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To provide ongoing monitoring of the patient, the ICON utilizes an infrared camera to track the patient’s head movement in real-time [20]. The patient is positioned in a head holder with a thermoplastic mask that is molded to the patient’s head. Four reflective markers are located on the head holder, and one additional marker is positioned on the patient’s nose. During treatment, the reflective markers are constantly monitored to see if the tolerance has been broken (Fig. 4a, b). A tolerance is determined by many factors, and is typically in the 1.0–2.0 mm range. If the tolerance is broken, the sources are retracted and the unit waits 30 s to see if the patient can return to within the tolerance. If not, the patient may be coached to bring them back into position with instructions on how to adjust the head, although this is not very feasible. Alternatively, a new CBCT [21, 22] could be acquired to determine new couch coordinates to apply and deliver the remaining dose.

Fig. 4. a Real-time tracking of a patient with a 1.5-mm tolerance. b Real-time tracking of a patient with a 2.0-mm tolerance, treatment was interrupted and resumed (yellow blocks).