ESTRO 2024 - Abstract Book

S3411

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

composite verification method, as recommended by the AAPM report TG-218. Angular dependence corrections for detector’s angular response were performed

Ten single ‐ isocenter multiple brain metastasis VMAT plans were elaborated by TPS Elekta Monaco® 6.1.2 and delivered by Elekta VersaHD Linac or a total of 48 targets. Dose prescription ranged between 18 and 20 Gy for each target and treatments were delivered in single fraction. The TPS Monaco strategy of high and low dose modulation was adopted for planning by combining high dose modulation to ensure target coverage with low dose modulation to give steeper dose gradients and fast delivery. The modulation degree of each plan was provided by the TPS Monaco according to the formula:

Modulation Degree = Total MU / [Sum of (Segment Area x Segment Mu)/Total Beam Area]

The agreement between measured and TPS dose distributions was evaluated using myQA® Platform software to calculate the Gamma Index with a global absolute dose difference of 3% and a distance-to-agreement of 1 mm criteria. No software interpolation of measurement results was necessary since the pixels are contiguous and there is no spacing between them. Tolerance and action limits were established, by using the Statistical control process (SPC) approach, according to the AAPM report TG-218 recommendations.

Results:

The Gamma Index results for the automatically co-registered planned and measured dose maps comparison showed a Passing rates ranged from 94.0% to 99.5%. Tolerance and action limits resulted equal to 95.7 % and 90.8% respectively, with no cases failing outside the control limit. The evaluated limits were in agreement with the universal values suggested by the TG 218. The modulation degrees of the evaluated plans were in the range 3.1 to 5.9.

Conclusion:

The IBA myQA® SRS detector analyzed in this study characterized by real time readout, small sensitive volume of the solid-state sensors and “film like” resolution, resulted to be suitable for single ‐ isocenter multiple brain metastasis SRS PSQA in an easy-to-use equipment, When the stricter criteria of 3%G/1mm for SRS is adopted, Tolerance and action limits evaluated for PSQA resulted in agreement with the universal values suggested by the TG 218.

Keywords: patient specific QA, SRS, detector resolution