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 (DAPw ) 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 cm2 field were acquired. A 6 MV calibration coefficient, ND,w,LAC , was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAPw . Additional calibrations in 10 × 10 cm2 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 cm2 field. ND,w,LAC was 163.7 mGy cm-2 nC-1 and differs by +3.0% from the coefficient derived in the 10 × 10 cm2 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 DAPw measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm2 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 DAPw measurements.

Integral small field output factor measurements using a transmission ionisation chamber.

Previous studies describe the use of a large area parallel-plate chamber, the PTW Bragg Peak chamber, for measuring dose-area product (DAP) and output factors in small megavoltage photon fields. However, in radiotherapy departments without protons, this detector would have to be purchased separately for this purpose. This work investigated the feasibility of alternatively using a large transmission ionisation chamber, the IBA round Stealth chamber (SC), for output factor measurements of stereotactic fields. This type of detector is more commonly found in radiotherapy departments as a reference chamber for water tank scanning of small fields, and hence DAP could be performed without an additional purchase. The SC's large sensitive area (diameter of 94 mm) measures the integral dose, also known as DAP, over the whole two-dimensional (2D) dose distribution of the small field. The measurements were performed using a 6 MV beam from an Elekta Infinity linear accelerator. Conversion of DAP to central axis point dose was performed using 2D dose maps from Gafchromic EBT3 films. The field sizes measured ranged from side length of 5 mm to 50 mm (all square). The resultant output factors were compared against measurements with a stereotactic diode. The small field output factors measured using SC + film were in good agreement with the stereotactic diode (within 2% for field sizes as small as 6 mm; 3% difference at 5 mm). The new proposed method showed that a transmission chamber like SC is a good alternative large-area parallel plate chamber to measure DAP and derive small field OFs. Furthermore, the feasibility of using 2D reconstructed dose maps from water tank profiles and hence filmless approach was investigated. Results showed that filmless conversion of DAP to central axis point dose is feasible using profiles. However, a large number of profiles are required (i.e. 15° increments (star pattern) are required for accurate 2D dose reconstruction), and hence the water tank scanning for this approach may be prohibitively time-consuming.