Split-dose Recovery Research Articles

Dose-response curve and split-dose recovery in human skin cancer

The data of 946 skin cancer patients treated with single doses or with 4, 8, 17, 40 or 47 fractions of X-rays were analysed using Strandquist's formalism and the linear-quadratic model. The analysis of tumour control in relation to tumour size and fractionation schedule shows a strong correlation between tumour size and tumour control rate. For small tumours, single dose irradiation was as effective as fractionated treatment. For large tumours, a straight line was easily fitted to the TCD50 and TCD90 values plotted logarithmically against time or number of fractions; the exponent for overall treatment time was 0.27 and for number of fractions it was 0.31. With the linear-quadratic model, an alpha/beta ratio of 13.8 Gy was calculated. On the assumption that the number of clonogenic cells in the tumour increases in proportion to tumour volume, Do values between 0.76 and 1.33 Gy were calculated and cellular survival curves were constructed. The estimated clonogenic fraction of tumour cells is about 10(-3); no influence of hypoxic clonogenic cells on local tumour control with single doses could be demonstrated.

Split-dose recovery in epithelial and vascular-connective tissue of pig skin

In the first 16 weeks after irradiation, two distinct waves of reaction can be observed in pig skin; the first wave (3-9 weeks) represents the expression of damage to the epithelium while the second is indicative of primary damage to the dermis, mediated through vascular injury. Following beta-irradiation with a strontium-90 applicator, a severe epithelial reaction was seen with little subsequent dermal effects. X-rays (250 kV) on the other hand, produced a minimal epithelial response at doses which led to the development of dermal necrosis after 10-16 weeks. Comparison of single doses with two equal doses separated by 24 h produced a D2-D1 value of 7.0 Gy at the doses which produced moist desquamation in 50% of fields (ED50) after strontium-90 irradiation. After X-irradiation comparison of ED50 doses for the later dermal reaction suggested a D2-D1 value of 4.5 Gy. Over this same dose range of X-rays the D2-D1 value for the first wave epithelial reaction was 3.5 Gy. These values of D2-D1 for epithelial and dermal reactions in pig skin were compared with published data and were examined in relation to the theoretical predictions of a linear quadratic model for tissue target cell survival. The results were broadly in keeping with the predictions of such a model.

Chronic damage after radiation therapy: Challenge to radiation biology

After local irradiation of the heart or the recto-sigmoid of rats, chronic radiation damage develops after several months, leading to well-defined clinical syndromes of fatal pericarditis and myocardial necrosis or fatal ileus. In both organs, the LD-50 is below 20 Gy, and a pronounced split dose recovery has been measured with fractionated irradiation. In both organs, the primary radiation damage appears to be to the capillaries, which then leads to secondary parenchymal atrophy. Damage to structured vessels and fibrosis was only seen in areas of necrosis, which in the gut might be precipitated by secondary trauma to the atrophic mucosa.

Implications of tissue target-cell survival-curve shape for values of split-dose recovery doses: late versus early effects.

Recent data from this laboratory on split-dose recovery for early and late effects in pig skin are consistent with the linear-quadratic model for cell survival, and with relative cell survival-curve shapes for early- and late-effect target cells where the early-effect cells have an initially steeper and straighter survival-curve than the late-effect cells.

Split-dose recovery is due to the repair of DNA double-strand breaks.

DNA double-strand breaks are the molecular lesions the repair of which leads to the reappearance of the shoulder observed in split-dose experiments. This conclusion is based on results obtained with the help of a diploid yeast mutant rad 54-3 which is temperature-conditional for the repair of DNA double-strand breaks. Two repair steps must be met to yield the reappearance of the shoulder on a split-dose survival curve: the repair of double-strand breaks during the interval between two doses and on the nutrient agar plate after the second dose. In yeast lethality may be attributable to either an unrepaired double-strand break (i.e. a double-strand break is a potentially lethal lesion) or to the interaction of two double-strand breaks (misrepair of double-strand breaks). Evidence is presented that the two cellular phenomena of liquid holding recovery (repair of potentially lethal damage) and of split-dose recovery (repair of sublethal damage) are based on the repair of the same molecular lesion, the DNA double-strand break.

Cytosar-U (ARA-C) induced changes in mouse jejunal crypt epithelial kinetics and radiosensitivity to gamma rays and fast neutrons

Pretreatment with the S phase specific cytotoxic agent Cytosine Arabinoside (Ara-C) protects the intestinal stem cells from gamma radiation injury by nearly tenfold. Studies were undertaken to test whether an altered cell age distribution could account for the reported duration and degree of Ara-C induced protection and to measure the degree of protection from the high energy neutrons of the Fermilab Cancer Treatment Facility. Twelve hours after treatmentwith Ara-C, B6CF 1/ANL mice wereexposed to increasing single doses of either 137Cs γ-rays or neutrons from the Fermilab accelerator, or a split dose of neutrons with intervals of 1, 2, and 3 hours. The number of regenerating microcolonies per jejunal circumference in Ara-C treated and irradiated animals was compared to irradiated controls. Another group of mice was given Ara-C but in the 12-hour interval between Ara-C and irradiation, colcemid was given every 3 hours to continuously block and kill cells in mitosis. The results suggest that Ara-C given 12 hours prior to neutron irradiation protects intestinal stem cells to nearly the same degree as it does from 137Cs γ-ray damage. Furthermore, the control split-dose recovery ratio to neutron irradiation at 1, 2, or 3 hours was 1.8 and was unchanged 12 hours after Ara-C. Colcemid reduced the crypt cell population to less than half the normal 250 cells per crypt; however, the cell survival curve was unaltered from the survival curve 12 hours after Ara-C. These results suggest that Ara-C recruits intestinal clonogenic stem cells, but inhibits their normal passage through DNA synthesis. These cells, responsible for intestinal mucosal regeneration, appear to be held in a radioresistant portion of the cell cycle for a period of about 10–16 hours after Ara-C.

Split dose recovery of a mouse tumour and its stroma during fractionated irradiation.

Abstract The shapes of tumour regrowth curves after irradiation are the combined effects of radiation damage to the tumour and to its stromal tissue. In the adenocarcinoma 284 both effects can be differentiated and studied simultaneously after fractionated irradiation. At doses per fraction between 2 and 6 Gy the stromal tissue showed much bigger split dose recovery than the tumour.

Evidence that progression of cells into S-phase is not a prerequisite for recovery between split doses of U.V.-light in synchronized and plateau phase cultures of Ehrlich ascites tumour cells.

The ability of Ehrlich ascites tumour cells (EAT-cells) to perform split-dose recovery after U.V. exposure was studied with unfed plateau phase as well as with synchronized cells selected from exponentially growing cultures. The cells were kept in balanced salt solution which inhibited the progression of the cells through the cell cycle. The results indicated that split-dose recovery occurred in EAT-cells in all phases of the cell cycle and that progression of the cells into S-phase was not a prerequisite for this type of repair. The second-dose survival curves of G1- and S-phase cells showed, 24 hours after the first U.V. exposure, a shoulder width comparable to that of singly irradiated cells. Second-dose survival curves for G2-cells showed, after the same time interval, a shoulder width smaller than that for singly exposed cells, presumably due to some cell division. The recovery time constant (t50 between 4 and 8 hours) increased with increasing U.V. exposure.

Recovery kinetics of radiation-induced chromosome aberrations in human G0 lymphocytes.

Declining yields of radiation-induced dicentric chromosomes in human G0 lymphocytes were observed in split-dose experiments with time intervals varied up to 8 h. In agreement with microdosimetric intratrack-intertrack interaction models, only the dose-squared yield component was reduced and approached an asymptotic value equal to one half of the corresponding single exposure yield. For 150 kV X-rays and 13 MeV electrons, at total doses up to 6 Gy, the time constant tau of the approximately exponential decline was practically dose- and quality-independent within a range of 100-180 min. For 10 kV X-rays, in the presence of a dominant linear yield component, only a small split-dose effect, but with a consistent tau-value, was observed for a total dose of 5 Gy. Since tau can be interpreted as the mean life time of "primary lesions" in chromatin fibres, its independence from absorbed dose and radiation quality means that radiation damage of the split-dose recovery mechanism can be excluded for doses up to 6 Gy. By correlating the observed split-dose reduction of the acentric fragment yield to the reduction of the dicentric yield, (1.64 +/- 0.03) acentrics/dicentric for 150 kV X-rays and (1.51 +/- 0.11) acentrics/dicentric for 13 MeV electrons were obtained. Acentrics formed in the course of dicentric formation as well as in other binary interactions of "primary lesions" are represented in these ratios. Post-irradiation recovery during time intervals between irradiation and cell stimulation up to 24 h did not occur. The relations to comparable results in cell lethality experiments are discussed, and a hypothesis of "fast" and "slow" binary interactions of "primary lesions" is put forward.

Influence of fractionation schedules on the response of a rat brain tumor to therapy with BCNU and radiation

The maximum dose of 1, 3 his(2-chloroethyl)-1-nitrosourea (BCNU) that produced minimal toxicity after intraperitoneal administration 2, 3, or 5×/wk for 2 weeks was found to be 3.0, 2.5, or 2.0 mg/kg/treatment, respectively. BCNU toxicity appeared to be related to the total dose administered, which indicated little if any split-dose recovery. Deaths after whole brain irradiation resulted predominantly from starvation because of the inability of the irradiated rats to eat food pellets. When placed on a diet of soft mash, virtually all rats survived irradiation schedules of 520 rad/day 2× /wk for 2 wk; 433 rad/day 3× /wk for 2 wk; and 260 rad/day 5× /wk for 2 wk. There was no additional toxicity when these maximum doses of BCNU and radiation were combined in 2 week fractionation schemes. The 2× /wk and 3× /wk BCNU + radiation schedules, which employed the maximum tolerable doses of each agent, were as effective as the 5× /wk combination schedule in treating day 12 intracerebral 9L brain tumors.

Rapid Recovery in Plateau-Phase Mammalian Cells

Rapid split-dose and rapid single-dose recovery was examined in two rodent cell lines in culture: hamster lung fibroblasts (V-79) and contact-inhibited mouse embryo fibroblasts (10T1/2). Cell turnover in density-inhibited plateau-phase 10T1/2 cell cultures is very low as compared with V-79 cells. Plateau cultures were exposed to cobalt-60 ..gamma.. rays at 85 to 90 rad/sec; 0 to 90 min of incubation at 37/sup 0/C was allowed either between two exposures or following a single exposure before subculture at low density. A rapid phase of recovery occurred in plateau-phase 10T1/2 cell cultures occurring within 4 to 10 min after irradiation, whereas little recovery occurred during this time in the V-79 cell line. Rapid recovery was not observed in either cell line following irradiation during exponential growth. When plateau-phase 10T1/2 cells were irradiated with a total dose of 1100 rad given as two 550-rad doses with exposures separated by 10 min, survival was enhanced by a factor of 5 over that seen following a single 1100-rad dose. When 10 min was allowed to elapse after a single dose before subculture, a twofold enhancement in survival occurred. This rapid recovery appeared to result from an additive effect of split-dose and potentially lethal damage repair.more » When rapid split-dose recovery had occurred, however, little further enhancement in survival took place when cells were placed under conditions which favored recovery of additional potentially lethal damage.« less

Interaction Between 2450-MHz Microwaves and Ionizing Radiation in Tribolium Confusum

The combined effect of single or fractionated /sup 137/Cs gamma radiation and 2 h of 2450-MHz microwave radiation at two different power levels, resulting in specific absorption rates of 680 or 760 W/kg in distilled H/sub 2/O, was investigated in the flour beetle, Tribolium confusum. The potentiating effect of microwave treatment after single gamma irradiations was observed only if applied within 60 min; in contrast, the potentiating effect of microwave treatment given before gamma-irradiation lasted at least 8 h. Microwave treatment also altered the kinetics of split-dose recovery. At the higher power level, split-dose recovery was abolished for at least 36 h; at the lower power level, split-dose repair of ionizing radiation damage was delayed for 8-10 h, and then, with longer interfraction intervals, survival increased. All of the foregoing observations essentially mimic those previously reported for appropriate hyperthermic treatment by means of immersion in hot water. These findings are in keeping with the hypothesis that heating, whether by microwaves or by water immersion, affects the repair capabilities of the beetle, either by damaging some of the enzymes that repair radiation-damaged DNA or by altering the cytostructural integrity of the DNA-chromatin membrane complex, rendering the DNA lesions less amenable to repair.

Effet du fractionnement de dose sur la survie de bourgeons axillaires etde cellules isolées de liseron irradiés aux rayons gamma

Split-dose recovery has been shown on bindweed (Calystegia=Convolvulus sepium [L.] R. Br.) nodes and isolated single cells in aseptic culture. Recovery from sublethal damage occurs within 2 h.

Small Intestinal Cryptogenic Cells in W/W v Mutant Mice

Crypt survival curves were obtained for normal and mutant ${\rm W}/{\rm W}^{{\rm v}}$ mice. This latter have stem cell defects in their germ cells, melanocytes, and haematopoietic tissue. The crypt survival curves were similar in shape and slope for normal and ${\rm W}/{\rm W}^{{\rm v}}$ mice suggesting that the number and split-dose recovery of intestinal crypt stem cells are not affected by the mutation in the ${\rm W}/{\rm W}^{{\rm v}}$ mice. This was further confirmed by a split-dose assay designed to estimate the single-cell survival curve shoulder size and hence the cellular multiplicity in the crypt survival curve, i.e., the number of cryptogenic (stem) cells per crypt.

Rat Skin Tumor Incidence Following Single and Fractionated Exposures to Proton Radiation

Experiments were conducted to establish the dose-response curve and degree of split-dose recovery for tumor induction and hair follicle survival in rat skin irradiated with protons. A proton beam from a cyclotron was modified to provide an approximately linear decrease in dose with depth in tissue to a maximum of about 1.3 mm. Six doses were given in the range from 75 to 750 rad. The tumor yield at 100 weeks after irradiation increased as $D^{2.8}$ . Split doses consisted of an initial dose of 150 rad followed 24 hr later by graded doses in the range from 150 to 600 rad. Split doses produced a sparing effect on follicle killing and on the yield of tumors. The magnitude of the reduction in tumor yield was consistent with complete recovery. Comparisons with previous results indicated that protons were 2.0-2.4 times as effective as electrons for tumor induction and that the shapes of the dose-response curves were generally similar.

Enhancement of Radiation Killing of Cultured Mammalian Cells by Cordycepin

Asynchronous populations of rat hepatoma cells (H4) in log-phase growth survived a 3-hour exposure to cordycepin (3'-deoxyadenosine), and RNA antimetabolite, in a simple exponential fashion with a 'DO' of 43.8 microM/l. When cordycepin-treated cells were exposed to X-irradiation, the resultant survival levels were much lower than one would expect were the agents simply additive. Patterns of X-ray survival of cells treated with cordycepin were dependent on drug concentration, the predominant effect being to decrease the DO of the X-ray survival curve. The increased sensitivity of cells exposed to cordycepin to subsequent X-ray treatment persists for longer than 4 hours after drug administration. Although immediate cordycepin post-treatment of X-irradiated cells is less effective than pre-treatment, the interaction is still significant. Cordycepin treatment did not appear to reduce split-dose recovery or to inhibit the rejoining of single-strand breaks as measured by DNA sedimentation in alkaline-sucrose gradients.

Effect of Metronidazole on the Split-dose Recovery of Yeast

(1978). Effect of Metronidazole on the Split-dose Recovery of Yeast. International Journal of Radiation Biology and Related Studies in Physics, Chemistry and Medicine: Vol. 34, No. 5, pp. 471-474.

Split-dose and Liquid-holding Recovery after X-irradiation in Diploid YeastSaccharomyces Cerevisiae

In diploid yeast, split-dose recovery (SDR) after X-irradiation was affected, if incubation between split doses was performed in the presence of the protein-synthesis inhibitor, cycloheximide. In exponentially-growing cell-cultures, early SDR was undisturbed but complete recovery was not achieved. Concomitantly the cells show a decresing ability to perform subsequent liquid-holding recovery (LHR). In stationary-phase cell-cultures, SDR was completely suppressed. The cells show, however, recovery from potentially lethal damage in the presence of cycloheximide during incubation between the dose-fractions. The experimental results suggest that in diploid yeast SDR after X-irradiation is an enzymatic process dependent on a functioning protein metabolism.

Split-dose and Liquid-holding Recovery after X-irradiation in Diploid YeastSaccharomyces Cerevisiae

Liquid-holding recovery (LHR) and the sparing effect of dose fractionation were investigated with 100 kVp-X-rays in diploid yeast from exponential and stationary phase. Exponential cells showed prompt and complete split-dose recovery. The ability to undergo LHR remained constant during 2-5 hours incubation after the first dose. Stationary-phase cells did not exhibit split-dose recovery (SDR) in a simple way: there was an increase in sensitivity after 3-5 hours followed by a gradual increases with time. It is suggested that stationary cells are starved of an essential co-factor which has to be synthesized de nova.

Split-dose Recovery for Radiation-induced Tumours in Rat Skin

Tumour-related recovery in rat skin was estimated from the dependence of tumour yield on time between split doses of electron radiation. Tumour yield versus dose was established at nine dose points, and at three points the dose was split into two equal fractions spaced 0-25, 3-2 or 6-3 hours apart. After irradiation the rats were observed periodically for at least 64 weeks, and at death the tumours were examined histologically. The dependence of yield on dose for single doses was consistent with a quadratic function up to a peak yield at about 1600 rad. The effect of split doses on tumour yield depended on the position on the dose--response curve. At the lowest split dose, the yield declined with a half-time of about 1-8 hours. At the intermediate split dose, an initial increase was followed by a decline with a half-time of about 3-9 hours. At the highest split dose, the tumour yield increased with time between exposures. Fractionation-induced increases in tumour yield were explained as a sparing effect on cell lethality, whereas tumour-related recovery per se was indicated at the lower two doses.