04 October 2017
This study investigates a large‐area plane‐parallel ionization chamber ( LAC) for measurements of dose‐area product in water ( DAP w) in megavoltage ( MV) photon fields.
Uniformity of electrode separation of the LAC ( PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high‐resolution micro CT. 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 2 field were acquired. A 6 MV calibration coefficient, N D ,w, LAC , was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAP w. Additional calibrations in 10 × 10 cm 2 fields at 6, 10, and 18 MV were performed.
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 2 field. N D ,w, LAC was 163.7 mGy cm −2 nC −1 and differs by +3.0% from the coefficient derived in the 10 × 10 cm 2 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV.
The LAC requires only small correction factors for DAP w measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm 2 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 w measurements.