Geometric Reproducibility of Fiducial Markers and Efficacy of a Patient-Specific Margin Design Using Deep Inspiration Breath Hold for Stereotactic Body Radiotherapy for Pancreatic Cancer

Abstract In patients undergoing SBRT for pancreatic adenocarcinoma, the reproducibility of tumor positioning between deep inspiration breath-hold (DIBH) is unclear. We characterize this variation with fiducials at simulation and treatment and investigated whether a patient-specific breath-hold margin (PSBH_margin) would help account for intrafraction variation at treatment. We analyzed 20 consecutive pancreatic cancer patients who underwent DIBH SBRT. At simulation, 3 additional breath-hold scans were acquired immediately after the contrast-enhanced planning CT and used to quantify average and maximum variations in the fiducial position (Sim_Varavg and Sim_Varmax), as well as to design ITV incorporating a PSBH_margin. At treatment, an average of 5 breath-hold CBCTs were acquired per fraction for each patient to quantify Tx_Varavg and Tx_Varmax. Various PTV margins on the GTV versus ITV were evaluated using CBCTs with the goal of achieving >95% of fiducials being covered at treatment. The Sim_Varavg and Sim_Varmax were 0.9 ± 0.5 mm and 1.5 ± 0.8 mm in LR, 0.9 ± 0.4 mm and 1.4 ± 0.4 mm in AP, and 1.5 ± 0.9 mm and 2.1 ± 1.0 mm in SI. The Tx_Varavg and Tx_Varmax were 1.2 ± 0.4 mm and 2.0 ± 0.7 mm in LR, 1.1 ± 0.4 mm and 1.8 ± 0.6 mm in AP, and 1.9 ± 1.0 mm and 3.1 ± 1.4 mm in SI. The ITV was increased by 21.0 ± 8.6% as compared to the GTV alone. The PTV margin necessary to encompass >95% of the fiducial locations was 2 mm versus 4 mm in both LR and AP and 4 mm versus 6 mm in SI for ITV and GTV, respectively. The inter-breath hold variation is not insignificant, especially in SI. Acquiring multiple breath-hold CTs at simulation can help quantify inter-breath hold reproducibility and design a PSBH_margin for treatment.