A device for marking a treatment isocenter on a patient's body includes a base including alignment indicia; a marker disposed over the base and positioned relative to the indicia for marking the isocenter on the patient's body; and an actuator for actuating the marker and causing a mark indicating the isocenter to be made on the patient's body. The actuator can include a button and a spring coupled to the marker. Compressing the actuator causes the marker to travel through an ink well prior to piercing the patient's skin. The device is disposable as intended for a single use.