Radiobiological intercomparison of the 160 MeV and 230 MeV proton therapy beams at the Harvard Cyclotron Laboratory and at Massachusetts General Hospital.

The purpose of this study was to determine the relative biological effectiveness (RBE) along the axis of two range-modulated proton beams (160 and 230 MeV). Both the depth and the dose dependence of RBE were investigated. Chinese hamster V79-WNRE cells, suspended in medium containing gelatin and cooled to 2 °C, were used to obtain complete survival curves at multiple positions throughout the entrance and 10 cm spread-out Bragg peak (SOBP).

The biological effectiveness of antiproton irradiation.

Background And Purpose: Antiprotons travel through tissue in a manner similar to that for protons until they reach the end of their range where they annihilate and deposit additional energy. This makes them potentially interesting for radiotherapy. The aim of this study was to conduct the first ever measurements of the biological effectiveness of antiprotons.

Radiobiology with heavy charged particles: a historical review.

Radiobiological studies using heavy charged particles followed closely the development of accelerators to produce beams of ever-increasing energy, driven primarily by the aspirations of physicists and chemists interested in the structure of matter. An impressive share of this development took place at Berkeley, beginning with the invention of the cyclotron by Ernest Lawrence in 1930. There followed a series of cyclotrons, synchrotrons and linear accelerators, culminating in the BEVALAC, which provided the first source of very heavy ions (helium to argon) to be used clinically, beginning in 1975.

Evidence for two forms of substructure in the cell survival curve--mechanisms and clinical consequences.

Recent laboratory studies have clearly demonstrated the presence of two types of fine structure in the radiation survival response of cultured mammalian cells: a) one type of substructure, observed at doses of a few Gy, is the result of the differential killing of subpopulations of cells of different, cell-cycle-related radiosensitivity; this substructure is strongly dependent on the cell-cycle distribution and is absent in tightly synchronized cell populations; b) the other type of substructure, found at lower doses (< 1 Gy), is expressed as a very sensitive (hypersensitive) response at very low doses followed by increased resistance as the dose increases until, by approximately 1 Gy, the response usually follows a standard linear-quadratic (LQ) function; it thus has the characteristics of a radiation-induced radioresistance and is assumed to reflect an inducible repair process. Although the linear-quadratic (LQ) model is widely used to describe the dose-effect response both in the laboratory and in the clinic, over the past 20 years there have been several reports of an anomalous departure from the simple LQ formalism, particularly at low doses. A review of these reports suggests that the observed anomalies are not so much a failure of the LQ formalism as a manifestation of the effects of the response substructure: mixed populations, a) and hypersensitivity, b) described above.

Cell-age heterogeneity and deviations from the LQ model in the radiation survival responses of human tumour cells.

Purpose: To determine whether some of the deviations from the simple linear-quadratic (LQ) theory in the radiation dose survival responses of asynchronous cultures of human tumour cell lines are caused by the presence of cell-age specific subpopulations which all individually follow LQ theory, but have different radiosensitivities.

Materials And Methods: Human tumour cells were synchronized by mitotic selection and their survival dose responses were measured at doses from 0.05 Gy to 12 Gy, using a high-precision survival assay.

Lack of correlation between G1 arrest and radiation age-response in three synchronized human tumour cell lines.

Purpose: To determine radiosensitivity as a function of cell age (the age-response) in three human tumour cell lines, and investigate the dependence of the age-response on G1 arrest and on cell-age heterogeneity in synchronized cell populations.

Materials And Methods: Variation in radiosensitivity throughout the cell cycle and G1 arrest was measured in mitotically selected populations of synchronized human tumour cells. In order to examine the effects of desynchronization and cell age heterogeneity on the measured age-response, a mathematical model was developed based on an existing kinetic model of the cell cycle.

Low-dose radiation sensitivity and induced radioresistance to cell killing in HT-29 cells is distinct from the "adaptive response" and cannot be explained by a subpopulation of sensitive cells.

Several reports using two different improved assays of clonogenicity have indicated the presence of a hypersensitive region in the radiation survival response at low doses, followed by an increase in radioresistance, in many mammalian cell lines. Mathematical modeling of these responses has suggested that it is unlikely that this effect can be explained by the presence of a small subpopulation of sensitive cells; however, this possibility cannot be excluded solely on the basis of those results. A second explanation has been offered which hypothesizes that a radiation-induced mechanism causes an increase in cellular radioresistance.

Substructure in the cell survival response at low radiation dose: effect of different subpopulations.

In order to obtain more accurate measurements of cell survival after low doses of radiation, we have used the cell sorter assay, in which a cell sorter is used to accurately count out the number of cells plated for colony formation. This method, combined with data averaging, permits measurements of survival with superior precision, which have revealed that there is substructure in the radiation response of asynchronously dividing Chinese hamster cells. The substructure, observed at doses of a few Gy, has features of a 2-component response, consistent with the presence of subpopulations of cells of different cell-cycle-related radiosensitivity.

A general cavity theory.

A cavity theory is used to relate the dose deposited in the cavity (sensitive volume of the detector) to that in the surrounding medium which may be of different atomic number or composition. Burlin proposed a general cavity theory to include all cavity sizes. The Burlin theory ignores all secondary-electron scattering effects which results in large discrepancies in dose to the cavity compared with the experimental results in high atomic number media.

Hypoxic cell sensitization: low-dose intrinsic radiosensitivity is predictive for etanidazole efficacy in a panel of human tumour cell lines.

We used a cell sorter assay to evaluate the efficacy of the hypoxic cell sensitizer etanidazole over 3-4 logs of cell inactivation, with particular attention to the clinically relevant low-dose survival region. Analysis of the radiation responses in a panel of six human tumour cell lines under conditions of hypoxia and hypoxia with etanidazole revealed both a cell-line and radiation dose-dependence in the sensitizing ability of this drug. Fits of the linear-quadratic (LQ) model to the low-dose region of cell survival indicate that sensitization of hypoxic cells by etanidazole results primarily from a modification of the beta parameter.

Low-dose hypersensitivity and increased radioresistance in a panel of human tumor cell lines with different radiosensitivity.

It is well known that cells of human tumor cell lines display a wide range of sensitivity to radiation, at least a part of which can be attributed to different capacities to process and repair radiation damage correctly. We have examined the response to very low-dose radiation of cells of five human tumor cell lines that display varying sensitivity to radiation, using an improved assay for measurement of radiation survival. This assay improves on the precision of conventional techniques by accurately determining the numbers of cells at risk, and has allowed us to measure radiation survival to doses as low as 0.

Substructure in the radiation survival response at low dose in cells of human tumor cell lines.

In earlier studies using asynchronously growing Chinese hamster cells, we observed substructure in the survival response at low doses. The substructure appeared to result from subpopulations of cells having different, cell cycle phase-dependent radiosensitivity. We have now applied the same flow cytometry and cell sorting technique to accurately measure the responses of cells of eight different asynchronously growing human tumor cell lines, representing a wide range in radiosensitivity.

Measurements of relative biological effectiveness of the 70 MeV proton beam at TRIUMF using Chinese hamster V79 cells and the high-precision cell sorter assay.

Measurements of relative biological effectiveness (RBE) have been made on the range-modulated 70 MeV proton beam at TRIUMF using a precise cell sorting survival assay. In this study, Chinese hamster V79-WNRE cells were suspended in medium containing liquid gelatin at 37 degrees C in irradiation tubes and the gel was allowed to solidify by cooling to 4 degrees C. Complete cell survival responses were measured at 11 positions with 2 mm spacing within a proton stopping peak width of approximately 2 cm.

Cytotoxic effect of RB 6145 in human tumour cell lines: dependence on hypoxia, extra- and intracellular pH and drug uptake.

Low pH and hypoxia are a common feature of many solid tumours. This study examined the effect of these two conditions on the cytotoxic properties of the bifunctional agent RB 6145, the prodrug of RSU 1069. The effect of acidic pH on RB 6145 toxicity was examined in six human tumour cell lines under hypoxic conditions and was found to have little effect in HT 29, A549, U373 and HT 144 cells.

The cytotoxicity of melphalan and its relationship to pH, hypoxia and drug uptake.

In the present in vitro studies we examined the effect of hypoxia and acidic pH, two important consequences of reduced blood flow in vivo, on the cytotoxicity of melphalan treatment in Chinese hamster V79-WNRE and SiHa human tumor cells. Cells were exposed to various concentrations of melphalan for 1 hr at 37 degrees C under oxic or hypoxic conditions; pH 6.6 or 7.

The effect of low pH and hypoxia on the cytotoxic effects of SR4233 and mitomycin C in vitro.

Purpose: We examined the effect of acidic pH and hypoxia on the cytotoxicity of SR4233 and mitomycin C in vitro.

Methods And Materials: The importance of tumor microenvironment to the response of solid tumors to cytotoxic treatment is well established. The bioreductive drug SR4233 has a very substantial selective toxicity for hypoxic cells.

The response of a human tumor cell line to low radiation doses: evidence of enhanced sensitivity.

The survival of asynchronous, exponentially growing DU-145 human tumor cells was measured after single doses of X rays in the dose range of 0.05-4 Gy using the cell sorting assay. When the response was modeled with the linear-quadratic (LQ) equation, a good fit to the data was observed for dose levels above 1 Gy; however, a region of enhanced sensitivity was observed at doses less than this.

The survival of asynchronous V79 cells at low radiation doses: modeling the response of mixed cell populations.

We have observed that when a single linear-quadratic (LQ) function is used to fit the radiation survival response of an asynchronously dividing population of V79 cells, a consistent misfit occurs at low doses. The data can be better described by fitting the low-dose and high-dose ranges separately, and there is evidence of a two-component response. The most obvious explanation is that we may simply be seeing the response of subpopulations of cells of different radiosensitivity: sensitive G1-, G2- and M-phase cells and resistant S-phase cells.

Substructure in the radiation survival response at low dose: asynchronous and partially synchronized V79-WNRE cells.

The radiation survival response of asynchronously-dividing populations of the cell line V79-WNRE indicates substructure in the low dose region that can be better characterized by separately fitting the data within the low and high dose regions to the linear-quadratic (LQ) equation. Flow cytometry and cell sorting techniques have been used to determine the response of both asynchronous populations and synchronous populations obtained by mitotic selection with or without an additional drug block. The statistically significant substructure which is present in the radiation response of asynchronous cells is absent in G1/S cells synchronized by mitotic selection followed by hydroxyurea (or aphidicolin) accumulation at the G1/S boundary.

The effect of pH on the aerobic and hypoxic cytotoxicity of SR4233 in HT-29 cells.

We have observed that low pH can substantially potentiate the cytotoxic effect of the bioreductive drug SR4233 in aerobic HT-29 human tumour cells. No such potentiation was observed under hypoxic conditions. This pH effect might be relevant both to the therapeutic effectiveness and to the normal tissue toxicity of this new agent.

Survival at low dose in asynchronous and partially synchronized Chinese hamster V79-171 cells.

In an earlier study using cell sorting techniques to define the radiation survival response of asynchronous Chinese hamster V79-171 cells more accurately, we found evidence of substructure in the response at low dose. In the present work we have attempted to show that this substructure arises from the subpopulations of sensitive (G1, G2 phase) and resistant (late S phase) cells which are present in asynchronously dividing cultures but which are not resolved by conventional survival assays. Partially synchronized cells were produced by exposure to 1 mM hydroxyurea for 12 h and were harvested 15 min later, yielding a population of viable cells at or just beyond the G1/S-phase boundary.

The effect of hypoxia and low pH on the cytotoxicity of chlorambucil.

Studies with mouse tumors have shown that the effectiveness of certain chemotherapeutic agents can be enhanced if they are used in appropriate combination with an anti-hypertensive drug such as hydralazine. This results in reduced tumor blood flow with, among other things, a consequent decrease in oxygenation and increase in acidity in the tumor tissue. The purpose of the present work was to determine to what extent hypoxia and low pH are involved in the mechanism of this effect for chlorambucil.

The radiation response of asynchronous cells at low dose: evidence of substructure.

Flow cytometry and cell sorting techniques have been used together with repeated measurement in an attempt to define better the radiation survival response of asynchronously dividing Chinese hamster V79-171 cells under aerobic and hypoxic conditions. Although the first two decades of cell inactivation have been examined, particular attention has been given to the low-dose range of a few grays, as used in individual radiation therapy treatments. A single linear-quadratic dose-response function was consistently unable to fit both the low-dose and high-dose data satisfactorily, suggesting a two-component response.

Dose dependence of the oxygen enhancement ratio (OER) in radiation inactivation of Chinese hamster V79-171 cells.

The dose dependence of the oxygen enhancement ratio (OER) has been examined through multiple measurements of the response of Chinese hamster V79-171 cells to low and high doses of radiation under aerobic and hypoxic conditions. In this series of experiments the cells were maintained at 37 degrees C throughout the gassing and irradiation periods, to simulate normal physiological conditions. Flow cytometry and cell sorting techniques were used to facilitate accurate measurement of cell survival throughout the dose range, but particularly at low dose.