Impact of Tumor Size on Local Control and Pneumonitis After Stereotactic Body Radiation Therapy for Lung Tumors.

PURPOSE: Stereotactic body radiation therapy (SBRT) is commonly used to treat primary or oligometastatic malignancies in the lung, but most of the available data that describe the safety and efficacy of SBRT are for smaller tumors. The purpose of this study was to evaluate the impact of tumor size, among other factors, on local control (LC) and radiation pneumonitis (RP) in patients who received lung SBRT.

METHODS AND MATERIALS: This retrospective study included 144 patients with 100 primary (57.1%) and 75 metastatic (42.9%) lung tumors treated with SBRT between 2012 and 2018. Measurements of tumor size, treatment volume, histology, and radiation dose were evaluated for association with LC. Additional factors evaluated for association with the development of symptomatic RP included volume of the lung, heart, and central airway exposed to relevant doses of radiation.

RESULTS: The median follow-up time was 15.0 months (interquartile range, 8.0-26.0 months). LC rates at 12 and 24 months posttreatment were 95.1% and 92.7%, respectively. LC at 1 year was higher for tumors <5 cm in diameter than for tumors >5 cm in diameter (98.2% vs 79.8%, respectively; P < .01). On univariate analysis, LC was associated with a smaller gross tumor volume (GTV) diameter (P < .01), GTV volume (P < .01), planning target volume (PTV) diameter (P < .01), PTV volume (P < .01), and larger PTV-to-GTV ratio (P = .04). Tumor histology and treatment intent were not correlated with LC. RP was associated with a higher ipsilateral lung mean lung dose (P = .02), V2.5 (P = .03), V5 (P = .02), V13 (P = .03), V20 (P = .05), V30 (P = .02), V40 (P = .02), and V50 (P = .03), and several similar total lung dose parameters and heart maximum point dose (P = .02). The optimal mean ipsilateral lung dose cutoff predictive of RP was 8.6 Gy.

CONCLUSIONS: A larger tumor size and smaller PTV-to-GTV ratio was associated with local recurrence of lung tumors treated with SBRT, but ipsilateral lung doses were most associated with symptomatic RP.