1. 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.
2. 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
3. Introduction 3
4. 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
5. 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
7. There is a point at which anincrease in the number offractions will no longerincrease survival—plateau inthe response 7
8. Repair● Base Excision Repair● Nucleotide Excision Repair● Homologous Recombination● Non Homologous End Joining 8
9. ● Base Excision Repair 9
11. Homologous Recombination 11
12. 12DNA-dependent protein kinase Cernunnos-XLF/XRCC4/DNA ligase IV
14. 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
15. Mammalian cellsmitotic shake-offgive 6.6Gy at various timesdo survival 15 Resistance in S : Conformation of DNA. Sensitivity at G2: little time to repair.
16. 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
17. 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
19. 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
20. 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
21. 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
23. 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
24. 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
26. 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
27. 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
28. 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
29. RadiosensitivityThe relative susceptibility of cells, tissues, organs, organisms, or other substances to the injurious action of radiation. NRC 29
30. Thank you 30
31. 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.
32. 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.