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

Abstract

PurposeThis 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.MethodsUniformity 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 DAP w. Additional calibrations in 10 × 10 cm2 fields at 6, 10, and 18 MV were performed.ResultsElectrode 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.ConclusionsThe 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 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 DAP w measurements.

Technical Note: Comprehensive evaluation and implementation of two independent methods for beam monitor calibration for proton scanning beam

“…As shown in this work, the heterogeneity correction factor for BPC2 is 0.978, which means that the DAP directly measured by BPC2 would be subject to a 2.2% systematic deviation if the chamber response heterogeneity was ignored. This systematic deviation is consistent with that reported in recent publications …”

“…This over‐response may be due to the fact that the size of the guard ring is actually larger by 0.4 mm than the specified value of 40.8 mm. Kupfer et al also observed a similar overresponse at the edge of BPC, and argue that this is due to insufficient width of the guard ring. When the BPC is calibrated using a large field, this over‐response must be taken into account.…”

“… is the beam quality correction factor for the FC for the reference proton field, which is available from IAEA TRS‐398 . is the chamber response heterogeneity (CRH) correction factor to take into account the nonuniform response of BPC …”

“…Hence, BPC has been widely used in proton beam therapy physics. Recently BPC has also been used to determine the dose of the small field dosimetry in photon therapy …”

“…For such a large sensitive area BPC, the heterogeneity of both the calibration field and the chamber response over the large active area must be taken into account . It has been reported that more than 3% error could be introduced if BPC's response heterogeneity is not taken into account in the cross‐calibration …”