A low oxygen-enhancement ratio for tumour-cell survival as compared with that for tumour-growth delay

Tumor hypoxia: its impact on cancer therapy

The presence of radiation resistant cells in solid human tumors is believed to be a major reason why radiotherapy fails to eradicate some such neoplasms. The presence of unperfused regions containing hypoxic cells may also contribute to resistance to some chemotherapeutic agents. This paper reviews the evidence that radiation resistant hypoxic cells exist in solid tumors, the assumptions and results of the methods used to detect hypoxic cells, and the causes and nature of tumor hypoxia. Evidence that radiation resistant hypoxic cells exist in the vast majority of transplanted rodent tumors and xenografted human tumors is direct and convincing, but problems with the current methodology make quantitative statements about the magnitude of the hypoxic fractions problematic. Evidence that radiation resistant hypoxic cells exist in human tumors is considerably more indirect than the evidence for their existence in transplanted tumors, but it is convincing. However, evidence that hypoxic cells are a significant cause of local failure after optimal clinical radiotherapy or chemotherapy regimens is limited and less definitive. The nature and causes of tumor hypoxia are not definitively known. In particular, it is not certain whether hypoxia is a chronic or a transient state, whether hypoxic cells are proliferating or quiescent, or whether hypoxic cells have the same repair capacity as aerobic cells. A number of new methods for assessing hypoxia are reviewed. While there are still problems with all of the new techniques, some of them have the potential of allowing the assessment of hypoxia in individual human tumors.

Hypoxic fractions of solid tumors: experimental techniques, methods of analysis, and a survey of existing data

Hypoxic fractions are measured by indirect techniques, which compare the response of tumors to large single doses of radiation given under normal aeration and artificial hypoxia. This paper reviews hypoxic fraction measurements and measurement techniques, giving particular attention to the biological, technical, and statistical aspects of the assays; the implicit assumptions underlying the analyses; and the dependence of the determinations on the assay conditions and the tumor and host characteristics. The three major hypoxic fraction assay techniques (paired survival curve, clamped tumor control, and clamped growth delay) share common biological assumptions. They require that the survival curves of naturally and artificially hypoxic cells have the same slope and intercept. They assume that the majority of the cells are either fully oxic or fully hypoxic. They assume that the methods used to induce artificial hypoxia leave no oxygenated regions and that tumor cells rendered artificially hypoxic are no less viable than cells in normally-aerated tumors. The universal validity of these assumptions is questionable. Each technique uses additional special assumptions and each may measure a different population of hypoxic cells. This paper reviews 92 hypoxic fraction determinations in 42 tumor systems. Radiobiologically hypoxic cells appear to be present in the majority of macroscopic solid rodent tumors. The hypoxic fraction was found to increase as the tumor size increased from microscopic to macroscopic; the dependence of hypoxic fraction on tumor size at macroscopic sizes was less clear. The site of tumor implantation, the use of anesthesia, and certain host characteristics may influence the hypoxic fraction. The hypoxic fraction generally did not depend on the tumor growth rate, transplantation history, or histology. These findings indicate that hypoxic cells are a common feature of solid tumors in rodents and provide no evidence that hypoxic cells should not be present in human tumors.

Recovery from sublethal damage by acutely hypoxic tumour cells in vivo and in vitro

The ability of acutely hypoxic tumour cells to recover from sublethal damage following irradiation in vivo and in vitro has been measured using a single tumour system. The methods of assay were tumour growth delay, local tumour control and tumour cell survival in vitro following treatment in vivo or in vitro. Tumours in vivo or cells in vitro rendered acutely hypoxic during irradiation were irradiated with either single doses or two doses 24 hours apart. Cells left in situ had a greater capacity for recovery than those treated either in vivo or in vitro and then assayed in vitro. It is suggested that tumours may not show a systematically reduced capacity for recovery relative to normal tissues, unless chronically hypoxic tumour cells have a reduced capacity for recovery and determine the response. However, the results imply that deductions as to the ability of tumour cells to recover from sublethal damage (whether chronically hypoxic or not), which rely on in vitro assays, may underestimate the extent of recovery.

Cure, regression and cell survival: a comparison of common radiobiological endpoints using an in vitro tumour model

Multicell spheroids of Chinese hamster V79-171 cells grown in suspension culture display many of the characteristics of solid tumours in vivo, and can be used as an in vitro tumour model. Two populations of spheroids differing in age and radiosensitivity were exposed to single doses of gamma-radiation and their response assayed by several techniques: (1) spheroids were reduced to single cells by trypsinization at various times post-irradiation, and viability of the single cells determined by colony formation; (2) entire spheroids were placed in petri dishes and observed for cellular outgrowth; (3)spheroid volume and cell content were monitored as a function of time after irradiation. It was found that spheroid volume changes could not be correlated with either the amount of radiation given or with the relative radiosensitivity. In contrast, the number of cells per spheroid, or cellularity, decreased exponentially with exposure dose at sufficiently long times after irradiation. Radiosensitivity was then quantified by calculating the per cent decrease of cellularity per rad. "Cure" of spheroids as defined by lack of outgrowth when entire spheroids were placed in petri dishes, correlated well with single cell survival, and was achieved 50 per cent of the time after 2,630 rads for the smaller spheroids and 3,750 rads for the larger ones. Since these spheroids contained an average of similar 7,600 and similar 30,700 cells respectively, comparison of cure data with single cell survival data showed that cures were achieved only when every cell was killed. This result may have significant therapeutic implications, since cells of the most radioresistant population of the spheroid, the chronically hypoxic internal cells, were capable of proliferation even when the spheroid was not reduced to single cells after irradiation.