measurements, an insert consisting of three detector positions that correspond to the centres of three assumed metastases was used, (MultiMet insert) with a PinPoint 3D ionization chamber (PTW Freiburg, Germany). MultiMet also contains three cylindrical volumes (made of bone equivalent material) positioned inside at different spatial angles (visible in planar MV, planar KV and CBCT images), to enable image registration with the reference data set.  For dose distribution measurements, an insert that accommodates a film was used (Film insert) with radiochromic film EBT3 and Radiochromic software (radiochromic.com). To check heterogeneity corrections and volume calculations of treatment planning system (TPS), an insert with non-homogeneities (consisting of cylinders of different sizes around a detector, interrupted at different points to enable image registration) was used. Planning CT was acquired in a GE Discovery RT with 120 kV and 1.25mm slice thickness, the TPS used was Monaco (V.5.51) and the plans were irradiated in a VERSA Agility Linac (Elekta) with an energy of 6 MV. IGRT (kV/kV and CBCT) was checked positioning the phantom bodies using the engraved visible lines that indicate defined translational and rotational shifts in all three spatial directions.

For IGRT using CBCT, differences between experimental and known shifts were smaller or equal than 0.5 mm and 0.5º. Higher differences were found when using Mosaiq (Elekta) fusion software (instead of Elekta-XVI software), due to rounding. Differences were lower or equal than 1.5 mm when using kV/kV IGRT. Differences in volume calculations (between TPS and Ruby phantom specifications) for heterogeneous inserts are lower than 1.5 %. Table 1 shows the results for point dose and dose distribution measurements. 

The modular QA Ruby phantom has been successfully used to check different steps of the radiotherapy process. Results for end to end of several Vmat treatments show a good performance of the whole radiotherapy procedure, with differences < 2.5 % for dose measurements and gamma passing rates > 97 % (2%, 2 mm).