Impact of Time-to-Treatment on Lung SBRT Outcomes: Histology Matters!

Abstract

<h3>Purpose/Objective(s)</h3> "Time-to-treatment" (TTT) is considered an indicator of quality care that has been postulated to affect lung cancer outcomes. The process by which an early stage medically inoperable lung cancer patient (pt) is deemed appropriate for lung stereotactic body radiation therapy (SBRT) typically involves a multi-disciplinary assessment over extended time. This study analyzed a large single institution registry to explore the impact of TTT on SBRT outcomes. <h3>Materials/Methods</h3> From an IRB-approved lung SBRT registry, we identified 1573 pts treated definitively for early-stage lung cancer between 2003 and 2020. TTT was defined as elapsed months between date of diagnosis (date of biopsy or date of PET CT in cases of radiographic diagnosis) and SBRT start date. Fine-Gray's Test was employed to evaluate the effect of TTT on local control (LC) and freedom from disease progression (FFP), and the long rank test was used for overall survival (OS). Cumulative incidence rates with death as competing event were calculated for LC and FFP. Actuarial analysis as used to calculate OS rates. <h3>Results</h3> Median follow-up was 22.9 months. Median age at SBRT start in years was 74.5 (range 35-98), median KPS was 80 (range 40-100), 52.3% of pts were female and 84.0% were white. Median tumor size and median PET SUV max were 2.2 cm (range 0.4-10.5) and 7.5 (range 0.8-56), respectively; 88.3% of pts were stage I. Of 75.7% pts with biopsies, 30.6% were adenocarcinomas (AC), 27.8% had squamous carcinoma (SqC). Median SBRT dose was 50 Gy in 5 fractions (40.5% cases). The 2-year rates of LC, FFP and OS for all pts were 6.5%, 76.3%, and 62.6%, respectively. Median TTT was 1.7 months (range 0.2-18.1). Using a cut-off of 1.5 months to compare patients receiving upfront vs delayed treatment, there was no difference in LC (p=0.5644), FFP (p=0.1976) or OS (p=0.3018) between the two groups. Of 1038 pts with histologic diagnosis, TTT was associated with OS but not LC or FFP for SqC pts (p=0.0422), and with no AC outcomes. An association of TTT with FFP for pts aged 70-79 (p=0.0302) was seen but otherwise no significant differences were noted in TTT outcomes for pts stratified by T stage, age in decades or KPS. Pt and tumor characteristics were balanced between cut-off cohorts. <h3>Conclusion</h3> Although TTT per se was not associated with altered lung SBRT outcomes in this cohort of nearly 1600 early-stage inoperable lung cancer pts, increasing TTT was found to have a negative association with OS for SqC pts on subset analysis. Previous studies have suggested impaired LC for SqC with SBRT although this was not seen in either of the TTT cut-off cohorts. Further work is warranted to determine how TTT may affect SBRT outcomes in the SqC population.