Kilovoltage intrafraction motion monitoring and target dose reconstruction for stereotactic volumetric modulated arc therapy of tumors in the liver

Purpose: To use intrafraction kilovoltage (kV) imaging during liver stereotactic body radiotherapy (SBRT) delivered by volumetric modulated arc therapy (VMAT) to estimate the intra-treatment target motion and to reconstruct the delivered target dose. Methods: Six liver SBRT patients with 2-3 implanted gold markers received SBRT in three fractions of 18.75 Gy or 25 Gy. CIV-to-PTV margins of 5 mm in the axial plane and 10 mm in the cranio-caudal directions were applied. A VMAT plan was designed to give minimum target doses of 95% (CTV) and 67% (PTV). At each fraction, the 3D marker trajectory was estimated by fluoroscopic kV imaging throughout treatment delivery and used to reconstruct the actually delivered CTV dose. The reduction in D-95 (minimum dose to 95% of the CTV) relative to the planned D-95 was calculated. Results: The kV position estimation had mean root-mean-square errors of 0.36 mm and 0.47 mm parallel and perpendicular to the kV imager, respectively. Intrafraction motion caused a mean 3D target position error of 2.9 mm and a mean D-95 reduction of 6.0%. The D-95 reduction correlated with the mean 3D target position error during a fraction. Conclusions: Kilovoltage imaging for detailed motion monitoring. with dose reconstruction of VMAT-based liver SBRT was demonstrated for the first time showing large dosimetric impact of intrafraction tumor motion. (C) 2014 Elsevier Ireland Ltd. All rights reserved.