Recombination effects on the liquid-filled ionization chamber array used in VMAT quality control
PH-0048
Abstract
Recombination effects on the liquid-filled ionization chamber array used in VMAT quality control
Authors: SOLÍS PRECIADO|, Noelia(1)*[noeliasolispreciado@gmail.com];Portas Ferradás|, Breixo Carmelo(1);Niebla Piñero|, Abel(1);Merino Gestoso|, José Ángel(1);Jiménez Vegas|, Diego(1);Alberto|, González Fernández(1);
(1)Hospital Universitario Nuestra Señora de Candelaria, Radiofísica, Santa Cruz de Tenerife, Spain;
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Purpose or Objective
VMAT quality assurance measurements are performed with a liquid-filled ionization chambers array because it allows a higher resolution than air-filled devices. However, it is accompanied by reduced collection efficiencies because of ion recombination effects. Ion recombination depends on the dose per pulse and pulse repetition frequency.
This work is aimed to characterize the variability in PTW Octavius 1000 SRS array efficiencies with pulse dose and pulse frequency, and to correct subsequently quality assurance measurements.
Material and Methods
This study was made in a Versa HD (Elekta) linear accelerator with 6 MV FFF and 10 MV FFF beams with Octavius 1000 SRS array and a 0.125 PTW 31010 air-filled ionization chamber as reference. They both were exposed to different dose rates through the variation of solid water depth and source to detector distance. Python software was developed to perform a simple correction method using the measurements files from SRS and DICOM RT Plan files from TPS. From dose and time at each interval from SRS file, together with MU that were found from TPS file as a function of gantry angle, MU/min and pulse dose were calculated.
Results
A linear relationship was obtained between pulse dose and collection efficiencies, the latter increasing as the pulse dose decreased in all cases. These fittings got worse as the pulse repetition frequency decreased, probably as a result of an increasing uncertainty in measurements as the pulse frequency was reduced.
Pulse frequency did not reveal a so clear relationship with collection efficiency, although in a general way, efficiency increased as pulse frequency decreased (Figures 1 and 2).
Conclusion
Liquid-filled ionization chambers collection efficiencies increase the lower the pulse dose and pulse frequency are. These parameters vary during an VMAT delivery so they will influence the verification of the patient treatments. It can be significant in SRS and SBRT where unflattened beams are employed and tolerance levels are strict.
Efficiencies corrections make the SRS measurements more consistent with those of the TPS. However, uncertainties in the way the pulse dose is calculated with the developed software are not compatible with clinical treatments. Therefore, a new more accurate correction method would be necessary.