This was a confusing curriculum topic. I think it refers to radiation carcinogenesis in particular, rather than the general carcinogenesis implied in the subtopics; these have been covered extensively elsewhere. So I have focused on radiation carcinogenesis.

Evidence for Human Carcinogenesis

Hall lists 'early' and 'recent' events which led to the understanding of radiation carcinogenesis.

I think the 'recent' events sound quite a long time ago!

Early events

• Skin cancer and leukaemia in early radiation workers (eg: Marie Curie)
• Lung cancer in uranium ore miners
• Bone tumours in radium dial painters
• Liver tumours in patients injected with radioactive thorium

I'm sure it sounded like a good idea at the time…

Recent events

• Survivors of atomic attacks in Hiroshima/Nagasaki
• Patients treated with radiotherapy for ankylosing spondylitis
• Increased leukaemia in radiologists from the 1920s
• Children who were treated with radiation for physiologically enlarged thymus glands, or for tines capitis
• Women treated with repeated iatrogenic pneumothorax, imaged with repeated fluoroscopy

Summary of human evidence

The various populations exposed to ionizing radiation have shown an increase in the number of cancers. The only group used for determination of risk is the atomic bomb survivors, as they were an otherwise healthy group of individuals of varying age and gender, and for whom relative whole body doses could be calculated.

Factors effecting risk of radiation carcinogenesis

Dose

Dose is an important factor in carcinogenesis. As dose increases, there is a linear increase in the relative risk of malignancy. At low effective doses (under 0.5 Sv) there appears to be a region of supralinearity where the relative risk is higher at these doses compared with higher doses.

High doses

The doses used in radiotherapy are a special case. There are conflicting reports of cancer risk, compounded by the higher second cancer risk expected in the population due to their genetic and environmental factors.
Recent studies have suggested that there is an elevated risk of malignancy. This includes analysis of cancer populations and comparisons between patients treated with and without radiation. The risk appears to plateau at about a factor of three, at doses approaching 10-20 Gy. Some studies suggest that the relative risk in certain tissues may be even higher.

Dose Rate

There is some evidence that lower dose rates, or fractionated doses, may cause less carcinogenesis than a single dose at a high dose rate. This effectively halves the risk for lower dose rates when compared with high dose rates.

For low doses or low dose rate, the risk of carcinogenesis is $4 \times 10^{-2}$ per Sv (for a working population). If the entire population is included, the risk is $5 \times 10^{-2}$ due to the presence of children.