ESTRO 2024 - Abstract Book

S3319

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

The dose-rate and field size dependence of the 1600 SRS detector array were quantified during initial commissioning. PSQC was then performed on Varian SRS Hyperarc ® and SABR plans which had been previously measured with Gafchromic EBT3 ® film and a PinPoint ionisation chamber, and were found to be in good agreement. PSQC was carried out twice for 8 different SABR plans and 20 SRS Hyperarc ® plans (17 single lesion and 3 multi lesion), once with the manufacturer’s recommended calibration procedure using a standard field size of 4 × 4 cm 2 and at the default (maximum) dose-rate [2], and a second time with a plan specific calibration procedure described by Decabooter et al [1]. This procedure recommends the device is calibrated using the average dose-rate of the treatment plan and a field size representative of the plan MLC apertures. Measured 3D dose distributions were compared with Eclipse calculated dose distributions using PTW Verisoft (v8.1) with gamma parameters of 5%, 1 mm applied throughout. Initial array testing showed a dose rate dependency of up to 7.1% for the 10FFF beams over a range 400 – 2400 MU/min. A field size dependency of 7.4% was noted when testing over a range of 1 – 10 cm 2 . With the device calibrated using the default dose-rate and a 4 x 4 cm 2 field, PSQC of SRS and SABR plans resulted in poor γ-index pass rates as low as 36% for a 10FFF single lesion brain plan. Inspection of the points of failure in PSQC results indicated that a dose difference rather than a distance to agreement was the root cause; the poorest results being on plans which had a materially lower average dose rate compared to the default dose-rate. By following the plan specific calibration procedure using the plan’s average dose-rate and a field size of 2 x 2 cm 2 the γ-index pass-rates for these plans were all ≥ 97%. This was in good agreement when compared with the ionisation chamber and film gold standard. Results:

Conclusion:

A plan specific calibration procedure provided a clinically acceptable solution to PSQC dosimetry with a PTW Octavius ® 1600 SRS detector array used with the 4D modular rotation phantom. Acceptable γ-index pass-rates were obtained for the PSQC of SABR and SRS Hyperarc ® plans using this adapted cross-calibration method. We have shown that this device can be successfully used for the PSQC of single and multi-lesion SRS and complex SABR plans.

This PSQC solution has now been in clinical use for 18 months.

Keywords: PSQC, PSQA, doserate

References:

[1] Operation and calibration of the novel PTW 1600 SRS detector for the verification of single isocenter stereotactic radiosurgery treatments of multiple small brain metastases. Esther Decabooter et al. Department of Radiation Oncology (Maastro clinic), GROW School for Oncology, Maastricht University Medical Centre, Maastricht, The Netherlands. Br J Radiol;94:20210473.