A virtual source model for Monte Carlo simulation of helical tomotherapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Current helical tomotherapy Monte Carlo (MC) simulations rely on full MC models requiring phase-space files (PSFs) for patient dose calculation.
• Calculating PSFs involves extensive modeling of the radiation source, jaws, and multileaf collimator (MLC), increasing computational complexity.
• A need exists for a more efficient and accessible MC simulation method for helical tomotherapy.

Purpose of the Study:

• To present a novel Monte Carlo (MC) simulation method utilizing a virtual source model (VSM) for helical tomotherapy.
• To enable accurate patient dose calculation without the necessity of generating phase-space files (PSFs).
• To validate the VSM against experimental measurements and established full MC models.

Main Methods:

• Developed a virtual source model (VSM) based on gold-standard beam data from a tomotherapy treatment planning station (TPS).
• Extracted TPS-generated sinograms from patient XML files to create fluence maps incorporating leaf open time, leaf filter, jaw penumbra, and leaf latency.
• Validated the VSM across diverse geometries, heterogeneous media, and delivery quality assurance (DQA) scenarios.

Main Results:

• VSM commissioning showed <1% agreement between measured and simulated percent depth doses (PDDs) and open beam profiles.
• The VSM leaf filter model accuracy was confirmed with a Picket Fence pattern, showing agreement with measurements.
• For heterogeneous phantoms, VSM achieved <2% agreement with a published full MC model; DQA plans for head and neck cases met 98% gamma passing criteria (2%/2 mm).

Conclusions:

• The VSM-based MC simulation method is feasible for helical tomotherapy, leveraging existing TPS beam models.
• The VSM demonstrates high accuracy, validated against measurements in homogeneous and heterogeneous media.
• This approach offers a computationally efficient and accurate alternative for patient dose calculation in helical tomotherapy.