Mean Arc Distance (MAD): a quantity to compare trajectory 4πsampling in single target cranial stereotactic...

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Planning

Background:

• C-arm linac-based radiotherapy utilizes 4π methods for improved dose delivery.
• Simultaneous couch and gantry rotations aim to spare organs-at-risk (OARs) and enhance dose compactness.
• Current methods often rely on heuristics and trajectory restrictions, lacking a metric for sampling efficiency.

Purpose of the Study:

• To develop a quantitative metric for comparing 4π space sampling in radiotherapy trajectories.
• To assess this metric's utility as a surrogate for dosimetric compactness and OAR sparing.

Main Methods:

• Evenly spaced sampling points were distributed across a 4π sphere.
• The mean arc distance (MAD) metric was defined to quantify the average distance to the nearest field.
• MAD's relationship with isodose volume was analyzed across 2,047 radiotherapy plans, including DCA and VMAT techniques.

Main Results:

• All isodose volumes above 10% of prescription dose decreased exponentially with decreasing MAD.
• Increased 4π sampling, indicated by lower MAD, led to absolute isodose volume reductions, particularly for larger targets.
• Conversely, very low isodoses (0-10%) increased as MAD decreased.

Conclusions:

• Mean arc distance (MAD) effectively quantifies 4π sampling efficiency in radiotherapy.
• MAD serves as a valuable tool for comparing dynamic trajectories and predicting normal tissue sparing.
• This metric shows promise for optimizing cranial radiotherapy and potentially other treatment sites.