Dose--volume metrics associated with radiation pneumonitis after stereotactic body radiation therapy for lung cancer.

Purpose To identify dose–volume factors associated with radiation pneumonitis (RP) after stereotactic body radiation therapy (SBRT) for lung cancer. Methods and Materials This study analyzed 74 patients who underwent SBRT for primary lung cancer. The prescribed dose for SBRT was uniformly 48 Gy in four fractions at the isocenter. RP was graded according to the Common Terminology Criteria for Adverse Events (CTCAE) v.3. Symptomatic RP was defined as grade 2 or worse. Optimal cut-offs dividing the patient population into two subgroups based on the incidence of symptomatic RP were sought using the following dose–volume metrics: PTV volume (ml), mean lung dose (Gy), and V5, V10, V15, V20, V25, V30, V35, and V40 (%) of both lungs excluding the PTV. Results With a median follow-up duration of 31.4 months, symptomatic RP was observed in 15 patients (20.3%), including 1 patient with grade 3. Optimal cut-offs for pulmonary dose–volume metrics were V25 and V20. These two factors were highly correlated with each other, and V25 was more significant. Symptomatic RP was observed in 14.8% of the patients with V25 p = 0.019). PTV volume was another significant factor. The symptomatic RP rate was significantly lower in the group with PTV p = 0.020). The patients were divided into three subgroups (patients with PTV p = 0.013). Conclusions Lung V25 and PTV volume were significant factors associated with RP after SBRT.