ESTRO 2024 - Abstract Book

S3332

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

measurements with an Exradin A1SL and Gafchromic EBT-XD film dosimetry, in the Cheese phantom. The diode array SRS MapCheck and associated StereoPHAN phantom (Sun Nuclear) are suitable devices for SRS/SBRT PSQA but they are usually used in conventional linear accelerators and the calibration procedures described in the manual do not include Tomotherapy. The aim of this study was to adapt both array and absolute dose calibration procedures to Tomotherapy and validate its clinical use.

Material/Methods:

For SRS MapCheck absolute dose calibration, a specific static field irradiation procedure has been created to mimic the 5x5cm2 field with 200 MU used in a conventional linac. Point dose measurements were performed with a calibrated PTW 31016 PinPoint ionization chamber placed at the centre of StereoPHAN phantom. SRS MapCheck was irradiated with the same procedure and the dose determined with the ionization chamber was assigned. Array calibration was performed in two steps: i) SRS MapCheck was positioned in the corresponding calibration fixture and irradiated in a TrueBeam STx with a flattened 6MV beam in anterior and posterior directions ii) SRS MapCheck was inserted in the StereoPHAN phantom and was irradiated in Tomotherapy with a 5x5cm2 field in anterior and posterior directions. Different tests have been designed to assess the diodes’ response equalization and dose calibration. The StereoPHAN phantom and SRS MapCheck were irradiated with anterior, lateral and oblique incidences. A helical IMRT plan was also created and delivered. The dose measured with SRS MapCheck was compared with the dose calculated in the Precision 2.0.1.1 treatment planning system. For validation, 10 clinical SRS plans already treated in Tomotherapy have been re-irradiated using SRS MapCheck and a PTW 31016 PinPoint, both in the StereoPHAN phantom. The results have been compared with the previous results of film and A1SL chamber in the Cheese phantom.

Results:

The array and absolute dose calibration were successfully adapted for Tomotherapy and verified through the irradiation of different beam angle configurations and a rotational plan. Differences to the treatment planning system calculated dose were within 1.5%. For clinical cases, the obtained gamma passing rates for SRS MapCheck were on average 99.9%±0.3% with 2% of maximum dose difference, 2 mm distance to agreement and 10% threshold criteria. For PinPoint measurements the mean dose difference was 1.1%±1.7%. These results are in close agreement with the ones previously obtained for film dosimetry and A1SL chamber measurements, considering the corresponding associated uncertainty.

Conclusion:

The SRS MapCHECK diode array in the StereoPHAN phantom after proper calibration has proved to be a useful tool for PSQA in Tomotherapy, efficiently replacing other more labour intensive verification methods.

Keywords: SRS/SBRT, patient specific QA, tomotherapy