SBRT treatment of abdominal and pelvic oligometastatic lymph nodes using ring‐mounted Halcyon Linac

Abstract Purpose/objectives This work seeks to evaluate the plan quality, treatment delivery efficiency, and accuracy of single‐isocenter volumetric modulated arc therapy (VMAT) of abdominal/pelvic oligometastatic lymph nodes (LNs) stereotactic body radiation therapy (SBRT) on Halcyon Linac. Materials and Methods After completing the in‐house multitarget end‐to‐end phantom testing and independent dose verification using MD Anderson’s single‐isocenter/multi‐target (lung and spine target inserts) thorax phantom, eight patients with two to three abdominal/pelvic oligometastatic LNs underwent highly conformal single‐isocenter VMAT‐SBRT treatment using the Halcyon Linac 6MV flattening filter free (FFF) beam. Targets were identified using an Axumin PET/CT scan co‐registered with planning CT images and a single‐isocenter was placed between/among the targets. Doses between 25 and 36.25 Gy in 5 fractions were delivered. Patients were treated every other day. Plans were calculated in Eclipse with advanced AcurosXB algorithm for heterogeneity corrections. For comparison, Halcyon VMAT‐SBRT plans were retrospectively generated for SBRT‐dedicated TrueBeam with a 6MV‐FFF beam using identical planning geometry and objectives. Target coverage, conformity index (CI), dose to 2 cm away from each target (D2cm) and dose to adjacent organs‐at‐risk (OAR) were evaluated. Additionally, various treatment delivery parameters including beam‐on time were recorded. Results Phantom measurements showed acceptable spatial accuracy of conebeam CT‐guided Halcyon SBRT treatments including compliance with MD Anderson’s single‐isocenter/multi‐targets phantom credentialing results. For patients, the mean isocenter to tumor center distance was 3.4 ± 1.2 cm (range, 1.5–4.8 cm). The mean combined PTV was 18.9 ± 10.9 cc (range, 5.6–39.5 cc). There was no clinically significant difference in dose to LNs, CI, D2cm and maximal doses to OAR between single‐isocenter Halcyon and Truebeam VMAT‐SBRT plans, although, Halcyon plans provided preferably lower maximal dose to adjacent OAR. Additionally, total monitor units, beam‐on time and overall treatment time was lower with Halcyon plans. Halcyon’s portal dosimetry demonstrated a high pass rate of 98.1 ± 1.6% for clinical gamma passing criteria of 2%/2 mm. Conclusion SBRT treatment of abdominal/pelvic oligometastatic LNs with single‐isocenter VMAT on Halcyon was dosimetrically equivalent to TrueBeam. Faster treatment delivery to oligometastatic LNs via single‐isocenter Halcyon VMAT can improve clinic workflow and patient compliance, potentially reducing intrafraction motion errors for well‐suited patients. Clinical follow‐up of these patients is ongoing.