Repair Of Sublethal Radiation Damage Research Articles

Improving the Therapeutic Ratio in Stereotactic Radiosurgery: Optimizing Treatment Protocols Based on Kinetics of Repair of Sublethal Radiation Damage

Sublethal damage after radiation exposure may become lethal or be repaired according to repair kinetics. This is a well-established concept in conventional radiotherapy. It also plays an important role in single-dose stereotactic radiotherapy treatments, often called stereotactic radiosurgery, when duration of treatment is extended due to source decay or treatment planning protocol. The purpose of this study is to look into the radiobiological characteristics of normal brain tissue and treatment protocols and find a way to optimize the time course of these protocols. The general problem is nonlinear and can be solved numerically. For numerical optimization of the time course of radiation protocol, a biexponential repair model with slow and fast components was considered. With the clinically imposed constraints of a fixed total dose and total treatment time, three parameters for each fraction (dose-rate, fraction duration, time of each fraction) were simultaneously optimized. A biological optimization can be performed by maximizing the therapeutic difference between tumor control probability and normal tissue complication probability. Specifically, for gamma knife radiosurgery, this approach can be implemented for normal brain tissue or tumor voxels separately in a treatment plan. Differences in repair kinetics of normal tissue and tumors can be used to find clinically optimized protocols. Thus, in addition to considering the physical dose in tumor and normal tissue, we also account for repair of sublethal damage in both these tissues.

Enhancement of radiation effect using beta-lapachone and underlying mechanism

Beta-lapachone (β-Lap; 3,4-dihydro-2, 2-dimethyl-2H-naphthol[1, 2-b]pyran-5,6-dione) is a novel anti-cancer drug under phase I/II clinical trials. β-Lap has been demonstrated to cause apoptotic and necrotic death in a variety of human cancer cells in vitro and in vivo. The mechanisms underlying the β-Lap toxicity against cancer cells has been controversial. The most recent view is that β-Lap, which is a quinone compound, undergoes two-electron reduction to hydroquinone form utilizing NAD(P)H or NADH as electron source. This two-electron reduction of β-Lap is mediated by NAD(P)H:quinone oxidoreductase (NQO1), which is known to mediate the reduction of many quinone compounds. The hydroquinone forms of β-Lap then spontaneously oxidizes back to the original oxidized β-Lap, creating futile cycling between the oxidized and reduced forms of β-Lap. It is proposed that the futile recycling between oxidized and reduced forms of β-Lap leads to two distinct cell death pathways. First one is that the two-electron reduced β-Lap is converted first to one-electron reduced β-Lap, i.e., semiquinone β-Lap (SQ)·- causing production of reactive oxygen species (ROS), which then causes apoptotic cell death. The second mechanism is that severe depletion of NAD(P)H and NADH as a result of futile cycling between the quinone and hydroquinone forms of β-Lap causes severe disturbance in cellular metabolism leading to apoptosis and necrosis. The relative importance of the aforementioned two mechanisms, i.e., generation of ROS or depletion of NAD(P)H/NADH, may vary depending on cell type and environment. Importantly, the NQO1 level in cancer cells has been found to be higher than that in normal cells indicating that β-Lap may be preferentially toxic to cancer cells relative to non-cancer cells. The cellular level of NQO1 has been found to be significantly increased by divergent physical and chemical stresses including ionizing radiation. Recent reports clearly demonstrated that β-Lap and ionizing radiation kill cancer cells in a synergistic manner. Indications are that irradiation of cancer cells causes long-lasting elevation of NQO1, thereby sensitizing the cells to β-Lap. In addition, β-Lap has been shown to inhibit the repair of sublethal radiation damage. Treating experimental tumors growing in the legs of mice with irradiation and intraperitoneal injection of β-Lap suppressed the growth of the tumors in a manner more than additive. Collectively, β-Lap is a potentially useful anti-cancer drug, particularly in combination with radiotherapy.

Abstract 5716: Interference with repair of sub-lethal radiation damage by ABT-888, an inhibitor of poly(ADP-ribose) polymerase (PARP), and theobromine

Abstract BACKGROUND: Upon DNA damage, PARP-1 and -2 use NAD to build a polymer of ADP-ribose on themselves, histones, and other proteins, altering chromatin and recruiting repair complexes to the damage. Inhibition of PARP activity sensitizes cancer cells or tumor xenografts to radiation therapy (RT) and inhibits DNA repair. RT is usually delivered as fractionated daily doses, allowing sub-lethal damage (SLD) to be repaired between doses. We asked if ABT-888, a specific PARP inhibitor, or theobromine (TB), which radiosensitizes via a PARP-independent mechanism, could inhibit SLD repair. METHODS: Human lung cancer cells A549 and H460 were plated at a range of cell densities, exposed to Cs-137 γ-radiation (0-8 Gy), as single doses or as multiples of 2 Gy/day, with or without drugs, then allowed to form colonies. DNA double-strand breaks (DSB) were estimated as γH2AX foci. RESULTS: In agreement with literature data, 2.5 μM ABT-888 produced a dose enhancement factor (DEF) of 1.4 at 10% survival for single radiation doses, and 10 μM was only modestly better (DEF 1.5). A549 cells showed marked repair of SLD (Table), such that the surviving fraction (SF) after 6 Gy was lower (0.13) than for 3 x 2 Gy (0.52). When ABT-888 or TB was added 1 h before the first irradiation, cells were sensitized to 6 Gy single dose or 3 x 2 Gy. The combination was more effective. ABT-888-sensitized cells responded to 3 x 2 Gy with similar killing as unsensitized cells to a single 6 Gy dose (p = 0.93). The combined effects are reflected in a decrease in mean inactivation dose (MID). Higher levels of γH2AX foci in drug-treated cells indicate inhibition of damage processing. CONCLUSIONS: Radiosensitization of human lung cancer cells by ABT-888 is at least partially via inhibition of SLD repair. Although more ABT-888 produces little additional benefit, addition of TB, which acts through a complementary mechanism, further improves the response. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5716. doi:1538-7445.AM2012-5716

Abstract 2677: Novel Hsp90 inhibitor, STA-9090, for combination with radiotherapy

Abstract Introduction: Radiation is accepted as an important standard therapy for locally unresectable cancers, and as such is given to approximately 60% of cancer patients. However, radio-resistance and repair of sublethal radiation damage can limit its efficacy. Recent studies have shown that Heat Shock Protein 90 (Hsp90), a molecular chaperone that mediates maturation and activation of client proteins, plays a critical role in establishing resistance to radiation therapy. Inhibiting Hsp90 has been reported to sensitize tumors to radiation, resulting in tumor growth suppression and augmenting therapeutic cell death induction. Unfortunately, many of the Hsp90 inhibitors currently in clinical trials exhibit hepatotoxicity as well as ocular toxicity, hindering their clinical use. Taken together, development of clinically acceptable Hsp90 inhibitors for combination with radiation could serve as an important strategy for improving radiotherapy success. STA-9090 is a second generation Hsp90 inhibitor that has shown potent preclinical activity and is currently in twelve Phase II trials across a broad range of indications. STA-9090 has demonstrated encouraging activity in a Phase II trial in patients with stage IIIB and IV non-small cell lung cancer. Importantly, STA-9090 has displayed a favorable safety profile with substantially lower incidence of hepatic or ocular toxicity than that reported for other Hsp90 inhibitors. Results: We evaluated the radiosensitizing potential of STA-9090 in vivo. Monotherapy treatment with either STA-9090 or 2 Gray (Gy) ionizing irradiation resulted in moderate reductions in human tumor growth rates in a mouse xenograft model. Combination of STA-9090 with 2 Gy irradiation resulted in substantial tumor regression. Increasing the dose of radiation in the combination arm to 4 Gy further enhanced tumor regression, resulting in a 50% reduction in tumor volume. In summary, STA-9090 offers a safe and effective strategy for improving the outcome of radiotherapy in human cancers. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2677. doi:10.1158/1538-7445.AM2011-2677

Loss of radiobiological effect of imaging dose in image guided radiotherapy due to prolonged imaging‐to‐treatment times

Increased use of cone beam CT guidance in image guided radiotherapy has prompted the inclusion of the imaging dose in treatment plans, thus using imaging beams to treat tumors. Sublethal radiation damage repair during tau(d), the time between imaging and treatment, could reduce the effectiveness of the imaging dose, resulting in tumor underdosage. The theoretical magnitude of this effect was quantified using radiobiological modeling. The therapeutic effective dose (TED), which, if delivered using only therapeutic beams, would result in the same tumor cell survival as for both the imaging and therapeutic beams, was derived using the generalized linear-quadratic model. The correction factor P(d) by which therapeutic dose can be scaled to compensate for sublethal damage repair was also derived. TED and P(d) are dependent on alpha/beta, sublethal damage repair half-time (T(r)), imaging dose (D(I)) and dose rate (D(I)), therapeutic dose (D(T)) and dose rate (D(T)), and tau(d). TED and P(d) were calculated as a function of tau(d), and each parameter was varied independently while holding the remaining parameters at their reference values. The reference values were based on prostate cancer cells and were D(p) = D(I)+D(T) = 1.8 Gy, D(I)/D(p) = 5%, D(I) = 0.33 Gy/min, D(T) = 1.0 Gy/min, alpha/beta = 3.1 Gy, T(r) = 16 min, and tau(d) = 0 min. Estimates of the expected values of TED and P(d), (TED) and (P(d)), were calculated using tau(d) and D(T) distributions from a few thousand prostate treatment fractions. For a typical tau(d) value of 5.0 min and all other parameters set to their reference values, TED was 0.5% lower than the prescription dose D(p). For tau(d) = 20 min and all other parameters at reference conditions, TED dropped by 5% relative to D(p) when D(I)/D(p) was 20% and by 2% relative to D(p) when alpha/beta = 1 Gy or T(r) = 5 min. TED/D(p) varied more with D(T) than D(I) when tau(d) < or = 20 min, varying by up to 1% over 0.05 < or = D(T) < or = 10 Gy/min and by less than 0.1% over 0.05 < or = D(I) < or = 2.0 Gy/min. Under the reference conditions, (TED) was lower than D(p) by 0.5%. For the extreme D(I)/D(p) = 20% and all other parameters at their reference values, setting alpha/beta = 1 Gy resulted in (TED) dropping below D(p) by 2.5% and setting T(r) = 5 min resulted in (TED) dropping below D(p) by 4%. For tumors with a T(r) of 16 min or greater and alpha/beta of 11 Gy, (TED) dropped below D(p) by 0.2% or less. For prostate tumors receiving a reasonable percentage of 5% of their total dose from imaging beams, the theoretical drop in (TED) relative to D(p) was 0.5%. This loss could be accounted for during treatment planning by scaling the therapeutic dose by the expected sublethal damage repair factor (P(d)). For nonprostate tumors with alpha/beta values of 11 Gy, the theoretical drop in (TED) relative to the reference TED was low at 0.2%.

Toward improving the therapeutic ratio in stereotactic radiosurgery: selective modulation of the radiation responses of both normal tissues and tumor

A review of the radiobiological factors that influence the response of the brain to radiation is provided in relation to stereotactic radiosurgery (SRS). The prospects for intervention after radiation treatment to selectively modulate the expression of late central nervous system (CNS) injury is considered, as well as an account of recent interest in the use of radiation enhancers to selectively increase the response of tumors to radiation. Brain necrosis in humans, after conventional irradiation, indicates that the risk of necrosis increases rapidly after an equivalent single dose of 12 or 13 Gy. When single-dose treatments are extended due to 60Co decay or planned extension of treatment times, account should be taken of the effects of the repair of sublethal radiation damage to DNA on the efficacy of treatment. Both repair capacity and repair kinetics will also influence tumor control, but parameters to quantify this effect have not yet been established. The volume of CNS tissue that has been irradiated affects the tissue response, but this effect is only significant for volumes less than 0.05 cm3. The gain obtained from irradiation of small volumes is reduced, however, when focal irradiation is given within a wider field of irradiation. Based on a vascular hypothesis explaining the pathogenesis of late CNS damage, approaches designed to selectively modulate the frequency of late CNS damage have been validated. Given the high intrinsic radioresistance of some tumors, as opposed to the presence of hypoxia, an interest has developed in the use of selective radiation enhancers in the treatment of tumors. The compound presently available has proved to be disappointing clinically due to toxicity at effective doses, when repeated administration is required. However, when given at high single doses it is less toxic and may be more effective. Less toxic radiation enhancers need to be developed.

Synergistic Effects of Radiation and β-Lapachone in DU-145 Human Prostate Cancer CellsIn Vitro

It has been reported that beta-lapachone (beta-lap), a bioreductive anti-cancer drug, synergistically interacts with ionizing radiation and that the sensitivity of cells to beta-lap is closely related to the activity of NAD(P)H:quinone oxidoreductase 1 (NQO1). Here we report the results of our studies of mechanisms underlying the synergistic interaction of beta-lap and radiation in killing cancer cells using the DU-145 human prostate cancer cell line. The clonogenic cell death caused by the combination of radiation and beta-lap was synergistic when beta-lap was administered 0-10 h after irradiation but not when it was given before irradiation. The expression and activity of NQO1 increased significantly and remained elevated for longer than 12 h after 4 Gy irradiation, suggesting that the long-lasting elevation of NQO1 sensitized the cells to beta-lap. Studies with split-dose irradiation demonstrated that beta-lap given immediately after irradiation effectively inhibited sublethal radiation damage (SLD) repair. Taken together, these results lead us to conclude that the synergistic interaction between beta-lap and radiation in killing cells is the result of two distinct mechanisms: First, radiation sensitizes cells to beta-lap by up-regulating NQO1, and second, beta-lap sensitizes cells to radiation by inhibiting SLD repair. The combination of beta-lap and radiotherapy is potentially promising modality for the treatment of cancer in humans.

Synergistic Effect of Ionizing Radiation and β-lapachone against RKO Human Colon Adenocarcinoma Cells

To reveal the interaction between beta-Lapachone (beta-lap) and ionizing radiation in causing cell death in RKO human colon adenocarcinoma cells, and to elucidate the potential usefulness of combined beta-lap treatment and radiotherapy for cancer treatment. The cytotoxicities of various treatments were determined in vitro using clonogenic and apoptotic cell death. The changes in cell cycle distribution were studied using flow cytometry and an in vitro kinase assay. The tumor growth was studied using RKO tumors grown s.c. in the hind leg BALB/c- nuslc nude mice. beta-Lap caused clonogenic cell death and rapid apoptosis in RKO cells in vitro, in a dose dependent manner. The repair of sublethal radiation damage was almost completely inhibited when cells were maintained in beta-lap during the interval between the two-dose irradiation. Flow cytometry study demonstrated that beta-lap induced apoptosis, independent of the cell cycle phase, and completely prohibited the induction of radiation-induced G2 arrest in irradiated cells. The prohibition of radiation-induced G2 arrest is unclear, but may be related to the profound suppression of the p53, p21 and cyclin B1-Cdc2 kinase activities observed in cells treated with beta-lap. The combination of beta-lap and radiation markedly enhanced the radiation-induced growth suppression of tumors. beta-Lap is cytotoxic against RKO cells, both in vitro and in vivo, and also sensitized cells to ionizing radiation by inhibiting sublethal radiation damage repair. beta-lap is potentially useful as a potent anti-cancer chemotherapy drug and potent radiosensitizer against cancer cells.

In vitro enhancement of tumor cell radiosensitivity by a selective inhibitor of Cyclooxygenase-2 enzyme: mechanistic considerations

Purpose : Selective cyclooxygenase-2 inhibitors have been reported to enhance the tumor response to radiation in vivo, but the cellular mechanisms underlying the radiosensitizing effect are not understood. In the present study, we investigated several possible mechanisms using a murine sarcoma cell culture system. Methods and Materials : Cells derived from a murine sarcoma, designated NFSA, were cultured in vitro and exposed to different (either single or split) doses of radiation with and without a pretreatment of SC-236 (4-[5-(4-chlorophenyl)-3-(trifluoromethyl)-1H-pyrazol-l-yl] benzene sulfonamide), a selective cyclooxygenase-2 (COX-2) inhibitor. The cells were assayed for clonogenic survival to determine the radiosensitizing effect of SC-236. In addition, MTT assay and TUNEL assay were performed to determine the effects of SC-236 and radiation on the cell survival and cell cycle distribution. RNase protection assay was performed on the total RNA extract using probes that encoded for selected cell cycle regulatory proteins, such as cyclins and cyclin-dependent kinases. To monitor the extent of COX-2 activity and its role in radiosensitization, the cellular content of prostaglandin E2, a major metabolite of COX-2 activity on arachidonic acid, was also determined. Results : The cell clonogenic survival assay showed that SC-236 significantly enhanced tumor cell radiosensitivity: 50 μM SC-236 increased it by a factor of 1.51 at the 0.1 cell survival level. Treatment with SC-236 (50 μM, 3 days) removed the “shoulder” region on the radiation survival curve, suggesting that the drug inhibited repair of sublethal radiation damage. The inhibition was confirmed by split-dose experiments where two doses (3 Gy each) of radiation were given 4 h apart. The cells exposed to radiation only repaired the damage by a factor of 1.44, whereas those treated with SC-236 plus radiation repaired it by a factor of 1.1 only. Whereas SC-236 induced apoptosis in these NFSA cells, radiation did not. No further increase in apoptosis was observed when the cells were exposed to both SC-236 and radiation, suggesting that SC-236 did not render tumor cells more susceptible to radiation-induced apoptosis. The RNase protection assay showed that SC-236 (50 μM, 3 days) inhibited the expression of cyclins A and B, as well as cyclin-dependent kinase-1. Inhibition of these cell cycle regulatory elements by SC-236 was associated with the arrest of cells in the radiosensitive G2-M phase (67%), determined by flow cytometry. Conclusions : SC-236 significantly enhanced radiosensitivity of tumor cells; the magnitude of sensitivity was dependent on the drug’s concentration. The likely mechanisms involve accumulation of cells in the radiosensitive G2-M phase of the cell cycle and inhibition of repair from sublethal radiation damage.

Repair between Dose Fractions: A Simpler Method of Analyzing and Reporting Apparently Biexponential Repair1

Increasing numbers of animal experiments in situ are reporting that repair of sublethal radiation damage in vivo slows down with time, usually described as two components of (monoexponential) repair. For repair of DNA strand breaks, plotting the reciprocal of proportion unrepaired as a function of time yielded straight lines. Two processes have been suggested as causing this: (1) a second-order process (bimolecular) instead of first-order (exponential) and (2) a skewed distribution of monoexponential rates. The present paper investigates whether such plots of hyperbolic or reciprocal repair are relevant for laboratory animal tissue results. Published repair data were reanalyzed from laboratory animal experiments that employed split doses or two fractions per day. Graphs are presented of the reciprocal proportion of damage remaining as a function of the interval between the two doses. If the reciprocal model applies, the graphs would be straight lines. Different animal data showed no inconsistency with straight reciprocal plots. These reciprocal plots describe well with one parameter tau, the first half-time, repair curves previously thought to be "biexponential", and to require three parameters. Straight reciprocal plots mean that in a constant time interval tau the unrepaired damage falls from 1 to (1/2), then from (1/2) to (1/3), then (1/3) to (1/4), etc. A much larger proportion of damage would therefore remain unrepaired at several half-times than is estimated by current mono- or biexponential models. The practical implications for clinical radiotherapy are important.

Late functional response of mouse urinary bladder to fractionated X-irradiation

Purpose: Late functional changes in mouse urinary bladder were studied after fractionated irradiation in order to define parameters of repair of sublethal radiation damage. Materials and methods: In the repair capacity study, 1 to 10 fractions were applied within 5 days. To assess the kinetics of repair, 4 fractions were applied with intervals of 0, 0.5, 1, 2, 4, or 8h. The endpoint used to establish dose-effect relationships was a reduction in the cystometrically measured bladder capacity by 50% of the individual pre-treatment value. Data analysis was performed with a linear-quadratic mixture model, which includes latent-time analysis in an effect-dependent manner. Results: The alpha/beta values were found to be 4.4Gy (95% confidence interval [2.0-8.4]) for the repair capacity data alone and 3.7Gy (1.7-6.7) in combination with the repair kinetics data. The halftime of repair, assuming mono-exponential repair kinetics, was 0.39h (95% CI [0.23-1.3]). The mean latent time decreased from around 40 weeks to 12 weeks, inversely dependent on the biologically effective dose. In animals experiencing an acute response during the first month after treatment, the incidence of severe late responses was significantly increased (p=0.0025). Conclusions: The present results confirm that the sparing effect in the mouse urinary bladder by dose fractionation is in the lower range of chronically responding tissues. The half-time of repair is in the lower range reported for mouse tissues.

Interactions of mild hyperthermia, cisplatin and split dose irradiation in human ovarian carcinoma cells.

Human ovarian cancer cells were evaluated to determine whether combination treatment with mild hyperthermia and cisplatin could inhibit repair of sublethal radiation damage. Human ovarian carcinoma cell lines A2780S (parental line) and A2780CP (cisplatin resistant variant) were used in this study. Cisplatin at concentrations of 1 or 3 microg/ml was given concomitantly with 1 h of heating at 40 degrees C either immediately before or after irradiation. Survival was determined using a colony-forming assay. Neither mild hyperthermia nor cisplatin treatments alone affected sublethal damage repair. The combined treatment showed an effect in both cell lines and was treatment sequence-dependent. The effect was greater in the parental cell line. The data show that combined treatment of cisplatin and hyperthermia may have clinical efficacy at cisplatin concentrations and hyperthermia temperatures which by themselves have little to no effect.

Irinotecan and radiation in vitro and in vivo

Topoisomerase I inhibitors have shown positive effects in combination with radiation therapy in some studies. Normally oxygenated and hypoxic human MCF-7 breast carcinoma-cells were exposed to irinotecan (100 mu M or 250 mu M) or to SN-38 (10 mu M or 25 mu M) for 1 h prior to, during and for 3 h after radiation. Irinotecan and SN-38 showed little or no radiation sensitization of normally oxygenated MCF-7 cells but were effective radiation sensitizers of hypoxic cells. Both irinotecan and SN-38 diminished or eliminated the shoulder of the radiation survival curves of both the normally oxygenated and hypoxic cells indicating inhibition of the repair of sublethal radiation damage to DNA. Irinotecan (20 mg/kg or 30 mg/kg) was administered to mice bearing the EMT-6 mammary carcinoma on days 7 through 11 just prior to fractionated radiation (5x3 Gray). The tumor growth delays obtained with the combination regimens were greater than expected for simple additivity of the two treatments. Treatment with irinotecan resulted in decreased expression of topoisomerase I mRNA and increased expression of topoisomerase II mRNA in EMT-6 tumor tissue. Irinotecan treatment did not alter the protein levels for topoisomerase I or II in the tumor tissue; however, the combination of radiation therapy and irinotecan administration resulted in decreased topisomerase I and increased topoisomerase II protein in the tumor tissue. These results suggest that with appropriate scheduling of a topoisomerase I inhibitor and a topoisomerase II inhibitor with fractionated radiation therapy maximal cyto-reduction can be achieved.

Dose reduction factors when increasing dose rate in LDR or MDR brachytherapy of carcinoma of the cervix

Background: The detailed summary of results from Bristol where 270 patients with carcinoma of the cervix were treated with either 75 cGy/h from manually loaded caesium or 150 cGy/h by remote afterloading (Newman, G. Increased morbidity following the introduction of remote afterloading, with increased dose rate, for cancer of the cervix. Radiother. Oncol. 39: 97–103, 1996) can be analysed to study the radiobiological factors that could have contributed to the different outcomes reported. The increases in grade 3 late complications from 4 to 22%, and in grade 2 + 3 from 12 to 32%, in spite of a reduction of 20% in dose, imply a rather large difference in biological effect between the two systems, which might or might not be due to dose rate differences. Purpose: The possibility that a significant part of the effect might have been due to the dose rate is investigated, with particular attention to what values of radiobiological parameters might explain it, or can be excluded. A similar set of clinical data from Tilsburg (Rodrigus, P., de Winter, K., Venselaar, J. and Leers, W.H. Evaluation of late morbidity in patients with carcinoma of the uterine cervix following a dose rate change. Radiother. Oncol. 42: 137–141, 1997), where dose rate was doubled from 54 to 107 cGy/h with a dose reduction of 20%, is also considered. Methods: Linear quadratic modelling is employed to calculate Biologically Effective Doses (or ERDs) corresponding to the clinical protocols used. Repair of sublethal radiation damage at a range of half-times is assumed, both for mono- and bi-exponential components. When the LDR is doubled it is called MDR in the present study. Results: The maximum ratios calculated for the BEDs of 16 Gy at MDR to 20 Gy at LDR were 1.06–1.15, assuming α β = 4−2 Gy , the latter being an unlikely extreme for rectal or urinary complications. These maxima occurred over a narrow range of t 1 2 values from 1.5 to 2.5 h. If t 1 2 were as long as 4–7 h or as short as 0.5–0.75 h, the biological effects would have been equal. The theoretically ideal dose reduction factors, calculated using the t 1 2 values derived from the clinical data, are in the range of 24–29% instead of 20%. Conclusions: Somewhat greater dose reduction factors for late complications were suggested by this analysis than the 20% that has been commonly used when the dose rate is increased, both from the Bristol and Tilsburg data. The Bristol data showed no loss of therapeutic ratio on changing from manual to remote afterloading. Due to the close-to-optimum choice of the dose reduction factor which was actually used, some values for the half-time of repair of late complications in gynaecological brachytherapy could be estimated and used to calculate the theoretical dose reduction factors.

Do cells repair precancerous lesions induced by radiation?

The most widely accepted point of view is that cells are endowed with the capacity to repair the primary lesions responsible for cancer induction. In radiobiology, this popular belief evolved from experiments of the same type as those that suggested the existence of sublethal radiation damage repair. The central problem with such data is that the cell-killing component of radiation damage may mask the effects associated with repair of precancerous lesions. The challenge is to separate the two processes that contribute to the observed tumor incidence after irradiation. using a recently developed stochastic model of radiation carcinogenesis allowing for cell death, we provide evidence that precancerous lesions are not subject to repair under certain experimental conditions.

Effect of Combined Natural Human β–lnterferon and Radiation on Human Tumor Cells

Background: This investigation is done to evaluate the extent of β-Interferon-induced radiosensitization and the underlying mechanisms by which β-Interferon alters the radiation response of two different human tumours in vitro. Material and Methods: Two human tumors were studied in vitro. One was the second passage of a human squamous cell carcinoma of the head and neck (ZMK-1), the other was the breast cancer cell line MCF-7. Cells were cultured by standard methods. β-Interferon was used in concentrations from 10 to 1,000 IE/ml medium, incubation times were 3 h (pulse) or for the remainder of the incubation for colony formation (continuous). Cells were either attached to their flasks or in suspension. The cells were irradiated with 200-kV X-rays at dose levels from 2 to 10 Gy. Evaluation was done by colony assay. Results: For MCF-7 cells we found an antiproliferative effect caused by β-Interferon depending on concentrations and incubation times. When combined with X-radiation we found no additional effects of β-Interferon for both kinds of cells irradiated in suspension. When the cells were irradiated attached to their flasks, β-Interferon caused a slight radiosensibilization with respect to the MCF-7 cells and a pronounced effect with respect to the ZMK-1 cells, the isoeffect enhancement ratio being 1.5, determined at the 10% survival level. Conclusion: These results demonstrate that under the influence of β-Interferon repair of sublethal radiation damage of MCF-7 and ZMK-1 cells is decreased by a short preirradiation incubation time. Additionally, lethal radiation of ZMK-1 cells is increased by β-Interferon.

A STUDY OF CISPLATIN-RADIOSENSITIZATION THROUGH INHIBITION OF REPAIR OF SUBLETHAL RADIATION-DAMAGE IN OVARIAN-CARCINOMA CELLS SENSITIVE AND RESISTANT TO CISPLATIN

Human ovarian carcinoma parental cells and cisplatin resistant variants were evaluated for the effect of cisplatin on radiosensitization and repair of sublethal radiation damage. The data showed that for 1 h cisplatin treatment (2 mu g/ml), radiosensitization was achieved only if the drug was given after irradiation. Cisplatin treatment was also able to inhibit repair of sublethal radiation damage (SLDR). The degree of inhibition was about the same in the resistant and the sensitive cell lines. In the parental line 4 mu g/ml cisplatin was required to inhibit SLDR while in the resistant line both 2 and 4 mu g/ml produced inhibition of SLDR and in the resistant cell line the degree of SLDR inhibition was greater for cisplatin after irradiation compared to cisplatin before irradiation.

Dose-rate effects between 0.3 and 30 GY/H in a normal and a malignant human cell line

Purpose : This study used continuous “intermediate” dose rate irradiation (0.3–30 Gy/h) to compare the capacity for and repair of sublethal radiation damage in different cell lines growing in tissue culture. Methods and Materials : Two human cell lines were studied; one was derived from normal human fibroblasts (AG1522) and the other from a squamous cell carcinoma of the uterine cervix (HTB-35). Dose-response curves for clonogenic survival were determined following irradiation of plateau-phase cultures at five different dose rates: 22.6, 6.12, 3.65, 1.04, and 0.38 Gy/h. Subculture following irradiation was delayed for 8–24 to allow for the full repair of “potentially lethal damage.” Results : A significant dose-rate effect was seen in both cell lines. For irradiation at the highest dose rate, survival at 2 Gy (SF2) and the α β ratio were similar for the two cell lines (approximately 0.7 and 8.0 Gy, respectively) but the half-time of repair of sublethal damage was estimated to be approximately five times longer in the normal human fibroblast line (154 min) than in the carcinoma (31 min) cell line. Conclusion : These results indicate that measuring the dose-rate effect between 0.3 and 30 Gy/h is a useful way to identify and quantify differences in sublethal damage repair between cell lines. To the extent that in vitro and in vivo repair parameters are similar, and that representative tumor biopsy specimens can be examined in this way, this approach may provide a prospective way of determining the dose rate (brachytherapy) or fractionation schedule that will optimize the therapeutic ratio.

Repair kinetics of radiation damage in the developing rat cervical spinal cord.

The kinetics of repair of sublethal radiation damage was examined in the cervical spinal cord of developing, 1-week-old rats. Split-dose irradiation treatments were given with time intervals of 0.5-96 h. The data, supplemented with fractionation data from previous experiments, were analysed using direct analysis based on the incomplete repair (IR) model. The best fit to the monoexponential repair model resulted in an estimated half-time of repair (T1/2) of 1.5 (1.3-1.8) h. No indications of a slow or second component of repair could be detected in the 1-week-old cervical spinal cord. This is in contrast with literature reports of experiments with the adult rat cervical spinal cord, suggesting bi-exponential repair, with 65% of the damage repaired with a slow repair T1/2 of about 4 h. Two-step methods, although statistically inferior to direct analysis, are still in use for analysis of repair experiments. A number of two-step methods used for data analysis in previous reports concerning sublethal damage repair, are dose (un)repaired, proportion of dose unrepaired, and proportion of damage unrepaired. It is argued that of the methods discussed, only analysis of the data expressing the results as proportion unrepaired damage (delta Eu) and using split-dose experiments, does not result in introduction of an artificial second repair T1/2 in tissues with a high fractionation sensitivity. Two-step analysis of the present data using delta Eu suggested monoexponential repair with a T1/2 value of 1.5 (SE 0.2) h.

Myelopathy and hyperfractionated accelerated radiotherapy: a radiobiological interpretation.

Where there is a rapid repopulation of tumor clonogens, a gain in local control should be realized by administering radiation treatments in a shorter overall time, thereby reducing the opportunity for proliferation of tumor cells during the treatment. Because of the differential effect of fractionation in early-versus late-responding normal tissues, decreased overall treatment time must be done in the context of keeping the fraction size small. The only feasible approach is to administer multiple fractions per day and use short interfraction intervals. Provided repair of sublethal radiation damage is largely complete within the interfraction interval, excessive late morbidity would not be expected. These radiobiological rationales of accelerated hyperfractionated radiotherapy thus suggest an altered fractionation scheme in which the overall treatment time is reduced, the dose per fraction is reduced, and multiple fractions are given per day.KeywordsPrincess Margaret HospitalSmall Fraction SizeSublethal DamageRepair KineticRadiation MyelopathyThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.