Introduction:
The relative biological effectiveness (RBE) of protons depends on the linear energy transfer or lineal energy. Measurements of lineal energy spectra are however rarely performed in conjunction with proton radiobiological experiments partly due to lack of suitable high spatial resolution microdosimeters. The aim of this study was therefore to apply a novel silicon based microdosimeter to measure microdosimetric spectra in a low energy proton beam used for radiobiological experiments. 

Method
A 3D silicon on insulator (SOI) detector “mushroom” with a sensitive volume array was used in a 15.2 MeV proton beam line. Multiple lineal energy spectra were measured along the Bragg curve by sequential introduction of polyamide absorbers with 16 µm thickness. The spectra were converted to tissue by a tissue conversion function and the microdosimetric spectra were compared to Monte Carlo simulations performed with GATE/GEANT4.

Results
The measured dose-mean lineal energy (yD) converted to tissue was 8.0 keV/µm at the entrance, approximately 13 keV/µm just prior to the Bragg Peak (BP), and reached 24 keV/µm at BP before rising to 35 keV/µm in the DDF (Fig 3). The measured yD matches well with the simulation apart from slightly higher measurements at the entrance. 

Conclusion
The obtained lineal energy spectra show that the novel 3D SOI “mushroom” detector is an excellent tool to characterize the radiation quality of proton beams for radiobiological experiments with low energy protons.

Microdosimetry with a 3D silicon on insulator (SOI) "mushroom" detector in a low energy proton beamline for radiobiological experiments