Fractionated Radiation And The

Dose-rate Effect

PARAG ROYDEPT OF RADIOTHARAPY

LOK NAYAK HOSPITAL

• Classifications of Radiation Damage• Potentially Lethal Damage Repair• Sublethal Damage Repair

• Mechanism of Sublethal Damage Repair• Repair and Radiation Quality• The Dose-Rate Effect• Examples of Dose-Rate Effect In Vitro and

In Vivo• The Inverse Dose-Rate Effect

Overview

Classifications of Radiation Damage

• Radiation damage to mammalian cells can operationally be divided.• (1) Lethal damage (LD)• Irreversible and irreparable• Leads irrevocably to cell death

• (2) Potentially lethal damage (PLD)• Can be modified by post-irradiation

environmental conditions• (3) Sublethal damage (SLD)• Can be repaired in hours unless additional SLD is

added.

Potentially Lethal Damage Repair

• Potentially lethal : under ordinary circumstances, it causes cell death.

• Manipulation of post-RT environment – PLD can be repaired

• Repaired if cells are incubated in a balanced salt solution.

• But this treatment does not mimic a physiologic condition

• If mitosis is delayed by suboptimal growth conditions, DNA damage can be repaired.

PLD Repair……• X-ray survival curves for

densityinhibited stationary-phase cells, subcultured (trypsinized and plated) either immediately or 6 or 12 hours after irradiation.

• Cell survival is enhanced if cells are left in the stationary phase after irradiation, allowing time for the repair of potentially lethal damage

PLD repair …..

PLD & Radioresistance• Radioresistance of certain types of human

tumors is linked to their ability to repair PLD• Radiosensitive tumors repair PLD inefficiently• Radioresistant tumors have efficient

mechanisms to repair PLD. • This is an attractive hypothesis, but never

been proven.

Sub Lethal Damage Repair

• SLD is the operational term• Increase in cell survival that

is observed if a given radiation dose is split into two fractions separated by a time interval.

• The increase in survival in a split-dose experiment results from the repair of sublethal radiation damage.

• In first few hours, prompt repair of SLD evident, but at longer intervals between the two split doses, the surviving fraction of cells decreases, reaching a minimum with about a 5-hour separation.

• “Age response function”- asynchronous population of cells exposed to a large dose of radiation, more cells are killed in the sensitive than in resistant phases. The surviving population of cells to be partly synchronized.

SLD Repair….

• Fig is a combination of 3 processes occurring simultaneously.• 1. the prompt repair of SLD.• 2. Reassortment• Progression of cells through the cell cycle.

• 3. Repopulation• Increase of surviving fraction resulting from cell

division.

SLD Repair……

Four R of Radiobiology• “Four Rs” of radiobiology• Repair• Reassortment• Repopulation• Reoxygenation

• The dramatic dip in the split-dose curve at 6 hrs caused by reassortment.• The increase in survival by 12 hrs because of

repopulation are seen only for rapidly growing cells.

• Dramatic dip in the curve at 6 hrs caused by reassortment.

• More repair in small 1-day tumors than in large hypoxic 6-day tumors.

• Repair is an active process requiring oxygen and nutrients.

• SLD repair in split dose experiment. • A: dose is delivered in two fractions there is an increase in cell

survival • B: The fraction of cells surviving a split dose increases as time

interval between the two dose fractions increases. interval in-creases from 0 to 2 hours, increase in survival results from SLD repair. In cells with a long cell cycle or that are out of cycle, there is no further increase in cell survival by separating the dose by more than 2 or 3 hours. In a rapidly dividing cell popu-lation, there is a dip in cell survival caused by reassortment.

Mechanism of SLD Repair

• Te repair of SLD is simply the repair of double-strand breaks.• Rejoin and repair of double-strand breaks.

• The component of cell killing that results from single-track damage is the same whether the dose is given in a single exposure of fractionated.• The same is not true of multiple-track damage.

Repair and Radiation Quality• The shoulder on the acute survival

curve and the amount of SLD repair indicated by a split-dose experiment vary with the type of radiation used.

• The effect of dose fractionation with x-rays and neutrons is compared in Figure

The Dose-Rate Effect• For x- or r-rays, dose rate is one of the

principal factors that determine biologic consequences of a given absorbed dose.• Lowered dose rate and extended exposure

time generally occur reduced biologic effect.• The classic dose-rate effect results from the

repair of SLD that occurs during a long radiation exposure.

Dose-Rate Effect……

• Continuous low-dose-rate(LDR) irradiation may be considered to be an infinite number of infinitely small fractions.• No shoulder, shallower than

for single acute exposures.

DRE In Vitro & In Vivo

• Survival curves for HeLa cells cultured in vitro and exposed to X-rays at high and low dose rates.• dose-rate effect from the repair of

SLD varies enormously among different types of cells• HeLa cells have small initial

shoulder.• As the dose rate is reduced, the

survival curve becomes shallower and the shoulder tends to disappear

• Chinese hamster cells• Broad shoulder, large dose-

rate effect.• There is a clear-cut

difference in biologic effect, at least at high doses, between dose rates of 1.07, 0.30, and 0.16 Gy/min.• Differences between HeLa

and hamster cells reflect differences in apoptosis.

DRE …….

• At LDR, the survival curves “fan out”.• Show greater variation of slope • Inherent radiosensitivity ( evident in

HDR)• And variant range of repair times of

SLD.

Dose Survival Curves…

• Response of mouse jejunal crypt cells irradiated with ɣ-rays from cesium-137 over a wide range of dose rates.

DRE……

Inverse Dose-Rate Effect

• Decreasing the dose rate results in increased cell killing.

• In HeLa cell, dose decreasing 1.54 to 0.37 Gy/h is almost as damaging as an acute exposure.• At low dose progress

through cell cycle and arrested in G2 (radiosen)• At higher dose they frozen

in phase of cell cycle (at start of radiation)• At higher dose rates, they

are “frozen” in the phase of the cycle they are in at the start of the irradiation.

Dose-Rate Effect • DRE resulting from repair of SLD,

redistribution, proliferation. • Dose-response curve for acute

exposures has broad initial shoulder.

• As dose rate is reduced, survival curve becomes progressively more shallow as more SLD is repaired, but cells are “frozen” in cell cycle

• As the dose rate is lowered further survival curve steepens again because cells progress through block in G2, but still cannot divide.

• further lowering of dose rate below this critical dose rate allows cells to escape the G2 block and divide

Take Home……• PLD repair can occur if cells are prevented from

dividing for 6 hours or more after irradiation• PLD repair is significant for x-rays but does not

occur after neutron irradiation.• Half-time of SLD repair in mammalian cells is about

1 hour, but it may be longer in late-responding normal tissues• SLD repair is significant for x-rays, but almost

nonexistent for neutrons.• dose rate is reduced, the slope of the survival

curve becomes shallower (D0 increases), and the shoulder tends to disappear.

Thank you