3D Dosimetric verification of volumetric-modulated arc therapy by portal dosimetry

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.

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.

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, 3 mm) revealed mean γ 〈γ_mean〉 = 0.37 [0.34,0.39], maximum 1% γ 〈γ_1%〉 = 0.72 [0.66,0.81] and percentage of points with γ ? 1 〈P_γ ? 1〉 = 99% [97%,100%]. For in vivo verification, the average isocentre dose ratio was −1.2% [−0.8%,−1.7%], 〈γ_mean〉 = 0.52 [0.40,0.64], 〈γ_1%〉 = 0.92 [0.76,1.08] and 〈P_γ ? 1〉 = 96% [93%,100%].

Conclusions

Our portal dosimetry method was successfully adapted for verification of VMAT treatments, pre-treatment as well as in vivo.