Long-term stability of a PTW 34070 large-area parallel ionization chamber in clinical proton scanning beams

Abstract

Purpose: In the modeling of beam data for proton therapy planning systems, absolute dose measurements are performed utilizing a Bragg peak chamber (BPC), which is a parallel-plate ionization chamber. The long-term stability of the BPC is crucial for ensuring accurate absolute dose measurement. The study aims to assess the long-term stability of the BPC in clinical proton pencil beam scanning delivery.

Methods: The long-term stability evaluation focused on the BPC-Type 34070 (PTW Freiburg, Germany), utilizing clinical proton scanning beams from December 2022 to November 2023. Monthly investigations were conducted to evaluate the response and cross-calibration factor of the BPC and a reference chamber, employing the spread-out Bragg peak (SOBP) field. Additionally, assessments were made regarding the BPC's response to monoenergetic beams, along with an examination of the impact of polarity and ion recombination on the BPC.

Results: The response and cross-calibration factor of the BPC varied up to 1.9% and 1.8%, respectively, while the response of the reference chamber remained within a 0.5% range. The BPC's response to the mono-energetic beams varied up to 2.0% across all energies, demonstrating similar variation trends in both the SOBP field and mono-energetic beams. Furthermore, the variations in polarity and ion recombination effect remained stable within a 0.4% range throughout the year. Notably, the reproducibility of the BPC remained high for each measurement conducted, whether for the SOBP field or mono-energetic beams, with a maximum deviation observed at 0.1%.

Conclusions: The response and cross-calibration factor of the BPC demonstrated significant variations, with maximum changes of 1.9% and 1.8%, respectively. However, the reproducibility of the BPC remained consistently high for each measurement. It is recommended that when conducting absolute dose measurements using a BPC, its response should be compared and corrected against the reference chamber for each measurement.

Keywords: Bragg peak chamber; calibration factor; long‐term stability; proton therapy; response.

FIGURE 1

Overview of setup configurations. The left illustrates the setup of the response and cross‐calibration factor between the BPC and reference chamber, Farmer ionization chamber (FC), and k _pol and k_s , to a SOBP field. The right illustrates the setup of the response of the BPC to mono‐energetic beams.

FIGURE 2

Temporal variation of the normalized FC and BPC response. Error bars indicate the standard deviation derived from the measurements, with results normalized relative to the values from Month 1.

FIGURE 3

Temporal variation of $N_{D,W,Q_{\mathrm{cross}}}^{\mathrm{BPC}}$. The central solid line represents the average value of $N_{D,W,Q_{\mathrm{cross}}}^{\mathrm{BPC}}$. The outer and inner dash lines are at ±1.0% and ±0.5% relative to the mean.

FIGURE 4

Temporal variation of relative dose of the BPC in the mono‐energetic beams. Error bars represent the standard deviations derived from the measurements. The doses were normalized to the values recorded in Month 1 for each energy level.

FIGURE 5

Temporal variations of k _pol and k_s of the BPC.