FLASH dose rate calculation based on log files in proton pencil beam scanning therapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Radiation therapy can cause side effects due to unnecessary irradiation of healthy tissues.
• FLASH radiotherapy, using ultra-high-dose-rate beams, shows potential for normal-tissue sparing.
• Accurate dosimetry is essential to confirm the mean and instantaneous dose rates of FLASH beams.

Purpose of the Study:

• To develop and verify a dosimetry method for measuring 2D/3D dose distributions and dose rates in FLASH radiotherapy.
• Utilize machine log files from built-in monitor chambers for dose and dose rate calculations.
• Enable stable and accurate measurement of average and instantaneous dose rates for FLASH effect verification.

Main Methods:

• Developed a mini-ridge filter for creating spread-out Bragg peaks (SOBP) with 3D printing.
• Created proton pencil beam line scanning plans for various field sizes.
• Calculated dose and average dose rates using direct and Monte Carlo (MC) simulation methods based on machine log files.
• Measured absorbed dose with an ionization chamber for comparison.
• Calculated instantaneous dose rates using MC simulation with 5 ms temporal resolution.

Main Results:

• Dose differences were within ±3% for most cases using direct (10/12) and MC (9/11) methods compared to ionization chamber measurements.
• Dose rate differences were within ±3% for most cases using direct (9/12) and MC (8/11) methods.
• MC simulation revealed significant instantaneous dose rate fluctuations (4.29–163 Gy/s) with a mean of 62 Gy/s.

Conclusions:

• Successfully developed methods using machine log files for calculating dose and dose rates in FLASH radiotherapy.
• Demonstrated the feasibility of verifying delivered FLASH beams using these dosimetry methods.
• These methods are crucial for advancing the understanding and application of FLASH radiotherapy.