Table 1. SCD₅₀ values for HCT-116 and HT29 spheroids, which had been exposed to different treatment conditions for 5 days before irradiation.

Treatment	HCT-116		HT29
	SCD50, Gy	DRF [95% CI]	SCD50, Gy	DRF [95% CI]
+ Arg (control)	12.8	–	16.0	–
– Arg	7.0	1.82 [1.66–2.00]	9.5	1.86 [1.75–1.97]
– Arg + 0.05 mM Cit	8.2	1.54 [1.43–1.66]	10.4	1.53 [1.47–1.60]
– Arg + 0.4 mM Cit	12.1	1.06 [1.00–1.13]	16.4	n.s.
+ Arg + Cav	11.6	1.10 [1.03–1.16]	15.6	n.s.
– Arg + Cav	5.7	2.24 [2.06–2.42]	6.2	2.61 [2.45–2.79]
– Arg + 0.05 mM Cit + Cav	6.0	2.11 [1.97–2.26]	7.0	2.27 [2.11–2.42]
– Arg + 0.4 mM Cit + Cav	7.3	1.69 [1.57–1.81]	11.3	1.38 [1.31–1.46]

+Arg – control medium, arginine-free medium supplemented with 0.4 mM arginine; –Arg – arginine-free medium; Cit – citrulline; Cav – 0.1 mM canavanine; SCD₅₀ – spheroid control dose 50; DRF – dose reduction factor with 95% confidence interval (CI) relative to SCD₅₀ in control medium; n.s. – not significant.