Use of a novel two-dimensional ionization chamber array for pencil beam scanning proton therapy beam quality assurance

Abstract

The need to accurately and efficiently verify both output and dose profiles creates significant challenges in quality assurance of pencil beam scanning (PBS) proton delivery. A system for PBS QA has been developed that combines a new two-dimensional ionization chamber array in a waterproof housing that is scanned in a water phantom. The MatriXX PT has the same detector array arrangement as the standard MatriXX(Evolution) but utilizes a smaller 2 mm plate spacing instead of 5mm. Because the bias voltage of the MatriXX PT and Evolution cannot be changed, PPC40 and FC65-G ionization chambers were used to assess recombination effects. The PPC40 is a parallel plate chamber with an electrode spacing of 2mm, while the FC65-G is a Farmer chamber FC65-G with an electrode spacing of 2.8 mm. Three bias voltages (500, 200, and 100 V) were used for both detectors to determine which radiation type (continuous, pulse or pulse-scanned beam) could closely estimate Pion from the ratios of charges collected. In comparison with the MatriXX(Evolution), a significant improvement in measurement of absolute dose with the MatriXX PT was observed. While dose uncertainty of the MatriXX(Evolution) can be up to 4%, it is < 1% for the MatriXX PT. Therefore the MatriXX(Evolution) should not be used for QA of PBS for conditions in which ion recombination is not negligible. Farmer chambers should be used with caution for measuring the absolute dose of PBS beams, as the uncertainty of Pion can be > 1%; chambers with an electrode spacing of 2 mm or smaller are recommended.

Figure 1

Setup picture of DigiPhant phantom with beam coming from the left direction and the detector array moving in four locations within the water tank.

Figure 2

Comparison of proton beam depth dose curve in water obtained using a Zebra multilayer ionization chamber with that obtained using the MatriXX PT and Evolution in the DigiPhant water phantom. The buildup material thickness corrections of 6.2 mm and 3.1 mm were provided by IBA dosimetry.

Figure 3

Percent difference of MatriXX PT and Evolution measured dose from the prescribed dose (10, 1, and 0.2 Gy equivalent at 16 cm depth) for depths of 5, 16, 18, and 19 cm of a pencil beam scanning beam that scans a cube with range of 20 cm, modulation of 8 cm, and field size of 10 cm.

Figure 4

Output measured using the MatriXX PT in Solid Water relative to that in water, at depths of 10 and 27 cm, for PBS beams with a range of 12 cm and modulation of 4 cm, and a range of 32 cm and modulation of 10 cm, respectively. Scanning field sizes of 10 cm and 20 cm were used for both depths.

Figure 5

Ionization collection efficiency of Farmer chamber FC65‐G and parallel chamber PPC40 used to measure the absolute dose of MatriXX PT. Q1/Q2 is the ratio of collected charge under bias voltage of 500 V to that collected under bias voltages of 200 V or 100 V. Measurements are performed at the depth of 16 cm of a pencil beam scanning beam that scans a cube with range of 20 cm, modulation of 8 cm, and field size of 10 cm. Data points indicate ${\mathrm{P}}_{\text{ion}}$ determined from the measured Q1/Q2 ratio using the continuous and pulse models. The curves (dash and solid curves) are calculated ${\mathrm{P}}_{\text{ion}}$ from the charge ratio collected under bias voltages between 500 V and 200 V, and between 500 V and 100 V, based on the formula from Weinhous et al., ⁽¹³⁾ using continuous (red and shortened as C) and pulsed radiation types (green and shortened as P).