An apparatus for monitoring a delivery of a radiation treatment to a lesion located in or near the prostate gland of a patient, the apparatus comprising: an ultrasound imaging device for acquiring signals representing one or more ultrasound images of an area of interest of the patient during the delivery of the radiation treatment, the radiation treatment including one or more beam-delivery paths directed to the area of interest of the patient; and a brace for accepting the imaging device, wherein: the brace is configured to cause a movement of the imaging device within an envelope outside the one or more beam delivery paths of the radiation treatment, the brace is configured to prevent the imaging device from interfering with the delivery of the radiation treatment to the patient, while the imaging device acquires the signals, and the brace is configured to provide adjustability to the imaging device according to multiple degrees of freedom relative to the patient; and a processor for receiving the signals from the imaging device and detecting a movement of at least one anatomical structure within the area of interest during the radiation treatment, characterized in that: the brace is secured to a patient-support platform such that the brace is configured to hold the imaging device in contact with the patient's perineum whilst causing a movement of the imaging device in a transperineal position between the patient's legs for transperineal imaging of the prostate gland. an ultrasound imaging device for acquiring signals representing one or more ultrasound images of an area of interest of the patient during the delivery of the radiation treatment, the radiation treatment including one or more beam-delivery paths directed to the area of interest of the patient; and a brace for accepting the imaging device, wherein: the brace is configured to cause a movement of the imaging device within an envelope outside the one or more beam delivery paths of the radiation treatment, the brace is configured to prevent the imaging device from interfering with the delivery of the radiation treatment to the patient, while the imaging device acquires the signals, and the brace is configured to provide adjustability to the imaging device according to multiple degrees of freedom relative to the patient; and the brace is configured to cause a movement of the imaging device within an envelope outside the one or more beam delivery paths of the radiation treatment, the brace is configured to prevent the imaging device from interfering with the delivery of the radiation treatment to the patient, while the imaging device acquires the signals, and the brace is configured to provide adjustability to the imaging device according to multiple degrees of freedom relative to the patient; and a processor for receiving the signals from the imaging device and detecting a movement of at least one anatomical structure within the area of interest during the radiation treatment, characterized in that: the brace is secured to a patient-support platform such that the brace is configured to hold the imaging device in contact with the patient's perineum whilst causing a movement of the imaging device in a transperineal position between the patient's legs for transperineal imaging of the prostate gland.