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For patients undergoing ICON mask fixation, the workflow has several variations (Fig. 2). High-definition brain imaging with MRI (or CT in patients who cannot have MRI) can be obtained in advance of or on the day of the stereotactic procedure. Intravenous contrast MRI imaging should include axial plane 1.5-mm slices starting at the C2 region and extending well above the vertex of the head. This is required since ICON can use the MRI or CT images to create the head configuration necessary for accurate dose and beam calculations for targets anywhere in the cranial vault down to the region of C2. However, the present design of ICON does not allow mask immobilization of targets in the skull base posteriorly.

Fig. 2. Components of mask-based SRS can be performed in a single day or over several days. Initial dose planning is performed when the imaging and CBCT date are available. On the day of treatment, a new CBCT scan is performed and the previous dose plan is co-registered to this CBCT scan. The system performs a 6D virtual couch rotation and offers the opportunity to review the plan. Once planning has been reviewed and approved the treatment (radiation delivery) begins (courtesy of AB Elekta).

Patients selected for mask SRS are generally calm and relaxed individuals with targets that are not extremely located in the cranial vault. For example, far anterior targets require that the head support moldable cushion (Fig. 3) and the thermoplastic mask (Fig. 4) itself be made with the patient’s head slightly extended. Similarly, for far posterior targets, the neck and support must be slightly flexed to reach the target. We frequently arrange for the MRI to be done at 06:30 on the morning of the procedure. Patients may receive 1 mg of lorazepam sublingually but that is the only sedation given (and runs the risk in some that they may fall asleep during beam on time and snore, leading to fiducial movement and ejection from treatment). The MRI imaging can be done after contrast administration (if creatine clearance permits) using a fast head coil. An entire head 1.5-mm axial SPGR sequence using the HSN coil of a 1.5 GE MRI scanner can be obtained in less than 5 min. In addition, we also obtain a 3-mm whole-head axial T2 scan, which is valuable to follow lesion response and detect edema in subsequent comparisons at follow-up.

Fig. 3. Moldcare head cushion. The cover material is polypropylene and the impression material is polystyrene beads. The bonding agent is hydraulic urethane resin. Spraying with water and massaging it leads to the hardening of this cushion. The head support is CT and CBCT compatible and can be used together with existing CT adapters.

Fig. 4. Thermoplastic hybrid mask. The top layer is 1.6-mm thick with microperforation to increase the bending stiffness, which provides a thin but stable mask. The reinforcement layer is 1.6-mm thick with microperforation.

The images are electronically transferred to the Leksell Gamma Knife planning system and imported. A pre-plan of the lesion is then created on the patient’s images. In addition, unexpected findings at this point may determine whether it is feasible to continue with a mask procedure or whether the patient needs to be converted to a frame-based procedure because of the projected length of beam on time (e.g., >30 min). During this time, the patient enters the Gamma Knife treatment room and a water spray molded neck and occiput support is molded for the individual patient. The thermoplastic mask has a small nose opening, which we often enlarge to mitigate the patient’s complaint of claustrophobia. It takes 5 min to heat the oven to 73°C. The mask needs about 7–10 min of heating. The mask is removed from the oven in the treatment room and gently molded over the patients’ head. It is attached to the mask assembly on the ICON bed and snapped into place. The patient rests comfortably, perhaps listening to music for next 10 min to allow the mask to harden. A small bump is placed under the knees to relax the spine. The bed level or position must be noted and fixed for patient comfort and taking into consideration the target location before the first CBCT.

Once dose planning is completed, the first stereotactic cone beam CT scan is obtained to define the patient stereotactic space [4]. The initial non-stereotactic MRI (used for pre-planning) is co-registered to this CT image and the treatment plan is converted to stereotactic space [5]. Initial studies have documented good agreement of CBCT-based and frame-based stereotactic space definition [6]. After review by the treatment team (surgeon, radiation oncologist, and physicist), the plan is finalized and exported to the treatment console. A second (treatment) CBCT is performed after the motion management detector system is in place. The dose plan is not co-registered to this CT image. The dose plan is reviewed again for any changes in margin dose, maximal dose, and conformity indices. Once approved, the dose plan is exported to the treatment consol. The patient is instructed not to move or even talk. The physicist in discussion with the radiation oncologist sets the acceptable variation in motion detection (0.5–3.0 mm depending on the target and the patient), and treatment can begin. The team monitors the motion detection variation on the screen (Fig. 5). If motion extends beyond the selected tolerance, the beams retract to the block position. If the motion does not return or drop below the threshold selected, after 30 s the patient leaves the focus position and the bed returns to the home position, after which the shielding doors close. A new CBCT must be performed and the remaining treatment plan again co-registered with the new stereotactic scan to correct for the patient motion. When the procedure is completed, the mask is removed, the patient sits up on the treatment couch and is then escorted to his or her outpatient room pending early discharge.

Fig. 5. The head movement is monitored in real-time during treatment using the IFMM system. The tolerance can be set between 0.5 and 3 mm. a A cooperative patient can be treated without significant head movement. b The treatment stops if the system detects head movement beyond the set threshold.