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4R's of Radiobiology

-Dr Abish AdhikariMD Resident,

NAMS/BPKMCH

2012.11.26

H. R. Withers, The four r's of radiotherapy, Adv. Radiat. Biol. 5 (1975) 241–247.

abishadh @ gmail . com

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Definition

The most important biological factors influencing the responses of tumours and normal tissues to fractionated treatment are often called the “four Rs”:

● Repair (few hours)

● Reassortment (few hours)

● Repopulation (5 – 7 weeks)

● Reoxygenation (hours to few days)

Radiation Biology: A Handbook for Teachers and Students

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Introduction

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1. Repair of Radiation Induced DNA Damage

● Radiation generates Highly reactive oxygen species (hROS), from water molecules. They are short-lived and rapidly interact with biomolecules in cells.

● Those that are generated within 2 nm of the DNA are more important in causing DNA damage.

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Types of Damage

● Lethal—irreversible, irreparable, leads to cell death

● Sublethal (SLD)—repaired in hours; if a second dose is given, can interact with more damage to create lethal damage; represents shoulder on cell survival curve.

● Potentially Lethal Damage (PLD)—can be modified by the post-irradiation environment.

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There is a point at which an increase in the number of fractions will no longer increase survival—plateau in the response

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Repair

● Base Excision Repair● Nucleotide Excision Repair● Homologous Recombination● Non Homologous End Joining

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● Base Excision Repair

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Homologous Recombination

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DNA-dependent protein kinase Cernunnos-XLF/XRCC4/DNA ligase IV

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2. Redistribution / Reassortment

● Cells exhibit differential radiation sensitivity while in the different phases of the cell cycle.

● Cells in mitosis are most sensitive to DNA damaging agents and cells in late S-phase being most resistant.

● With multiple doses, cells progress through to a new phase of the cell cycle (sensitive)

● “Sensitization due to re-assortment” causes therapeutic gain.

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Mammalian cellsmitotic shake-offgive 6.6Gy at various timesdo survival

Resistance in S : Conformation of DNA. Sensitivity at G2: little time to repair.

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Experiments of Warren Sinclair: Survival curves during cell cycle

----- calculated for -hypoxic conditions of-Mitotic cells

M>G2>G1>early S>late S for sensitivity

Shoulder vs no shoulder

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3. Repopulation

● Damage and cell death occur during the course of the treatment may induce an increased rate of cell proliferation.

● Most important in early-responding normal tissues (e.g., skin, GIT)

● True for tumours as well. Accelerated repopulation can occur in the later part of a course of fractionated therapy. (HNSCC: 3 - 4 wks)

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Repopulation

● Influences local tumor control in HNSCC or cervical cancer.

● Local control is reduced by ~0.5% for each day that overall treatment time is prolonged.

● Rationale for accelerating fractionated radiation therapy.

● Overall treatment time would be expected to be less important for slower-growing tumors such as prostate or breast cancer.

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Accelerated Repopulation

● Clonogenic cells which survive radiation treatment may repopulate the tumor quickly.

Hermens & Barendsen (1969)

● Following a single radiation dose of 15 – 35 Gy there was cleare evidence of acclerated repopulation. Speed increased with bigger doses.

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4. Reoxygenation

● Sensitivity to radiation increases with oxygen.

● Tumors under 1 mm in size are fully oxic, but tumors over this size develop regions of hypoxia.

●

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Reoxygenation Mechanisms

● Reopening of temporarily occluded blood vessels (minutes).

● Reduced respiration of lethally damaged cells (minutes to hours).

● Resorption of dead cells leads to decreased distance from capillaries to tumor cells, improving their oxygen supply (days).

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OER

● Oxygen Enhancement Ratio● Ratio of radiation doses in hypoxic and

aerated conditions to get the same biological effect.

● X-Rays/γ-Rays : maximum ~ 2.5-3.0● OER is less than 2 in G1 phase.● ~1 for High LET radiations (alpha particles)

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Summary

● Reassortment, Repair, Reoxygenation are all benefits of fractionation.

● Repopulation is the negative associated with fractionation of radiation.

● Repair occurs in normal cells and tumor cells.● Reassortment occurs in cycling cells—mostly

tumor but some normal cells● Reoxygenation occurs only in tumor cells.● Repopulation occurs in the tumor cells.

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But

● Repair and Repopulation tend to make the tissue more resistant to second dose of radiation.

● Reassortment and Reoxygenation tend to make it more sensitive.

● The overall sensitivity of the tissue depends on:

The Fifth 'R' : Radiosensitivity

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Radiosensitivity

Seibert, 1996.

● In 1906 Bergonie and Tribondeau realized that cells were most sensitive to radiation when they are:

● Rapidly dividing

● Undifferentiated

● Have a long mitotic future

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The relative susceptibility of cells, tissues, organs, organisms, or other substances to the injurious

action of radiation.NRC

Radiosensitivity

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Thank you

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Cause of Radiosensitivity

● High metabolism of the tumor cells was early recognized as a prominent factor in radiosensitivity.

● Radiosensitivity can be judged by rate of growth.

● Increased or unstable vascularity also goes with rapid growth,

● So that three factors are generally combined to render rapidly growing tumors sensitive to radiation. Annual Meeting, at Chicago, Dec. 3–7, 1928.

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Radiosensitivity

● 17 tumor types were placed in 5 categories.● Categories A to E with decreasing sensitivity.● A: Lymphoma, Myeloma, Neuroblastoma.● B: Medulloblastoma, SCLC● C: Breast, Bladder, Cervix● D: Pancreas, Colo-Rectal, Squamous Lung.● E: Melanoma, Osteosarcoma, Glioblastoma, RCC

The radioresponsiveness of human tumours and the initial slope of the cell survival curve. Deacon J, Peckham MJ, Steel GG. Radiother Oncol. 1984 Dec;2(4):317-23.