The significance of PTV dose coverage on cancer control outcomes in early stage non-small cell lung cancer patients treated with highly ablative stereotactic body radiation therapy.

Abstract

We evaluated whether patients with early-stage non-small-cell lung cancers (NSCLCs) treated with stereotactic body radiation therapy (SBRT) without full prescription dose coverage of the planning target volume (PTV) had inferior outcomes. The SBRT regimen was 54 Gy in three fractions. Dosimetric constraints were as per the Radiation Therapy Oncology Group 0236 guidelines. All patients underwent four-dimensional CT (4D-CT) simulation. The internal target volume (ITV) was defined using 4D-CT, and the PTV was defined as a 6-mm longitudinal and a 3-mm axial expansion from the ITV. If normal tissue constraints were beyond tolerance, ITV-based dosing was employed where priority was made for full ITV coverage at the expense of PTV coverage. Patients with and without full PTV dose coverage were compared, and control rates were estimated using Kaplan-Meier analysis. 120 NSCLC cases were evaluated with 81% having adequate PTV dose coverage. ITV and PTV were significantly larger in the cohort with inadequate PTV dose coverage (p = 0.0085 and p = 0.0038, respectively), and the mean ITV and PTV doses were higher in patients with adequate PTV dose coverage (p = 0.002 and p < 0.0001, respectively). The 3-year local control rate was 100% for both cohorts. There was no difference in 3-year regional control (p = 0.36), disease-specific survival (p = 0.79) or overall survival (p = 0.73). When delivering a highly ablative SBRT regimen for early-stage NSCLC, full-dose coverage of the ITV is sufficient for local control. Our data are among the first to evaluate the utility of PTV margins in a highly ablative SBRT regimen and suggest that when dosing constraints cannot be met, full tumouricidal dose coverage of the ITV is sufficient for local control.