***Chapters 25 & 27 Flashcards
What type of radiation are neutrons?
Neutrons are high LET, indirectly ionizing radiation.
Describe the survival curve for high LET radiation compared to low LET.
High LET radiation has a steeper survival curve with a smaller shoulder region.
What factors increase RBE?
RBE is higher for:
- Lower dose (more efficient at killing than high dose)
- Lower dose rate
- Higher LET
- Tissues with higher sublethal damage repair
For what reasons might we want to treat with neutrons?
Tumors with lots of sublethal damage repair behave more like late-responding tissues than early-responding tissues. Since neutron RBEs are much higher for late effects than early effects, slowly proliferating tumors are more damaged by neutrons than rapidly proliferating ones are.
What types of radiosensitivity factors become less important with high LET radiation therapy?
Radiosensitivity for the following are reduced with high LET:
- hypoxia
- cell cycle position (S vs G2/M)
- Q cells vs P cells
Slow growing radiation resistant tumors should show net therapeutic gain when treated with high LET radiation.
What is the definition of therapeutic ratio?
The ratio of the tumor response to the normal tissue response for a given dose.
Therapeutic ratio= Tumor response/Normal tissue response
When is it a good option to use high LET? When tumor cells are more radiosensitive than critical normal cells, or when they are less radiosensitive?
When tumor cells are less radiosensitive.
If tumor cells are less radiosensitive than critical normal cells, high LET RT could reduce difference in radiosensitivity and therefore for the same level of normal tissue damage, ‘sensitize’ tumor cell population.
What are the radiobiological advantages to using protons?
There are no radiobiological advantages to using protons. RBE and OER is same as for 250 kV x-rays.
What are the physical advantages to using protons?
The Bragg peak is advantageous because you can tailor the beam to cover the tumor volume while minimizing dose to surrounding organs by varying the energy.
What is meant by cell cycle specific or cell cycle non-specific agents?
This refers mainly to chemotherapy. Cell cycle specific drugs are mainly effective during a particular phase of the cell cycle, such as S or M.
How does oxygen and hypoxia affect chemotherapy response?
Oxygen enhancement can go either way with chemo drugs. More oxygen can either help or hinder the tumor control depending on the drug.
How do chemo drugs target cancer?
a therapeutic gain requires a differential betweeen tumor and normal tissue. This may be achieved by exploiting some of these characteristics:
- rapid proliferation
- cell age distribution
- hypoxia
- genetic instability
- pH
- elevated specific pathways
