In comparison with conventional radiotherapy techniques,12C beam therapy has its significant advantage in cancer treatment because the radiation dose are mostly concentrated near the Bragg peak region and damage to normal tissues along the beam path is thus greatly reduced.In-beam PET provides a way to monitor dose distribution inside human body since several kinds of positron-emitting nuclei are produced through the interaction between 12C beam and body matters.In this work,we study the quantitative relationship between the spatial location of the Bragg peak and the spatial distribution of positrons produced by positron-emitting nuclei.Monte Carlo package GATE is used to simulate the interactions between the incident 12C beam of different energies(337.5,270.0 and 195.0 MeV/u) and various target matters(water,muscle and spine bone).Several data post-processing operations are performed on the simulated positron-emitting nuclei distribution data to mimic the impacts of positron generation and finite spatial resolution of a typical PET imaging system.Simulation results are compared to published experimental data for verification.In all the simulation cases,we find that 10C and 11C are two dominant positron-emitting nuclei,and there exists a significant correlation between the spatial distributions of deposited energy and positrons.Therefore,we conclude that it is possible to determine the location of Bragg peak with 1 mm accuracy using current PET imaging systems by detecting the falling edge of the positron distribution map in depth direction. In contrast to conventional radiotherapy techniques, 12C beam therapy has its significant advantage in cancer treatment because the radiation dose are mainly concentrated near the Bragg peak region and damage to normal tissues along the beam path is greatly reduced. In-beam PET provides a way to monitor dose distribution inside human body several several kinds of positron-emitting nuclei are produced through the interaction between 12C beam and body matters. In this work, we study the quantitative relationship between the spatial location of the Bragg peak and the spatial distribution of positrons produced by positron-emitting nuclei. Monte Carlo package GATE is used to simulate the interactions between 12C beam of different energies (337.5, 270.0 and 195.0 MeV / u) and various target matters (water, muscle and spine bone) post-processing operations are performed on the simulated positron-emitting nuclei distribution data to mimic the impacts of positron generatio n and finite spatial resolution of a typical PET imaging system. Simulation results are compared to published experimental data for verification. All of the simulation cases, we find that 10C and 11C are two dominant positron-emitting nuclei, and there exists a significant correlation between the spatial distributions of deposited energy and positrons.Therefore, we conclude that it is possible to determine the location of Bragg peak with 1 mm accuracy using current PET imaging systems by detecting the falling edge of the positron distribution map in depth direction.