Measurement of secondary neutrons produced at proton therapy, a simulation study

At proton therapy due to protons interaction with devices located in front of the beam and used for modulation and also with patient body atoms, secondary particles are produced such as neutrons. Until now, many research studies have been done working on the topics of radiation protection and keeping patient body safe against secondary neutrons produced in front of the therapeutic beam, but there are rare studies on neutrons produced inside patient body. In this research, we have quantitatively investigated the presence and neutrons flux inside tumor volume and surrounding soft tissues, while their measurements is impossible experimentally using neutrons detectors. To do this investigation, a simulation environment has been used by means of Monte Carlo FLUKA code. A spherical tumor with 2.5 cm radius has been assumed inside a cubic phantom assigned with soft tissue equivalent matter. In this work, proton as therapeutic beam has been simulated with the parameters used clinically while there are uniform dose distribution onto tumor volume at all dimensions. Neuron flux between tumor and normal tissues borders and also the number of total neutrons exited from patient body have been calculated. In general, energy deposition of neutrons produced inside patient body is very low in comparison with energy deposition of protons as therapeutic beam and this is due to low effective atomic number of soft tissue equivalent phantom.