CMD Radiation Physics & Radiobiology 2

Question 1

Which effect describes the increased radiosensitivity of cells in the presence of molecular oxygen?

Accepted Answer

Oxygen effect

Answer

The oxygen effect refers to the 2–3 fold increase in radiation sensitivity when cells are well-oxygenated compared to hypoxic conditions, because oxygen fixes radiation-induced free radical damage.

Question 2

In conventional fractionation for radiation therapy in the US, the standard daily fraction size is:

Accepted Answer

1.8–2.0 Gy

Answer

Conventional fractionation uses 1.8–2.0 Gy per fraction, delivered once daily, five days per week, balancing tumor control with normal tissue sparing.

Question 3

Which quantity describes the rate of energy deposition by a charged particle per unit path length?

Accepted Answer

Linear Energy Transfer (LET)

Answer

LET (keV/μm) describes the energy deposited per unit length along a charged particle's track and determines the biological effectiveness of different radiation types.

Question 4

The 'Four Rs of Radiobiology' include repair, reassortment, repopulation, and:

Accepted Answer

Reoxygenation

Answer

Reoxygenation is the fourth R, where hypoxic tumor cells become oxygenated between fractions as adjacent cells die, increasing overall radiosensitivity.

Question 5

Which cell cycle phase is most radiosensitive?

Accepted Answer

M phase (mitosis) and G2 phase

Answer

Cells are most radiosensitive in M and late G2 phases because DNA repair mechanisms are suppressed and chromosomal aberrations are most lethal at this stage.

(CMD) Certified Medical Dosimetrist Practice Test

(CMD) Certified Medical Dosimetrist Practice Test

CMD Radiation Physics & Radiobiology 2