TG-36 Fetal Dose Effects

Question 1

What are the sources of radiation outside a treatment volume?

Answer 1

1.) Leakage from treatment head
2.) Radiation scattered from collimators and beam modifying devices
3.) Radiation scattered from within the patient
4.) >10 MV photo neutrons that are produced in the treatment room and the LINAC equipment

Question 2

At what distance are scatter in the patient and head leakage approximately equal?

Answer 2

30cm

Question 3

At what distance does head leakage dominate?

Answer 3

> 30 cm

Question 4

Scatter from wedges increases the dose near the beam edge by a factor of what?

Answer 4

2 to 5

Question 5

At 10 cm from the field edge, the dose is __% the central axis dose

Answer 5

1%

Question 6

At 30 cm from the field edge, the dose is __% the central axis dose

Answer 6

0.2%

Question 7

In and near the beam neutrons are a negligible relative amount of dose as compared to the total dose (T/F)

Answer 7

True

Question 8

At greater distances from the primary beam, the relative dose from neutrons may jump to ___%

Answer 8

40%

Question 9

At the time of TG-36, the NCRP considered the biological risk to the fetus from neutrons as negligible (T/F)

Answer 9

True

Question 10

Pregnant patients should be treated with beams less than ___MV

Answer 10

10 MV

Question 11

What are adverse effects that radiation may cause to a fetus?

Answer 11

1.) Malformations
2.) Severe mental retardation
3.) Subsequent development of cancer

Question 12

What is the occupational dose to the mother for the entire pregnancy?

Answer 12

5 mSv

Question 13

Below ___cGy there is very little risk to the fetus at any gestational phase

Answer 13

5 cGy

Question 14

At ___ to ___cGy, the doses are so low that the risk is uncertain

Answer 14

5 to 10 cGy

Question 15

At ___ to ___cGy, there is significant risk to the fetus especially during the first trimester

Answer 15

10-50 cGy

Question 16

Above ___ cGy, there are very high risks to the fetus in any trimester

Answer 16

50 cGy

Question 17

___ cGy is enough to kill 50% of fetuses

Answer 17

100 cGy

Question 18

Risk of radiation during the Preimplantation (first week) gestational age

Answer 18

death of embryo

Question 19

Risk of radiation during the embryonic (1-8 weeks) gestational age

Answer 19

malformation of specific organs, growth retardation, sterility

Question 20

Risk of radiation during the early fetal (8-15 weeks) gestational age

Answer 20

severe mental retardation, and small head syndrome

Question 21

Risk of radiation during the mid fetal (15-25 weeks) gestational age

Answer 21

Severe mental retardation

Question 22

Risk of radiation during the late fetal (>25 weeks) gestational age

Answer 22

Probably subsequent cancer development

Question 23

Methods to decrease dose to the fetus while treating a pregnant patient

Answer 23

1.) Modify the field size and angle
2.) Use 3D planning instead of IMRT
3.) Use lower energy (less penetrating, easier shielding w/ less internal scatter)
4.)Design special lead shielding bridge over patients abdomen (50% reduction using 4 or 5 HVL)
5.) Don’t perform excess image alignment

Question 24

How could you verify dose to the fetus?

Answer 24

1.) Phantom measurements prior to treatment
2.) In-vivo dosimetry (top of uterus, mothers navel, cervix)