M. R. Raju

Radiobiology of α particles. III: Cell inactivation by α-particle traversals of the cell nucleus

Cell inactivation after exposure to collimated 3.5-MeV alpha particles in three hamster cell lines, V79, CHO-10B, and HS-23, one mouse cell line, C3H 10T1/2, and a human skin fibroblast cell line were studied. Several parameters were investigated for each cell line. Theoretical calculations were performed to find the distribution of energy deposited in the nuclear volume for each cell line. The mean number of alpha-particle traversals required to induce a lethal lesion varied between two for HS-23 cells and six for C3H 10T1/2 cells. The number of traversals per unit area and the total track length of alpha particles that inactivated a cell were found to be nearly constant for the hamster and mouse cell lines. These quantities were found to be lower for the human skin fibroblast cell line. The RBE values for all cell lines were found to be about 3.8 at 10% survival. Thus cell lines that are more sensitive to alpha radiation are also more sensitive to gamma radiation. The average number of alpha-particle traversals producing a single lethal lesion is greater than one. The passages of alpha particles through the cell nucleus that do not kill the cell may lead to carcinogenic effects.

Radiobiology of ultrasoft X rays. I. Cultured hamster cells (V79)

Ultrasoft X rays (approximately less than keV) provide a useful probe for the study of the physical parameters associated with the induction of biological lesions because the spatial scale of their energy depositions is of nanometer dimensions, comparable to that of critical structures within the cell. We report on cell-killing experiments using cultured hamster cells (V79) exposed to carbon K (0.28 keV), aluminum K (1.5 keV), copper K (8.0 keV), and 250 kVp X rays, under oxic and hypoxic conditions, and as a function of cell-cycle phase. Our principal results are: RBE increases with decreasing X-ray energy; OER decreases with decreasing X-ray energy; and cell-cycle response is similar for all X-ray energies. Our RBE results confirm earlier observations using ultrasoft X rays on mammalian cells. The shapes of fitted curves through the data for each energy are statistically indistinguishable from one another, implying that the enhanced effectiveness is purely dose modifying. The results reported herein generally support the view that single-track effects of radiation are predominantly due to very local energy depositions on the nanometer scale, which are principally responsible for observed radiobiological effects.

Proton Radiobiology, Radiosurgery and Radiotherapy

This review briefly traces the historical developments of proton radiobiology, radiosurgery and radiotherapy for the benefit of young researchers and clinicians entering into this field. In preparing to use protons in radiosurgery and radiotherapy, radiobiological effects of protons were studied extensively by various groups, including the University of California at Berkeley, the University of Uppsala, Massachusetts General Hospital, and the Harvard Cyclotron Laboratory. Considerable work on proton radiobiology was also done because protons are a major component of the radiation environment in space. The biological effects of proton beams were found to be quantitatively and qualitatively similar to conventional radiations used in radiotherapy. The relative biological effectiveness (RBE) of protons suitable for large-field radiotherapy, compared with 60Co gamma-rays, is generally in the range 1.0-1.25, and remains the same with depth of penetration, except for the descending portion of the depth-dose curve. Also, unlike other heavier charged particles and neutrons, the RBE of high-energy protons, which are suitable for large-field radiotherapy, compared with 60Co gamma-rays, is generally found to be independent of the fraction size in in vivo experiments. The oxygen enhancement ratio for high-energy protons is not significantly different from that of X-rays. An RBE = 1.1, compared with 60Co gamma-rays, is generally used in the clinical application of protons; however, the radiobiological data on mouse, rat, rabbit and primate suggest that the gastrointestinal tissues may be relatively more sensitive to protons. About 13,000 patients have been treated with protons at about 15 facilities around the world. Nearly half of these patients were neurosurgical patients treated with stereotactic radiosurgery. The pioneering efforts at the Harvard Cyclotron in collaboration with the Massachusetts General Hospital and the Massachusetts Eye and Ear Infirmary were responsible for the development of proton treatment for choroidal melanoma and for the tumours of the skull base and spine. There has been extensive confirmation of these results by other groups, especially the groups at Lawrence Berkeley Laboratory and Paul Scherrer Institute. The first medically dedicated proton facility is in operation at Loma Linda University in California. The construction in the USA of another proton treatment facility at Massachusetts General Hospital has been decided upon, and there are plans for many more worldwide.

Radiation tolerance of the rat rectum to fractionated X-rays and pi-mesons

A 2.5 cm segment of the rectum of female F344 rats was irradiated with up to 10 fractions of X-rays or pions. Transient signs of acute proctitis were followed by chronic rectal injury starting at about 2 months. Recto-vaginal fistulas were frequently observed in animals with severe chronic injury. Two patterns of chronic injury were observed, an early type consisting of deep ulcers and fistulas, and a late type characterized by vascular injury, fibrosis and mucosal cysts. In a 4-fraction X-ray experiment, the influence of a low-residue diet was compared with a regular diet, showing no significant differences in pathology or survival. Isoeffective doses were determined for the occurrence of severe rectal injury at 250 days. Fitting the data to an LQ-model yielded an alpha/beta ratio of 13 Gy for pions, and 6.5 Gy for X-rays. Increasing the overall treatment time from 9 days to one month gave a significant rise in isoeffective doses for chronic injury. This suggests that the more delayed types of injury did not develop independently from the acute mucosal changes. The RBE of pions for rectal injury at 250 days was 1.2 for single doses, increasing to about 1.4-1.5 at a dose per fraction of 4 Gy. These values are similar to those obtained in other acute and late responding tissues.

Repair of potentially lethal damage in Chinese hamster cells after X and alpha irradiation.

Recovery from potentially lethal damage (PLD) of cultured Chinese hamster cells (line CHO) after exposure to X rays and to α particles was investigated using both exponentially growing and plateau-phase cells (nutritionally induced stationary phase). The results indicated that there was no recovery from PLD after exposure to α particles from plutonium. Experiments were also conducted to determine if recovery phenomena were dependent on cell concentration. Recovery from PLD after X-ray exposure was not dependent on cell concentration. Cell-to-cell contact also does not seem to be an important factor in PLD recovery. The recovery curve shape after exposure to X rays, both for exponentially growing and plateau-phase CHO cells, remained identical; however, the magnitude of repair by plateau-phase cells was higher than by exponentially growing cells, indicating that cells in G1 may be primarily involved in PLD recovery.

Radiobiology of ultrasoft X-rays II. Cultured C3H mouse cells (10T1/2)

In the first paper of this series (Radiat. Res. 110, 396-412 (1987], using V79 cells, we reported that the relative biological effectiveness (RBE) of ultrasoft X rays was found to increase with decreasing energy, and the oxygen enhancement ratio (OER) was found to decrease with decreasing energy. In this report, we present RBE and OER results for 10T1/2 cells that are known to grow uniformly flat and are considerably thinner than V79 cells. Thus the variation in dose across the cell nucleus is considerably reduced. The OER results agree well with our earlier V79 results. However, the RBE values for 10T1/2 cells compared to V79 cells are systematically less for all soft X rays and especially for 0.28 keV carbon-K (1.3 compared to 3.4 for V79 cells). Some plausible explanations are presented to reconcile the apparent discrepancy between V79 and 10T1/2 results.

Radiobiology of ultrasoft X-rays III. Normal human fibroblasts and the significance of terminal track structure in cell inactivation

Ultrasoft characteristic X rays from carbon (0.28 keV) are severely attenuated as they pass through biological material, causing a nonuniform distribution of dose to cell nuclei. Complications of studying ultrasoft X rays can be minimized in this context by using cells with very thin cytoplasm and nuclei (e.g., less than the attenuation length of the X rays), and which exhibit a more nearly exponential dose response to cell killing, such as normal human fibroblasts compared with V79 cells. Using this cell system, we report the relative biological effectiveness (RBE) of A1-K and C-K X rays to be near unity. Previous studies of cell inactivation by characteristic carbon X rays gave RBEs of 3 to 4, supporting the idea that localized energy depositions from secondary electrons and primary track ends represent the principal mode of biological action for other low-LET radiations. In part, the reported high RBEs result from the use of mean dose to describe energy deposited within the cell nuclei by these poorly penetrating radiations. Implicit in the use of mean dose is that cellular damage varies linearly with dose within a critical target(s), an assumption that is of questionable validity for cells that exhibit pronounced curvilinear dose responses. The simplest interpretation of the present findings is that most energy depositions caused by track-end effects are not necessarily more damaging than the sparsely ionizing component.

Radiobiology of ultrasoft X rays. IV. Flat and round-shaped hamster cells (CHO-10B, HS-23)

The results reported earlier in this series indicated that the relative biological effectiveness (RBE) of ultrasoft X rays decreases with decreasing cell thickness, approaching unity for the thinnest cells used, plateau-phase human skin fibroblasts (HSF). The possible dependence of RBE on the configuration of the cell nucleus is investigated further in this paper using two CHO cell lines that attach well and have similar intrinsic radiosensitivities to 60Co gamma rays. One of the lines forms monolayers similar to V79 cells, while the other remains more spherical during growth. We find an increasing RBE with decreasing X-ray energy for both of these cell lines, consistent with our results using V79 cells. Also consistent with our results obtained with 10T1/2 and HSF cells, we find an increasing RBE with increasing cell thickness. The possible dependence of RBE on radiosensitivity and the use of the concept of mean dose for ultrasoft X rays is discussed.

Age Response for Line CHO Chinese Hamster Cells Exposed to X-Irradiation and Alpha Particles from Plutonium

The age response of Chinese hamster cells (line CHO) synchronized by mitotic selection and by mitotic selection with hydroxyurea resynchronization was studied for X rays and alpha particles from pl...

Radiobiology of alpha particles. I. Exposure system and dosimetry

A 238Pu alpha-particle exposure apparatus was designed and constructed for use in radiobiological studies with cultured cell systems. The system provides a wide dynamic range of absorbed doses and a uniform radiation field. Average dose rate in air was measured with a small-volume ionization chamber. Estimates of dose rate at the cell surface were obtained from measurements taken with a silicon surface barrier detector. Particle fluence uniformity and fluence rate were measured using track etch procedures. The design and dosimetric characterization of the apparatus are discussed.