Practical robustness evaluation in radiotherapy - A photon and proton-proof alternative to PTV-based plan evaluation

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Planning

Background:

• The static dose cloud approximation in photon therapy planning is insufficient for intensity-modulated proton therapy due to range errors.
• Existing methods do not adequately account for uncertainties in proton beam range.

Purpose of the Study:

• To develop a robust evaluation method applicable to both photon and proton therapy.
• To create a PTV-less approach consistent with historical PTV-based evaluations.

Main Methods:

• A multi-scenario simulation was used, extending 3D probability distributions to 4D to include range errors.
• Voxel-wise minimum and maximum dose distributions were calculated to assess CTV coverage and hot spots.
• Acceptance criteria were calibrated against PTV-based criteria from historical photon plans.

Main Results:

• CTV D98 and voxel-wise minimum dose strongly correlated with scenario average D98 (R² > 0.99).
• Voxel-wise minimum dose effectively visualized CTV coverage, aligning with PTV-based photon therapy evaluations.
• Robustness criteria showed strong correlation with PTV D98 and D2, requiring minimal corrections.

Conclusions:

• A practical, PTV-less robustness evaluation method was developed and implemented for photon and proton therapy.
• The method provides accurate dose assessment without the static dose cloud approximation.
• This approach ensures consistency with PTV evaluations while improving accuracy for proton treatments.