• Clinical stage I NSCLC (T1 or T2a, N0, M0) by CT performed within 90 days of screening.

‣ PET/CT is required within 90 days of screening except under circumstances where the clinical picture suggests, or biopsy confirms, a low-grade adenocarcinoma.

⁃ Biopsy is strongly encouraged. In the event biopsy is not performed, rationale must be provided for performing empiric treatment.

⁃ Patients with hilar or mediastinal lymph nodes ≤ 1 cm and no abnormal hilar or mediastinal uptake on PET will be considered N0. Pre-treatment mediastinal lymph node sampling by any technique is allowed but not required. Patients with \> 1 cm hilar or mediastinal lymph nodes on CT or abnormal PET (including suspicious but nondiagnostic uptake) are be eligible only if directed tissue biopsies of all abnormally identified areas are negative for cancer.

• First primary NSCLC on the ipsilateral side.

• At least 18 years of age.

• Clinically eligible for either treatment (surgical resection or SBRT). Because this is a pragmatic study and treatment decisions are at the discretion of the treating physicians and their patients, patients must be eligible for either treatment. To be considered eligible for either treatment, patients must have:

‣ ECOG performance status ≤ 2

⁃ No home oxygen use

⁃ FEV1 and DLCO ≥ 40% predicted

⁃ No symptomatic congestive heart failure as documented by NYHA I-II functional classification

⁃ Been deemed operable by a thoracic surgeon, per clinical visit or review of the medical record and as documented by e-mail, tumor board, study meetings, or other acceptable source documentation. In addition to operability, the surgeon must define what anticipated surgical approach and procedure would be undertaken.

⁃ Been deemed treatable by a radiation oncologist with 10 or fewer fractions of SBRT, per clinical visit or review of the medical record and as documented by e-mail, tumor board, study meetings, or other acceptable source documentation. In addition to suitability for SBRT, the radiation oncologist must define which dose and fractionation would be undertaken.

• Ability to understand and willingness to sign an IRB-approved written informed consent document.

• Agrees to receive treatment for clinical stage I NSCLC (either surgical resection or SBRT).