A pencil beam algorithm for intensity modulated proton therapy derived from Monte Carlo simulations.

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Biology

Background:

• Intensity modulated proton therapy (IMPT) requires high accuracy, especially in heterogeneous tissues.
• Pencil beam algorithms are crucial for IMPT dose calculations but face challenges with scatter and nuclear interactions.
• Spot weight optimization in IMPT can amplify positional and distortion errors of the Bragg peak.

Purpose of the Study:

• To develop and validate a pencil beam algorithm tailored for the accuracy demands of IMPT.
• To address challenges in dose calculation for heterogeneous geometries in IMPT.
• To improve the accuracy of proton therapy dose distributions by incorporating advanced corrections.

Main Methods:

• A pencil beam algorithm was developed and tuned for IMPT accuracy requirements.
• Multiple raytracing with fluence-weighted sub-spots was used for heterogeneity corrections.
• Monte Carlo simulations were performed to derive nuclear interaction corrections, including long-range products.
• Energy-dependent stopping power ratios and scatter from beamline accessories were implemented.
• The algorithm was benchmarked against Monte Carlo simulations for dose distribution accuracy.

Main Results:

• The algorithm achieved 3%/1 mm agreement with Monte Carlo simulations in simple heterogeneous phantoms.
• Principal shortcomings of pencil beam algorithms became evident in more complex phantom geometries.
• The impact of these limitations on IMPT dose distributions was demonstrated using clinical examples.

Conclusions:

• The developed pencil beam algorithm shows good accuracy in simple heterogeneous phantoms for IMPT.
• Limitations exist for complex geometries, highlighting areas for future algorithmic improvement in proton therapy.
• The study provides insights into the performance and limitations of pencil beam algorithms in clinical IMPT scenarios.