Adaptive margins for online adaptive radiotherapy.

Area of Science:

• Radiation Oncology
• Medical Physics
• Image-Guided Therapy

Background:

• Conventional radiotherapy uses constant planning target volume (PTV) margins, which may not account for daily anatomical changes.
• Online adaptive radiotherapy (ART) allows for daily plan adjustments based on updated imaging and contouring.

Purpose of the Study:

• To introduce and evaluate a novel method for updating radiotherapy margins dynamically each fraction.
• To explore the benefits of adaptive margins based on accumulated dose and patient treatment history.

Main Methods:

• Developed a methodology for implementing adaptive isotropic and anisotropic margins during treatment.
• Simulated treatment histories (up to 20 fractions) with varying systematic/random errors and dose distributions.
• Compared adaptive margins against standard constant margins using the van Herk approach.

Main Results:

• Adaptive margins reduced population mean margins to 70% (isotropic) and 55% (anisotropic) of constant margins.
• Margin reduction was greater with longer fractionation and inhomogeneous dose distributions, primarily by mitigating systematic errors.
• Adaptive margins improved the minimum CTV dose for patients receiving underdosing, benefiting up to 10% of cases.

Conclusions:

• Adaptive margins offer a significant reduction in PTV margins for most patients in online ART.
• This approach can improve dose coverage and potentially reduce toxicity without compromising safety.
• Dynamic margin adaptation is a promising strategy for optimizing radiotherapy delivery.