2197 - Assessment of Patient Reported Clinical Outcomes of Risk-Adapted Stereotactic Body Radiotherapy (SBRT) Treatment of Peripherally Located Lung Tumors

Purpose/Objective(s): Lung SBRT became a standard of care for localized non-small cell lung cancer (NSCLC) or metastatic lung cancer patients. However, radiation-induced chest wall pain or rib fracture is a concern for peripherally located lung SBRT patients. Herein, we report our long-term tumor local control (TLC) rates and toxicity profiles in lung SBRT patients treated per RTOG0813 protocol’s criteria and 6MV flattening filter free (FFF) volumetric modulated arc therapy (VMAT) plans.

Materials/Methods: In this IRB approved retrospective study, we have included 123 inoperable lung SBRT patients with either primary NSCLC (n=92) or isolated thoracic metastatic lesion (n=31) treated with a risk-adapted 50–55 Gy in 5 fractions prescribed to 70-80% isodose line. Patients were immobilized on VacLoc bag and 4D-CT based highly conformal SBRT plans were generated via 6FFF beam, non-coplanar partial VMAT arcs and AcurosXB algorithm. Treatments were delivered every other day via pre-treatment CBCT guidance and 6dof couch corrections. Plan quality and delivery efficiency were reported. Outcomes reported include TLC rates, chest wall pain, rib fracture and pulmonary toxicity on physical exam followed by post SBRT diagnostic CT scans per CTCAE v5 criteria. Median follow up interval was 16.2 ± 12.8 (3–66) months.

Results: Mean planning target volume (PTV) was 33.2 ± 31.1 (4.7–201.3) cc. All lung SBRT plans met RTOG0813 criteria for tumor coverage, conformity, and organs at risk (OAR) sparing: average maximum dose to skin (18.5 Gy), ribs (46.7 Gy) and dose to 1 cc of ribs (35.0 Gy). Mean values of conformity and gradient indices, D_2cm and lung V_20Gy were 1.02 ± 0.05, 4.2 ± 0.9, 51.8 ± 5.8% and 2.6 ± 2.2%. Average beam on time was 4.5 ± 1.2 min. All patients tolerated lung SBRT treatment. Mean couch time including CBCT imaging & set up correction was < 15 min. Of 123 lung SBRT patients treated, 112 had an adequate post-treatment chest CT scan to assess treatment response; among them TLC was achieved in 101/112 (90.2%) as reported by their tumor shrinkage in follow up CT scan; 18 (16%) patients had locoregional nodal failure; 19 (17%) metastatic patients had new distant lesions. 17 (15.2%) patients who were smokers, had COPD showed asymptomatic radiographic changes in lungs with no Grade 3+ toxicity; 6 (5.4%) patients reported chest wall pain within 3 months of SBRT and 4 (3.6%) patients presented with rib fracture at 4–48 months; these patients were managed with steroids or gabapentin.

Conclusion: Our risk-adapted SBRT delivery of peripheral lung lesions via 6FFF VMAT was safe, fast, efficacious, and convenient treatment with promising TLC rates, acceptable acute and late toxicity profiles. Fast delivery of VMAT lung SBRT could reduce intrafraction motion error due to coughing, shortness of breath or back pain making target miss unlikely also improving patient comfort and clinic workflow. Kaplan-Meier curves for longer follow up time including dosimetric factors associated with rib fracture or chest wall pain in this cohort is warranted.