Dosimetric Parameters Related to Local Control and Rate of Distant Metastases in NSCLC SBRT Patients

Abstract

Previous studies have suggested that the dose immediately outside the PTV may impact the incidence of distant metastases after stereotactic body radiation therapy (SBRT) for patients with early-stage non-small cell lung cancer. In particular, Diamant et al have reported a correlation between the mean EQD2 of a 30mm shell around the PTV and both local control and the rate of distant metastases in lung SBRT patients. In this study, we review this parameter and others in a series of patients with radiographically presumed or biopsy proven early-stage non-small cell lung cancer treated at our institution with SBRT between 2014 and 2019.We reviewed the dosimetry, local control and rate of distant metastases for a series of 155 patients with 165 lesions treated with SBRT at our institution. Dosimetric parameters obtained included the prescribed dose, minimum and mean doses to the PTV, conformity index, and the mean EQD2 to a 30mm shell around the PTV, as reported by Diamant et al. Time to local progression and distant metastasis were defined from date of last fraction of SBRT to date of event, with event-free patients censored at last radiographic follow-up. All plans were calculated in the Varian Eclipse treatment planning system using the AAA algorithm and treated on conventional Linac with either IMRT or VMAT delivery. Analysis of the data was performed using IBM SPSS for Windows (version 27), with analysis of time to event via Kaplan-Meier method. Univariate analysis of outcomes based on the above dosimetric parameters was performed utilizing the log-rank test. A P-value < 0.05 was considered to be statistically significant.No significant correlation was seen between the mean EQD2 dose to a 30mm shell around the PTV and either the local control or the rate of distant metastases in these patients. Positive predictors of reduced incidence of distant metastases were the prescribed dose of 54 Gy (vs 45-50 Gy, P < 0.01) and PTV < 22 cc (vs. ≥ 22 cc, P = 0.01). In the 45-50 Gy cohort, PTV mean dose ≥ 53 Gy (vs. < 53 Gy, P = 0.04) was associated with reduced incidence of distant metastases. Improved local control was also noted with prescribed dose of 54 Gy (vs 48-50 Gy, P = 0.01). In the 45-50 Gy cohort, PTV mean dose ≥ 53 Gy (vs. < 53 Gy, P = 0.03) was associated improved local control. No additional factors examined were significantly correlated with local control.We failed to replicate the correlation of the rate of distant metastases with dose outside the PTV as reported by Diamant et al. Higher prescribed and mean dose to the PTV was correlated with both local control and reduced incidence of distant metastases. References.