MARKERLESS DYNAMIC TUMOR TRACKING USING DIAPHRAGM AS A SOFT-TISSUE ANATOMICAL SURROGATE FOR LIVER TUMORS

Liver cancer is the sixth most common cancer worldwide and radiation therapy has been a non-surgical alternative for treating patients with advanced liver cancer. Dynamic tumor tracking (DTT) has been developed to treat intra-fractionally moving liver tumors with radiation therapy. However, one of the most challenging aspects of DTT is detecting the internal tumor position, which usually relies on implanted markers as a surrogate for tumor position. This thesis addresses two questions: 1) The uncertainty in marker-based DTT by using in-vivo EPID images, and 2) The feasibility of eliminating implanted markers by using soft tissue surrogates for the liver target.
The first part of the thesis investigates the uncertainty in marker based DTT for Stereotactic Ablative Body Radiotherapy (SABR) treatments of the liver. The study utilized in-vivo EPID images to assess the target localization uncertainty for DTT. Phantom and patient EPID images were acquired during non-coplanar 3DCRT-DTT delivered on a Vero4DRT linac, and the absolute differences between the measured center of mass
(COM) of the markers relative to the aperture-center were reported in pan, tilt, and 2D-vector directions at the isocenter plane. The study found that marker based DTT uncertainty could be evaluated in-vivo on a field-by-field basis using EPID images, which can contribute to PTV margin calculations for DTT.
The second part of the thesis focuses on investigating the feasibility of using soft tissue surrogates for liver tumors during markerless dynamic tumor tracking (MDTT) radiotherapy. The study aimed to demonstrate the pre-clinical feasibility of MDTT using the diaphragm as an anatomical motion surrogate for liver radiotherapy. KV X-ray images from phantom and ten patients undergoing liver radiotherapy were used to analyze the relative motions of the diaphragm and gold markers. The study found that the diaphragm positions correlated well with markers' 
positions and could be used as a reliable surrogate for motion management. The study also utilized the Brainlab Vero4DRT linac MDTT module, originally designed for lung tracking, and applied it in a novel way to demonstrate the effectiveness of using the diaphragm as a surrogate for liver tumor motion management during MDTT radiotherapy.