Purpose: To evaluate the feasibility of boron neutron capture therapy (BNCT) for breast cancer occurs during pregnancy. Methods: computational models of pregnant women at 3- and 6-month gestational ages were used with two various simulated tumors in their left breasts. The Monte Carlo simulation of irradiating the tumors by thermal and epithermal output beams of in-hospital neutron irradiator (IHNI), were then performed in five directions. The optimum treatment plans as a combination of the irradiation directions and output beams were then assessed using an optimization code written in this work. Results: According to the results of this study, the total irradiation time of ≤10 minutes is needed to deliver a prescribed dose of RX = 24.4 Gy-Eq to gross tumor volume (GTV) in a BNCT single fraction. The dosimetric properties and volume metrics of the optimized treatment plans were obtained and the dose volume histogram (DVH)-based metrics were compared with those from conventional radiotherapy (RT). It has been shown that the dose to both target volume and organs at risk (OARs) were within clinically acceptable dose constraints throughout the course of a single fraction BNCT. Moreover, the fetal dose (~4.8 mGy-Eq) is well below the threshold for secondary cancer incidence (10 mGy) at first trimester of pregnancy, while for second trimester of pregnancy is much higher (~35.5 mGy-Eq). Conclusion: Regarding the DVH metrics for GTV, maternal OARs and the fetus, the studied treatment modality is an appropriate alternative treatment especially for breast cancer occurring at the first trimester of pregnancy.

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