Patients with early-stage non–small-cell lung cancer (NSCLC) who are unable to undergo surgery can be offered radiation therapy (RT). Previously, conventional RT was offered; however, newer techniques such as stereotactic body RT (SBRT) have become available. The objective of the present systematic review was to investigate the effectiveness of RT with curative intent in patients with early-stage medically inoperable NSCLC. MEDLINE, EMBASE, and the Cochrane Library were searched for studies comparing stereotactic RT with curative intent compared with observation or other types of RT for early-stage, medically inoperable, NSCLC. Comparisons of radiation dosing or fractionation schedules for SBRT were included. We include 4 systematic reviews and 52 observational studies. The evidence suggests that SBRT compared with observation or other forms of RT, such as accelerated hypofractionated RT, 3-dimensional conformal RT, conventional fractionated RT, external beam RT, proton beam therapy, and carbon ion therapy, could have similar or improved results in survival or local control, with similar or fewer adverse effects. Evidence also suggests that local tumor control and survival were associated with the biologically effective dose (BED) for SBRT. Several studies suggested a cutoff of approximately 100 BED correlated significantly with patient outcomes. The presented evidence suggests that SBRT compared with other forms of RT is a reasonable treatment option for patients with medically inoperable early-stage NSCLC.