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Objective.A low-energy x-rays camera-based system has been introduced to image secondary electron bremsstrahlung x-rays, enabling external visualization of proton beam profiles during irradiation. To date, this approach has been demonstrated only in phantom studies and simulations, and has not yet been successfully applied in patients. To overcome this limitation, it is essential to establish the feasibility of SEB x-ray imaging in a clinical condition.Approach.We first optimized the energy window and threshold settings for range estimation by acquiring SEB x-ray images of a water phantom comparable in size to the human lower abdomen, using a cerium doped yttrium aluminum perovskite (YAP(Ce)) x-ray camera with a pinhole collimator. We then performed the firstin vivoSEB x-ray measurement by imaging proton beams during treatment of a patient of prostate cancer with two opposed fields, each delivering an RBE-weighted dose of 1.5 Gy.Main results.In the patient study, SEB x-ray distributions were observed in the measured images using the same energy window as that established in the water phantom study. From these images, the beam ranges were estimated to be within approximately 1 cm of the values calculated by the treatment planning system using the optimized profile threshold. Dynamic imaging at 1 s time intervals was achieved, enabling temporal evaluation of count rate curves.Significance.We present, for the first time, measurements of SEB x-ray images acquired in a patient during proton beam irradiation. These preliminary results indicate thatin vivoSEB x-ray imaging could be a promising approach for clinical real-time beam monitoring in proton therapy.Ethics approval.Ethics approval was obtained from the institutional review board of Kobe Proton Center: approval number: 06-10.