Direct megavoltage photon calibration service in Australia

Characterisation of a synthetic diamond detector for end-to-end dosimetry in stereotactic body radiotherapy and radiosurgery

BACKGROUND AND PURPOSE

Synthetic diamond detectors offer real time measurement of dose in radiotherapy applications which require high spatial resolution. Additional considerations and corrections are required for measurements where the diamond detector is orientated at various angles to the incident beam. This study investigated diamond detectors for end-to-end testing of Stereotactic Body Radiotherapy (SBRT) and Stereotactic Radiosurgery (SRS) in the context of dosimetry audits.

MATERIAL AND METHODS

Seven individual diamond detectors were investigated and compared with respect to warm up stability, dose-rate dependence, linearity, detector shadowing, energy response, cross-calibration, angular dependence and positional sensitivity in SBRT and SRS.

RESULTS

Large variation in the cross calibration factors was found between the seven individual detectors. For each detector, the energy dependence in the cross calibration factor was on average <0.6% across the beam qualities investigated (Co-60 Gamma Knife, and MV beams with TPR20,10 0.684-0.733). The angular corrections for individual fields were up to 5%, and varied with field size. However, the average angular dependence for all fields in a typical SRS treatment delivery was <1%. The overall measurement uncertainty was 3.6% and 3.1% (2σ) for an SRS and SBRT treatment plan respectively.

CONCLUSION

Synthetic diamond detectors were found to be reliable and robust for end-to-end dosimetry in SBRT and SRS applications. Orientation of the detector relative to the beam axis is an important consideration, as significant corrections are required for angular dependence.

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.