A test of equal effect per fraction in the kidney of the mouse.

Measurements of renal damage in the mouse were made to determine if there was an equal effect per fraction during a course of repeated 240-kVp X-ray doses. An X-ray dose of 2 Gy was given 2, 8, 14, or 20 times with interfraction intervals of 12 h. Some animals were also irradiated with twenty 2-Gy doses using a 5-h interfraction interval. The underlying effect per fraction (-logeSF of the notional target cell population) was determined from the additional top-up dose of d(4)-Be neutrons needed to produce measurable renal impairment assessed by decreased clearance from the plasma of [51Cr]EDTA and by a reduction in the hematocrit at 25, 29, 33, and 39 weeks after treatment. There was no significant influence of the time of assay on the values of underlying effect measured. A mean value of underlying effect was therefore calculated for the two different assays of each mouse, from the measurements at the four times. This gave approximately 40 estimates (one for each animal assessed) with each assay of the effectiveness of 2-Gy fractions in each of the four fractionation schedules, a total of 321 determinations in the study with 12-h intervals. Regression analysis showed that there was no significant trend in underlying effect per fraction with number of fractions, i.e., the damage per fraction was constant regardless of the number of fractions used. With underlying effect normalized to 1 unit of damage for a single 2-Gy dose, the slope of this plot was -0.0013 per fraction2 +/- 0.0097 (95% CL). The assumption of equal effect per fraction was therefore not invalidated in the kidney of the mouse. With a 5- instead of a 12-h interfraction interval, the 20-fraction schedule was 7% more effective as measured by the two assays analyzed together; this was significant at P = 0.0001. This shows that 5 h is not sufficient time between fractions for full repair to occur in the kidney, and underlines the need for intervals of at least 6 h between the doses in clinical radiotherapy using more than one fraction per day. The data are consistent with an alpha/beta ratio approximately 1.6 Gy, with a repair half-time approximately 1.3 h. However, these experiments were not designed to determine these parameters and their values should be regarded only as rough estimates.     

Analysis of cell survival after multiple fractions per day and low-dose-rate irradiation of two in vitro cultured rat tumor cell lines

Two rat tumor cell lines which differ significantly in radiosensitivity, a rhabdomyosarcoma (R-1) and a ureter carcinoma (RUC-2), were treated with multiple fractions per day and low-dose-rate gamma radiation. The purpose of these experiments was to investigate (i) the influence of fraction size and interfraction interval on repair of sublethal damage (SD) and (ii) whether low-dose-rate irradiation can be simulated by giving multiple fractions per day which might be applied in clinical treatments. In both cell lines, multiple doses were given at 1- to 4-hr intervals. SD repair was at a maximum in 2 hr but did not reach the theoretically expected level. For both cell lines, survival at higher total doses was different from that theoretically expected if repair of SD was assumed to be completed and at the maximum level. To account for the observation that less than complete repair of SD occurred, theoretical survival curves were calculated with the assumption of a constant but less than 100% level of SD repair. Experimental data correlated well with these calculated curves. There were only very small differences in survival after the different multiple fractions per day regimens. Survival after irradiation at a dose rate of 1.00 Gy/hr was found to be similar to that after multiple fractions per day.     

Tissue repair and repopulation in the tumor bed effect

These experiments were designed to study the kinetics and magnitude of cell repair and repopulation in tissues whose damage results in the tumor bed effect. The right hind thighs of mice were irradiated with single doses or two equal gamma-ray fractions. Interfraction intervals ranging from 30 min to 24 h (to measure the kinetics of repair from sublethal damage) and 6 and 12 weeks (to determine the extent of repopulation) were used. One day after the second radiation dose 5 X 10(5) FSA tumor cells were inoculated into the center of the irradiated field. Radiation dose-response curves were obtained by calculating the time required for tumors to reach 12 mm diameter. No recovery occurred within 6 h of the radiation delivery as measured by this assay. Some recovery, 3.2-4.6 Gy above a single radiation dose, occurred when the interval between two fractions was 24 h. With increasing interfraction intervals of 6 and 12 weeks further dose sparing occurred in the amount of 5.0-6.9 and 7.5-8.3 Gy, respectively. The data suggest that repopulation is the major contributor to the radiation dose-sparing recovery of stromal tissue and that some proliferative response may occur as early as 1 day after the first irradiation.     

Multifraction radiation response of mouse lung

The response of mouse lung to repeated doses of 60Co gamma-rays of as low as 115 cGy per fraction was measured using death from pneumonitis between 80 and 120 days after irradiation as the endpoint. A fractionation interval of 3 h was maintained for most regimens but in the longer experiments some 12 h intervals were introduced for logistic reasons. The longest overall duration (for a 43 fraction regimen) was 8 days. The total doses required to produce 50 per cent mortality increased continuously as dose/fraction was decreased, even from 160 to 115 cGy per fraction. Of clinical relevance, the steepness of the isoeffect curve over the dose range 115-500 cGy indicates that the lung shows greater sparing from dose fractionation than is characteristic of more rapidly-responding normal tissues, resembling, in this respect, other more slowly-responding tissues such as spinal cord. The plot of the reciprocal of the LD50 values as a function of dose per fraction was non-linear, suggesting that a linear quadratic dose response model may not be appropriate or that repair of cellular injury in lung is not complete in 3 h, or both.     

Loss of repair capacity in density-inhibited cultures of C3H 10T1/2 cells during multifraction irradiation

Multifraction survival curves for slowly cycling, density-inhibited C3H 10T1/2 cells were shown previously to bend toward lower survival levels with increasing total dose, even for doses per fraction as small as about 2.0 Gy. In an attempt to explain this, we tested the capacity of cells to repair potentially lethal damage (PLD) as fractionation progressed. Plateau-phase cultures were exposed to repeated doses of 4.0 Gy of 137Cs gamma rays delivered at 12-hr intervals. After zero, three, five, and seven fractions, some cultures were put aside, incubated for 12 hr at 37 degrees C, irradiated with a single dose of 9.0 Gy, and subsequently returned to a 37 degrees C incubator. At 0, 2, 4, 6, and 12 hr after the 9.0 Gy dose, cultures were trypsinized and plated for a survival assay. Following three fractions of 4.0 Gy, cells were able to repair PLD as well as those receiving a single dose of 9.0 Gy without prior fractionation. Following five fractions, cells were less able to repair PLD, and after seven fractions, only a very small amount of PLD repair was detectable using this method of measurement.     

A model for optimizing normal tissue complication probability in the spinal cord using a generalized incomplete repair scheme

The purpose of this study was to determine the treatment protocol, in terms of dose fractions and interfraction intervals, which minimizes normal tissue complication probability in the spinal cord for a given total treatment dose and treatment time. We generalize the concept of incomplete repair in the linear-quadratic model, allowing for arbitrary dose fractions and interfraction intervals. This is incorporated into a previously presented model of normal tissue complication probability for the spinal cord. Equations are derived for both mono-exponential and bi-exponential repair schemes, regarding each dose fraction and interfraction interval as an independent parameter, subject to the constraints of fixed total treatment dose and treatment time. When the interfraction intervals are fixed and equal, an exact analytical solution is found. The general problem is nonlinear and is solved numerically using simulated annealing. For constant interfraction intervals and varying dose fractions, we find that optimal normal tissue complication probability is obtained by two large and equal doses at the start and conclusion of the treatment, with the rest of the doses equal to one another and smaller than the two dose spikes. A similar result is obtained for bi-exponential repair. For the general case where the interfraction intervals are discrete and also vary, the pattern of two large dose spikes is maintained, while the interfraction intervals oscillate between the smallest two values. As the minimum interfraction interval is reduced, the normal tissue complication probability decreases, indicating that the global minimum is achieved in the continuum limit, where the dose delivered by the "middle" fractions is given continuously at a low dose rate. Furthermore, for bi-exponential repair, it is seen that as the slow component of repair becomes increasingly dominant as the magnitude of the dose spikes decreases. Continuous low-dose-rate irradiation with dose spikes at the start and end of treatment yields the lowest normal tissue complication probability in the spinal cord, given a fixed total dose and total treatment time, for both mono-exponential and bi-exponential repair. The magnitudes of the dose spikes can be calculated analytically, and are in close agreement with the numerical results.     

Equivalence of pulsed-dose-rate to low-dose-rate irradiation in tumor and normal cell lines

To determine whether different fractionation schemes could simulate low-dose-rate irradiation, ovarian cells of the carcinoma cell lines A2780s (radiosensitive) and A2780cp (radioresistant) and AG1522 normal human fibroblasts were irradiated in vitro using different fraction sizes and intervals between fractions with an overall average dose rate of 0.53 Gy/h. For the resistant cell line, the three fractionation schemes, 0.53 Gy given every hour, 1.1 Gy every 2 h, and 1.6 Gy every 3 h, were equivalent to low dose rate (0.53 Gy/h). Two larger fraction sizes, 2.1 Gy every 4 h and 3.2 Gy every 6 h, resulted in lower survival than that after low-dose-rate irradiation for the resistant cell line, suggesting incomplete repair of radiation damage due to the larger fraction sizes. The survival for the sensitive cell line was lower at small doses, but then it increased until it was equivalent to that after low-dose-rate irradiation for some fractionation schemes. The sensitive cell line showed equivalence only with the 1.6-Gy fraction every 3 h, although 0.53 Gy every 1 h and 1.1 Gy every 2 h showed equivalence at lower doses. This cell line also showed an adaptive response. The normal cell line showed a sensitization to the pulsed-dose-rate schemes compared to low-dose-rate irradiation. These data indicate that the response to pulsed-dose-rate irradiation is dependent on the cell line and that compared to the response to low-dose-rate irradiation, it shows some equivalence with the resistant carcinoma cell line, an adaptive response with the parental carcinoma cell line, and sensitization with the normal cells. Therefore, further evaluation is required before implementing pulsed-dose-rate irradiation in the clinic.     

Split-dose sparing of gamma-ray-induced pulmonary endothelial dysfunction in rats

The purpose of this study was to determine whether radiation-induced pulmonary endothelial dysfunction exhibits split-dose sparing. Rats were sacrificed 2 months after a range of 60Co gamma-ray doses (0-40 Gy) delivered to the right hemithorax in either a single fraction or in two equal fractions separated by 24 h. Pulmonary angiotensin converting enzyme (ACE) activity, plasminogen activator (PLA) activity, and prostacyclin (PGI2) and thromboxane (TXA2) production served as indices of lung endothelial function. There were dose-dependent decreases in ACE and PLA activity and increases in PGI2 and TXA2 production after both single and split-dose exposures. The D2-D1 values determined from the two-fraction minus single-fraction isoeffective doses were 3.9 Gy for ACE activity, 7.2 Gy for PLA activity, 4.8 Gy for PGI2 production, and 4.7 Gy for TXA2 production. Thus these data demonstrate that over the present range of radiation doses approximately 4-7 Gy is repairable as subeffective endothelial damage during the 24-h interval between fractions. These values agree with previously published estimates of split-dose sparing in mouse lung based on lethality and breathing rate assays.     

Effects of split-dose X irradiation on rat salivary gland function.

The effect of a single local dose of 15 Gy on salivary gland function in male Albino Wistar rats was compared with the effect of two doses of 7.5 Gy. The intervals chosen were 0-24 h and 1 week. Before and 1-30 days after the last radiation dose, samples of parotid and submandibular saliva were collected simultaneously after stimulation of the glands with pilocarpine. Irradiation with the single dose resulted in an increased lag phase and potassium concentration, and a decreased flow rate and sodium concentration. The rate of secretion of amylase was decreased during Days 1-6, increased at Day 10, and was decreased again at Day 30. With two dose fractions, substantial dose-sparing effects on lag phase, flow rate, and secretion of amylase were observed for both the very early (0-6 days postirradiation) and later (6-30 days postirradiation) effects. These effects were maximal when the interval between the fractions was 6 h. A significant dose-sparing effect on electrolytes was observed for the later effects only, again with a maximum for the 6-h interval. The dose-sparing observed for the very early effects cannot be explained satisfactorily by repair of sublethal damage (SLD), redistribution of cells over the cell cycle, or repopulation of salivary gland tissue between the doses. In contrast to the earlier dose-sparing effects, the split-dose recovery seen for later damage may be attributed, in part, to SLD repair in providing for greater reproductive survival of intercalated ductal cells and enhanced tissue regeneration.     

Radiosensitization in multifraction schedules. I. Evidence for an extremely low oxygen enhancement ratio

Stable monolayers of contact-inhibited C3H 10T1/2 cells were used in multifraction radiation experiments to measure the oxygen enhancement ratio (OER) at low doses/fraction under conditions where cell cycle effects (repopulation, redistribution) were minimal. Consistent with there being a dose-dependent reduction in the OER at low doses, an extremely low OER of 1.34 was measured after 20 fractions of 1.7 Gy every 12 h. The sparing effects of fractionating radiation doses were not apparent for cells irradiated under hypoxic conditions (i.e., multifraction survivals were lower than acute single-dose values) until doses exceeding 15 Gy were reached. This result suggested a deficiency in the recovery from sublethal and/or potentially lethal damage might exist after hypoxic irradiations, thereby reducing the OER. The capacity to repair potentially lethal damage was found to be nearly the same after hypoxic as compared to aerobic irradiations. However, there was an apparent absence of sublethal damage repair by 10T1/2 cells between two hypoxic irradiations which could be a major contributing factor to the extremely low OER value measured in this multifraction schedule.     

Radiation-induced renal damage: the effects of hyperfractionation

The response of mouse kidneys to multifraction irradiation was assessed using three nondestructive functional end points. A series of schedules was investigated giving 1, 2, 4, 8, 16, 32, or 64 equal X-ray doses, using doses per fraction in the range of 0.9 to 16 Gy. The overall treatment time was kept constant at 3 weeks. Kidney function was assessed from 19 to 48 weeks after irradiation by measuring changes in isotope clearance, urine output, and hematocrit. The degree of anemia (assessed from the hematocrit measurements) is a newly developed assay which is an early indicator of the extent of renal damage after irradiation. All three assays yielded steep dose-effect curves from which the repair capacity of kidney could be estimated by comparing the isoeffective doses in different schedules. There was a marked influence of fractionation, with increasing dose being required to achieve the same level of damage for increasing fraction number, even between 32 and 64 fractions. The data are well fitted by a linear quadratic dose-response equation, and analysis of the data in this way yields low values (approximately 3.0 Gy) for the ratio alpha/beta. This would suggest that hyperfractionation , using extremely small X-ray doses per fraction, would spare kidneys relative to tumors and acutely responding tissues.     

Vascular response to radiation injury in the rat lung.

Changes in relative left-to-right lung blood flow ratios were followed as an index of vascular radiation injury in left-hemithorax-irradiated Sprague-Dawley rats. Single doses of 11 to 21 Gy gamma radiation resulted in a dose-dependent decrease in relative blood flow to the irradiated lung from 3 to 5 weeks after exposure during the development of pneumonitis. Blood flow returned to near normal by 5 weeks after lower doses (11-13.5 Gy). After a single dose of 15 Gy the left-to-right blood flow ratio recovered to 75% of normal at 12 weeks and leveled off. Following 18 Gy irradiation a second period of reduced flow began 16 weeks after exposure. After 21 Gy irradiation flow to the irradiated side remained low for 1 year after exposure. Rats that received a single dose of 18 Gy to the left hemithorax were also treated with one or two of the following drugs: captopril, cyproheptadine, dexamethasone, diethylcarbamazine, penicillamine, or theophylline. Dexamethasone was most effective at preventing the decrease in blood flow to the irradiated lung when treatment was continued through the pneumonitis period and dose was not tapered until 8 weeks after radiation exposure. All other drugs and drug combinations were, for the most part, virtually ineffective after the pneumonitis period. There was a relatively poor correlation with earlier vascular permeability surface area product studies. This suggests that endothelial damage, as well as damage to other cell types, contributes to the development of post-irradiation fibrosis in the lung.     

Split-dose exposure to N-methyl-N'-nitro-N-nitrosoguanidine in BALB/3T3 C1 a31-1-1 cells: evidence of DNA repair by alkaline elution without changes in cell survival, mutation and transformation rates

Dose fractionation of a direct-acting chemical carcinogen, the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), was studied for its concurrent effects on survival, DNA damage and repair, ouabain resistance (Ouar) mutations and neoplastic transformation, in the mouse embryo cell line BALB/3T3 C1A31-1-1. MNNG doses of 0.5, 1 and 2 micrograms/ml were added to the cells either as a single exposure or in two equal fractions separated by 1, 3 or 5 h intervals. No significant difference in cytotoxicity was found when single and split-dose treatments were compared. No recovery from sublethal damage was therefore found in this cell line by split-dose administration of MNNG, although such an effect was found when the same cell line was treated with single and split doses of X-rays. Repair of DNA damage as measured by alkaline elution was studied up to 24 h after a single MNNG exposure (0.5 micrograms/ml). DNA repair was rapid during the first 5 h after treatment and slow thereafter. DNA damage detected after split doses of MNNG at 1 and 5 h intervals was significantly lower than after a corresponding single dose. With both single and split doses, rejoining of single-strand breaks (ssb) was nearly complete after 24 h of repair time. Ouar mutation and neoplastic transformation frequencies were determined for single and split doses of MNNG with the second treatment being given during (1 h) or after (5 h) the period of rapid DNA repair. No significant differences in either effect were detected for dose splitting at any tested dose.     

Quantitative regularities and peculiarities of the development of the cerebral form of radiation sickness at fractionated irradiation

Several peculiarities in manifestations of cerebral form of radiation sickness have been revealed at a fractionated double irradiation with equal and unequal doses per fraction and different intervals between the fractions. A reliable increase in average lifespan of rats irradiated with (100 + 100 Gy) equal doses at 10 and 60 min intervals between two fractions compared to the single radiation exposure to 200 Gy has been obtained. Lifespan of rats irradiated with a total dose greater than 200 Gy in most cases of double exposures with 10 min interval was reliably less than that for animals after a single exposure. The influence of the first dose on the reduction of animal average lifespan increased with fraction dose increasing from 150 to 300 Gy and was most pronounced at the total exposure dose of 400 Gy. Reaction of rats on the repeated irradiation was significantly weakened in comparison with the reaction on the first exposure. At a study of capacitation the interval of 30 min appeared to be more favorable compared to 10 min interval. Importance of a dose value in the first fraction has been demonstrated: the higher this value the worse the capacity of the rats 3 hours after the repeated exposure.     

Differential processing of ultraviolet or ionizing radiation-induced DNA-protein cross-links in Chinese hamster cells

The yield and repairability of DNA-protein cross-links have been compared after gamma- or U.V.-irradiation of Chinese hamster V79-379 lung fibroblasts. Using a filter-binding assay, cross-linked DNA can be specifically isolated after doses between 10 and 100 Gy of gamma-radiation and fluences between 20 and 300 J/m2 of U.V.-radiation. After ionizing radiation, the majority of DNA cross-linked to protein is released with biphasic kinetics, requiring 1 h for removal of 50 per cent of the cross-linked DNA and 24 h for 90 per cent release. In these cells, U.V.-induced cross-linked DNA is not removed; on the contrary, the yield of apparent DNA-protein complexes increases during postirradiation incubation. Prior gamma-irradiation, to initiate the associated repair system, does not stimulate release of U.V.-induced cross-linked DNA. Inhibition of protein synthesis by cycloheximide affects neither the removal of gamma-ray-induced cross-linked DNA nor the increase in U.V.-induced cross-linked DNA. 3-Aminobenzamide, an inhibitor of poly(ADP-ribose) polymerase, slows the second phase of release after gamma-irradiation as well as the increase in apparent cross-links after U.V.-irradiation. Thus, even though both types of DNA-protein cross-links can be detected by the same assay, their structures or other factors must be substantially different, since the repair system for one type does not recognize the other.     

Method for probing cells in radiation-induced G2 arrest: demonstration of potentially lethal damage repair

Chinese hamster ovary cells were arrested in the G2 phase of the cell cycle by X-irradiation. When subsequently treated with 5 mM caffeine the arrested population progressed into mitosis as a synchronous cohort where it was harvested by mitotic cell selection. This procedure provides a means to isolate cell populations treated in G2, for the investigation of G2 arrest. Comparisons were made of the number of cells retrieved from G2 arrest with the number suffering arrest, as determined by flow cytometry and by matrix algebraic simulations of irradiated cell progression. The retrieved population was not significantly less than expected for doses up to 3.5 Gy, indicating that the retrieval process does not favour the isolation of any population subset below this dose. Cell populations retrieved from arrest at varying intervals (0-3 h) after irradiation (0-3.5 Gy) showed an increase in survival with increase in interval, consistent with repair of potentially lethal damage. The repair curves (surviving fraction vs time) were each described by a single exponential. G2 cells that were brought to mitosis without a period of arrest exhibited the same radiation response as cells irradiated in mitosis.     

Progressive development of radiation damage in mouse kidneys and the consequences for reirradiation tolerance

The aim of this study was to investigate the influence of protracted overall treatment times on the development and repair of renal irradiation injury in mice. Functional kidney damage was measured, from the proportion of 51CrEDTA remaining in the plasma at 30 min after injection of the tracer. Damage was assessed at monthly intervals for up to 14 months after two equal doses of X-rays given in 1 day, 1 month or 6 months. There was no difference between the time of onset or rate of development of damage after two fractions in 1 day or 1 month, but there was a time lag of 7-15 weeks (depending on dose) before the development of damage after 2F given in 6 months. After this time lag the rate of progression of damage was the same for 2F/6 months as for 2F in the shorter intervals. There was therefore no indication of any increase in total tolerated dose for the kidney when the treatment time was protracted, although the time scales for onset of this damage differed. Tolerance of mouse kidneys to reirradiation at 6 months after single doses of 6-12 Gy was also assessed. All of the previously irradiated animals developed a more severe renal impairment after reirradiation than did the age-matched control mice. The most severe damage occurred in mice which received the highest initial radiation doses, but doses of only 6 Gy were sufficient to markedly reduce the tolerance to reirradiation. It was concluded from these studies that no additional dose-sparing (tissue recovery) took place in the kidneys during a 6-month interval. This was true even when the initial radiation dose alone was insufficient to cause measurable renal dysfunction.     

Evidence for reduced capacity for damage accumulation and repair in plateau-phase C3H 10T1/2 cells following multiple-dose irradiation with gamma rays

The effects of multiple-dose gamma irradiation on the shape of survival curves were studied with mouse C3H 10T1/2 cells maintained in contact-inhibited plateau phase. The dose-fractionation intervals included 3, 6, and 24 h. Following three fractionated doses (5 Gy per dose) of exposures, cells responded to further irradiation by displaying a survival curve with a much reduced shoulder width (Dq) compared to that of the survival curve measured in cells irradiated with single-graded doses alone. The effect on the mean lethal dose (D0) was small and appeared to be significant. The effect on reduction of Dq could not be completely overcome by lengthening the fractionation intervals from 3 to 6 h or 24 h, times in which repair of sublethal damage (SLD) measured by simple split-dose scheme and potentially lethal damage (PLD) measured by postirradiation incubation was completed. Other experiments showed that pretreatments of cells with fractionated irradiation appeared to slow down the cellular repair processes of SLD and PLD. Therefore, the observed change in the shape of survival curves after fractionation treatments may be attributed to a reduction of the cells' capacity for damage accumulation by an enhancement of the lethal expression of SLD and PLD. Although the molecular mechanism(s) is not known, the results of this study indicate that the acute graded dose-survival curve cannot be used a priori to extrapolate and reliably predict results of hyperfractionation. It is probable that for a nondividing or slowly dividing cell population, such an extrapolation may lead to an underestimation of cell killing. Furthermore, the findings of this investigation appear to support an interpretation, alternative to the high-linear energy transfer (LET) track-end postulate, for the effects on cell survival seen at low doses or low dose rates.     

Effects of split-dose irradiation on survival and oncogenic transformation induced by 31 MeV protons in C3H10T1/2 cells

Survival and oncogenic transformation were studied in C3H10T1/2 cells exposed to 31 MeV protons. Total doses of 0.5, 1 and 7 Gy were delivered as single and two equal fractions with various time intervals up to 10 h between doses. With split doses as compared with single doses to a total dose of 7 Gy, survival increased by a factor of 2.5 +/- 0.2, whereas the frequency of transformation per surviving cell declined by a factor of 3.1 +/- 0.5. Maximal split-dose recovery occurred within the first 5 h for both endpoints. Further, the transformation frequency decreased by factors of 3.1 +/- 0.6 and 1.5 +/- 0.3 respectively for total doses of 0.5 and 1.0 Gy split into two equal fractions. The data for 1 and 7 Gy are compatible with data in the literature for other low LET radiations.     

A comparison of chromosome repair kinetics in G(0) and G(1) reveals that enhanced repair fidelity under noncycling conditions accounts for increased potentially lethal damage repair

Potentially lethal damage (PLD) and its repair were studied in confluent human fibroblasts by analyzing the kinetics of chromosome break rejoining and misrejoining in irradiated cells that were either held in noncycling G(0) phase or allowed to enter G(1) phase of the cell cycle immediately after 6 Gy irradiation. Virally mediated premature chromosome condensation (PCC) methods were combined with fluorescence in situ hybridization (FISH) to study chromosomal aberrations in interphase. Flow cytometry revealed that the vast majority of cells had not yet entered S phase 15 h after release from G(0). By this time some 95% of initially produced prematurely condensed chromosome breaks had rejoined, indicating that most repair processes occurred during G(1). The rejoining kinetics of prematurely condensed chromosome breaks was similar for each culture condition. However, under noncycling conditions misrepair peaked at 0.55 exchanges per cell, while under cycling conditions (G(1)) it peaked at 1.1 exchanges per cell. At 12 h postirradiation, complex-type exchanges were sevenfold more abundant for cycling cells (G(1)) than for noncycling cells (G(0)). Since most repair in G(0)/G(1) occurs via the non-homologous end-joining (NHEJ) process, increased PLD repair may result from improved cell cycle-specific rejoining fidelity of the NHEJ pathway.