Complex Multi-site Stereotactic Body Re-irradiation With CT-Guided Online Adaptive Radiotherapy

Online adaptive radiotherapy optimizes a patient's treatment plan to their daily anatomy to account for inter-fraction motion. Daily target and organ-at-risk (OAR) delineation allows for optimized treatments and has been shown to have favorable outcomes in the abdominal region. Adaptive radiotherapy also has the potential to support fine control of dose in re-irradiation to OARs. Herein, we describe a complex multi-site re-irradiation case utilizing CT-guided adaptive radiotherapy. A 46-year-old man with metastatic hepatocellular carcinoma presented for re-irradiation of four metastatic lesions to the right acetabulum, T11, S2, and a gastrosplenic lymph node (gsLN). The right acetabulum, T11, and S2 lesions previously received 20 Gy in five fractions. For the current course, he was prescribed 35 Gy (T11, right acetabulum, and gsLN) and 30 Gy (S2) in five fractions. An equivalent dose in 2 Gy fractions (EQD₂) was employed to assess cumulative doses for critical OARs and guide planning. The re-irradiated lesions were treated with stereotactic body radiation therapy (SBRT), and the gsLN was treated with adaptive radiotherapy. An isotoxic approach was utilized to create the scheduled and adapted plans for the gsLN. Adapted plans were created on the patient's daily anatomy as visualized on kilovoltage cone beam computed tomography and compared against the scheduled plan. Dose-volume histogram objectives were used to compare the plans, and the superior plan was chosen for delivery. The adapted plan was used for all five fractions and met all critical OAR constraints while maintaining target coverage. The use of the scheduled plan would have resulted in stomach and/or esophagus constraint violations on all five fractions. This resulted in reduced EQD₂ doses of 6.4 and 12.3 Gy for the esophagus and stomach, respectively. We report the successful treatment of a patient undergoing tri-site SBRT re-irradiation with concurrent CT-guided adaptive radiotherapy to a gsLN. The adaptive treatment allowed us to meet critical OAR constraints while maintaining target coverage. Few studies have described the use of CT-guided adaptive radiotherapy in re-irradiation cases, and the potential benefit for these complex cases is evident.