Cell, Tissue and Tumor Kinetics

Cell, Tissue and Tumor Kinetics Proliferation Kinetics: rate of growth of a population, change in total cell number. Adult tissues are in homeostasis. Children (and tumors) grow. [Image removed due to copyright considerations] I. Quantitative Assessment of Parts of the Cell Cycle Mitotic Index (MI): proportion of cells in mitosis (count directly) MI = TM/TC where: TM = time for mitosis TC = total cell cycle time Assumes: • All cells in the population are dividing • All cells have the same cell cycle times [Image removed due to copyright considerations] • Cells are uniformly distributed around the cell cycle (probably not) Labeling Index (LI): proportion of cells in S phase LI = TS/TC • Same assumptions as for MI. • Can be determined by pulse-labeling (10-30 min) cells with DNA precursors (3H-thymidine or with BrdUrd), fixing, staining as appropriate and counting labeled cells. • Note that both MI and LI are ratios, not actual duration of phases of the cell cycle. Percent-labeled-mitoses. Label cells in one part of the cell cycle. [Image removed due to copyright considerations] Observe passage of these cells through another part of the cell cycle. Requires staining and scoring multiple samples from the original population. b ab bc bd TC TG1 first labeled mitotic figures appear = TG2 = TM = TS = ce = peak 1 - peak 2 = TC – (TS – TG2 – TM) Real data shows asymmetric peaks. [Image removed due to copyright considerations] Typical values for duration of parts of cell cycle: TC 10 h–10 days TG1 1–150 h TS 6-10 h TG2 1-2 h TM 1 h Cell population has a variety of cell cycle times. If TC is long, variability is greater; second peak may not be distinct. • TS is similar in all cell lines • TG1 varies widely and is the major contributor to the widely different TC values observed Growth Fraction (GF) Not all cells in a tumor are actively growing. • Proliferating cells (P) • Quiescent cells (Q) Growth fraction is ratio of the number of proliferating cells (P) to the total number of cells. GF = P/(Q+P) • Determined by continuous labeling of cells for ~one cell cycle time (TC). • GF = fraction of cells labeled. • Growth fraction for experimental animal tumors is usually 30-50%. Or use of antibody (Ki-67) that binds to a nuclear antigen in cycling cells. Because not all cells in a tumor are in the growth fraction, the actual (measured) tumor volume doubling time (TD) is usually longer than the cell cycle time. Normal adult tissues show no net growth: growth fraction balanced by cell loss. Flow cytometry Provides a direct measure of all phases of the cell cycle. [Image removed due to copyright considerations] Potential Doubling Time (Tpot) The parameter of most immediate relevance to clinical radiotherapy. • Tpot is the cell cycle time/fraction of cycling cells: Tpot = TC/GF • Tpot does not take cell loss into account TS and LI can be determined by the % labeled mitoses approach, but this is not practical in humans. An average Tpot can be estimated for tumors using flow cytometry with the dual fluorescence method: Tpot = (TS/LI) • • • • • Inject a tracer amount of BrdUrd 4-8 hours later obtain tumor biopsy Stain with antibody to BrdUrd (green) Stain DNA with propidium iodide (red) Disaggregate for flow cytometry analysis. Tpot is being studied as a potential predictive assay. [Image removed due to copyright considerations] Objective is to identify patients whose tumors are growing rapidly. These tumors might benefit from an accelerated fractionation schedule. [Image removed due to copyright considerations] LI is the proportion of cells that show green fluorescence (BrdUrd labeled). TS is calculated from mean red fluorescence of S cells relative to G1 and G2 cells (assumes DNA content in S phase cells increases linearly through S phase). Relative Movement (RM) RM = (FL-FG1)/(FG2-FG1) FL = the mean red fluorescence of the BrdUrd labeled cells FG1 and FG2 are the mean red fluorescence of the G1 and G2 populations TS = 0.5t RM − 0.5 Cell Loss • Tumors usually grow much more slowly than predicted by knowledge of TC and GF because of cell loss. • Tumor growth is the net result of cell growth and cell loss. • Cells can be lost due to death from: o inadequate nutrition (necrosis, hypoxia, reflects inability of the vascular supply to keep up with tumor growth) o apoptosis o immunological surveillance o metastasis o exfoliation. Cell loss factor, φ: φ = 1 – (Tpot/TD) or TD = Tpot/(1 - φ) Example: If TC = 22h, GF = 0.6, and φ = 0.9, the TD = 366 hrs • In experimental animal tumors, cell loss factors range from 0 to 90%. • Carcinomas tending to have high cell loss factors (>70%) and sarcomas tending to have low cell loss factors (<30%). • This might be attributable to the origin of carcinomas from continuously renewing epithelial tissues, where the cell loss factor is 100%. • This might also account for the differing responses of sarcomas and carcinomas to radiation. [Image removed due to copyright considerations] Summary: Potential doubling time Cell loss Tumor doubling time Tpot = TC GF φ =1 − Tpot TD = Tpot TD (1 − φ ) Tpot = TS LI (flow cytometry) V. Volume Doubling Time, TD Three factors determine tumor growth • Cell cycle time of proliferating cells • Growth fraction • Cell loss fraction Cell cycle times for tumor cells (TC) ~ 1-5 days Tumor volume doubling times (TD) ~40-100 days Example: Initial tumor diameter = 0.5 cm. V = (4/3)π r3 100 days later the tumor had grown to 4 cm. What is the tumor doubling time? Calculation: • Diameter increased by a factor of 8. • 83 = 512-fold increase in tumor volume. • How many doublings occurred during the 100 days? • 2n = 512; • n = 9 doublings • Nine doublings in 100 days = 1 doubling every 11 days [Image removed due to copyright considerations] Relationship between tumor weight, number of cells it contains, and the number of doublings per cell. (assumes 109 cells per gram tissue) Tumor growth tends to be Gompertzian Doubling time of the tumor increases progressively as the tumor gets bigger. Cell cycle time is probably constant This implies that GF, cell loss factor, vary as a function of tumor size. [Image removed due to copyright considerations] Kinetics of Human Tumors Some parameters • TC usually between 15 and 125 h, modal value of 48 h • TS usually between 10 and 24 h, modal value of 16 h. • TC usually 3 x TS, so LI gives an approximate value of GF. • LI varies between 0.1% and 40%, so GF varies between 0.3% and 100%. • Cell loss factors high, usually in excess of 50%. Thus, cell loss may be the most important factor in determining the pattern of growth of human tumors. • Volume doubling times range from 18 to over 200 days, with an average median doubling time estimated to be 66 days. Generally, the cell cycle time of malignant cells is shorter than that of their normal tissue counterparts. Metastases appear to grow more rapidly than the primaries in the same individual. • Tumors with high GF tend to be more radiosensitive. • GF, LI, cell loss factor determine response to chemotherapy (acting on S phase cells) • Chemotherapy cures observed only in tumors with high LI. Kinetics After Irradiation If cells are growing normally and have adequate nutrition, etc. (e.g., exponentially growing cells in culture or in a tumor), immediately after irradiation their cell cycle will increase, largely due to division delay. • Subsequently, the loss of some cells in a tumor or non-growing normal tissue may cause the remaining cells to show accelerated growth or repopulation. • This may result from shortening of the TC, increased growth fraction, and/or decreased cell loss. [Image removed due to copyright considerations] Tumor behavior after irradiation will depend, at least in part, on the cell loss factor, GF and TC of the tumor.