Table 2. Optimal SI threshold value to classify unfavorable tumor voxels at different imaging times.

Summary of ROC Analysis for Optimal SI and Universal Threshold

Ultimate treatment outcome	Local tumor control (LC)			Disease-specific survival (DSS)
MRI Time	Pre-RT	20–25 Gy	40–50 Gy	Pre-RT	20–25 Gy	40–50 Gy
Max AUC (Area under the curve)	0.66	0.78	0.80	0.66	0.71	0.69
Time-specific optimal SI threshold	2.1	2.1	2.0	1.8	2.1	1.9
*p-value (time-specific threshold)	0.02	8.0·10−5	3.7·10−5	0.009	0.001	0.006
*p-value (universal threshold of 1.9)	0.025	1.4·10−4	4.2·10−5	0.006	0.002	0.006

Optimal SI thresholds to define at-risk voxels, determined by ROC analysis, varied at different treatment time points. Threshold values ranged from 1.8–2.1 (row 5) before treatment (#1), and at 2–2.5 weeks (#2) or 4–4.5 weeks into treatment, respectively. These time-specific optimal SI thresholds correlated significantly with treatment outcomes (row 6). Significant correlation remains when a single universal SI threshold (1.9) is applied to define the unfavorable voxels for all three time points (row 6).

^*Mann-Whitney U test.