CANCER AND LOW DOSE RESPONSES IN VIVO: IMPLICATIONS FOR RADIATION - - PowerPoint PPT Presentation

CANCER AND LOW DOSE RESPONSES IN VIVO: IMPLICATIONS FOR RADIATION PROTECTION

Ron Mitchel

Radiation Biology and Health Physics Branch Atomic Energy of Canada Limited, Chalk River Laboratories, Chalk River, ON, K0J 1J0 Canada

The Linear Dose-Risk Function

(risk of cancer from irradiation) R = risk in exposed tissue D = absorbed dose R D R = • D

?

Linear No Threshold Hypothesis Implies:

• 1. Risk is determined by the physics
• 2. Biological inputs to the risk

are either constant with dose

• r irrelevant at all doses

Radiation at Low Doses

• Dose = energy/unit mass
• Radiation deposits energy in

tracks

• The lowest dose a cell can receive

is one track

• At doses < 1 track/cell, not all

cells are hit. Those that are hit still receive 1 track

Linear No Threshold Hypothesis is Accepted by Regulatory Agencies ISSUE

Is the LNT hypothesis true for cancer risk at low doses??

Radiation Protection: LNT Hypothesis

• Dose is a surrogate for risk
• Risk per unit dose is constant without a

threshold, overall and for each tissue

• Dose (risk) is additive (normalized

using Sv) and can only increase

• At low doses and dose rates risk is

reduced 2 fold (DDREF=2)

Cancer

Does the LNT approach adequately predict risk at all doses for:

• Normal individuals
• Cancer prone individuals

A radiation exposure is a change in the environment that creates a stress The Basic Rule of Biology In a Changing Environment: “Adapt or Die”

ADAPTIVE RESPONSE

Exposure

• f cells or animals to radiation

at a low dose and dose rate induces mechanisms that protect against the detrimental effects

• f other events or agents,

including radiation

Radiation-Induced Chromosome Breaks

Broken Chromosomes in Micronuclei

Micronucleus frequency (%)

2 0 4 0 6 0 8 0 1 0 0 1 2 0

Control 4 Gy 1 mGy - 3h - 4 Gy 500 mGy - 3h - 4 Gy 1 mGy 500 mGy 5 mGy 25 mGy 100 mGy 5 mGy - 3h - 4 Gy 25 mGy - 3h - 4 Gy 100 mGy - 3h - 4 Gy

Ability to Repair Broken Chromosomes in Cells Adapted by Exposure to Low Doses

No Adaptation in Human Cells at High Dose Rate

0.77 Gy/min

60Co

Broome, Brown and Mitchel. Radiat. Res. 158, 181-186 (2002)

0.1 mGy 5 mGy 4 Gy 0.1 mGy + 3h + 4 Gy 5 mGy + 3h + 4 Gy 10 20 30 40 50 60 70 80

Dose Micronucleus Frequency

Broome, Brown and Mitchel. Radiat. Res. 158, 181-186 (2002)

Sub-critical Dose for Adaptation in Human Cells

2 4 6 8 10 12 14 16 18 0-3-0 1-3-0 10-3-0 100-3-0 100-6-0 0-3-4 1-3-4 10-3-4 100-3-4

Adapting (mGy)- Interval (h)- Challenge (Gy) BNCs with micronuclei (%)

Adaptation in Whitetail Deer Cells

Ulsh, Miller, Mallory, Mitchel, Morrison, and Boreham J Environ Radioact 74, 73-81 (2004)

1 2 3 4 1 2 3 4 5 10 15 20 25

Frequency of Unrepaired Chromosomes

Environmental Adaptation in Frogs in vivo

• M. Stuart, AECL, unpublished
• 1. Control
• 2. Adapting dose (1-100 mGy )
• 3. Test Dose (4 Gy) 4. Adapting + test dose

Background Background + 1 mGy/y 3H

Adaptation to radiation shown in:

• Single cell organisms
• Insects
• Plants
• Lower vertebrates
• Mammalian cells including human

This is an Evolutionarily Conserved Response

Treatment Transformation Frequency (x 10-4) Control 3.7 4 Gy (high dose rate) 41 100 mGy (low dose rate) +24h + 4 Gy (high dose rate) 16

Low Doses Protect Cells Against Malignant Transformation by High Doses

1.8 0.53 0.42 0.53 Control 1.0 mGy 10 mGy 100 mGy Transformation Frequency (x 10-3) Treatment

The Influence of Low Doses On the Risk of Spontaneous Malignant Transformation

Transformation in Human Cells

• J. L. Redpath and R.J. Antoniono,
• Radiat. Res. 149, 517-520 (1998)

Dose (mGy)

200 400 600 800 1000

Transformation Frequency (x10-5)

2 4 6 8

5 1 0 1 5 2 0 2 5

3 G y 1 0 c G y + 2 4 h + 3 G y

Apoptosis Frequency I n d i v i d u a l

A ( e x p t 1 ) A ( e x p t 2 ) B C x

Low Doses Sensitize Non-Dividing Human Lymphocytes to Apoptosis

Number of Individuals Sensitivity Increase 18 27.5 ± 5.7 % 8 7.0 ± 3.0 %

Increased Sensitivity for Radiation-Induced Apoptosis in Human Lymphocytes Previously Exposed to a 10 cGy Adapting Dose

Is This Genetic Variation?

The Percentage of Human Lymphocytes Expressing IL-2 Receptors 24 h After Stimulation

BYSTANDER EFFECT

Control Cells Irradiated Cells (10 mGy) 50% Control Cells + 50% Irradiated Cells 7.7 ± 4.1 17.8 ± 3.3 p<0.01 22.6 ± 4.8 p<0.01

• Y. Xu, C.L. Greenstock, A. Trivedi and R.E.J. Mitchel

Radiation and Environmental Biophysics 35: 89-93 (1996)

DO THESE RADIATION-INDUCIBLE ADAPTIVE PROCESSES PRODUCE PROTECTIVE EFFECTS IN VIVO??

y = -39x + 378 R2 = 0.99 y = -32x + 583 R2 = 0.99

200 400 600 800 1 2 3 4

Dose (Gy)

D ays at R isk (M ean +/- S .E .)

LOSS OF LIFE FROM HIGH DOSE EXPOSURE IN NORMAL AND Trp53 +/- MICE

Trp53+/+ Trp53+/-

y = -41x + 629 R2 = 0.99 y = -47x + 413 R2 = 0.99

200 400 600 800 1 2 3 4

Dose (Gy) Days at Risk (Mean +/- S.E.)

LOSS OF LIFE FROM HIGH DOSE EXPOSURE IN NORMAL AND TRP53 +/- MICE WITH CANCER

• R. E. J. Mitchel et al. unpublished

Trp53+/+ Trp53+/-

LOSS OF LIFE IN NORMAL AND Trp53 +/- MICE WITH LYMPHOMAS

y = -46x + 379 R2 = 0.94 y = -48x + 625 R2 = 0.91

200 400 600 800 1 2 3 4

Dose (Gy)

D ays at R isk (M ean +/- S .E .)

Trp53+/+ Trp53+/-

SKIN TUMORS IN MICE

Protection by Radiation Against Chemical Tumor Initiation

2.04 0.39 MNNG Beta Radiation (0.5 Gy) Beta + MNNG Tumors per Animal Initiation Treatment

• Fig. 4A Mitchel et al.

TIME (days)

200 400 600 800 1000

SURVIVAL PROBABILITY

0.0 0.2 0.4 0.6 0.8 1.0

Myeloid Leukemia in Genetically Normal Mice

Control and 1 Gy ML Neg. 1 Gy ML Pos. 100 mGy + 24h + 1 Gy ML Pos.

Trp53+/+

0.2 0.4 0.6 0.8 1 50 150 250 350 450 550

Time (days) Survival Probability Lymphomas

4 Gy acute 10 mGy + 4 Gy acute 100 mGy + 4 Gy acute 0 Gy

Lymphomas in Cancer-Prone Mice

Trp53 +/-

Lym phom a Latency

Tum or Latency (days)

200 400 600 800

Number of Tumors

10 20 30 40 0 Gy Trp53 +/- 10 m Gy Trp53 +/- 100 m Gy Trp53 +/- 0 Gy Trp53 +/+

Tumor Latency (days)

200 300 400 500 600 700

Number of Spinal Osteosarcomas

5 10 15 20 25

• Spinal Osteosarcomas in Trp53+/- Mice

Bottom Line Low doses of low LET radiation, at low dose rate, reduce, not increase, risk in vivo

IMPLICATIONS for LNT

• High dose responses cannot be

extrapolated to low doses

• At low doses in vivo, cancer risk

is not proportional to dose

• Dose thresholds for increased risk

exist in both normal and cancer prone individuals

• Cancer susceptibility modifies

threshold (zero risk) dose

IMPLICATIONS FOR DOSE ADDITIVITY

• Radiation protection assumes

dose (i.e risk) additivity (Sv)

• If some doses protect,

doses are not additive

• Further complicated if some doses

protect some but not all organs

IMPLICATIONS FOR DDREF

• Assumed DDREF = 2

for increased risk

• If risk from low dose/dose rate

0 then DDREF may =

IMPLICATIONS FOR Wt

• Tissue Weighting Factors (Wt)

are not constant with dose

• Wt changes from positive values

through zero to negative values as dose decreases

• Cancer proneness and/or

a second dose modify Wt

IMPLICATIONS FOR WR

• Radiation weighting factors (WR)

for high LET are based on the RBE ratio with low LET

• BUT: Low doses of low LET are

protective in vivo (negative risk)

• THEREFORE: At low doses WR and

dose in Sv have no meaning

IMPLICATIONS FOR “ALARA”

Preventing an exposure that would induce an adaptive response will INCREASE risk!

THE BIG QUESTION

Regulations are based on human epidemiological data: So Why is the accepted human epidemiological data inconsistent with data from all other organisms???

Statistically significant increases in cancer risk in humans can be only detected down to about 100mGy

• 100 mGy is very close to the transition point

between protection and harm in rodent and human cells and in mice.

• The assumptions of the LNT

hypothesis and radiation protection practices are not compatible with the

• bservations in vitro or in vivo
• Environmental assessments must

Environmental assessments must consider real effects consider real effects

• A new approach to radiation protection

at low doses is needed

RADIATION PROTECTION CONCLUSIONS

POTENTIAL REGULATORY SOLUTION

Adopt “Linear With Threshold” hypothesis