Volumetric modulated arc therapy (VMAT) has recently emerged as a new clinical modality for conformal radiation therapy. Current clinical practice of VMAT implicitly assumes that patient position and anatomy remain the same over the course of cancer treatment and that the treatment plan can be precisely realized as expected by the delivery machine. However, none of these assumptions are guaranteed in reality and the actual dose received by a patient is generally unknown after delivery. In this work, we develop a pragmatic method to calculate the actual dose delivered to the patient in VMAT using pre-treatment CBCT, which provides up-to-date patient anatomy, and systematic log-files, which record on-treatment machine parameters during the delivery. Phantom studies show that the proposed method can reliably reconstruct the actually delivered dose and reveal the dosimetric impact of patient setup errors in VMAT. This technique offers an objective means for VMAT dose verification and will find applications in future adaptive arc radiation therapy.

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