Table 3: Multivariable analysis association between pre-SRT serum calcium and BCR of prostate cancer after SRT.

Model adjustment Association of pre-SRT serum calcium and BCR of prostate cancer after SRT
Pre-SRT serum calcium as a continuous variable
(0.5 mg/dL increase)
Moderate versus low pre-SRT serum calcium High versus low pre-SRT
serum calcium
RR (95% CI)P valueRR (95% CI)P valueRR (95% CI)P value

No adjustment for other variables0.96 (0.74, 1.25)0.760.94 (0.59, 1.50)0.791.08 (0.66, 1.75)0.76
Adjusting individually for risk factors for BCR
 Pathologic tumor stage0.99 (0.75, 1.30)0.910.89 (0.55, 1.42)0.611.12 (0.69, 1.82)0.65
 Gleason score0.94 (0.74, 1.25)0.680.96 (0.59, 1.56)0.861.15 (0.69, 1.92)0.59
 Pre-SRT PSA0.95 (0.73, 1.24)0.700.93 (0.58, 1.49)0.761.10 (0.68, 1.78)0.71
 SRT dose0.98 (0.74, 1.28)0.860.91 (0.57, 1.45)0.681.10 (0.68, 1.79)0.70
Adjusting for all four risk factors for BCR 0.95 (0.71, 1.28)0.750.91 (0.55, 1.48)0.691.21 (0.72, 2.04)0.47
Adjusting for all four risk factors for BCR and:
 Preoperative PSA0.93 (0.67, 1.28)0.650.88 (0.53, 1.47)0.631.11 (0.64, 1.94)0.72
 Age at SRT initiation0.98 (0.72, 1.32)0.870.93 (0.57, 1.52)0.761.26 (0.74, 2.13)0.39
 Time from RP to SRT initiation0.96 (0.71, 1.29)0.760.90 (0.55, 1.48)0.691.24 (0.73, 2.09)0.43
 Time from serum calcium
 measurement to SRT initiation
0.95 (0.70, 1.28)0.740.89 (0.54, 1.45)0.641.21 (0.72, 2.03)0.48
 Surgical margin0.93 (0.69, 1.26)0.630.88 (0.54, 1.44)0.611.16 (0.69, 1.96)0.57
 Pre-SRT hormone therapy0.94 (0.70, 1.26)0.660.86 (0.53, 1.40)0.551.20 (0.72, 2.02)0.48
 Year of start of SRT0.95 (0.70, 1.28)0.730.90 (0.55, 1.47)0.671.19 (0.70, 2.01)0.51

Relative risks, 95% confidence intervals, and P values result from the Cox proportional hazards regression models. Low pre-SRT serum calcium was considered to be ≤9.0 mg/dL. Moderate pre-SRT serum calcium was considered to be >9.0 mg/dL and ≤9.35 mg/dL. High pre-SRT serum calcium was considered to be >9.35 mg/dL. BCR: biochemical recurrence. RR: relative risk. CI: confidence interval. SRT: salvage radiation therapy. RP: radical prostatectomy.