Evaluating dose coverage and conformity in stereotactic ablative body radiotherapy (SABR) plans

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Current conformity metrics for stereotactic ablative body radiotherapy (SABR) have limitations.
• Accurate assessment of dose prescription and gradient near the planning target volume (PTV) edge is crucial for effective treatment.
• Automation of these checks can improve efficiency and consistency in radiotherapy planning.

Purpose of the Study:

• To develop an automated spatial assessment methodology for evaluating dose prescription and dose gradient from the PTV edge in SABR.
• To introduce a novel metric, the 'Internal PTV contour' distance, for assessing PTV size and shape relative to dose distribution.
• To establish baseline data and automation acceptability criteria using a cohort of SABR patients.

Main Methods:

• A Python-based script was developed to calculate linear distances from the PTV edge to specified isodose lines.
• Measurements were taken every 15 degrees on axial slices and along the central axis in the coronal plane.
• A new metric, 'Internal PTV contour' distance, was introduced and analyzed in 134 SABR patients.

Main Results:

• Median distances from the PTV edge to the 100% and 90% isodose lines were quantified in both axial and coronal planes.
• Dose gradient criteria were established for lung and non-lung SABR using a polynomial fit of median distance versus 'Internal PTV contour'.
• Acceptability criteria were defined as Median Distance + 1 mm for the 80% isodose and Median Distance + 2 mm for the 50% isodose.

Conclusions:

• The developed in-house script provides a high-quality, automated spatial assessment of PTV dose coverage and gradient.
• The novel 'Internal PTV contour' distance metric demonstrates a strong correlation with dose gradient.
• This methodology enhances the accuracy and automation of quality assurance in SABR planning.