Elemental analysis of tissue in proton therapy using photopeak count of prompt gamma spectrum

Online elemental analysis of irradiated tissue in proton therapy is a major part of this treatment, since it leads to accurate estimation of the depth of Bragg peak, proton range, and distribution of stopping power of irradiated tissue. Given the fact that each element has unique characteristic prompt gamma spectrum, recording and analyzing of the emitted gamma-ray spectra is one of the methods used for online elemental analysis of irradiated tissue. In the present study, the prompt gamma lines of most abundant elements of human tissues (^12C, ¹⁶O, ^20Ca, and ¹⁴N) are obtained using Geant4 tool kit. Moreover, the linear dependency of 7.12, 3.91 and 2.31 MeV prompt gamma counts to the mass of ¹⁶O, ^20Ca, and ¹⁴N are investigated in five phantoms. The gamma lines of 2.31, 3.91 and 7.12 MeV are selected to determine the mass of nitrogen, calcium, and oxygen in the irradiated volume. Furthermore, for two test phantoms, the mass of elements is estimated using linear fitting and is compared with predefined mass of these elements in the test phantoms. The results showed that the relative errors of estimated mass of calcium, nitrogen, and oxygen are less than 6%, which leads to the range estimation deviation of about 1 mm.