Fluence-corrected pencil beam algorithm for carbon ion therapy patient quality assurance in solid phantoms

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Imaging

Background:

• Solid phantoms simplify carbon ion therapy quality assurance (QA) but introduce dose deviations due to algorithms calculating dose in water.
• Accurate patient-specific QA is crucial for effective carbon ion therapy, particularly at Heavy Ion Medical Machine (HIMM) centers.

Purpose of the Study:

• To develop and validate a fluence-corrected pencil beam algorithm (PBA) for accurate dose calculation in solid phantoms.
• To enable precise patient-specific QA directly within solid phantoms, overcoming limitations of water-based calculations.

Main Methods:

• Modeled carbon ion beams using TOPAS Monte Carlo simulations to match clinical commissioning data.
• Derived energy- and depth-dependent fluence correction (FC) factors (FCFs) from simulated depth-dose data in solid phantoms and water.
• Validated FCFs experimentally and integrated them into the PBA for QA workflow comparisons.

Main Results:

• Simulated FCFs increased with energy and depth, showing good agreement with experimental measurements (within ±0.5%).
• Gamma pass rates significantly improved for depths > 100 mm using the fluence-corrected PBA (workflows 2-4), reaching up to 99% under 2 mm/2% criteria.
• Workflow 2 (phantom CT with FC) and workflow 4 (virtual CT with phantom SPR and FC) yielded comparable results.

Conclusions:

• Fluence correction factors (FCFs) are essential for carbon ion therapy QA in solid phantoms, particularly beyond 100 mm depth.
• The developed fluence-corrected PBA or a simplified correction table can be used on virtual phantoms for efficient QA.
• This advancement supports easier and more accurate patient-specific QA in carbon ion therapy.