(P008) Pretreatment FDG Uptake of Nontarget Lung Tissue Correlates With Symptomatic Pneumonitis Following Stereotactic Ablative Radiotherapy (SABR)

Aadel A. Chaudhuri, MD, PhD, Michael S. Binkley, BA, Justin N. Carter, BA, Maximilian Diehn, MD, PhD, Billy W. Loo, MD, PhD; Stanford University School of Medicine

INTRODUCTION: Radiation pneumonitis (RP) is the most common toxicity associated with radiation therapy to the lung. It can be debilitating and even life-threatening, especially since many patients with lung malignancies have other pulmonary comorbidities. Here, we report our institution’s experience in using pretreatment positron emission tomography-computed tomography (PET-CT) to calculate baseline nontarget lung fluorodeoxyglucose (FDG) avidity, which we have found to be significantly associated with symptomatic RP following stereotactic ablative radiotherapy (SABR).

METHODS: We retrospectively reviewed outcomes in 73 patients with lung tumors treated with SABR. All patients underwent whole body PET-CT and four-dimensional (4D) chest CT simulation scan within 2 weeks prior to treatment. Gross tumor volume (GTV) was contoured on axial CT slices using lung windows. Respiration-induced motion was assessed by 4D CT, used to design the internal target volume (ITV), and a 0.5-cm margin was added to form the planning target volume (PTV). Total lung minus PTV and ipsilateral lung minus PTV were contoured using lung windows. All contouring was done using Varian Eclipse and MIM software. We used the simulation PET-CT scans to calculate the standard uptake value (SUV) at the 85th (SUV85), 90th (SUV90), and 95th (SUV95) percentiles and mean SUV for total lung minus PTV and ipsilateral lung minus PTV using MIM and compared these values between patients who experienced symptomatic RP and those who did not. All strata were compared using Student’s t-test and Cox regression for univariate and multivariate analyses.

RESULTS: Median follow-up time was 15 months (range: 4–45 mo). A total of 70 patients (95.9%) were treated for T1 to T3 non–small-cell lung cancer, while 3 (4.1%) were treated for oligometastatic disease from non-lung primary cancers. Sixty-six patients (90.4%) were treated for one lung tumor, and seven patients (9.6%) were treated for two lung tumors. Sixteen patients (21.9%) experienced grade ≥ 2 RP, 3 of whom experienced grade ≥ 3 RP. Median time to pneumonitis was 4.95 months (range: 2.5–24 mo). Analysis of simulation PET-CT scans from all patients revealed that those who experienced grade ≥ 2 RP had significantly higher total lung minus PTV SUV85 (P = .0005), SUV90 (P = .0014), and SUV95 (P = .0078) than those with no symptomatic pneumonitis. We saw similar results when we analyzed ipsilateral lung minus PTV, with significantly higher baseline SUV85 (P = .0021), SUV90 (P = .0036), SUV95 (P = .0108), and mean SUV (P = .0012) in patients who experienced grade ≥ 2 RP. A significant association was observed between grade ≥ 2 RP and total lung minus PTV SUV85 on univariate (hazard ratio [HR] = 28.9; 95% confidence interval [CI], 3.66–216.61; P = .001) and multivariate (HR = 25.22; 95% CI, 3.3–192.94; P = .002) analysis.

CONCLUSION: Patients with higher nontarget lung FDG avidity appear to be at greater risk forradiation pneumonitisfollowing SABR.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org