Radiation Flashcards
Cells in this phase(s) are most resistant to radiation
Late S-phase
Cells in this phase(s) are most sensitive to radiation
Late G2 and M phases
Which are more sensitive to radiation, normoxic or hypoxic cells?
Normoxic cells are up to 3x more sensitive to XRT than hypoxic
What does 1 Gy equal?
1 Gy = one joule absorbed per kilogram tissue
Photons interact w tissue primarily by which effect?
The Compton effect
The Compton effect produces high energy electrons which have 2 effects
- cause ionization events either to critical molecules (direct effect) or
- from water molecules located w/in nanometers of critical molecules (indirect effect)
Hyperfractionated radiation is what?
Hyperfractionated radiation: dose per fraction is reduced and total dose is increased
Accelerated fraction radiation is what?
Accelerated fraction radiation: overall time of treatment is reduced but dose per fraction and total dose are unchanged
Hypofractionated radiation is what?
Hypofractionated radiation: administration of high doses per fraction given in a small # of fractions to a lower dose
Name the 4 R’s of radiation therapy
o Repair of DNA damage
o Redistribution of cells in the cell cycle
o Reoxygenation of tumor cells
o Repopulation of tumor and normal tissues
Describe accelerated repopulation.
Accelerated Repopulation:
When radiation treatment lasts longer than 4 weeks, tumors may repopulate faster than otherwise.
What are the 4 criteria for radiation-induced neoplasia?
4 Criteria for Radiation-Induced Neoplasia:
- Malignancy must arise in radiation field
- Sufficient latency must have elapsed bt tx & tumor (usu > 1y)
- Original and new tumors must be different
- Tissue in which new tumor forms must have been normal tissue before RT exposure
The sensitivity of a tumor/tissue to radiation is shown as a graph of what?
The sensitivity of a tumor/tissue to radiation is shown as a graph of the radiation dose (D) vs the surviving fraction (S)
- S (D) = e-(aD+BD2)
- S = surviving fraction at dose D
- Alpha (a) and beta (B) are constants that vary according to the tissue with
- “a” corresponding to the cell death that increases linearly with dose and
- “B” corresponding to cell death that increases in proportion to the square of the dose (aka the quadratic component)
What is the alpha beta ratio?
The a/B ratio is a useful # that is the dose in Gy when cell kill from the linear and quadratic components of the cell survival curve is equal
• Cells w a higher a/B ratio have a more linear appearance on a log scale
• Cells w a lower a/B ratio have a parabolic shape
Do most early-responding tissues have a HIGH or a LOW a/B ratio?
Most EARLY responding tissues/tumors have a HIGH a/B ratio (more linear curve)
At LOW-dose fractions, are cells with a LOW a/B ratio radiation sensitive or resistant?
At low-dose fractions, cells w a low alpha/Beta ratio are radiation RESISTANT
At low-dose fractions, are cells with a high a/B ratio radiation sensitive or resistant?
At low-dose fractions, cells w a high alpha beta ratio (parabolic curve) are radiation SENSITIVE
Give two examples of megavoltage RT:
Both cobalt and linear accelerators are megavoltage RT
ISsthe absorption of megavoltage or orthovoltage RT dependent on tissue composition? What is the effect of this?
Orthovoltage is preferentially absorbed by bone, increasing chance of bone necrosis (limit tx to small, superficial tumors); Megavoltage is minimally dependent on tissue composition
Define CTV.
Clinical target volume (CTV) = GTV + an expansion based on known behavior of tumor accounting for microscopic disease
Define PTV.
Planning target volume (PTV) = CTV + expansion for internal margin (IM) accounting for variations in size/shape relative to anatomic landmarks (bladder filling, respiratory movements) and set-up margin (SM)
- SM margins vary based on tumor location bc some sites (HEAD) more amenable to rigid immobilization
- IM expansion is impacted more by RT device
