In-vivo quality assurance of dynamic tumor tracking (DTT) for liver SABR using EPID images.

Abstract

To assess dynamic tumor tracking (DTT) target localization uncertainty for in-vivo marker-based stereotactic ablative radiotherapy (SABR) treatments of the liver using electronic-portal-imaging-device (EPID) images. The Planning Target Volume (PTV) margin contribution for DTT is estimated. Phantom and patient EPID images were acquired during non-coplanar 3DCRT-DTT delivered on a Vero4DRT linac. A chain-code algorithm was applied to detect Multileaf Collimator (MLC)-defined radiation field edges. Gold-seed markers were detected using a connected neighbor algorithm. For each EPID image, the absolute differences between the measured center-of-mass (COM) of the markers relative to the aperture-center (Tracking Error, (ET )) was reported in pan, tilt, and 2D-vector directions at the isocenter-plane. An acrylic cube phantom implanted with gold-seed markers was irradiated with non-coplanar 3DCRT-DTT beams and EPID images collected. Patient Study: Eight liver SABR patients were treated with non-coplanar 3DCRT-DTT beams. All patients had three to four implanted gold-markers. In-vivo EPID images were analyzed. Phantom Study: On the 125 EPID images collected, 100% of the markers were identified. The average±SD of ET were 0.24±0.21, 0.47±0.38, and 0.58±0.37mm in pan, tilt and 2D directions, respectively. Patient Study: Of the 1430 EPID patient images acquired, 78% had detectable markers. Over all patients, the average±SD of ET was 0.33±0.41mm in pan, 0.63±0.75mm in tilt and 0.77±0.80mm in 2D directions The random 2D-error, σ, for all patients was 0.79mm and the systematic 2D-error, Σ, was 0.20mm. Using the Van Herk margin formula 1.1mm planning target margin can represent the marker based DTT uncertainty. Marker-based DTT uncertainty can be evaluated in-vivo on a field-by-field basis using EPID images. This information can contribute to PTV margin calculations for DTT.