Hall Book Ch 5 (Fractionated Radiation and the Dose-Rate Effect) Flashcards
Radiation damage to mammalian cells can operationally be divided into three categories:
1) lethal damage, which is irreversible and irreparable and by definition, leads to irrevocably to cell death.
2) Potentially lethal damage (PLD), the component of radiation damage that can be modified by post irradiation environmental conditions; and environmental conditions.
3) Sublethal damage (SLD), which, under normal circumstances, can be repaired in hours unless additional SLD is added(e.g., from a second dose of radiation) with which it can interact to form lethal damage (SLD repair, therefore, is manifested by the increase in survival observed if a dose of radiation is split into two fractions separated by a time interval.
The component of radiation damage that can be modified by manipulation of
the post-irradiation conditions is known as ( ).
PLD (Potentially Lethal Damage Repair)
PLD (Potentially Lethal Damage) repair can occur if cells are prevented from dividing for ( ) after irradiation; this is manifested as an increase in survival.
This repair can be demonstrated in vitro by keeping cells in saline or plateau phase for ( ) after irradiation and in vivo by delayed removal and assay of animal tumors or cells of normal tissues.
6 hours or more, 6 hours
PLD (Potentially Lethal Damage Repair) repair is significant for ( ) but does not occur after ( ) irradiation.
It has been suggested that resistant human tumors (e.g., ) owe their resistance to large amounts of PLD repair. This is still controversial.
x-rays, neutron, melanoma
SLD (Sublethal Damage) repair is an operational term that describes the increase in survival if a dose of radiation is split into ( ) fractions separated in time.
The half-time of SLD repair in mammalian cells is about ( ), but it may be longer in late-responding normal tissues in vivo.
two, 1 hour
SLD repair occurs in ( ) tissues in vivo as well as in cells cultured in vitro.
The repair of SLD reflects the repair of DNA breaks before they can interact to form (
).
tumors and normal, lethal chromosomal aberrations
SLD (Sublethal Damage) repair is significant for x-rays but is almost nonexistent for (
).
neutrons
Dose-Rate Effect
If the radiation dose rate is reduced from about 1 Gy per minute to ( ), there is a reduction in the cell killing from a given dose because ( ) repair occurs during the (
).
0.3 Gy per hour, SLD (Sublethal Damage), protracted exposure
As the dose rate is reduced, the slope of the survival curve becomes shallower ( ), and the shoulder tends to ( ).
D0 increases, disappear
In some cell lines, an ( ) effect is evident (i.e., reducing the dose rate increases the proportion of cells killed) owing to the ( ), which is a sensitive phase of the cycle.
inverse dose-rate, accumulation of cells in G2
In general, cells are most radiosensitive in the ( ) and ( ) phase and most resistant in ( ) phase.
M, G2, late S
Brachytherapy
Implanting sources into or close to a tumor is known as brachytherapy (from the Greek word brachy, meaning “short”) or endocurietherapy (from the Greek word endo, meaning “within”).
Intracavitary radiotherapy involves placing radioactive sources into a body cavity close to a tumor. The most common example is the treatment of carcinoma of the uterine cervix.
Intracavitary brachytherapy at LDR may take ( ) days with a dose-rate of about (
).
1 to 4, 50 cGy per hour
To an increasing extent, LDR intracavitary brachytherapy is being replaced by ( ) intracavitary brachytherapy, delivered in ( ).
HDR, 3 to 12 dose fractions
Interstitial therapy involves implanting radioactive sources ( ) into the tumor and adjacent normal tissue for a period of ( ) days.
However, such techniques are little used nowadays.
Permanent implants can be used with radionuclides (such as ( )) that have relatively short half-lives.
Several novel radionuclides are being considered as sources for brachytherapy. Most emit low-energy photons, which simplifies the problems of radiation protection.
directly, 7 to 10, iodine-125 or palladium-103
