System (including exposure conditions and acute αa or LDRb)

End Point

Dose Range, mGy

Curve Shape

Frequency of Events per Viable Cell per Milligray

Comments and References

Human blood lymphocytes stimulated with PHA

Genomic instability chromosomal aberrations

1000–3000 (acute)


(3–10) × 10−5

Analyzed at 51–57 d after irradiation (Holmberg and others 1998)

Hamster embryo cells

Malignant transformation

30–1500 (acute)


4 × 10−6

LNT > ~30 mGy (Borek and others 1983)

C3H 10 T1/2 mouse cells (six labs)

Malignant transformation

250–5000 (acute)


8 × 10−8

LNT > ~250 mGy (Mill and others 1998)

Hela X skin fibroblast human hybrid cell system

Malignant transformation

0–1000 (acute)


4 × 10−8

Threshold at ~300 mGy dependent on time of trypsinization after irradiation (Redpath and others 2001)

NOTE: LDR = low dose rate; PCC = premature chromosome condensation; PHA = phytohemagglutinin.

aAcute indicates that doses were delivered at high dose rate (e.g., 0.1 Gy/min.), and α-component signifies the value of α in the linear-quadratic relationship.

bLDR indicates that the doses were delivered at low dose rates less than 0.01 Gy/min.

cLNT signifies a linear, no-threshold dose-response relationship.

dDDREF is defined and illustrated in Figures 2-1 and 2-8.

Three mutation experiments have yielded a linear dose-response relationship. First, the loss of an antigen marker on human chromosome 11 integrated in Chinese hamster cells and exposed to four different doses from 250 to 1500 mGy yielded a linear dose-response relationship with a slope of 7 × 10−6 mutants per viable cell per milligray (Puck and Waldren 1987). The relatively high frequency is due to the large target size because of the large distance between the antigen marker and essential genes on chromosome 11 (see “Induction of Gene Mutations in Somatic Cells”).

Second, human lymphoblast cells (TK6) exposed to one acute dose or to daily doses of 10, 25, 50, or 100 mGy for up to one month, with samples taken every 5 d, yielded a linear dose-response relationship for induction of HPRT or TK mutations (Figure 2-7). Over a total dose range of 50–2000 mGy, the slope for HPRT mutations was 6 × 10−9 mu-

FIGURE 2-5 Dicentric yields as a function of dose; ●, Pohl-Ruling and others (1983); , Lloyd and others (1992), experiment 1; □ experiment 2. SOURCE: From Lloyd and colleagues (1992).

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