A radioresistant variant derived from a human neuroblastoma cell line is less prone to radiation-induced apoptosis.

By subjecting radiosensitive human neuroblastoma IMR 32 cells to a regime of fractionated X-irradiation, a radioresistant variant, XRIMR 32, was obtained. Radiation resistance of XRIMR 32 cells was demonstrated by clonogenic and spheroid regrowth delay assays. The XRIMR 32 cultures were phenotypically unstable, with the resistant phenotype being lost after 3 passages in the absence of radiation-selective pressure, but a monoclonal cell line (clone F) was established that maintained its resistance over 35 passages without irradiation. Flow cytometry showed that exponentially growing IMR 32, XRIMR 32, and clone F cells all had very similar cell cycle distributions. Studies of initial DNA damage and repair, using the technique of neutral filter elution, revealed no differences between these lines. Chromosomal damage, as measured by micronucleus frequency following irradiation, was also seen to be very similar. However, studies of apoptosis following irradiation showed significantly higher levels of apoptosis in IMR 32 cells, compared to the resistant lines. This was true at all time points studied between 6 and 42 h after irradiation. p53 status was examined in the IMR 32 and clone F cells. No mutations were detected in exons 5-8 of the cDNA. Both lines showed increased p53 expression after irradiation. These data are consistent with the evolution of cellular resistance as a possible mechanism for the evolution of cellular radioresistance during protracted radiation regimes. However, the molecular mechanism responsible for the increased radioresistance remains to be discovered.

Tumor spheroid model for the biologically targeted radiotherapy of neuroblastoma micrometastases.

Neuroblastoma is a pediatric malignancy with a poor prognosis at least partly attributable to an early pattern of dissemination. New approaches to treatment of micrometastases include targeted radiotherapy using radiolabeled antibodies or molecules which are taken up preferentially by tumor cells. Multicellular tumor spheroids (MTS) resemble micrometastases during the avascular phase of their development. A human neuroblastoma cell line (NBl-G) was grown as MTS and incubated briefly with a radiolabeled monoclonal antibody (131I-UJ13A) directed against neuroectodermal antigens. Spheroid response was evaluated in terms of regrowth delay or proportion sterilized. A dose-response relationship was demonstrated in terms of 131I activity or duration of incubation. Control experiments using unlabeled UJ13A, radiolabeled nonspecific antibody (T2.10), radiolabeled human serum albumin, and radiolabeled sodium iodide showed these to be relatively ineffective compared to 131I-UJ13A. The cell line NBl-G grown as MTS has also been found to preferentially accumulate the radiolabeled catecholamine precursor molecule m-[131I]iodobenzylguanidine compared to cell lines derived from other tumor types. NBl-G cells grown as MTS provide a promising laboratory model for targeted radiotherapy of neuroblastoma micrometastases using radiolabeled antibodies or m-iodobenzylguanidine.

The effect of differing radiotherapeutic schedules on the response of glottic carcinoma of the larynx.

Laryngeal tumours, especially T1N0M0 and T2N0M0 lesions, are readily controlled by radiotherapy. Studies have shown that control varies with the dose of radiotherapy delivered to the tumour. Other factors, including the dose per fraction and the time over which the treatment schedule is delivered are also important. The varying biological effectiveness of a number of different dose fraction time schedules used in the management of laryngeal tumours of different stages are considered, the end points being tumour control and associated morbidity. Special attention has been given to the length of time over which the schedule is delivered. Of the schedules examined the results would suggest that a dose of 60 Gy given in 25 fractions over a period of 35 days is the best of the six schedules studied for T1, T2, T3 and T4 lesions with minimal associated morbidity. It is possible, however, that the poor results shown on the Kaplan-Meier curves for patients treated with the schedule of 60 Gy in 30 fractions over a period of 42 days could be due to geographical misses of the tumours as 56% were treated without a beam directed shell. The poor result obtained when patients were treated with the schedule of 60 Gy given in 30 fractions over 49+ days may be due to tumour repopulation occurring during the rest period though the possibility of geographical misses may contribute to the poor tumour control results. Mathematical modelling using linear quadratic analysis suggests that the shorter the period of time over which the treatment is given the better chance of achieving tumour control irrespective of the stage of the disease. These models were developed for patients treated with a beam directed shell thus excluding those patients who are most likely to be at risk from a geographic miss of the tumour. Linear quadratic analysis of the treatment data suggests that the ratio alpha/beta for tumour cells is estimated in the region of 13 Gy. For T1 lesions the tumour doubling time is in the order of 6 days, with longer doubling times for the more advanced stages. The analysis provides some support for investigative use of accelerated treatment schedules. This analysis also shows the importance of using beam directed shells when treating small fields especially in the head and neck region.

N-myc gene copy number in neuroblastoma cell lines and resistance to experimental treatment.

The N-myc oncogene is amplified in approximately 30% of neuroblastomas. It is well established that cases of neuroblastoma with amplified N-myc have markedly poorer prognosis than those in which N-myc copy number is not elevated. The mechanism for this association is not known but may be related to cellular resistance to radiation or cytotoxic drugs. Seven human neuroblastoma cell lines were used to investigate the relationship between N-myc copy number or expression and sensitivity to ionising radiation and to cisplatin. N-myc copy number was assessed by Southern blotting and hybridisation using the p-Nb1 probe. The signal produced by DNA from the cell lines was compared with that of single copy N-myc from normal human placental DNA. A range of N-myc copy numbers from 1 to 800 was found. Expression levels of N-myc mRNA were compared by "dot blotting" and subsequent hybridisation to the p-Nb1 probe. Radiosensitivity was assessed by surviving fraction at 2 Gy (SF2) following 60Co gamma irradiation. Values ranged from 0.13 to 0.52. Sensitivity to cisplatin was indicated by comparison of isoeffective concentrations (concentration required to produce 1 log cell kill). These ranged from 7.5 to 13 microM. Cisplatin studies showed a correlation between N-myc copy number (though not expression) and resistance to this drug. If this relationship is causal it may explain why treatment fails in those patients with an elevated N-myc copy number. However, no correlation was found between N-myc copy number or expression and sensitivity to radiation. It is possible that N-myc amplification confers resistance to some but not all treatments used in the therapy of neuroblastoma. Further investigations along these lines may lead to the identification of agents which are most appropriate for the treatment of neuroblastoma with amplified N-myc gene.

Multi-modality megatherapy with [131I]meta-iodobenzylguanidine, high dose melphalan and total body irradiation with bone marrow rescue: feasibility study of a new strategy for advanced neuroblastoma.

New therapeutic approaches are needed for advanced neuroblastoma as few patients are currently curable. We describe an innovative strategy combining [131I]meta-iodobenzylguanidine ([131I]mIBG) therapy with high dose chemotherapy and total body irradiation. The aim of combining these treatments is to overcome the specific limitations of each when used alone to maximise killing of neuroblastoma cells. Five children received combined therapy with [131I]mIBG followed by high dose melphalan and fractionated total body irradiation. Autologous bone marrow transplantation was undertaken in 3 patients and allogeneic in 2 patients. One patient received additional localised radiotherapy to residual bulk disease. One patient is alive without relapse 32 months after treatment. 4 patients relapsed after remissions of 9, 10, 14 and 21 months. These results indicate that this combined modality approach is feasible and safe, but further evaluation is necessary to establish whether it has advantages over conventional megatherapy using melphalan alone.

Radiobiological modeling of combined targeted 131I therapy and total body irradiation for treatment of disseminated tumors of differing radiosensitivity.

PURPOSE: A model is presented for calculating combinations of targeted 131I and total body irradiation, followed by bone marrow rescue, in the treatment of tumors of different radiosensitivity. The model is used to evaluate the role of the total body irradiation component in the optimal combination regime as a function of the radiosensitivity of the tumor cells. METHODS AND MATERIALS: A microdosimetric model was used to calculate absorbed dose in small tumors and micrometastases when uniformly targeted by the radionuclide 131I. Cell kill was calculated from absorbed dose using an extended version of the linear quadratic model. The addition of varying total doses of total body irradiation, assuming 2 Gy fractions, was also calculated using the linear quadratic model. The net cell kill from combined modality (targeted 131I and total body irradiation) was computed for varying proportions of the two components, for a range of tumor sizes, restricting the total radiation dose to within tolerance for a full-course TBI regime (approximately 14 Gy total) in all cases. The calculations were repeated for a range of presumed tumor uptakes of the targeting agent and for a range of tumor radiosensitivities, typical of those reported for tumor cells of differing type in culture. Optimal regimes were identified as those predicted to yield a high probable tumor cure rate (evaluated using a Poisson statistical model) for all tumor sizes. RESULTS: The analysis supports earlier model studies which predicted that systemic combination treatment with targeted 131I and total body irradiation would be superior to either component used alone. The intrinsic tumor radiosensitivity is found to be a factor which influences the optimal combination of the 131I and external beam total body irradiation components. The total body irradiation component is greater in optimal regimes treating radio-resistant than radiosensitive tumors. However, an obligatory total body irradiation component is also predicted for more radiosensitive tumors; the analysis suggests that the total body irradiation component should in no circumstances be less than 2 x 2 Gy, whilst practical arguments exist in favor of higher doses. CONCLUSION: Total body irradiation is an obligatory component for effective systemic treatment of disseminated malignant tumors to which 131I can be selectively targeted. Clinical studies applying this strategy to the treatment of neuroblastoma by 131I targeted by meta-iodo-benguanidine (mIBG), total body irradiation and bone marrow rescue are now in progress.

Optimum combination of targeted 131I and total body irradiation for treatment of disseminated cancer.

PURPOSE: Radiobiological modeling was used to explore optimum combination strategies for treatment of disseminated malignancies of differing radiosensitivity and differing patterns of metastatic spread. The purpose of the study was to derive robust conclusions about the design of combination strategies that incorporate a targeting component. Preliminary clinical experience of a neuroblastoma treatment strategy, which is based upon general principles obtained from modelling, is briefly described. METHODS AND MATERIALS: The radiobiological analysis was based on an extended (dose-rate dependent) formulation of the linear quadratic model. Radiation dose and dose rate for targeted irradiation of tumors of differing size was in part based on microdosimetric considerations. The analysis was applied to several tumor types with postulated differences in the pattern of metastatic spread, represented by the steepness of the slope of the relationship between numbers of tumors present and tumor diameter. The clinical pilot study entailed the treatment of five children with advanced neuroblastoma using a combination of 131I metaiodobenzylguanidine (mIBG) and total body irradiation followed by bone marrow rescue. RESULTS: The theoretical analysis shows that both intrinsic radiosensitivity and pattern of metastatic spread can influence the composition of the ideal optimum combination strategy. High intrinsic radiosensitivity generally favors a high proportion of targeting component in the combination treatment, while a strong tendency to micrometastatic spread favors a major contribution by total body irradiation. The neuroblastoma patients were treated using a combination regimen with an initially low targeting component (2 Gy whole body dose from targeting component plus 12 Gy from total body irradiation). The treatment was tolerable and resulted in remissions in excess of 9 months in each of these advanced neuroblastoma patients. CONCLUSIONS: Radiobiological analysis, which incorporates simple models of metastatic spread, emphasizes the importance of the total body irradiation component in a targeting/total body irradiation combination strategy. However, the analysis favors a larger targeting component than is used in clinical practice at present. A cautious escalation of the 131I mIBG component in the combination treatment of advanced neuroblastoma appears justified.

Relationships between tumor size and curability for uniformly targeted therapy with beta-emitting radionuclides.

UNLABELLED: Targeted radionuclide therapy is a new form of radiotherapy that differs in some important respects from external beam irradiation. One of the most important differences is due to the finite range of ionizing beta particles emitted as a result of radionuclide disintegration. The effects of particle range have important implications for the curability of tumors. METHODS: We used a mathematical model to examine tumor curability and its relationship to tumor size for 22 beta-emitting radionuclides that may have therapeutic potential. The model assumed a uniform distribution of radionuclide throughout. RESULTS: For targeted radionuclide therapy, the relationship between tumor curability and tumor size is different from that for conventional external beam radiotherapy. With targeted radionuclides, there is an optimal tumor size for cure. Tumors smaller than the optimal size are less vulnerable to irradiation from radionuclides because a substantial proportion of the disintegration energy escapes and is deposited outside the tumor volume. CONCLUSION: We found an optimal tumor size for radiocurability by each of the 22 radionuclides considered. Optimal cure diameters range from less than 1 mm for short-range emitters such as 199Au and 33P to several centimeters for long-range emitters such as 90Y and 188Re. The energy emitted per disintegration may be used to predict optimal cure size for uniform distributions of radionuclide.

Radiobiological modelling of the treatment of leukaemia by total body irradiation.

PURPOSE: Total body irradiation (TBI) has been used as part of the conditioning regimen before bone marrow transplantation or stem cell re-infusion for more than 30 years. A wide variety of regimens have been used, and no single one has emerged as the best. Experimental evidence suggests a diversity of radiosensitivities of leukaemia cells in culture, which may correlate with a significant variation of leukaemic cell radiosensitivities between patients. The purpose of this project was to compute leukaemic cell killing by different schedules and determine whether a "best treatment" could be devised for individual patients. METHODS: We have developed a mathematical model for leukaemic cell killing by alternative TBI schedules, applied to a patient population with diverse leukaemic radiosensitivities. We considered 13 schedules in clinical use, and 14 theoretical schedules calculated (by the linear-quadratic model) to be iso-effective for risk of radiation pneumonitis. When each schedule of treatment is applied to the patient population, a distribution of leukaemic cell kills (log cell kill values) can be obtained for that schedule. The leukaemic kill distribution was also computed for optimized individual scheduling, each individual being treated by the schedule that was most effective for that patient. Using available data on the clinically observed dose response relationship for acute myeloid leukaemia, the model was extended to provide leukaemia cure probabilities for each of the schedules and for the individualized strategy. RESULTS: The computer simulations show that each schedule, applied to the treatment of a radiobiologically diverse patient population, results in a broad distribution of leukaemic log kill values, with a mean of 3-5 for most schedules (i.e. 10(-3)-10(-5) surviving fraction of leukaemic cells), and a broad variation (1-10 log kill) amongst patients. The distributions generated by the various schedules were found to be overlapping, implying that many of the schedules would be difficult to distinguish reliably in clinical trials. Individualized optimum treatment is possible if radiobiological parameters are known for each patient and would improve the leukaemic log kill distribution by about 1 log on average, corresponding to an increase of leukaemia cure probability of several percent overall. For some individual patients, however, optimal scheduling could make a large difference to treatment outcome. CONCLUSIONS: The use of many different clinical treatment schedules may be continuing because outcomes are similar when these diverse schedules are applied to unselected patient populations. The measurement of individual leukaemic cell radiosensitivity would allow individualized scheduling, which could result in modest increases in overall curability, but substantial improvements in survival or duration of remission for individual patients.

Modelling the enhancement of fractionated radiotherapy by gene transfer to sensitize tumour cells to radiation.

BACKGROUND AND PURPOSE: Several strategies now exist for the use of gene transfer methodologies to sensitize tumour cells to radiation. These include the transfection of genes synthesizing cytokines, p53 gene replacement and methods based on the use of HSV-tk and gancyclovir. Very recently, the sequencing of radioprotector or repair genes, such as ATM, Ku80 and XRCC2, has made it possible to consider the design of gene transfer strategies resulting in protector gene knock-out. Selectivity of transfected gene expression might be achieved by use of tissue-specific promoters or by the trophism of viral vectors. The purpose of this study was to evaluate the probable efficacy of such strategies. METHODS: We have modelled gene transfer-mediated radiosensitization of tumour cells during radiotherapy, focusing on anti-protector gene strategies, to explore the role of transfection frequency, sensitizing efficacy, transfection stability, untransfectable subpopulations, the timing of gene therapy and the treatment schedule structure. RESULTS: We predict a substantial therapeutic benefit of gene transfer treatment (with at least weekly transfection) which modifies cellular radiosensitivity by a factor of 1.5 or more, despite modest efficiency of cellular transfection (e.g. 50%), transient retention of the transfected gene (e.g. 2-day half-life) and the existence of a small minority (e.g. 1%) of untransfectable cells. CONCLUSIONS: The analysis shows repeated administration of gene transfer treatment to be obligatory and implies that the existence of untransfectable minority subpopulations (i.e. cells inaccessible to the vector) will be the major limiting factor in therapy. Experimental work is needed to confirm these predictions before clinical studies begin.

The linear-quadratic transformation of dose-volume histograms in fractionated radiotherapy.

BACKGROUND AND PURPOSE: Dose-volume histograms (DVHs) are often used in radiotherapy to provide representations of treatment dose distributions. DVHs are computed from physical dose and do not include radiobiological factors; therefore, the same DVH will be computed for a treatment plan whatever fractionation regimen is used. However, dose heterogeneity resulting from variation of daily treatment dose within the volume will have biological effects due to spatial heterogeneity of fraction size as well as total dose. The purpose of the paper is to present a radiobiological (LQ) transformation of the physical dose distribution which incorporates fraction size effects and may be better suited to the prediction of biological effects. METHODS: An analytic formula is derived for the linear-quadratic transformation of a normal distribution of dose to give the corresponding distribution of biologically equivalent dose given as 2 Gy fractions. This allows LQ-transformed DVHs to be computed from physical DVHs. The resultant LQ-DVH depends on the assumed value of the relevant alpha/beta ratio. It is a modified dose distribution (corrected for spatial heterogeneity of fraction size) but does not incorporate time factors or volume effects. RESULTS: The analysis shows that the LQ-transformed distribution is always broader than the distribution of physical dose. Radiobiological 'hot spots' and 'cold spots' are further from the mean than physical distributions would indicate. The difference between conventional DVHs and LQ-transformed DVHs is dependent on the fractionation regimen used. LQ-DVHs for a single dose distribution (treatment plan) can be computed for different fractionation regimens with some simplifying assumptions (e.g. no time-factor-dependence of late effects). Regimens calculated to be radiobiologically equivalent at a single point nevertheless result in non-equivalent LQ-DVHs when spatial variation of daily treatment dose is included. The difference is especially important for tumour sites (such as breast and head and neck) for which considerable dose heterogeneity may occur and for which different treatment regimens are in use. CONCLUSIONS: LQ-DVHs should be computed in parallel with conventional DVHs and used in the evaluation of treatment plans and fractionation regimens and in the analysis of high-dose side-effects in patients.

The curability of tumours of differing size by targeted radiotherapy using 131I or 90Y.

A mathematical model has been used to investigate the relationship of curability to tumour size and cell number for spherical tumours treated with targeted 131I or 90Y, assuming uniform uptake of radionuclide throughout the tumour. The analysis shows that, for any given cumulated activity per unit mass of tumour, cure probability is greatest for tumours whose diameter is close to an optimum value which depends on the path length of the emitted beta-particle. Smaller tumours are less curable because of inefficient absorption of radiation energy, and larger tumours are less curable because of greater clonogenic cell number. The lesser curability of very small tumours is a feature of targeted radiotherapy using long-range beta-emitters which does not occur with external beam irradiation. The predicted inefficiency of sterilisation of microscopic tumours poses a problem for targeted radiotherapy which is analogous to "geographic miss" in conventional radiotherapy. The implication is that small micro-metastases could escape sterilisation by radionuclides administered at activity levels sufficient to eradicate larger tumours. It is suggested that single agent targeted radiotherapy should not be used for treatment of disseminated malignancy when multiple tumours of differing size, including micrometastases, may be present. The analysis implies that an advantage might result from the use of a panel of several radionuclides (including short-range emitters) or from combining targeted radiotherapy using long-range beta-emitters with external beam irradiation or some other modality to which microscopic tumours are preferentially vulnerable.

Strategies for systemic radiotherapy of micrometastases using antibody-targeted 131I.

A simple analysis is developed to evaluate the likely effectiveness of treatment of micrometastases by antibody-targeted 131I. Account is taken of the low levels of tumour uptake of antibody-conjugated 131I presently achievable and of the "energy wastage" in targeting microscopic tumours with a radionuclide whose disintegration energy is widely dissipated. The analysis shows that only modest doses can be delivered to micrometastases when total body dose is restricted to levels which allow recovery of bone marrow. Much higher doses could be delivered to micrometastases when bone marrow rescue is used. A rationale is presented for targeted systemic radiotherapy used in combination with external beam total body irradiation (TBI) and bone marrow rescue. This has some practical advantages. The effect of the targeted component is to impose a biological non-uniformity on the total body dose distribution with regions of high tumour cell density receiving higher doses. Where targeting results in high doses to particular normal organs (e.g. liver, kidney) the total dose to these organs could be kept within tolerable limits by appropriate shielding of the external beam radiation component of the treatment. Greater levels of tumour cell kill should be achievable by the combination regime without any increase in normal tissue damage over that inflicted by conventional TBI. The predicted superiority of the combination regime is especially marked for tumours just below the threshold for detectability (e.g. approximately 1 mm-1 cm diameter). This approach has the advantage that targeted radiotherapy provides only a proportion of the total body dose, most of which is given by a familiar technique. The proportion of dose given by the targeted component could be increased as experience is gained. The predicted superiority of the combination strategy should be experimentally testable using laboratory animals. Clinical applications should be cautiously approached, with due regard to the limitations of the theoretical analysis.

Radiobiological considerations in the treatment of neuroblastoma by total body irradiation.

Neuroblastoma is a radiosensitive neoplasm for which total body irradiation (TBI) is presently under clinical consideration. Collated data on the radiobiology of human neuroblastoma cells in vitro (11 cell lines derived from seven patients) indicates moderate cellular radiosensitivity and low capacity for accumulation of sublethal damage (median survival curve parameters: Do = 104 cGy, Dq = 32 cGy, n = 1.36). Mathematical studies incorporating these parameters suggest that low dose fractionated TBI is unlikely to achieve significant levels of tumour cell kill. When high dose TBI is used in conjunction with bone marrow rescue a tumour "log cell kill" of 4-5 should be achievable. This effect would be additional to that achieved by chemotherapy. The optimum schedule for exploitation of radiobiological differences between neuroblastoma cells and the dose-limiting normal tissues has a hyperfractionated structure. Twice-daily treatments with fraction sizes in the region 120-150 cGy seems appropriate. Single dose treatments at high dose rate are contraindicated. Fractionated TBI with bone marrow rescue may be curative for some patients in clinical remission who are presently destined to relapse.

The relative effectiveness of analogues of cisplatin in the experimental chemotherapy of human non-small-cell lung cancer and neuroblastoma grown as multicellular spheroids.

We compared cisplatin (cis-DDP) and two of its analogues, carboplatin (JM8, CBDCA) and iproplatin (JM9, CHIP) for their ability to retard the growth of multicellular tumour spheroids. The spheroids were derived from two human tumours, a neuroblastoma and a non-small-cell lung cancer. To produce a given level of regrowth delay in lung cancer spheroids, carboplatin and iproplatin were required at concentrations approximately 10 times that of cis-DDP. In the neuroblastoma spheroid experiments, iproplatin and cis-DDP produced the same level of regrowth delay when iproplatin was present at a concentration greater than 10 times that of cis-DDP. Carboplatin also required much higher concentrations than cis-DDP to produce equivalent regrowth delay in neuroblastoma. The dose-response curve produced by carboplatin on neuroblastoma spheroids displayed a pronounced shoulder in the low-dose region; this phenomenon was not seen with cis-DDP. These findings may have implications for the clinical use of these drugs and in particular would support a role for carboplatin in the treatment of lung cancer, since total free-drug exposure of patients to carboplatin may be up to 16-fold greater than with cis-DDP. However, one must be cautious about generalizing on the basis of results from only two cell lines as well as applying in vitro data to clinical situations.

Targeting radiation to tumours.

Biologically targeted radiotherapy entails the preferential delivery of radiation to solid tumours or individual tumour cells by means of tumour-seeking delivery vehicles to which radionuclides can be conjugated. Variant forms of this are the binary strategies (neutron capture therapy, photodynamic therapy) in which cell killing by the targeting moiety is dependent on activation by an external radiation beam. Monoclonal antibodies have attracted attention for some years as potentially selective targeting agents, but advances in tumour and molecular biology are now providing a much wider choice of molecular species. General radiobiological principles may be derived which are applicable to most forms of targeted radiotherapy. These principles provide guidelines for the appropriate choice of radionuclide in specific treatment situations and its optimal combination with other treatment modalities. In the future, the availability of gene targeting agents will focus attention on the use of Auger electron emitters whose high potency and short range selectivity makes them attractive choices for specific killing of cancer cells whose genetic peculiarities are known.

The radiobiology of targeted radiotherapy.

Targeted radiotherapy consists of biologically selective irradiation of malignant cells by means of radionuclides attached to tumour-seeking molecules. A variety of clinical strategies for targeted radiotherapy may be used, for which different normal tissues will be critical. A large number of radionuclides exist, emitting nuclear particles with a range of path lengths from nanometres to millimetres. An important feature of normal-tissue radiobiology is the dose-rate effect, which is especially marked for late-responding tissues. Radiobiological calculations imply that tolerance dose for targeted radiotherapy using low-LET emitters will depend strongly on the effective half-life of the radionuclide, which will be affected by pharmacokinetics and may vary between patients. Some strategies designed to improve the therapeutic radio (e.g. accelerated clearance of radionuclide) may have modulating effects on the tolerance dose. Tumour response will be governed by the 'four Rs' (repair, repopulation, reoxygenation, redistribution) as well as by mechanisms peculiar to targeted radiotherapy. Analysis based on the extended linear quadratic model predicts that dose-rate effects will be of major importance for only a minority of tumours. Most of the radiation dose to tumour will usually be delivered over a time-scale of a few days. This might give insufficient time for tumour reoxygenation, making the use of hypoxic sensitizers appropriate. A special feature of targeted radiotherapy is the complex relationship between tumour curability and tumour size for different radionuclides. For long-range beta-emitters, microscopic tumours may be operationally resistant because of inefficient absorption of radionuclide disintegration energy in small volumes. Short-range emitters will be more efficient in sterilization of micrometastases but sterilization of larger tumours may require an unattainable degree of homogeneity of radionuclide distribution. Optimal use of targeted radiotherapy may require it to be combined with external-beam irradiation or chemotherapy. Experimental studies will be necessary to investigate those features of targeted radiotherapy which differ from external-beam irradiation. Future directions may include targeted radiotherapy of minimal numbers of tumour cells detected by use of molecular probes. Such applications call for use of short-range alpha-emitters and Auger emitters whose radiobiology will become increasingly important.

The dose-response relationship for cancer incidence in a two-stage radiation carcinogenesis model incorporating cellular repopulation.

PURPOSE: To investigate the role of cellular repopulation in the dose-response relationship for radiation carcinogenesis resulting from high doses of radiation. METHOD: A two-stage mathematical model of radiation carcinogenesis was developed and used to explore the effects of differing assumptions about repopulation by surviving normal stem cells and by one-stage mutants. RESULTS: Characteristically, cancer incidence at any fixed time after irradiation increases with radiation dose, reaches a peak and then declines with dose (the decline reflecting radiation cell-killing). The optimal dose for cancer incidence, and the incidence level at this dose, are strongly influenced by repopulation kinetics. If repopulation does not occur, or is impaired owing to radiation damage to tissues, the highest value of cancer incidence is reduced, and this value occurs at a lower dose than if repopulation had been complete. A similar result is found if repopulation by one-stage mutants is impaired relative to unmutated cells, or if tissue recovery is assisted by immigration of unirradiated cells. CONCLUSIONS: Differing repopulation kinetics can account for differing dose-response relationships after large doses of radiation. These findings are relevant to the occurrence of 'second tumours' following radiotherapy and to the interaction of radiation with other agents.

Absence of delayed chromosomal instability in a normal human fibroblast cell line after 125I iododeoxyuridine.

PURPOSE: To seek the delayed appearance of chromosomal abnormalities in human fibroblasts exposed to the Auger electron emitter 125I. MATERIALS AND METHODS: Normal untransformed human fibroblasts, HF19, were exposed to a concentration of [I125]IUdR, which allowed the survival of 37% of clonogens. Chromosomal analysis using both conventional Giemsa and fluorescence in situ hybridization (FISH) was undertaken on non-clonal bulk cultures from 2 to 39 days after treatment. RESULTS: The data show a declining level of unstable aberrations in the progeny of HF19 fibroblasts exposed to [I125]IUdR, eventually reaching control levels. CONCLUSIONS: The results provide evidence that [125I]IUdR does not induce ongoing chromosomal instability in long-term culture, and gives further support to the use of Auger-electron emitting radionuclides in the treatment and diagnosis of tumours.