L 6: O2 effect and RE-O2

Question 1

Rapidly growing cells in vitro

Answer 1

smaller OER
2.5 at lower doses

Question 2

Variation of OER with cell cycle

Answer 2

G1 Phase: Lower OER than S
As G1 phase cells are more radiosensitive.

Question 3

OER
O2 enhancement ratio

Answer 3

• The ratio of doses under hypoxic to aerated conditions necessary to produce the same level of killing.
• Normal value is close to 3.5
• x-rays/gamma rays = 2.5-3.0
• OER is never less than 1 as hypoxic cells are always looking to oxygenate.
• OER is lower for single hit mechanisms
• Depends on type of radiation and dose of radiation - look at the figure.
• High LET is not dependent on O2 as it is direct action and not indirect action with hydroxyl molecule involvement.

Question 4

How does O2 work?

Answer 4

Damage produced by free radicals in DNA can be repaired under hypoxia but the damage is made permanent or irreparable in presence of O2.

Question 5

RT sensitivity in presence of O2

Board question - picture

Answer 5

• 3 times more sensitive
• Maxs out at 20/40 mmHg in venous blood.
• 3mmHgor 0.5% O2.

Question 6

Acute Hypoxia

Answer 6

• temporary closing of blood vessels/random
  intermittent hypoxia

Question 7

Chronic Hypoxia

Answer 7

• This is due to the distance of tumor cells from the blood vessels
• Necrosis results from chronic hypoxia & necrotic center (dead cells) in many tumors.
• Hypothesized a portion of tumor would have dividing cells but have ↓02 and thus be resistant to radiation

Question 8

Hypoxic cells

Answer 8

• They are hard to treat with radiation.
• Hypoxic cells form a rim between necrotic center and viable cells in the periphery.
• decreased apoptotic potential & an increased metastatic potential.
• Hypoxic cells have an increased mutation frequency compared to oxic cells.

Question 9

Bi-Phasic Survival Curve

Answer 9

• The bi-phasic curve is due to resistance caused by hypoxic cells.
• This is the reason single dose is not a good idea for killing cancer cells.

Question 10

Re-Oxygenation

Answer 10

• Takes 6 hours to re-oxygenate that is the reason we wait atleast 6 hrs prior to next fraction
• Hypoxia confers resistance to X-rays/gamma rays – also to chemotherapeutic drugs.
• Human tumors that do not respond to radiotherapy may not re-oxygenate.
• Optimal fractionation regimen depends on reoxygenation.
• Survival goes down with increasing hypoxia for soft tissue sarcomas.
• PIMONIDAZOLE is the stain used for detecting hypoxic cells

Question 11

Evidence for Hypoxia in Human Tumors In Vivo

Boards question

Answer 11

• survival curves in vivo and in vitro are similar to those in which hypoxia is proven to play a role histology suggests hypoxia
• oxygen probe binding suggests hypoxic areas
• nitroimidazoles (hypoxic) bind
• Hb levels pretreatment are prognostic indicators in squamous carcinoma of head & neck, carcinoma of cervix, bronchus, and transitional cell carcinoma of bladder
• Predictive assays: oxygen probes

Question 12

Hypoxia Markers

Answer 12

• 2 Nitro-imidazole radio sensitizers, they bind irreversably to macromolecules in hypoxic cells.
• Pimonidazole

Question 13

Advantages of hypoxic markers when compared to O2 probes

Answer 13

1. They provide relative O2 concentrations.
2. They make it possible to distinguish between viable and necrotic tissue.
3. They make it possible to distinguish between acute and chronic hypoxia.

Question 14

Carbogen

Answer 14

95% O2 + 5% Co2

Question 15

What happens when a cell is Hypoxic

Answer 15

As hypoxia goes up metastatic potential goes up.

Question 16

Angiogenesis growth factors

Answer 16

• VEGf
• PDGF-BB
• FGF2 (bFGF)—proangiogenic agent that is synergistic with VEGF, may reduce endothelial cell apoptosis.
• Endostatin—inhibitor of angiogenesis, induces endothelial cell apoptosis and migration.
• FGF7 (KGF)—aids in angiogensis by stimulating epithelial cell growth, receptor is a therapeutic target.
• IL8—increases angiogenesis and metastasis.
• Heparin—inhibitor of angiogenesis
• HIF-1alpha—major transcription factor of VEGF

Question 17

HIF-alpha

Answer 17

Question 18

Summary

Answer 18

• Hypoxic cells are more resistant to the effects of ionizing radiation than oxic cells.
• Hypoxic cells are found in tumor cords with a radius of 200um.
• Tumor necrosis results from chronic hypoxia.
• Local control of tumors correlates with pre-treatment oxygen concentrations.

Question 19

ARCON Trial

Answer 19

Accelerated radiotherapy with carbogen and nicotinamide

• Carbogen breathing - Overcome chronic hypoxia
• Nicotinamie: Overcome acute hypoxia
• Accelerated treatment : Overcome proliferation
• Hyperfractionted dose: Minimize late effects to normal tissue