Room: Room 202
Purpose: To evaluate the impact of a soft-tissue alignment strategy for frame-based intracranial stereotactic radiosurgery (SRS), as facilitated by a custom cone-beam computed tomography (CBCT) acquisition template.
Methods: A custom CBCT acquisition template was created on the Varian TrueBeam platform using a full 360-degree rotation and a beam energy of 80 kVp. Initial patient alignments were based on fixed localization points on the head frame used for all intracranial SRS cases. CBCT acquisition and alignment preceded each unique treatment isocenter. All CBCTs were assessed for intracranial tissue visibility in close proximity to the treatment target. Clinical alignment focused on intracranial tissues, when visible, and on proximal bony anatomy otherwise. For all cases with intracranial tissue visibility, differences between bony alignment and final tissue alignment were quantified.
Results: 142 unique CBCT volumes were collected from 100 patients since the inception of our current SRS program in late 2016. Intracranial tissues were directly visualized in close proximity to the target in 75% of all CBCT volumes, including volumes from metastatic patients, trigeminal neuralgia patients, and arteriovenous malformation patients. The average displacement (Â± standard deviation) between the head-frame based patient alignment and the final CBCT alignment was 2.7 mm (Â± 1.9 mm). For patients with visualized intracranial tissues, the average difference between the tissue alignment and the bony anatomy alignment strategies was 0.21 mm (Â± 0.16 mm). The maximum observed difference between tissue- and bony-alignment strategies was 1.0 mm.
Conclusion: Direct visualization and alignment of intracranial tissues was possible in the majority of SRS cases. Anatomic alignments (as opposed head-frame alignments) often resulted in shifts larger than our 1 mm planning margin. Discrepancies between tissue- and bony-alignment strategies were smaller, though still relevant for some patients. Clinics should consider their patient alignment strategy when choosing their planning margins for SRS.