Radiobiology of fractionated treatments: the classical approach and the 4 Rs Vischioni Barbara MD, PhD Centro Nazionale Adroterapia Oncologica
Radiobiology It is fundamental in radiation oncology
Radiobiology in radiation oncology
First fractionation experiments In multifraction radiotherapy schemes the dayly patients treatment dose is of 1.8-2 Gy
Radiobiology in radiation oncology It contributes to the definition of optimal radiotherapy schemes for patients
Tumor control Sigmoid curve Each radiation dose destroys the same proportion of clonogenic cells. The success of a radiotherapy scheme depends on the distruction of all the surviving clonogenic cells within the tumor 100% Tumor control probability% 0% dose
Therapeutic gain Normal tissue complication probability compared to tumor control probability Therapeutic gain when the 2 curves are separated
REPAIR 4R in radiobiology (Whiters 1975) REPOPULATION REDISTRIBUTION REOXYGENATION
Radiation effect DNA Liysosomes, endoplasmic reticulum, cytoplasmic and nuclear membrane, etc.) proteins
Radiation effect
Radiation effect at the DNA level Base damage Nucleotide damage SSB DSB Bulky lesions 1 2 Gy extensive base damage 1000 SSB 50 DSB Single Strand Breaks Base Damage Double Strand Breaks Appr. 30% of cells dies and the rest has been repired or are able to survive with a damaged genome Bulky Lesions
Cell fate after radiation Error-free repair Faulty repair No repair The damage is totally removed The damage causes mutations not lethal or lethal but in the longterm The damage is lethal for the cell Cell survival Neoplasia Cell death
First R: Repair Damaged DNA is enzimatically repaired after each fraction of a multifraction radiotherapy scheme Repair mechanisms in normal tissues works much better than in tumor tissues. It is convenient to fractionate the dose since more cells of the healthy tissue than tumor cells will survive after each fraction Therapeutic index: Healthy tissue tolerance dose Tumor lethal dose
Single Strand Break (SSB) repair Error-free mechanism of repair Unrepaired SSBs contribute to DBS damage
Double strand break (DSB) repair Homologous Recombination Non-Homologous End joining
Clonogenic activity study
In vitro test for clonogenic activity
Clonogenic activity study Cell survival curves considers Radiation dose Cell clonogenic activity (surviving fraction of irradiated clonogenic cells) The shape of the curve is characteristic for each cell population and express specific radiosensitivity
Clonogenic activity study Dose response curve depends on: Cell population type Radiation quality Oxygen level and temperature drugs
Cell fate after IR lymphocytes/ endothelial cells fibroblasts/ pneumocytes many normal cells many tumor cells Apoptosis Permanent arrest Reversible arrest and DNA repair Short-term arrest and attempted DNA-repair Resumed proliferation Attempt to resume proliferation Gudkov, Nature 2003 / modified OK Mitotic catastrophe
Mathematical models of the radiobiological effect Radiobiological models can help to predict clinical outcomes when treatment parameters are altered They have assumptions: Cell death after radiation connected to abrogation of cell reproductive activity At least one DSB in DNA is responsible for cell death
Cell survival curves and the linear-quadratic model
Cell survival curves and the linear-quadratic model component Linear variation with dose (Gy -1 ) Lethal damage DSB Especially for cells with impaired DDR machinery Predominant for high LET radiation component Quadratic variation with dose (Gy -2 ) Damage can be repaired SSB Especially for cells with good DDR machinery
/ ratio / ratio is the dose at which the linear component of the damage is equal to the quadratic component / ratio defines the bending of the survival curve / ratio high Lethal damage Curve linear at origin / ratio low Damage can be repaired Curve with shoulder at the beginning
/ ratio / ratio high Early responding normal tissues Proliferating tissues skin Mucosae Bone marrow Fast growing tumor / ratio low Late responding normal tissues Tissues not proliferating kidney liver Central nervous system Slow growing tumor
/ ratio and isoeffect relationship / ratio high No fractionation sensitivity / ratio low Fractionation sensitivity
Linear-quadratic model and BED D = dose totale d = dose per frazione BED (biologically equivalent dose) To calculate isoeffect relationship To compare different fractionation schemes To sum up doses given to the same patients with different fractionation
Cell survival curves and the linear-quadratic model
Radiobiological basis of fractionation / RATIO high / Ratio- early reacting tissues squamous cell ca acute normal tissues -- total dose Low / Ratio- Late reacting tissues late normal tissues -- total dose and dose/fraction
Altered fractionation schemes Hypofractionation Hyperfractionation - low dose/fraction - higher total dose - more fraction/day (6 h) Continuous Hyperfractionation - less total time (accelerated)
Radiobiological basis of fractionation Large dose/fraction (hypofractionation) increase the RT effect Less in the tissues with high / RATIO Less damage can be repaired within each fraction Large dose/fraction more toxic to tissues with low / ratio compared to tissues with high / ratio
Radiobiological basis of fractionation Small dose/fraction (hyperfractionation) has reduced effect in the tissues with low / RATIO More damage can be repaired within each fraction Small dose/fraction protects tissues with low / ratio compared to tissues with high / ratio
Fractionation sensitivity of different tumors in the clinical setting Tumor fractionation sensitivity Definition Optimal fractionation schedule Clinical level of evidence Reference Low / ratio of ca higher than that of late responding healthy tissues More, smaller-sized fr. with higher total dose, or fr. given over a shorter time course-> improves LC, same late tox, more acute tox. Level I evidence for improved therapeutic ratio in head and neck and lung ca Nguyen et al.,2002 Overgaard et al., 2003 Saunders et al., 1999 Moderate to high / ratio of ca similar or slightly higher than that of late responding healthy tissues Fewer, larger-sized fractions might achieve same LC and late toxicity as conventional fractionation Level II evidence for therapeutic ratio equivalent to conventional scheme in BREAST CA Yarnold et al., 2005 Owen et al., 2006 Whelan et al., 2002 START A, 2008 START B, 2008 High / ratio of ca lower than that of late responding healthy tissues Fewer, larger-sized fr-> improve LC with similar or reduced late and acute tox effects Level III evidence for therapeutic ratio equivalent to conventional fr. In prostate ca Fowler, 2005
Dose fractionation and the 4 R 1. Repair of the damage 2. Repopulation: For tumour cells this repopulation partially counteracts the cell killing effect of radiotherapy The repopulation time of tumour cells appears to vary during radiotherapy - at the commencement it may be slow (e.g. due to hypoxia), however a certain time after the first fraction of radiotherapy repopulation accelerates. Repopulation must be taken into account when protracting radiation e.g. due to scheduled (or unscheduled) breaks such as holidays. Also normal tissue repopulate - this is an important mechanism to reduce acute side effects from e.g. the irradiation of skin or mucosa
3. Redistribution Dose fractionation and the 4 R 4. Reoxygenation: at each fraction oxygenated cells will be killed and hypoxic cells will replace the dead cells in more oxygenated parts of the tumor progressively reducing the final tumor mass
New frontiers to increase the therapeutic gain: hadrontherapy No fractionation sensitivity Effect not dependent on cell cycle, oxygenation
New frontiers to increase the therapeutic gain: radiogenomics Research on factors that increase sensitivity to different fraction size and radiation type Allow to add drugs to treatment