The first national IMRT dosimetry inter comparison in Switzerland using a thorax phantom with inhomogeneities

In 2008, the first IMRT (intensity modulated radio therapy) dosimetry intercomparison of the SGSMP (Swiss Society of Radiobiology and Medical Physics) using a thorax phantom has been carried out. It was the aim to check the ability of the calculation algorithms used in practice to take into account inhomogeneities. Additionally, the feasibility of the selected procedure had to be tested. All 23 institutions performing radio-oncology in Switzerland participated in the intercomparison. Thereby 30 plan-measurement combinations have been used. The intercomparison results are presented here grouped by the applied calculation algorithms. Following Knoos et al. [1] and Fogliata-Cozzi et al. [2], the algorithms have been additionally classified as “type a” or ”type b”. As opposed to ”type a” algorithms, “type b” algorithms take into account the three dimensional density distribution and are able to treat the electron transport in an approximate way as well as the secondary photon transport. TLD (thermo luminescence dosimeter) and ionisation chamber measurements demonstrate that ”type b” algorithms generally show a better agreement between calculations and measurements. But there are still differences within the two groups of algorithms. The results demonstrate that the performed intercomparison is feasible and suggests that cancer patients in Switzerland get a suitable radiation therapy in any of the centers offering this treatment modality.