Commissioning small fields in lung using Monte Carlo corrected film measurements

Area of Science:

• Medical Physics
• Radiotherapy
• Dosimetry

Background:

• Current radiotherapy commissioning often measures dose in solid water, not directly in lung tissue.
• Accurate dose characterization in lung material is vital for lung stereotactic body radiotherapy (SBRT) uncertainty analysis.
• Film dosimetry in lung phantoms requires corrections for precise dose measurement.

Purpose of the Study:

• To develop and present a method for correcting film measurements in lung material for SBRT commissioning.
• To improve the accuracy of dose delivered in lung tissue compared to treatment planning system (TPS) calculations.
• To establish a generic approach for correcting small field lung measurements using Monte Carlo simulations.

Main Methods:

• Monte Carlo (MC) simulations were used to derive medium-dependent correction factors (k_med).
• Simulations accounted for differences between film calibration in water and measurement in lung material at 6 and 10 MV.
• A novel approach applies corrections as a function of isodose level for a nominal field size, simplifying the process.

Main Results:

• Central axis dose agreement improved across all field sizes, with a 7% enhancement for a 6MV 1x1 cm^2 field.
• Position-dependent corrections significantly improved gamma assessment passing rates (5%/1mm and 3%/1mm).
• Uniform central axis corrections showed minimal improvement in passing rates.

Conclusions:

• MC simulations provide a validated method for accurate dose correction in lung materials during radiotherapy.
• The presented generic, isodose-level-based correction approach enhances the accuracy of lung SBRT commissioning.
• This method allows for more reliable comparisons between film measurements and TPS dose calculations in heterogeneous lung environments.