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Pencil beam scanning dose calibration at reduced source-to-axis distance
Nicolas Depauw1, Hanne Kooy1, Brian Winey1
1Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA.
Medical Physics
|May 8, 2022
Summary
Pencil beam scanning (PBS) in proton therapy requires careful consideration of beam angle (τ) to prevent systematic errors. Using gigaprotons (Gp) instead of monitor units (MUs) standardizes data and improves accuracy.
Area of Science:
- Medical Physics
- Radiation Oncology
- Particle Therapy
Background:
- Pencil beam scanning (PBS) utilizes ionization signals (MUs or Gp) for dose calculations.
- Beam deflection by the nozzle at a source-to-axis distance (τ) can cause systematic errors if not accounted for.
Purpose of the Study:
- To analyze the impact of varying source-to-axis distances (τ) on PBS proton therapy.
- To address pathlength changes, dose uniformity, Monte Carlo (MC) simulations, and pencil beam algorithms (PBA).
- To investigate beam shape alterations from circular to oval projections.
Main Methods:
- Description of coordinate systems for PBS delivery.
- Analysis of beam pathlength and dose distribution based on τ.
- Evaluation of Gp and MU in MC simulations and PBAs.
Main Results:
- Intermediate-τ machines (e.g., 59–120 cm for 24x24 cm² fields) require pathlength corrections in MC simulations and PBAs.
- Small-τ machines necessitate corrections for beam shape projection onto voxels.
- Specific τ ranges are defined for intermediate and small machines based on field size.
Conclusions:
- Identifying and correcting for pencil beam angle effects is crucial for optimizing proton therapy gantry design.
- Standardizing data interchange with gigaprotons (Gp) reduces systematic errors associated with beam angle.
