RBE Estimation of Proton Radiation Fields Using a DeltaE-E Telescope

A new monolithic silicon DeltaE−E telescope was evaluated in unmodulated and modulated 100 MeV proton beams used for hadron therapy. Compared to a classical microdosimetry detector, which provides one-dimensional information on lineal energy of charged particles, this detector system provides two-dimensional information on lineal energy and particle energy based on energy depositions, collected in coincidence, within the DeltaE and E stages of the detector. The authors investigated the possibility to use the information obtained with the DeltaE−E telescope to determine the relative biological effectiveness (RBE) at defined locations within the proton Bragg peak and spread-out Bragg peak (SOBP). An RBE matrix based on the established in vitro V79 cell survival data was developed to link the output of the device directly to RBE(alfa), the RBE in the low-dose limit, at various depths in a homogeneous polystyrene phantom. In the SOBP of a 100 MeV proton beam, the RBE(alfa) increased from 4.04 proximal to the SOBP to a maximum value of 5.4 at the distal edge. The DelatE−E telescope, with its high spatial resolution, has potential applications to biologically weighted hadron treatment planning as it provides a compact and portable means for estimating the RBE in rapidly changing hadron radiation fields within phantoms.