According to the Indiana University protocol guidelines, dose-limiting toxicity (DLT) was any grade 3 cardiac or pulmonary toxicity or any grade 4 toxicity attributed to the therapy. Thirty-seven patients were treated using a standard dose escalation protocol with 3 patient cohorts with minimum 1 month between dose levels to assess toxicity. Patients were categorized into separate independent dose escalations according to tumor volume, T1 vs. T2 (≤ 5 cm) vs. T2 (> 5 to ≤ 7 cm). Grade 3 pneumonitis was seen at a dose of 14 x 3 = 42 Gy total in one T2 patient with a 7-cm tumor and transient grade 3 hypoxia was seen at 16 x 3 = 48 Gy total in one patient. Additional patients were treated at each of these levels without further toxicity observed. Twenty-one patients had mild to moderate fibrosis distal to the treated lesion appear on chest x-ray after treatment. Nine of these patients had a decline of an element of their pulmonary function tests (FEV1, FVC, DLCO, or PO2) by 10-20% of predicted which returned back to baseline values with follow-up in all but two. The timing of onset of this toxicity was generally acute to subacute (< 1 month in most cases). The maximum tolerated dose (MTD) was not reached on this trial for patients with T1 tumors and smaller T2 tumors (≤ 5 cm). Dose-limiting pneumonitis or pericarditis occurred in 2/5 patients with larger T2 tumors (>5 to ≤ 7 cm) at a dose of 24 x 3 = 72 Gy defining the MTD for this subgroup at 22 x 3 = 66 Gy. Patients treated at a dose of 22 Gy per fraction times three fractions had follow up of over 24 months without late toxicity for all T-stage tumor categories. Treatment failure within the PTV has been observed in 8 of 26 patients treated at doses of up to 20 x 3 = 60 Gy. However, all but one of these local failures occurred at doses of 16 x 3 = 48 Gy or lower.31