Oxygen Effect & Reoxygenation

Question 1

Oxygen Enhancement Ratio

Answer 1

Ratio of hypoxic to aerated doses needed for same biological endpoint

Question 2

OER and LET relationship

Answer 2

Oxygen effect is more pronounced in low-LET radiations and non-existent (OER = 1) in high-LET radiations

Question 3

Chronic Hypoxia

Answer 3

Limited diffusion distance of approximately 70 micrometers through respiring tissues. Cells this distance from blood vessel are hypoxic.

Tumors may outgrow blood supply and have O2 starved regions

Question 4

Acute Hypoxia

Answer 4

Blood vessels can be temporarily shut down, resulting in intermittent hypoxia in tissues, and then rapidly re-open to supply tissues with O2

Question 5

Reoxygenation during fractionated treatments

Answer 5

As cells die, hypoxic cells within the tumor obtain more oxygen which will make the next dose fraction more effective

Question 6

OER for photons

Answer 6

About 3 at high doses

About 2 at lower doses (<2 Gy)