Hall 12 - Effects of RT on Embryo & Fetus

Question 1

When is the most radiosensitive stage of an embryo?

Answer 1

Preimplantation (0-9 days)
Most sensitive to lethal effects of RT
Threshold dose 0.1 Gy
»prenatal death

Question 2

What is the effect of RT on embryos in organogenesis?

Answer 2

10 days to 6 weeks
Structural (congenital) malformations

Threshold dose 0.1 Gy

*Does not increase the risk of prenatal death, but increases the risk of neonatal death

Question 3

What is the effect of RT on embryos in the fetal period?

Answer 3

6 weeks+
Permanent growth/mental retardation

Threshold dose 0.2 Gy

Question 4

When is the highest risk for mental retardation in utero from RT exposure? What is the risk and IQ decline per Gy?

Answer 4

8-15 weeks (fetal period)
40% per Gy
25 IQ points per Gy

*At 16-25 weeks, risk is 4x lower (10% per Gy)

Question 5

What are the risks of mental retardation in utero from RT exposure?

Answer 5

0-7 weeks = microcephaly
8-15 weeks = both
16-25 weeks = mental retardation

Question 6

At what time will a fetal thyroid incorporate radioactive iodine? What happens?

Answer 6

Week 10
Cretinism

Question 7

Once a pregnancy is declared, what is the maximum permissible dose to the fetus?

Answer 7

0.5 mSv/month subsequent to the declaration