2395 - Feasibility and Safety of Subsequent Course(s) of Single-Isocenter/Multi-Lesion (SIML) Stereotactic Body Radiation Therapy (SBRT) to Synchronous Primary or Oligometastatic Lung Lesions

D. Pokhrel¹, E. C. Amoah², and R. C. McGarry²; ¹University of Kentucky, Department of Radiation Medicine, Lexington, KY, ²University of Kentucky, Lexington, KY

Purpose/Objective(s): SIML lung SBRT is becoming a standard of care for multiple primary tumors or unresectable oligometastatic (= 5) lesions. After the first course of SBRT, some patients present with synchronous new lung lesions or metasynchronous disease. In these cases, in our institute, patients receive subsequent course(s) of lung SBRT via SIML method. We present feasibility and safety of treating multiple courses of lung SBRT via SIML method. Materials/

Methods: Thirteen patients (n=9, 2 courses; n=4, 3 courses) with oligometastatic lung lesions (n=11, two lesions; n=2; 3 lesions, total 28 lesions) in course 1; (n=12, two lesions; n=1; 3 lesions, total 27 lesions) in course 2, and (n=4, 2 lesions, total 8 lesions) in course 3 were treated with 4D-CT based highly conformal SIML lung SBRT plans via 6MV-FFF co/non-coplanar VMAT arcs and Acuros-XB dose for heterogeneity corrections. Common prescriptions were 50 Gy/5 and 54 Gy/3 fractions prescribed to 70-80% isodose line to each lesion. Mean tumor volumes were 15.4 cc (course 1) and 21.5 cc (course 2) and 11.7 cc (course 3). Mean tumor distance-to-isocenter was < 6.5 cm in each course. Conebeam CT-guided SIML treatments were delivered every other day (15 min, patients door-to-door time) via PerfectPitch couch corrections. For each of 53 total fractions (in courses 1 and 2), inter-fractional positioning errors were analyzed. Tumor local control (LC) rates based on post-treatment CT and toxicity profiles based on CTCAE v5 were reported.
Results: For all successive SBRT courses, SIMT lung VMAT plans met RTOG-0618/0813 criteria for target coverage and dose to critical organs. For both successive courses average inter-fractional couch shifts from the patient’s marks were < 2 mm (maximum up to ±6 mm) in the vertical, longitudinal, and lateral directions and rotational corrections were < 0.5^o (maximum up to ±2.9^o) for pitch, roll, and yaw, respectively. Median follow-up interval was 23.3 months (range, 3.2-60 months) and 10.6 months (range, 0-44.2 months) in courses 1 and 2, respectively. Median interval between courses 1 and 2 was 12.5 months (range, 1.7-32 months). LC was achieved in 26/28 (93%) and 26/27 (96.3%) of treated lesions in courses 1 and 2. In each course, 5 patients (38.5%) experienced grade 1 asymptomatic pneumonitis on average 5 months after SBRT. No rib fracture, chest wall pain or esophagitis occurred in any patient. However, 6 (46 %) patients died due to distant metastases or other medical conditions. Clinical follow up of SIML course 3 is ongoing.
Conclusion: Highly conformal SIML lung SBRT treatments for synchronous lung cancers or multiple lung metastases with chronologically separate treatment courses had an excellent tumor LC rate (~95%) and low toxicity profile. Rapid SIML lung SBRT improved patient’s compliance and comfort who had difficulty lying still at treatment position in contrast to long traditional individual isocenter treatments. Significantly reduced treatment time, reducing intrafraction motion errors and improving clinic workflow.