Sensitivity of postplanning target and OAR coverage estimates to dosimetric margin distribution sampling parameters

Area of Science:

• Radiation oncology
• Medical physics
• Image-guided therapy

Background:

• Dosimetric margin (DM) and dosimetric margin distribution (DMD) are crucial for accounting for geometric uncertainties in radiation therapy.
• Coverage (Q) quantifies the probability of achieving desired dose metrics, essential for post-planning evaluation of treatment plans.
• Accurate coverage estimation is vital as static margins may not fully capture the impact of interfraction and intrafraction uncertainties.

Purpose of the Study:

• To evaluate coverage probabilities for 28 prostate cancer treatment plans.
• To determine optimal dosimetric margin distribution (DMD) sampling parameters for accurate post-planning coverage estimates.
• To assess the impact of sampling techniques (angular vs. isotropic) on coverage accuracy.

Main Methods:

• Interfraction setup uncertainties were simulated for 28 prostate cancer plans (79.2 Gy prescribed dose, 10 mm CTV-to-PTV margin).
• Dosimetric margins for CTV, bladder, and rectum were calculated using angular and isotropic sampling techniques with varying angular intervals (ω) and radial step sizes (δ).
• Coverage estimation accuracy (ΔQ) was quantified as a function of sampling parameters and technique.

Main Results:

• Coverage estimation accuracy is dependent on sampling parameters (ω, δ) and technique.
• Achieving target ΔQ < 1% and OAR ΔQ < 3% requires ω = 20°, δ = 1 mm.
• Higher accuracy (target ΔQ < 0.5%, OAR ΔQ ≈ 1%) is obtained with ω = 10°, δ = 0.5 mm.
• Isotropic sampling demonstrated better accuracy than fixed angular sampling as the number of sampling points decreased.

Conclusions:

• Post-planning coverage estimates are essential for accurate assessment of geometric uncertainties.
• Finer sampling of the dosimetric margin distribution (DMD) leads to more precise coverage evaluations.
• DMD sampling with ω = 10° and δ = 0.5 mm is recommended for adequate planning purposes in prostate cancer radiotherapy.