Is the Ptv Margin Important for Local Control in Stage I Non-Small Cell Lung Cancer (nsclc) Patients Treated with Definitive Sbrt?

We sought to determine if stage I NSCLC patients treated with stereotactic body radiation therapy (SBRT) without full prescription dose coverage to the PTV had inferior local control outcomes. From February 2009 to January 2013, consecutive Stage I NSCLC patients treated at a single institution with SBRT with curative intent were retrospectively analyzed. The SBRT regimen was 54Gy in 3 fractions for peripheral tumors and 50Gy in 4 fractions for central tumors. Dosimetric constraints were derived from RTOG 0236 for peripheral tumors and previously established 50Gy in 4 fractions constraints for central tumors. The ITV was defined using a 4D-CT and PTV was defined as a 6mm craniocaudal and a 3mm radial expansion around the ITV. If normal tissue constraints were beyond defined tolerance thresholds, then priority was made for full ITV coverage at the expense of PTV coverage. For our analysis, patients who had ≤ 94.5% prescription dose coverage to the PTV were compared to patients with >94.5% prescription dose coverage to the PTV. Control and survival rates were estimated with Kaplan-Meier analysis. The Cox model was also used for comparison analysis. One hundred three patients with Stage I NSCLC were assessed with median post-treatment follow-up of 23.2 months. For the entire patient population, the three-year local, regional, and distant control rates were 96.2%, 85.1%, and 88.3%, respectively, and the three year overall survival rate was 81.1%. There were 42 patients who had ≤ 94.5% prescription dose coverage to their PTV. The average %PTV prescription dose coverage in the cohort with ≤ 94.5% PTV prescription dose coverage was 87.1% and average the %PTV prescription dose coverage in the cohort with >94.5% PTV prescription dose coverage was 96.2%. All patients evaluated had ≥95% coverage of their ITV with the prescription dose. The two year local control rate was 100% for the patients with ≤94.5% PTV prescription dose coverage and 93.3% for patients with >94.5%% PTV prescription dose coverage (p=0.969). Our data suggests that in definitive SBRT for stage I NSCLC, full tumoricidal dose coverage for the ITV is sufficient for local control regardless of PTV coverage.