Boron neutron capture therapy (BNCT) is a binary radiotherapy combining biochemical targeng with neutron irradiaon. However, monitoring the boron distribuon is a fundamental problem in BNCT. Prompt gamma rays emi"ed by boron capture reacon can be used to address the issue. Materials and Methods: The general-purpose Monte Carlo toolkits Geant4 and MCNP were used for the simulaons. A cubic phantom with so) ssue was used to study the prompt gamma emission during BNCT. The Chinese hybrid phantom

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... h arbitrary tumors was constructed and used to acquire the 0.478 MeV prompt gamma rays in BNCT. Tomographic images were reconstructed with the maximum likelihood expectaon maximizaon (MLEM) algorithm. Results: Comparison between MCNP and Geant4 showed a similar gamma rays emission rate in so) ssue. Up to 30 gamma ray peaks were found in the simulaon, and 0.478 MeV prompt gamma ray from boron was clearly observed. The single brain tumor with variable diameter from 1 cm to 4 cm in the heterogeneous anthropomorphic phantom was each me found to be recognizable in the reconstructed image. Furthermore, in a paent with four tumors, the variable distance between the source and the tumors leads to a neutron a"enuaon thus resulng in an inhomogeneous number of prompt gammas. Conclusion: The SPECT system for a heterogeneous phantom in BNCT was simulated with Geant4. The results show that BNCT-SPECT is valid for the reconstrucon of the boron capture interacon posion for a heterogeneous paent.