Analysis of Dose-Volume Metrics Associated with Radiation Pneumonitis after Stereotactic Body Radiotherapy for Lung Cancer

Abstract

The objective was to find dose-volume factors associated with radiation pneumonitis (RP) after stereotactic body radiotherapy (SBRT) for lung cancer. Seventy-four patients who underwent SBRT for primary lung cancer were analyzed in the present study. Fifty-five were male, and nineteen were female. A median of age was 77 years old (range, 63-88). Chemotherapy was not administered unless disease progression was confirmed. Prescription dose for SBRT was uniformly 48 Gy in 4 fractions at the isocenter. Irradiation was performed with 6-MV X-ray beams, which were shaped into the planning target volume (PTV) plus 5-mm margins, from a linear accelerator (Clinac 2300 C/D; Varian). Dose distribution was re-calculated with the analytical anisotropic algorithm in Eclipse (Varian). RP was graded according to the CTCAE version 3, and high-grade RP was herein defined as Grade 2 or worse. Optimal cut points dividing the patient population into two subgroups based on incidence of high-grade RP were searched using the recursive partitioning method in the following dose-volume metrics: PTV volume (cc), mean lung dose (Gy), V5, V10, V15, V20, V25, V30, V35, and V40, where Vd was defined as a relative volume (%) of the lung other than PTV which received more than a threshold dose d Gy. RP Grade 2 or worse was observed in 15 patients (20.2%), including 1 patient of Grade 3, with a median follow-up duration of 31 months. Optimal cut points for pulmonary doses were found in V25 and V20. These factors were highly correlated with each other (R = 0.99), and V25 was more significant than V20. High-grade RP rate was 14.8% in the patients with V25 less than 4.2%, while the rate was 46.2% in the remainder (p = 0.02). PTV volume was found as the other significant factor. High-grade RP rate was significantly lower in the group with PTV volume less than 37.7 cc compared with the larger PTV group (11.1% vs. 34.5%, p = 0.02). Correlation between lung V25 and PTV volume was not strong (R = 0.58). When dividing the patient population into three subgroups: (1) PTV < 37.7 cc; (2) PTV ≥ 37.7 cc and V25 < 4.2%; and (3) PTV ≥ 37.7 cc and V25 ≥ 4.2%, incidence rates of high-grade RP were 11.1%, 23.5%, and 50.0%, respectively (p = 0.01). Lung V25 and PTV volume were suggested as significant factors associated with RP after SBRT.