4Rs of Radiobiology -Dr Abish Adhikari MD Resident, NAMS/BPKMCH 2012.11.26 abishadh @ gmail . com 1H. R. Withers, The four rs of radiotherapy, Adv. Radiat. Biol. 5 (1975) 241–247.
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 2
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. 4
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. 5
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. 14
Mammalian cellsmitotic shake-offgive 6.6Gy at various timesdo survival 15 Resistance in S : Conformation of DNA. Sensitivity at G2: little time to repair.
Experiments of Warren Sinclair: Survival curves during cell cycle Shoulder vs no shoulder ----- calculated for -hypoxic conditions of -Mitotic cellsM>G2>G1>early S>late S for sensitivity 16
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) 17
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. 19
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. 20
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.● 21
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). 23
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) 24
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. 26
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 27
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 28
RadiosensitivityThe relative susceptibility of cells, tissues, organs, organisms, or other substances to the injurious action of radiation. NRC 29
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 31 radiation. Annual Meeting, at Chicago, Dec. 3–7, 1928.
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 32 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.