Commissioning of a PTW 34070 large-area plane-parallel ionization chamber for small field megavoltage photon dosimetry.

PURPOSE

This study investigates a large-area plane-parallel ionization chamber (LAC) for measurements of dose-area product in water (DAP ) in megavoltage (MV) photon fields.

METHODS

Uniformity of electrode separation of the LAC (PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high-resolution microCT. Signal dependence on angle α of beam incidence for square 6 MV fields of side length s = 20 cm and 1 cm was measured in air. Polarity and recombination effects were characterized in 6, 10, and 18 MV photons fields. To assess the lateral setup tolerance, scanned LAC profiles of a 1 × 1 cm field were acquired. A 6 MV calibration coefficient, N , was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAP . Additional calibrations in 10 × 10 cm fields at 6, 10, and 18 MV were performed.

RESULTS

Electrode separation is uniform and agrees with specifications. Volume-averaging leads to a signal increase proportional to ~1/cos(α) in small fields. Correction factors for polarity and recombination range between 0.9986 to 0.9996 and 1.0007 to 1.0024, respectively. Off-axis displacement by up to 0.5 cm did not change the measured signal in a 1 × 1 cm field. N was 163.7 mGy cm nC and differs by +3.0% from the coefficient derived in the 10 × 10 cm 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV.

CONCLUSIONS

The LAC requires only small correction factors for DAP measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm fields, but beam incidence must be kept as close to normal as possible. Calibration in 10 × 10 fields is not recommended because of the LAC's over-response. The accuracy of relative point-dose measurements in the field's periphery is an important limiting factor for the accuracy of DAP measurements.