Table A1. Subgroup Interaction Effects Model.

Covariate	Combined Effect Subgroup Model According to TTI, Hazard Ratio (95% CI)
	≤ 30 d	31-60 d	61-90 d	> 90 d
Overall AJCC stage group
I-II	Reference	1.17 (1.12 to 1.23)	1.54 (1.41 to 1.68)	1.52 (1.33 to 1.72)
III-IV	Reference	1.02 (0.99 to 1.07)	1.08 (1.02 to 1.14)	1.29 (1.20 to 1.40)
Primary tumor site
Oral tongue	Reference	0.88 (0.84 to 0.93)	0.95 (0.88 to 1.04)	1.09 (0.98 to 1.23)
Larynx/hypopharynx	Reference	1.04 (1.01 to 1.09)	1.16 (1.09 to 1.24)	1.35 (1.23 to 1.48)
Oropharynx	Reference	1.39 (1.32 to 1.46)	1.24 (1.12 to 1.38)	1.34 (1.16 to 1.56)
Treatment modality
CRT	Reference	0.93 (0.89 to 0.97)	0.94 (0.88 to 1.01)	1.22 (1.11 to 1.35)
RT	Reference	1.07 (1.02 to 1.13)	1.28 (1.17 to 1.40)	1.30 (1.16 to 1.46)
Surgery	Reference	1.06 (1.01 to 1.12)	1.25 (1.14 to 1.37)	1.24 (1.08 to 1.43)

NOTE. Subgroup effects derived from interaction testing between covariates and time to treatment initiation (TTI). Each hazard ratio represents the combined effect of a covariate and TTI compared with TTI ≤ 30 days within each subgroup. Only covariates identified with significant interaction were included in the combined effects model, which include stage (I-II or III-IV), primary tumor site (tongue, larynx/hypopharynx, or oropharynx), and treatment modality (chemoradiation [CRT], radiation [RT], or surgery alone).

Abbreviation: AJCC, American Joint Committee on Cancer.