Room: AAPM ePoster Library
Purpose: To investigate potential dosimetric errors in the calculations of the treatment planning system (TPS) provided for an image-guided small animal irradiator and its impact in the dose calculation results of experiments pertaining to animal models of tumor biology and therapy.
Methods: In this study, the comparison of cone and motorized variable collimator (MVC) inserts at kilovoltage energies for different field sizes were performed using EBT3 film. Field size of 5 mm x 5 mm and 10 mm x 10 mm were used when comparing the performance of cone and MVC inserts regarding beam spectrum changes and 2D dose distribution evaluation. The beam spectrum for the usage of different inserts was performed by measuring the percent depth dose (PDD) curves at 33 cm SSD, up to a depth of 60 mm, with 0.5 cm measurement increments using film. Measured 2D dose distribution were compared with the dose distribution calculated by the TPS using DoseLab (Mobius Inc). The backscatter of low-energy photons from the couch for field sizes of 3-20 mm were measured using film and compared to the TPS calculations for a 20 mm thick water phantom.
Results: PDD measurements between the MVC and cone insert differ the most at the surface due to greater electron contamination from the MVC. Backscatter from the couch was measured to be 12-17% for the aforementioned field sizes with the TPS underestimating backscatter by 5%. For 2D dose distribution, gamma tests comparing measurement from cone and MVC inserts with TPS fail to exceed 70% (going as low as 59.3%) using 2%/1mm criteria.
Conclusion: This study addresses dosimetric issues of the treatment planning system used in a small animal image-guided irradiator in calculating kV small field dosimetry. The inaccuracies are worth discussing to improve radiation research in the future.