Markerless dynamic tumor tracking (MDTT) radiotherapy using diaphragm as a surrogate for liver targets

Abstract

PurposeTo assess the feasibility of using the diaphragm as a surrogate for liver targets during MDTT.MethodsDiaphragm as surrogate for markers: a dome‐shaped phantom with implanted markers was fabricated and underwent dual‐orthogonal fluoroscopy sequences on the Vero4DRT linac. Ten patients participated in an IRB‐approved, feasibility study to assess the MDTT workflow. All images were analyzed using an in‐house program to back‐project the diaphragm/markers position to the isocenter plane. ExacTrac imager log files were analyzed. Diaphragm as tracking structure for MDTT: The phantom “diaphragm” was contoured as a markerless tracking structure (MTS) and exported to Vero4DRT/ExacTrac. A single field plan was delivered to the phantom film plane under static and MDTT conditions. In the patient study, the diaphragm tracking structure was contoured on CT breath‐hold‐exhale datasets. The MDTT workflow was applied until just prior to MV beam‐on.ResultsDiaphragm as surrogate for markers: phantom data confirmed the in‐house 3D back‐projection program was functioning as intended. In patients, the diaphragm/marker relative positions had a mean ± RMS difference of 0.70 ± 0.89, 1.08 ± 1.26, and 0.96 ± 1.06 mm in ML, SI, and AP directions. Diaphragm as tracking structure for MDTT: Building a respiratory‐correlation model using the diaphragm as surrogate for the implanted markers was successful in phantom/patients. During the tracking verification imaging step, the phantom mean ± SD difference between the image‐detected and predicted “diaphragm” position was 0.52 ± 0.18 mm. The 2D film gamma (2%/2 mm) comparison (static to MDTT deliveries) was 98.2%. In patients, the mean difference between the image‐detected and predicted diaphragm position was 2.02 ± 0.92 mm. The planning target margin contribution from MDTT diaphragm tracking is 2.2, 5.0, and 4.7 mm in the ML, SI, and AP directions.ConclusionIn phantom/patients, the diaphragm motion correlated well with markers’ motion and could be used as a surrogate. MDTT workflows using the diaphragm as the MTS is feasible using the Vero4DRT linac and could replace the need for implanted markers for liver radiotherapy.