A Portal imager-based patient dosimetry system

Abstract

A technique for the in vivo dose verification of intensity modulated radiation therapy (IMRT) has been developed. An electronic portal image, calibrated in terms of absolute dose, is acquired for each radiation field following transmission through the patient at the time of treatment. For an IMRT field, the portal image signal is back-projected through a model of the patient in order to calculate the dose at the isocentric plane perpendicular to the beam central axis. The IMRT in vivo dose verification technique was adapted for volumetric modu- lated arc therapy (VMAT) treatments when a single dosimetric image is acquired over an arc. The patient dose along axis of gantry rotation can be directly related to the signal along the vertical axis of EPIs in integrated mode. In this novel VMAT in vivo dosimetry technique, the portal image signal is back-projected through a rotationally averaged model of the patient to calculate a 1D in vivo dose along the axis of gantry rotation. A research ethics board clinical study was approved and transmission portal images were acquired at regular intervals from human subjects. Portal image-derived isocenter point doses were in good agreement with treatment planning system (TPS) calculations for IMRT (mean difference δ=0.0%, standard deviation of the differences σ=4.3%) and VMAT (δ=1.1%, σ=1.7%). The one-dimensional (VMAT) and two-dimensional (IMRT) reconstructed doses were further analyzed by calculating mean dose differences and γ−evaluation pass-rates, which were also shown to be in good agreement with TPS calculations. The portal image-based in vivo dosimetry techniques were shown to be clinically feasible, with reconstruction times on the order of minutes for the first fraction and less than one minute for each fraction thereafter.