Solid state diode – Ionization chamber method for measuring out-of-field neutron dose in proton therapy

In proton therapy neutrons are introduced to out-of-field regions inside the patient. Clinicians would like to know the absorbed dose being deposited by neutrons separately to that from protons, so as to be able to directly apply their own dose equivalent weighting factors based on their opinion of the biological risk posed by neutrons in this region. The purpose of this study is to investigate a novel approach to experimentally separating the proton and neutron contributions to the absorbed dose in out-of-field regions. The method pairs specially designed silicon PIN diodes with a standard clinical ionization chamber. The sensitivity of the Si diode to non-ionizing energy losses in silicon is exploited, and can be quantified by measuring the shift in forward voltage for a fixed injection current, pre and post irradiation. The mathematical relations that describe the response of the diode and the ionization chamber can be solved simultaneously to give the contributions to the absorbed dose from protons and neutrons separately. Experimental measurements were made at the Loma Linda University Medical Center (LLUMC), Loma Linda, and Massachusetts General Hospital (MGH), Boston, proton therapy facilities. Experimental separation of the partial proton and neutron contributions to the absorbed dose measured at positions lateral to a typical prostate therapy treatment field delivered to a Lucite phantom was successfully performed and compared with results from a GEANT4 simulation. The experimental results matched well with simulation confirming the validity and promise of the novel approach.