Abstract
MATERIALS AND METHODS
Several modifications to our existing approach were implemented to make the method applicable to VMAT: (i) gantry angle-resolved data acquisition, (ii) calculation of the patient transmission, (iii) compensation for detector 'flex' and (iv) 3D dose reconstruction and evaluation.
CONCLUSIONS
Our portal dosimetry method was successfully adapted for verification of VMAT treatments, pre-treatment as well as in vivo.
RESULTS
Planned and EPID-(Electronic Portal Image Detector)-reconstructed dose distributions show good agreement for pre-treatment verification of two prostate, a stereotactic lung and a head-and-neck VMAT plan and for in vivo verification of VMAT treatments of prostate and lung cancer. Averaged over pre-treatment verifications, planned and measured isocentre dose ratios were -1.2% (range [-4.7%,1.8%]). 3D gamma analysis (3% maximum dose, 3mm) revealed mean gamma gamma(mean)=0.37 [0.34,0.39], maximum 1% gamma gamma(1%)=0.72 [0.66,0.81] and percentage of points with gamma1 P(gamma)(1)=99% [97%,100%]. For in vivo verification, the average isocentre dose ratio was -1.2% [-0.8%,-1.7%], gamma(mean)=0.52 [0.40,0.64], gamma(1%)=0.92 [0.76,1.08] and P(gamma)(1)=96% [93%,100%].
BACKGROUND AND PURPOSE
To demonstrate the feasibility of back-projection portal dosimetry for accurate 3D dosimetric verification of volumetric-modulated arc therapy (VMAT), pre-treatment as well as in vivo.