TABLE 4.

INSTRUMENTAL VARIABLE ANALYSIS OF THE EFFECTIVENESS OF RADIATION THERAPY FOR UNRESECTED STAGE I AND II NON–SMALL CELL LUNG CANCER

	Health Care Service Area		Probability of RT Use
Adjusted Survival*	High Utilization†	Low Utilization	High Utilization	Low Utilization	IV Estimator‡ (95% CI)
Overall
12-mo	58.8%	56.6%	64.7%	50.4%	15.6% (2.0%–33.9%)
24-mo	38.1%	35.6%			18.1% (2.2%–38.0%)
Lung cancer-specific
12-mo	64.4%	61.2%	64.7%	50.4%	21.6% (0%–29.6%)
24-mo	45.0%	40.9%			26.3% (0.2%–36.4%)

Definition of abbreviation: IV = instrumental variable; RT = radiation therapy.

^*Adjusted survival was calculated using Cox regression adjusting for sociodemographic characteristics, comorbidities, and stage.

^†Represent Health Care Service areas with high use of radiation therapy.

^‡The IV estimator was calculated as the difference between adjusted survival in the highest and lowest utilization areas, divided by the difference in the probability of undergoing radiation therapy among lung cancer patients in these regions.