Denoising proton reference dosimetry spectrum using a large area ionization chamber-physical basis and type A...

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Pencil beam scanning proton therapy can experience dose fluctuations due to intra-spill spot positional drift.
• Noisy reference dosimetry measurements against energy can lead to errors in monitor unit calibration.

Purpose of the Study:

• To investigate the impact of smoothing reference dosimetry measurements on type A uncertainty.
• To assess the effectiveness of fitting techniques in reducing dosimetry noise.

Main Methods:

• Reference dosimetry (Dw/MU) measured with a PTW 34045 chamber across 98 energy layers over five days.
• Simultaneous measurements using a PTW 34089 large area ionization chamber (LAIC) for single spot irradiations (MspotLAIC).
• Linear and quadratic fitting of Dw/MU and MspotLAIC data to create a de-noised reference dosimetry curve.

Main Results:

• Repeatability of reference dosimetry measurements showed up to 2.3% relative difference.
• Linear and quadratic fits demonstrated high correlation (R² > 0.95).
• De-noised curve had lower maximum type A uncertainty (0.69%) but higher average uncertainty (0.50%) compared to measurements (0.88% and 0.43%, respectively).

Conclusions:

• Fitting LAIC charges to the reference dosimetry curve provides a method to mitigate large errors in specific energy layers.
• While reducing peak uncertainty, the smoothing technique increases the average type A uncertainty across all energies.
• The fitted reference dosimetry curve should be applied cautiously in clinical practice.