Dosimetric impact of arc simulation angular resolution in single-isocentre multi-target stereotactic radiosurgery

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiosurgery

Background:

• Volumetric Modulated Arc Therapy (VMAT) is a common technique for Single Isocenter Multiple Target (SIMT) Stereotactic Radiosurgery (SRS).
• The angular resolution of arc simulation in VMAT planning can impact dosimetric accuracy, particularly for small and peripherally located targets.

Purpose of the Study:

• To evaluate the dosimetric impact of different arc simulation angular resolutions (0.5°, 1°, 2°) in VMAT-based SIMT SRS.
• To determine the dependence of these impacts on target size, isocenter distance, number of arcs, and arc type.

Main Methods:

• A phantom study was conducted using various target sizes (0.5-2 cm) and distances (2.5-7.5 cm) with conformal arc and VMAT plans.
• Clinical validation involved 32 patients with brain metastases, comparing treatment plans recalculated at 1° and 2° resolutions.
• Key dosimetric parameters including target volume within planning target volume (TV_PIV), conformity index (CI_Paddick), and gradient index (GI) were analyzed.

Main Results:

• Phantom studies showed that a 1° angular resolution significantly reduced deviations in TV_PIV, Dmean, and GI for small targets at greater distances compared to 2° resolution.
• Clinical results corroborated phantom findings, with maximum TV_PIV and GI deviations of 2.76% and 0.65% for the smallest targets.
• TV_PIV was identified as the most sensitive dosimetric parameter affected by angular resolution (p=0.01).

Conclusions:

• A 1° angular resolution is recommended for VMAT-based SIMT SRS to enhance dosimetric accuracy, especially for small, distally located targets.
• The choice of angular resolution is critical for optimizing treatment planning in complex SRS cases.
• Further investigation into the clinical significance of minor dosimetric variations is warranted.