Dose-response relationships for human tumors: implications for clinical trials of dose modifying agents

Completeness of reporting of radiation therapy planning, dose, and delivery in veterinary radiation oncology manuscripts from 2005 to 2010

Surrounding a shift toward evidence-based medicine and widespread adoption of reporting guidelines such as the Consolidated Standards of Reporting Trials (CONSORT) statement, there has been a growing body of literature evaluating the quality of reporting in human and veterinary medicine. These reviews have consistently demonstrated the presence of substantive deficiencies in completeness of reporting. The purpose of this study was to assess the current status of reporting in veterinary radiation oncology manuscripts in regards to treatment planning methods, dose, and delivery and to introduce a set of reporting guidelines to serve as a standard for future reporting. Forty-six veterinary radiation oncology manuscripts published between 2005 and 2010 were evaluated for reporting of 50 items pertaining to patient data, treatment planning, radiation dose, delivery of therapy, quality assurance, and adjunctive therapy. A mean of 40% of checklist items were reported in a given manuscript (range = 8-75%). Only 9/50 (18%) checklist items were reported in > or = 80% manuscripts. The completeness of reporting was best in regards to a statement of prescription radiation protocol (91-98% reported) and worst in regards to specification of absorbed dose within target volumes and surrounding normal tissues (0-6% reported). No manuscripts met the current International Commission of Radiation Units and Measurements (ICRU) dose specification recommendations. Incomplete reporting may stem from the predominance of retrospective manuscripts and the variability of protocols and equipment in veterinary radiation oncology. Adoption of reporting guidelines as outlined in this study is recommended to improve the quality of reporting in veterinary radiation oncology.

Improving the morbidity of anorectal injury from pelvic radiotherapy

OBJECTIVE

Radiation anorectal injury due to pelvic radiotherapy for non intestinal cancer is a significant cause of morbidity which may limit the treatment dose required. Conservative treatment options are of limited value and surgery is reserved only for the most severe complications. This review addresses radioprotection of the anorectum and aims to increase awareness amongst surgeons of the strategies that have been in practice in order to minimize the side-effects of radiotherapy while preserving its therapeutic efficacy.

METHODS

This review is based on a literature search (Medline and NLM PubMed) with manual cross-referencing of all articles related to anorectal radiation injury.

RESULTS

Optimization of radiation dose, the use of radioprotective agents and improvement in radiation delivery are the main areas of development. There are few data on the potential of altered fractionation schedules in reducing anorectal injury. A few phase I and II studies suggest that the pharmacological agents amifostine and misoprostol could be beneficial in limiting radiation damage but larger phase III studies are awaited.

CONCLUSION

The introduction of 3-dimensional conformal radiation therapy and intensity modulated radiation therapy has been the most significant advance in reducing radiation morbidity.

Portal imaging for evaluation of daily on-line setup errors and off-line organ motion during conformal irradiation of carcinoma of the prostate

PURPOSE

To use portal imaging to measure daily on-line setup error and off-line prostatic motion in patients treated with conformal radiotherapy to determine an optimum planning target volume (PTV) margin incorporating both setup error and organ motion.

RESULTS

A total of 2549 portal images from 33 patients were acquired over the course of the study. Of these patients, 23 were analyzed for setup errors while the remaining 10 were analyzed for prostatic motion. Setup errors were characterized by standard deviations of 1.8 mm in the anterior-posterior (AP) direction and 1.4 mm in the superior-inferior (SI) direction. Displacements due to prostatic motion, with standard deviations of 5.8 mm AP and 3.3 mm SI, were found to be more significant than setup errors.

CONCLUSIONS

Taking into account both setup errors and target organ motion, optimum PTV margins to ensure 95% coverage are 10.0 mm AP and 5.9 mm SI. The portal imaging protocol established in this study allows radiation therapists to accept or adjust a treatment setup based upon daily on-line image matching results. The successful localization of radiopaque fiducial markers on a significant number of portal images acquired in the study gives hope that more accurate on-line targeting verification may soon be possible through the visualization of the prostate itself as opposed to the surrounding bony structures of the pelvis.

Comparison of radiation side-effects of conformal and conventional radiotherapy in prostate cancer: a randomised trial

BACKGROUND

Radical radiotherapy is commonly used to treat localised prostate cancer. Late chronic side-effects limit the dose that can be given, and may be linked to the volume of normal tissues irradiated. Conformal radiotherapy allows a smaller amount of rectum and bladder to be treated, by shaping the high-dose volume to the prostate. We assessed the ability of this new technology to lessen the risk of radiation-related effects in a randomised controlled trial of conformal versus conventional radiotherapy.

METHODS

We recruited men with prostate cancer for treatment with a standard dose of 64 Gy in daily 2 Gy fractions. The men were randomly assigned conformal or conventional radiotherapy treatment. The primary endpoint was the development of late radiation complications (> 3 months after treatment) measured with the Radiation Therapy and Oncology Group (RTOG) score. Indicators of disease (cancer) control were also recorded.

FINDINGS

In the 225 men treated, significantly fewer men developed radiation-induced proctitis and bleeding in the conformal group than in the conventional group (37 vs 56% > or = RTOG grade 1, p=0.004; 5 vs 15% > or = RTOG grade 2, p=0.01). There were no differences between groups in bladder function after treatment (53 vs 59% > or = grade 1, p=0.34; 20 vs 23% > or = grade 2, p=0.61). After median follow-up of 3.6 years there was no significant difference between groups in local tumour control (conformal 78% [95% CI 66-86], conventional 83% [69-90]).

INTERPRETATION

Conformal techniques significantly lowered the risk of late radiation-induced proctitis after radiotherapy for prostate cancer. Widespread introduction of these radiotherapy treatment methods is appropriate. Our results are the basis for dose-escalation studies to improve local tumour control.

[Retrospective analysis of the reproducibility of the daily set-up of the irradiation fields of cancer of the rectum using a megavoltage imaging system]

PURPOSE

To evaluate a ionization chamber on-line portal imaging system in routine clinical radiotherapy of rectum cancers.

PATIENTS AND METHODS

Megavolt portal images were obtained using a fast electronic megavoltage radiotherapy imaging system in 13 cases of pelvic fields. A total of 208 portal images and 13 simulator films were used to determine the values of set-up deviations in the X-Z-directions of a fixed co-ordinate system, and of the rotation fields (R).

RESULTS

Mean standard deviations of the difference between simulation and treatment images were 3.2 mm and 0.9 mm for X and Z, 3.6 mm for the rotation fault. The standard deviations were, respectively, 7.1 mm, 7.1 mm and 1.5 mm. The cumulative frequency distributions revealed that 80% and 95% of the absolute differences were less than 10 mm and 17.5 mm, respectively.

CONCLUSION

These results indicate the difficulty of a reproducible daily set-up. A weekly control could be proposed in order to increase the quality of pelvic site treatment. The introduction of masks to improve the set-up is also discussed.

Quantifying the position and steepness of radiation dose-response curves

Radiation dose-response curves are of fundamental importance both in practical radiotherapy and as the basis of more theoretical considerations concerning the potential benefit to be gained from modified dose-fractionation schedules or of the effects of dosimetric and biological variability. The steepness of the dose-response curve is a key parameter and quantitative measures of steepness derived from clinical data are strongly needed. Unfortunately, there are many ambiguities associated with quantifying the steepness of radiation dose-response curves and these are identified and discussed in the present paper. The following problems are reviewed. (1) In the literature, various descriptors of 'steepness' are reported. We focus on the normalized dose-response gradient, gamma, and the dose-response slope, theta. The mathematical properties and the relationship between these are discussed. (2) Steepness estimates depend on the mathematical model used to describe the dose response relationship. Three standard formulations are considered: the Poisson, the logistic and the probit dose-response model. The magnitude of the model dependence is influenced by the range of the empirical dose-response data available, and is most pronounced for data concentrated around very low or very high response levels. (3) Reparametrizations of the standard models in terms of position and steepness are given, and it is pointed out that some previously published formulas are only approximations. (4) The method of analysis can influence the steepness estimate. An analysis of a specific data set shows that the use of the least-squares method rather than the preferred maximum likelihood method may influence both the steepness estimate and its confidence interval. (5) Dose-response data generated with a fixed number of fractions rather than a fixed dose per fraction will produce steeper dose-response curves. The approximation involved in describing such a set of dose-response data by a position and a single steepness parameter is discussed. (6) The importance of specifying the statistical uncertainty of the steepness estimate is stressed. All of these problems are illustrated by a practical example, in which dose-response data from the literature are re-analysed.

Radiation dose-response of human tumors

PURPOSE

The dose of radiation that locally controls human tumors treated electively or for gross disease is rarely well defined. These doses can be useful in understanding the dose requirements of novel therapies featuring inhomogeneous dosimetry and in an adjuvant setting. The goal of this study was to compute the dose of radiation that locally controls 50% (TCD50) of tumors in human subjects.

METHODS AND MATERIALS

Logit regression was used with data collected from single institutions or from combinations of local control data accumulated from several institutions treating the same disease.

RESULTS

90 dose response curves were calculated; 62 of macroscopic tumor therapy, 28 of elective therapy with surgery for primary control. The mean and median TCD50 for gross disease were 50.0 and 51.9 Gy, respectively. The mean and median TCD50 for microscopic disease control were 39.3 and 37.9 Gy, respectively. At the TCD50, an additional dose of 1 Gy controlled an additional 2.5% (median) additional patients with macroscopic disease and 4.2% (median) additional patients with microscopic disease. For both macro- and microscopic disease, an increase of 1% of dose at the TCD50 increased control rates approximately 1% (median) or 2-3% (mean). A predominance of dose response curves had shallow slopes accounting for the discrepancy between mean and median values.

CONCLUSION

Doses to control microscopic disease are approximately 12 Gy less than that required to control macroscopic disease, and are about 79% of the dose required to control macroscopic disease. The percentage increase in cures expected for a 1% increase in dose is similar for macroscopic microscopic disease, with a median value of approximately 1%/% and a mean of approximately 2.7%/%.

Radiosensitivity and cell cycle redistribution of cultured human tumour cells during fractionated daily 2-Gy irradiations

Exponentially growing SQ-5 human lung squamous cell carcinoma cells were irradiated in vitro at 2-Gy per fraction per day continuously for 14 days. The number of total cells continued to increase exponentially till day 5, and reached a plateau level thereafter. The cell cycle distribution changed marginally for the first 5 days, and showed a prominent G2/M accumulation at day 7. Plating efficiencies decreased exponentially with increasing fractionation while the total clonogenic cell number remained constant until day 4. Radiosensitivity at each fraction was stable until day 9, but significantly increased at day 11. A comparison of plating efficiencies between the immediate and 24-h delayed assays revealed that the capacity of cells to spare 2-Gy damage increased with the number of 2-Gy fractions. These results suggest that sublethal damage could accumulate during multifraction daily irradiations, while repair of potentially lethal damage and/or proliferation could rather increase.

Quantifying the effect of dose inhomogeneity in brachytherapy: application to permanent prostatic implant with 125I seeds

PURPOSE

To quantitate the influence of dose inhomogeneity on brachytherapy efficacy.

METHODS AND MATERIALS

A computed tomography-based system of planning, implementation and evaluation was used to generate tumor-specific dose-volume histograms of eight permanent 125I implants of prostate cancers. The radiobiological effect was then assessed, voxel by voxel, in terms of the biologically effective dose and the associated cell inactivation. The overall cell survival of the entire target volume was then computed. To evaluate the influence of inhomogeneity, the dose-volume histogram was modified in an iterative fashion, with the corresponding surviving fraction calculated after each step. Specifically, the volume in the highest dose bin was combined with that in the next bin to give a new frequency distribution from which cell survival was generated. Tumor control probability (TCP) was also used as an endpoint, using the same iterative procedure.

RESULTS

Doses 20-30% higher than D99 (the dose that covered 99% of the target volume) contributed to additional cell inactivation, but still higher doses did not further increase cell kill. With homogeneous irradiation at D99 as a reference, we defined the inhomogeneity enhancement factor as the ratio of the biologic effective dose of the actual implant to that of the reference dose distribution. The calculated enhancement factors were inversely dependent on tumor potential doubling time (Tp), about 1.2-1.3 for a Tp of 30 days, and between 1.3 and 1.7 if Tp = 10 days, with higher values for implants with low D99. Dose inhomogeneity enhanced TCP. For implants with high control probabilities does significantly higher (> 20%) than the D99 value did not further enhance the tumor control probabilities. In contrast, for implants with relatively low tumor control and D99 values, the control probability continued to increase with doses significantly higher than D99, up to a dose of 2 x D99. The underlying reasons were the incorporation of patient "population averaging" in the calculation and the saturation of tumor control dose response at about 120 Gy.

CONCLUSION

Dose heterogeneity in implants increased tumor cell kill and local control probability, although doses > 20% higher than the prescription dose is wasted. The increase the beneficial effect of dose inhomogeneity may be greatest when most needed.

Conformal radiotherapy at the Royal Marsden Hospital (UK)

Conformal radiotherapy seeks to allow increased intensity of radiation by reducing the volume of normal tissues within the treatment volume. Techniques have developed secondary to improvements in three-dimensional imaging and accessible treatment technology is based on computer-controlled multileaf collimators to create an irregular radiation beam shape. Preliminary clinical work in the Royal Marsden Hospital seeks to quantify the toxicity reduction achievable by conformal techniques in the context of a prospective randomized pelvic radiotherapy trial which has now recruited 240 patients. The data accumulated during this trial will allow comparison of conformal and conventional radiotherapy and also analysis of the impact of dose and volume of a particular organ on both acute and late toxicity. Assessments have revealed that conformal techniques reduced significantly the treatment volume of normal tissues, e.g. by a mean of 54% for rectum and 42% for bladder. However, a relationship between volume and acute toxicity has not been established. Late toxicity is currently being analysed. Dose escalation trials in thoracic and in pelvic tumours are planned.

Precision in reporting the dose given in a course of radiotherapy

A knowledge of the precise dose given in a course of radiotherapy is vital to the interpretation of the result. Despite this, an acceptable level of reporting was found in only 72 (36%) of 200 papers published in the two leading journals of radiation oncology. Analysis of the treatment data of the cases with head and neck tumours in the pilot study of CHART showed that the mean of the minimum tumour doses given was 5.1% lower than the mean of those at the intersection points. Had the same total dose been prescribed to the intersection point instead of the minimum there would have been a similar lowering of dose. There is evidence from published clinical data and a suggestion from an analysis of the CHART pilot study data that a dose difference as small as 5% may lead to real impairment or enhancement of tumour response, as well as altering the risk of morbidity. Inadequate reporting may lead to a false interpretation of a study and to its wrongful application. It is strongly recommended that it should be editorial policy to publish only those papers where the radiation dose is fully described.

The use of on-line image verification to estimate the variation in radiation therapy dose delivery

PURPOSE

On-line radiotherapy imaging systems provide data that allow us to study the geometric nature of treatment variation. It is more clinically relevant to examine the resultant dosimetric variation. In this work, daily beam position as recorded by the on-line images is used to recalculate the treatment plan to show the effect geometric variation has on dose.

METHODS AND MATERIALS

Daily 6 MV or 18 MV x-ray portal images were acquired using a fiberoptic on-line imaging system for 12 patients with cancers in the head and neck, thoracic, and pelvic regions. Each daily on-line portal image was aligned with the prescription simulation image using a template of anatomical structures defined on the latter. The outline of the actual block position was then superimposed on the prescription image. Daily block positions were cumulated to give a summary image represented by the block overlap isofrequency distribution. The summary data were used to analyze the amount of genometric variation relative to the prescription boundary on a histogram distribution plot. Treatment plans were recalculated by considering each aligned portal image as an individual beam.

RESULTS

On-Line Image Verification (OLIV) data can differentiate between systematic and random errors in a course of daily radiation therapy. The data emphasize that the type and magnitude of patient set-up errors are unique for individual patients and different clinical situations. Head and neck sites had the least random variation (average 0-100% block overlap isofrequency distribution width = 7 mm) compared to thoracic (average 0-100% block overlap isofrequency distribution width = 12 mm) or pelvic sites (average 0-100% block overlap isofrequency distribution width = 14 mm). When treatment delivery is analyzed case by case, systematic as well as random errors are represented. When the data are pooled by anatomical site, individuality of variations is lost and variation appears random. Recalculated plans demonstrated dosimetric deviations from the original plans. The differences between the two dosimetric distributions were emphasized using a technique of plan subtraction. This allowed quick identification of relative "hot and cold spots" in the recalculated plans. The magnitude and clinical significance of dosimetric variation was unique for each patient.

CONCLUSIONS

OLIV data are used to study geometric uncertainties because of the unique nature for individual patients. Dose recalculation is helpful to illustrate the dosimetric consequences of set-up errors.

Radiotherapy update

Images

The role of uncertainty analysis in treatment planning

The role of uncertainty analysis in 3-D treatment planning systems was addressed by four institutions which contracted with NCI to evaluate high energy photon external beam treatment planning. Treatment plans were developed at eight disease sites and the effects of uncertainties assessed in a number of experiments. Uncertainties which are patient-site specific included variations in the delineation of target volumes and normal tissues and the effects of positional uncertainties due to physiological motion and setup nonreproducibility. These were found to have a potentially major impact on the doses to the target volumes and to critical normal tissues which could result in significantly altered probabilities of tumor control and normal tissue complications. Other uncertainties, such as the conversion of CT data to electron densities, heterogeneities and dose calculation algorithms' weaknesses, are related to physical processes. The latter was noted to have the greatest potential contribution to uncertainty in some sites. A third category of uncertainty related to the treatment machine, the consequences of compensator misregistration, are exclusive to the site and the treatment portal. Because conventional treatment planning systems have not incorporated uncertainty analysis, tools and techniques had to be devised for this work; further development in this area is needed. Many of the analyses could not have been done without full 3-D capabilities of the planning systems, and it can be anticipated that the availability of uncertainty analysis in these systems which allow nontraditional beam arrangements will be of great value.

Further analysis of the time factor in squamous cell carcinoma of the tonsillar region

Recently, Bataini et al. reported that overall time was the major treatment-related determinant of local control in 465 squamous carcinomas of the tonsillar region. They did not, however, quantify the relationship or relate it to the doubling time of tumorigenic cells, except qualitatively. This note reports an attempt at that quantification.

Effect of normobaric oxygen on tumor radiosensitivity: fractionated studies

The sensitizing ability of 100% normobaric oxygen was investigated in a mouse mammary carcinoma (CaNT) using a variety of fractionated regimens. Both regrowth delay and local control were used as assays of tumor response. With both assays, there was a similar and significant increase in radiosensitivity for all fractionated schedules. Enhancement ratios ranged from 1.24 to 1.45, the highest increase being observed with a 30 fraction schedule given in an overall time of 6 weeks. Thus, in CaNT tumors normobaric oxygen is a far more efficient radiosensitizer in fractionated treatments than the oxygen-mimetic compound misonidazole; an oxygen effect being observed at doses per fraction as low as 1.8 Gy. These results suggest strongly that normobaric gases could play an important role in the clinical management of tumors where hypoxia may limit the outcome of radiotherapy.

The proportion of stem cells in murine tumors

Considerable evidence suggests that tumors contain only a minority of cells which are capable of regrowing the tumor (ie. tumor stem cells). Since all tumor stem cells must be killed if treatment is to be successful, the number of stem cells in a tumor can be expected to be an important determinant of curability. We have attempted to examine the proportion of stem cells in a variety of murine tumors by making measurements of three different parameters which might be expected to be related to stem cell content: (a) the radiation dose required to control the tumor (TCD50); (b) the number of cells required to transplant the tumor (TD50) and (c) the in vitro plating efficiency. An inverse correlation has been demonstrated between measured TCD50 and TD50 values for two independent groups of murine tumors of varying histopathological type. An inverse correlation was also obtained between the TD50 value and in vitro plating efficiency for a group of spontaneous murine mammary tumors. These correlations most likely reflect underlying differences in the stem cell content of the tumors, and indicate that there is a wide range (2-3 orders of magnitude) of stem cell proportions in different murine tumors, even those which have been transplanted a number of times.

Sensitizers and radiation dose fractionation: results and interpretations

Misonidazole is generally regarded as having been a clinical failure as a radiation sensitizer. It is hoped that the newer sensitizers SR-2508 and Ro 03-8799 will give better results because single dose studies with animal tumors have indicated that these two drugs give higher enhancement ratios than misonidazole at clinically tolerated doses. Other factors may also have influenced the clinical efficacy of misonidazole, however, particularly reoxygenation during the course of the fractionated treatments. In this paper reoxygenation in animal tumors and experimental studies in which fractionated radiation doses have been combined with sensitizers are reviewed. It is concluded that, even for dose fractions of 2 Gy, reoxygenation may not completely eliminate the influence of hypoxic cells on tumor response, when large total doses are given. Problems associated with tumor heterogeneity are also discussed to highlight the desirability of selecting the most suitable patients for clinical studies. Poorly reoxygenating tumors, rapidly growing tumors and tumors in patients in whom oxygen delivery to tissue is compromised are those whose control is most likely to be improved by combining radiation sensitizers with conventional treatment. However effective sensitizers should also allow fractionation schedules to be modified, to achieve a therapeutic gain, by taking advantage of differences in repair or repopulation between the tumor and critical normal tissue, without having to consider possible detrimental effects on reoxygenation.

Potential for increasing the differential response between tumors and normal tissues: can proliferation rate be used?

Rapid proliferation of malignant cells has not previously been emphasized as a major source of failure to control tumors. Evidence is presented that the effective doubling times of clonogenic cells in human tumors during multifraction radiotherapy are in the range of a few days, that is, similar to the pre-treatment Potential Doubling Times and much shorter than Volume Doubling Times. Evidence from animal tumor studies leads to the same conclusion. Accelerated fractionation should be considered for individual human tumors whose LI is measured (e.g., by flow cytometry and the BUdR antibody) and found to be too high.

Transmission blocks: clinical and biological rationales

This paper describes the clinical and biological rationales for the use of transmission blocks. Clinical advantages over the use of full-thickness blocks applied part way through the course of therapy include the use of only one set of fields, blocks, and beam calculations, and less complex chart recording. There is a net saving in time required for the preparation and treatment of the patient. There is also a quality assurance advantage since the impact of a potential error in block positioning is reduced. In terms of biological advantages, it is demonstrated that the linear-quadratic iso-effect model can be applied to predict an improvement of up to 10% in the therapeutic ratio if transmission blocks are used instead of full-thickness blocks.

Sensitizers and protectors in radiotherapy

The rationale for hypoxic cell radiosensitizers, of which misonidazole (MISO) is the most widely used drug, is based on the premise that hypoxic cells limit the cure rate of tumors to conventional radiotherapy. There is evidence that this is the case for tumors of the head and neck and possibly also for carcinoma of the cervix and lung. Despite numerous trials, however, MISO has not shown a significant clinical benefit. However, it can be demonstrated that little or no effect would have been expected at the doses of MISO used. New and more efficient radiosensitizers are now available which are more likely to produce significant improvements in local control with radiotherapy. The sulfhydryl radioprotector WR-2721 protects normal more than malignant tissues in animals from damage by radiation and chemotherapy agents. Evidence for protection of bone marrow and kidney has now been obtained for cyclophosphamide and cisplatin, respectively, in Phase I clinical trials. Sensitizers and protectors demonstrate that chemical modification of the therapeutic index is possible for both radiation and chemotherapy treatment of cancer.

A review of alpha/beta ratios for experimental tumors: implications for clinical studies of altered fractionation

Clinical interest in the use of more and smaller dose fractions in radical radiotherapy has been stimulated by recent reviews of experimental results with normal tissues. It has been found that if the dose per fraction is reduced (i.e., in hyperfractionation) there is sparing of late responding normal tissues relative to those which respond early. This phenomenon can be understood in terms of the shapes of the underlying dose effect relationships, which can be described using the linear quadratic equation. The ratio (alpha/beta) of the linear (alpha) and quadratic (beta) terms is a useful measure of the curviness of such dose effect curves. Low alpha/beta values (1.5 to 5 Gy) have been observed for late responding normal tissues and indicate that radiation damage should be greatly spared by the use of dose fractions smaller than the 2 Gy used in conventional radiotherapy. By contrast the high alpha/beta values (6-14 Gy) observed for acutely responding normal tissues indicate that the response is relatively linear over the dose range of clinical interest. Hence less extra sparing effect is to be expected if lower doses per fraction are administered. If tumors respond in the same way as acutely responding normal tissues then hyperfractionation might confer a therapeutic gain relative to late responding normal tissues. We have reviewed published results for experimental tumors irradiated in situ and either assayed in situ or after excision. The alpha/beta ratios were usually at least as high as those for acutely responding normal tissues, and 36/48 tumors gave values greater than 8 Gy. Low values of less than 5 Gy were obtained for only 4/48 tumors. There are considerable technical problems in interpreting these experiments, but the results do suggest that hyperfractionation might confer therapeutic gain relative to late responding normal tissues on the basis of differences in repair capability. In clinical practice more efficient reoxygenation, cell cycle redistribution and decreased overall treatment time might also confer therapeutic gain.

Clinical trials with hypoxic cell sensitizers: time to retrench or time to push forward?

Results of world-wide clinical trials with misonidazole are discussed. An attempt is made to assess the reasons for the lack of positive results and the cost-benefit analysis is critically reviewed. The data on the clinical investigations of the second generation misonidazole analogues SR-2508 and RO-03-8799 are presented. Emphasis is placed on future work such as tumor selection for clinical trials, reduction of drug toxicity and methods to increase the drug radiosensitizing properties. Because of the large amount of knowledge, experience, productivity and good scientific clinical data accummulated with nitroimidazoles over the past five years, it is recommended that this is the time to push forward with the work on the newest, more efficient compounds.