EUD-based margin selection in the presence of set-up uncertainties

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Biology

Background:

• Geometric uncertainties, including systematic and random errors, significantly impact radiation therapy treatment plan design.
• Accurate modeling of clinical target volumes (CTV) and organs at risk (OAR) is crucial for effective treatment planning.

Purpose of the Study:

• To assess the impact of geometric uncertainties on radiation therapy treatment plan design.
• To evaluate the effectiveness of margin adjustments and dose escalation in mitigating these uncertainties.

Main Methods:

• Numerical simulations were performed incorporating systematic and random errors.
• A four-field conformal treatment plan was designed in a cubic phantom with a CTV and abutting OAR.
• Variations in field size, prescribed dose, and systematic set-up uncertainty were analyzed using dose-volume histograms (DVH) and equivalent uniform dose (EUD).

Main Results:

• A 10 mm margin (CTV-Planning Target Volume) was adequate for CTV dose distribution integrity under considered systematic uncertainties.
• Reducing margins without reducing set-up errors necessitated increased isocenter dose to compensate for EUD loss.
• A 3.5 Gy dose increase on a 6 mm-margin plan achieved equivalent CTV EUD while reducing OAR EUD by 1 Gy under specific uncertainty conditions (2 mm random, 4 mm systematic).

Conclusions:

• Directional sensitivity to geometric uncertainties and required margin sizes depend on beam geometries and relative structure positions.
• Modest dose escalation, supported by the EUD concept, can lead to plans that better achieve clinical objectives.
• A plan quality index like EUD5% facilitates meaningful statistical comparisons between different treatment strategies.