Purpose: Proton therapy has been proposed as an alternative to photon beam therapy in stereotactic body radiation therapy (SBRT) of early‐stage NSCLC cancer. We investigated in the present study how proton range uncertainties can influence proton SBRT treatment quality. Methods: Ten medically inoperable patients with stage I NSCLC were included in the current study. The treatment strategy for the SBRT tumors at MGH is to use either i)proton passive scattering or ii)photon 3DCRT for tumors with motion amplitude of less 5mm. The SBRT proton treatment planning uses either 2 or 3 co‐planar beams. While, SBRT photon treatment planning uses 2 or 3 non‐coplanar arcs, with a total of 8–10 beams. In the case of the proton beams an additional 3.5%+2mm is added to account for range uncertainties in the proximal and distal ends of the spread‐out‐Bragg‐peak, SOBP. Dosimetric comparisons were performed by defining a high‐ and low‐dose region representing volumes receiving more and less than 50% of the prescription dose, respectively. Results: In high‐dose regions, the volume receiving ≥95% of the prescription dose was larger for proton than for photon SBRT, i.e., 46.5 cc versus 33.5 cc (p=0.009), respectively. The conformity indices were 2.46 and 1.56, respectively. For tumors in close proximity to the chest‐wall, the volume of the chest‐wall receiving more‐than‐30Gy (V30) was 7 cc larger for protons than for photons (p=0.06), respectively. In low‐dose regions, the lung V5 and maximum esophagus dose were smaller for protons than for photons (p=0.019 and p<0.001, respectively). Conclusions: Protons generate larger high‐dose regions than photons because of range uncertainties. This can result in nearby healthy organs (such as chest‐wall) receiving close to treatment dose. Future proton studies should focus on approaches to reduce dose range uncertainties and identify clinical subgroups of patients who will most benefit from the lack of exit dose.