Survival characteristics of mammalian cell lines after single or multiple exposures to roentgen radiation under oxic or anoxic conditions

Treatment modelling: the influence of micro-environmental conditions

The interest in theoretical modelling of radiation response has grown steadily from a fast method to estimate the gain of new treatment strategies to an individualisation tool that may be used as part of the treatment planning algorithms. While the advantages of biological optimisation of plans are obvious, accurate theoretical models and realistic information about the micro-environmental conditions in tissues are needed. This paper aimed to investigate the clinical implications of taking into consideration the details of the tumour microenvironmental conditions. The focus was on the availability of oxygen and other nutrients to tumour cells and the relationship between cellular energy reserves and DNA repair ability as this is thought to influence the response of the various hypoxic cells. The choice of the theoretical models for predicting the response (the linear quadratic model or the inducible repair model) was also addressed. The modelling performed in this project has shown that the postulated radiobiological differences between acute and chronic hypoxia have some important clinical implications which may help to understand the mechanism behind the current success rates of radiotherapy. The results also suggested that it is important to distinguish between the two types of hypoxia in predictive assays and other treatment simulations.

Radioprotective action of extracellular adenosine on bone marrow cells in mice exposed to gamma rays as assayed by the micronucleus test

The frequency of micronucleated polychromatic erythrocytes (PCEs) in mouse bone marrow was assessed after administration of dipyridamole and/or adenosine monophosphate (AMP) to nonirradiated mice or to mice irradiated 15 min later with a sublethal dose of 6.5 Gy gamma rays. In nonirradiated mice, the administration of the drugs increased the frequency of micronucleated PCEs significantly (by 108%). In contrast, in irradiated mice, the number of radiation-induced micronucleated PCEs was significantly decreased if the mice had been pretreated with dipyridamole or AMP alone (by 24% after administration of each of the compounds) and in particular after administration of the drugs in combination (by 36%).

Superfractionation as a potential hypoxic cell radiosensitizer: prediction of an optimum dose per fraction

PURPOSE

A dose "window of opportunity" has been identified in an earlier modeling study (1) if the inducible repair variant of the LQ model is adopted instead of the pure LQ model, and if all survival curve parameters are equally modified by the presence or absence of oxygen. In this paper we have extended the calculations to consider survival curve parameters from 15 sets of data obtained for cells tested at low doses using clonogenic assays.

METHODS AND MATERIALS

A simple computer model has been used to simulate the response of each cell line to various doses per fraction in multifraction schedules, with oxic and hypoxic cells receiving the same fractional dose. We have then used pairs of simulated survival curves to estimate the effective hypoxic protection (OER') as a function of the dose per fraction.

RESULTS

The resistance of hypoxic cells is reduced by using smaller doses per fraction than 2 Gy in all these fractionated clinical simulations, whether using a simple LQ model, or the more complex LQ/IR model. If there is no inducible repair, the optimum dose is infinitely low. If there is inducible repair, there is an optimum dose per fraction at which hypoxic protection is minimized. This is usually around 0.5 Gy. It depends on the dose needed to induce repair being higher in hypoxia than in oxygen. The OER' may even go below unity, i.e. hypoxic cells may be more sensitive than oxic cells.

CONCLUSIONS

If oxic and hypoxic cells are repeatedly exposed to doses of the same magnitude, as occurs in clinical radiotherapy, the observed hypoxic protection varies with the fractional dose. The OER' is predicted to diminish at lower doses in all cell lines. The loss of hypoxic resistance with superfractionation is predicted to be proportional to the capacity of the cells to induce repair, i.e. their intrinsic radioresistance at a dose of 2 Gy.

Hyperfractionation as an effective way of overcoming radioresistance

PURPOSE

To model the influence of hypoxic radioprotection in fractionated treatments over a range of fraction sizes. To determine whether there is a "therapeutic window" of dose per fraction where hypoxic radioresistance could be reduced, and if so, where it occurs in different cell lines.

MATERIALS AND METHODS

A mathematical model has been used to simulate the response of cells to low doses of radiation, in the region of clinical interest. We have used the inducible repair variant of the linear quadratic (LQ) equation, with a hypersensitive region (alphaS) at low doses that gradually transforms to the accepted "resistance" in the shoulder region (alphaR). It contains two new parameters, the ratio alphaS/alphaR, and D(C). We have accepted that the "induction dose" D(C) is modified by anoxia to the same extent as the other parameters. We have initially modeled using theoretical parameters and then checked the conclusions with 14 sets of published experimental data for cell lines investigated for inducible repair.

RESULTS

We have computed the clinical hypoxic protection (OER') as a function of dose per fraction in simulations of clinical fractionated schedules. We have identified a therapeutic window in terms of dose per fraction at about 0.5 Gy, where the OER' is minimized, regardless of the precise cell survival curve parameters. The minimum OER' varies from one cell line to another, falling to about 1.0 if alphaS/alphaR = 6-10 and even far below 1.0 if alphaS/alphaR > or = 20.

DISCUSSION

Hyperfractionation using 0.5 Gy fractions may therefore be more effective than oxygen mimetic chemical sensitizers, since it could even make some tumor cells more sensitive than oxic normal tissues. The tumor lines that benefit most from this type of sensitization are those with the highest intrinsic oxic radioresistance, i.e. those with high SF2 values.

New insights into factors influencing the clinically relevant oxygen enhancement ratio

BACKGROUND AND PURPOSE

This paper deals with the variations in the oxygen enhancement ratios that could be observed (OER') when comparing oxic and hypoxic cells in different types of fractionated experiments as a consequence of the non-linearity of the underlying cell survival curves. Calculations have been made of the OER' that would be obtained for fractionated irradiations with a series of small doses to allow the comparison of isoeffective doses in oxic and hypoxic conditions. Two styles of fractionated experiment were modelled. In one, the dose per fraction was kept constant in the oxic and hypoxic arms of the experiment, necessitating more fractions in hypoxia to achieve the same level of cell kill. In the other the number of fractions was kept constant and the fraction size was varied to obtain equal levels of damage. The first is the relevant design for the clinic, whereas the second is the design most commonly used in animal studies.

MATERIALS AND METHODS

Three models of the survival curve were used to simulate the response of cells to radiation injury, all based on the linear quadratic model, but with various added assumptions. A simple classical LQ model is compared with two models in which the concept of inducible repair is added. In one of these the induction dose for 'switching on' the more resistant response is assumed to be increased in hypoxia and in the other it is assumed to be independent of the oxygen tension.

RESULTS

These calculations show a clear and previously unsuspected dependence of the measured OER' on the design of the fractionated experiment. The values obtained in the clinical and animal types of study differ considerably with all three models. The direction and magnitude of that difference depends critically on the assumptions about the fine structure of the survival curve shape. The authors suggest that the inducible repair version with an oxygen-dependent induction dose is probably the most relevant model. Using this, the measured OER' is reduced at doses around 2 Gy for the clinically relevant design of constant sized fractions to the oxic and hypoxic cells. It may even, in certain model assumptions, fall below unity resulting in an increased sensitivity, not resistance, from the hypoxia.

CONCLUSIONS

These calculations indicate the urgent need for more knowledge about the fine structure of the low dose region of the survival curves for human tumour cells and especially for comparisons in the presence and absence of oxygen. The extent of the hypersensitivity at very low doses, the trigger dose needed to induce the repair and its oxygen modification may be dominant factors in determining the response of tumour cells to clinically relevant fractionation schedules.

A 100-year Nordic perspective on the dose-time problem in radiobiology

The therapeutic use of x-rays began almost immediately after their discovery by Röntgen, and within a few years two Swedish physicians could report the first successful treatment of human skin cancer by radiotherapy. Almost from the start it was clear that the biological effect of ionizing radiation depended critically on the exact distribution of the dose in time. The present paper reviews the historical development of dose-time concepts in radiotherapy as seen from a Nordic perspective. Among the topics reviewed are the discussion of single versus fractionated doses, Strandqvist's thesis and the development of power-law biological dose formulas, the effect of dose per fraction and of overall treatment time. It is only within the last 10-15 years that biologically and clinically important dissociation between the radiobiology of early- and late-responding human normal tissues has been appreciated. Biological developments have led to the proposal of altered fractionation schedules, hyperfractionation and accelerated fractionation, that are currently undergoing clinical trial.

Backscatter radiation at tissue-titanium interfaces. Analyses of biological effects from 60Co and protons

It has been claimed that implanted metals can cause backscatter radiation in radiation therapy with a dose enhancement at the bone-metal and tissue-metal interfaces on the beam entrance side. Theoretical calculations and experimental measurements with ionization chambers have indicated that such effects might be significant. Titanium implants are increasingly used in oral and maxillo-facial surgery for reconstruction purposes. A more detailed knowledge of backscatter-induced effects is therefore desired when head and neck cancers in patients with implants are treated with radiotherapy. We have made comparisons of cell survival after irradiation of two types of cultured cells grown directly on titanium metal and on plastic control supports. The cell cultures were irradiated with either 60Co photons or range modulated protons. No significant differences in the colony-forming capacity were found between the irradiated cells grown on titanium and those grown on plastic control supports. This was the case for both radiation types and the results were also observed to be dose-independent. The only observed phenomena were that the two cell-lines differed in radiosensitivity and that protons gave higher biological effects than gamma radiation. The results show that there were no significant changes in cell survival at the interface between the tissue equivalent medium and titanium support indicating that a dose increase induced by backscatter radiation, which possibly could demolish the osseointegration or induce osteoradionecrosis, are minimal when high energy photons or range modulated protons are applied.

Irradiation therapy with multiple small fractions per day in urinary bladder cancer

In a clinical trial 168 patients with carcinoma of the bladder, T2-T4, were randomized to one of two treatments; 1 Gy 3 times a day to a total of 84 Gy or 2 Gy once a day to a total of 64 Gy. Local eradication of the tumour in the bladder cystoscopically and cytologically at 6 months after completion of treatment and patient survival were analyzed. The results favoured significantly the patients treated with 84 Gy. All patients were followed 5-9 years. The survival was significantly improved in patients with T3 lesions treated with 84 Gy (p less than 0.01). Complications in the bowel requiring surgical treatment were not significantly different between the two groups of patients. The results indicate a therapeutic gain by hyperfractionated radiotherapy in comparison to conventional fractionated radiotherapy.

Survival curves of glutathione synthetase deficient human fibroblasts: correlation between radiosensitivity in hypoxia and glutathione synthetase activity

The role of intracellular non-protein bound sulphydryl compounds (NPSH), and in particular that of glutathione (GSH), in the response of cells to ionizing radiation under different O2 concentrations has been assessed using cell strains deficient in glutathione synthetase and exhibiting different NPSH levels. The cell strains used originated from patients with 5-oxoprolinuria and from their relatives (heterozygotes and proficient homozygotes). No correlation has been found between NPSH and GSH concentrations and radiosensitivity under oxic, aerobic and hypoxic conditions. However, a highly significant correlation has been observed between radiosensitivity under hypoxic conditions (and therefore the oxygen enhancement ratio) and the glutathione synthetase activity, suggesting that synthesis of GSH is required after irradiation. In order to explain our results we postulated, beside radical processes, the existence of a GSH-dependent enzymatic repair mechanism for N2 type damage. Hypoxic radio-sensitivity measured with survival curves would result from the interaction of both competition and biochemical repair processes.

Radiation dose dependence of the sensitization by oxygen and oxygen mimic sensitizers

New and old evidence is discussed which suggests that the oxygen enhancement ratio (OER) is decreased at lower radiation doses as compared with that of higher radiation doses. In addition, there is evidence that cells irradiated in severe hypoxia have an impaired ability to recover from sublethal damage. However, there is also contrary evidence to these observations. Thus the suggestions that oxygen is not strictly a dose modifying agent have been controversial from the very beginning. It is hoped that new experimental undertakings currently performed in many different laboratories will resolve these issues as well as explain the controversy which has lasted more than 25 years in this field of radiobiologic research.

Post-irradiation diethyldithiocarbamate-inhibition of CuZn superoxide dismutase reduces clonogenic survival of Chinese hamster V-79 cells

The survival of Chinese hamster cells was reduced when they were temporarily treated with diethyldithiocarbamate (DDTC), an efficient inhibitor of CuZn superoxide dismutase (SOD), before radiation exposure. The dose-modifying factor amounted to about 1.3, and was the same for irradiation under hypoxic and aerobic conditions. Treatment of the cells with DDTC in the early post-irradiation hours was, however, equally effective. The results suggest that inhibition of CuZn SOD does not enhance the immediate radiation damage, but increases damage in the first hours of the post-irradiation period, possibly through mechanisms interfering with potential lethal damage.

Multiple daily fractions of radiation in the palliation of pain from bone metastases

Twenty consecutive patients have been treated for palliation of pain from bony metastases with radiotherapy using a regime of multiple daily fractions. Patients received up to three treatments daily with 3-4 hour intervals between fractions. Pain was relieved in all cases and pain relief took place earlier in the treatment than with conventional daily regimes. Nausea was mild in most cases. Some factors in its aetiology are discussed. Acute skin reactions were minimal. The total dosage required and the advantages and disadvantages of the method are discussed.

Lack of oxygen effect in glutathione-deficient human cells in culture

The frequency of X-ray-induced DNA breaks was determined in human cell lines which are deficient in glutathione synthetase and have a greatly reduced glutathione content. Hydroxyapatite chromatography was used for the estimation of the DNA breaks in cell cultures, which were derived either from lymphoblasts transformed by infection with EB virus or from fibroblasts. The dose-effect relationship for the induction of breaks when radiation exposure was made in argon, was similar to that found when exposure was made in air. In control cultures with normal glutathione content, the induction of breaks was enhanced when irradiation was made under aerobic, instead of anaerobic, conditions. Treatment of the glutathione-deficient cells with the hypoxic radiosensitizer misonidazole did not enhance the induction of breaks by radiation delivered either in air or in argon. In control cultures, radiation induction of breaks was enhanced by misonidazole under anaerobic but not under aerobic conditions. When the glutathione-deficient cells were pretreated with cysteamine however, irradiation in the absence of oxygen resulted in a decreased frequency of DNA breaks.

Sensitivity of cells in exponential and stationary growth phase to combined treatment with radiation and quinacrine

The effect of Quinacrine on the recovery of cells from sublethal radiation injury was analysed by testing cellular survival after exposure to low and high radiation doses, and after irradiation with split doses. Quinacrine was found to inhibit recovery in cell cultures in exponential growth phase but not, or only slightly, in cells in stationary growth. This finding is related to the inhibitory effect of Quinacrine on the repair of radiation induced DNA strand breaks.