Lateral head flexion as a noncoplanar solution for ring gantry stereotactic radiosurgery

Area of Science:

• Radiotherapy physics and technology
• Medical imaging and image registration
• Radiation oncology

Background:

• Ring gantry radiotherapy devices offer simplified design and cost-efficiency but are limited by axial plane beam delivery.
• Noncoplanar beam delivery is crucial for optimizing stereotactic radiosurgery (SRS) but is challenging with current ring gantry systems.
• Increasing beam entrance angles is key to expanding the achievable solid angle for SRS treatments.

Purpose of the Study:

• To investigate the potential of lateral head flexion to increase beam entrance angles for ring gantry SRS applications.
• To evaluate the impact of head flexion on treatment plan quality, including conformity and dose spillage.
• To assess the feasibility of incorporating lateral head flexion into SRS delivery on ring gantry devices.

Main Methods:

• Seven healthy volunteers underwent MRI scans in neutral, left lateral flexion, and right lateral flexion head positions.
• Lateral flexion scans were co-registered to neutral scans using rigid registration to compute head pitch, roll, and yaw.
• Single-fraction SRS plans (21 Gy) were generated for both coplanar (neutral) and noncoplanar (flexion) geometries using Varian Halcyon 2.0 and Varian Edge systems.
• Plan quality was assessed using conformity index (CI), R50, R10, and gradient measure (GM).

Main Results:

• Lateral head flexion provided average pitch, yaw, and roll angles of up to 4.9°, 16.9°, and 2.8°, respectively.
• Ring gantry plans with head flexion showed significant improvements: 7.3% higher CI, 13% lower R50, 32% lower R10, and 7.8% lower GM compared to coplanar plans.
• Varian Edge plans also demonstrated benefits from head flexion, with a 3.0% higher CI, 6.8% lower R50, 29% lower R10, and 5.0% lower GM.

Conclusions:

• Lateral head flexion effectively increases beam entrance angles, leading to improved plan conformity and reduced normal tissue dose in SRS.
• The observed range of motion during lateral flexion is comparable to a 15° couch kick, offering substantial geometric advantages.
• This head flexion technique presents a viable strategy for enhancing SRS treatments delivered with ring gantry systems.