The effect of hyperthermia on radiation-induced carcinogenesis

Ten groups of mice were exposed to either a single (30 Gy) or multiple (six fractions of 6 Gy) X-ray doses to the leg. Eight of these groups had the irradiated leg made hyperthermic for 45 min immediately following the X irradiation to temperatures of 37 to 43 degrees C. Eight control groups had their legs made hyperthermic with a single exposure or six exposures to heat as the only treatment. In mice exposed to radiation only, the postexposure subcutaneous temperature was 36.0 +/- 1.1 degrees C. Hyperthermia alone was not carcinogenic. At none of the hyperthermic temperatures was the incidence of tumors in the treated leg different from that induced by X rays alone. The incidence of tumors developing in anatomic sites other than the treated leg was decreased in mice where the leg was exposed to hyperthermia compared to mice where the leg was irradiated. A systemic effect of local hyperthermia is suggested to account for this observation. In mice given single X-ray doses and hyperthermia, temperatures of 37, 39, or 41 degrees C did not influence radiation damage as measured by the acute skin reactions. A hyperthermic temperature of 43 degrees C potentiated the acute radiation reaction (thermal enhancement factor 1.1). In the group subjected to hyperthermic temperatures of 37 or 39 degrees C and X rays given in six fractions, the skin reaction was no different from that of the group receiving X rays alone. Hyperthermic temperatures of 41 and 43 degrees C resulted in a thermal enhancement of 1.16 and 1.36 for the acute skin reactions. From Day 50 to Day 600 after treatment, the skin reactions showed regular fluctuations with a 150-day periodicity. Following a fractionated schedule of combined hyperthermia and X rays, late damage to the leg was less than that following X irradiation alone. Mice subjected to X rays and hyperthermic temperatures of 41 and 43 degrees C had a lower median survival time than the mice treated with hyperthermia alone. This effect was not associated with tumor incidence.     

Low-level X-radiation effects on functional vascular changes in Syrian hamster cheek pouch epithelium during hydrocarbon carcinogenesis

Effects of repeated low-level X radiation on functional microvascular changes in hamster cheek pouch epithelium during and following carcinogenesis by 7,12-dimethylbenz[a]anthracene (DMBA) were studied. Prior studies showed enhancement of such carcinogenesis by repeated 20 rad head and neck X-radiation exposures, and it was proposed that one possible mechanism was radiogenic alteration of the functional microvasculature in a manner which favored subsequent tumor development. Hamsters were treated with either radiation, DMBA, radiation + DMBA, or no treatment. Animals were sacrificed at 3-week intervals from 0 to 39 weeks after treatments began. Pouch vascular volume and permeability changes were studied by fractional distributions of radiotracers and were analyzed by a variety of statistical methods which explored the vascular parameters, treatment types, elapsed time, presence of the carcinogen, and histopathologic changes. All treatments resulted in significant changes in vascular volume with time, while only DMBA treatments alone resulted in significant changes in vascular permeability with time. Prior to the appearances of frank neoplasms, volumetric changes in DMBA only and radiation only groups were similar, while volume changes in DMBA + radiation groups increased slowly to a peak later than in other groups and then declined steadily to levels similar to the radiation only group. As in prior studies, there were significant vascular volume differences between DMBA and DMBA + radiation groups of tumor-bearing cheek pouches. DMBA maxima were significantly higher than those of DMBA + radiation. Radiation significantly affected DMBA-associated vascular volume and permeability changes during carcinogenesis. Several possible explanations for the relationship of these changes to the enhancement of DMBA carcinogenesis include: radiation blocking normal capillary proliferative and/or dilatory responses to inflammation secondary to neoplastic changes; radiation-induced focal increases in the pericapillary connective tissue histohematic barrier, stimulating angiogenesis but reducing nutrient diffusion; radiation exposures sensitizing vascular endothelium to subsequent angiogenic stimulation from premalignant tissues; DMBA vascular and epithelial effects partially or completely blocking radiation effects on epithelial and/or endothelial cells; and radiation damage to vessel walls partially or fully inhibiting normal physiologic mechanisms of repairing DMBA damage to the vessels.     

The effects of ionizing radiation on the pulmonary vasculature of intact rats and isolated pulmonary endothelium

We studied the effects of ionizing radiation on the morphology of the pulmonary circulation using an in vivo rat model and an in vitro pulmonary artery endothelial cell model. Gamma radiation was given as either an acute (30 Gy) or fractionated (5 X 6 Gy) dose to one hemithorax of rats. An acute 30-Gy dose delivered resulted in a 70% decrease in pulmonary arterial perfusion, using technetium-99m microaggregated albumin (99mTc-MAA), in the irradiated lung by 2-3 weeks after irradiation. Pulmonary microradiographs, using a barium sulfate perfusion method, obtained 2-3 weeks after irradiation demonstrated widespread loss of capillary filling and segmentation of the vessels. Histologic examination demonstrated intact capillaries, suggesting that the alterations in pulmonary perfusion were at the precapillary level. Similar abnormalities in lung perfusion and morphology were found after delivery of fractionated doses of radiation, but the onset of the changes was delayed, occurring 4-6 weeks postirradiation. Using cultured pulmonary endothelial cell monolayers, cell sloughing and retraction from the surface substrate were observed within 24 h after in vitro delivery of 30 Gy. Similar findings occurred in monolayers given fractionated doses (5 X 6 Gy) of radiation 2-3 days after the final dose. The in vivo animal and in vitro endothelial cell models offer a useful means of examining the morphologic alterations involved in radiation lung vascular damage.     

Repopulation kinetics of rat rhabdomyosarcoma tumors following single and fractionated doses of low-LET and high-LET radiation

Measurements were made of clonogenic cell survival in rat rhabdomyosarcoma tumors as a function of time following in situ irradiation with single or fractionated doses of 225-kVp X rays or with 557-MeV/u neon ions in the distal position of a 4-cm extended-peak ionization region. Single doses of 20 Gy of X rays or 7 Gy of peak neon ions reduced the initial surviving fraction to approximately 0.025 for each modality. Daily fractionated doses (four fractions in 3 days) of either peak neon ions (1.75 Gy per fraction) or X rays (6 Gy per fraction) achieved a cell survival of approximately 0.02-0.03 after the fourth dose of radiation. In the single-dose experiments, significant 5- and 10-fold decreases in the fraction of clonogenic cells were observed between the third and fourth days after irradiation with peak neon ions and X rays, respectively. After the sixth day postirradiation, the residual clonogenic cells exhibited a rapid burst of proliferation leading to doubling times for the surviving cell fractions of approximately 1.5 days. Radiation-induced growth delay was consistent with the cellular repopulation dynamics. In the fractionated-dose experiments with both radiation modalities, a large delayed decrease in cell survival was observed at 1-3 days after completion of the fractionated-dose schedule. Cellular repopulation was consistent with postirradiation tumor volume regression and regrowth for both radiation modalities. The extent of decrease in survival following the four-fraction radiation schedule was approximately two times greater in X-irradiated than in neon-ion-irradiated tumors that produced the same survival level immediately after the fourth dose. Mechanisms underlying the marked reduction in cell survival 3-4 days postirradiation are discussed, including the possible role of a toxic host cell response against the irradiated tumor cells.     

Dose-rate effects of mammary tumor development in female Sprague-Dawley rats exposed to X and gamma radiation

Mammary tumour development was followed in two experiments involving a total of 2229 female Sprague-Dawley rats exposed to various doses of X or gamma rays at different dose rates. The data for another 462 rats exposed to tritiated water in one of these experiments were also analyzed. The incidence of adenocarcinomas and fibroadenomas at a given time after exposure increased linearly in proportion to total radiation dose for most groups. However, no significant increase in adenocarcinomas was observed with chronic gamma exposures up to 1.1 Gy, and the increase in fibroadenomas observed with chronic gamma exposures at a dose rate of 0.0076 Gy h-1 up to an accumulated dose of 3.3 Gy was small compared to that observed after acute exposures. The incidence of all mammary tumors increased almost linearly with the log of dose rate in the range 0.0076 to 26.3 Gy h-1 for 3 Gy total dose of gamma rays. The effects of X rays appeared to be less influenced by dose rate than were the effects of gamma rays.     

Characterization of the adaptive response to ionizing radiation induced by low doses of X rays to human lymphocytes

In previous studies we have shown that low doses of radiation from incorporated tritiated thymidine can make human lymphocytes less susceptible to the genetic damage manifested as chromatid breakage induced by a subsequent high dose of X rays. We have also shown that this adaptive response to ionizing radiation can be induced by very low doses of X rays (0.01 Gy; i.e., 1 rad) delivered during S phase of the cell cycle. To see if a low dose of X rays could induce this response in cells at other phases of the cell cycle, human lymphocytes were irradiated with 0.01 or 0.05 Gy before stimulation by phytohemagglutinin (G0) or with 0.01 Gy at various times after stimulation (G1), followed by 1.5 Gy (150 rad) at G2 phase. Although G0 lymphocytes failed to exhibit an adaptive response, G1 cells irradiated as early as 4 h after stimulation did show the response. Experiments were also carried out to determine how long the adaptive response induced by 0.01 Gy could persist. A 0.01-Gy dose was delivered to lymphocytes in the first S phase, followed by 1.5 Gy in the same or subsequent cell cycles. Lymphocytes receiving a 1.5-Gy dose at 40, 48, or 66 h after stimulation exhibited an adaptive response, whereas those receiving a 1.5-Gy dose at 90 or 114 h did not. Duplicate cultures containing bromodeoxyuridine showed that at 40 h all the lymphocytes were in their first cell cycle after stimulation, at 48 h half of the lymphocytes were in their first cell cycle and half in their second, and at 66 h 80% of the lymphocytes were in their third cell cycle. Thus the adaptive response persists for at least three cell cycles after it is induced by 0.01 Gy of X rays. In other experiments, the time necessary for maximal expression of the adaptive response was determined by delivering 0.01 Gy at hourly intervals 1-6 h before the 1.5-Gy dose. While a 4-h interval was enough for expression of the adaptive response, shorter intervals were not.     

Carcinogenesis in laboratory mice after low doses of ionizing radiation

Experimental data on the incidence of solid tumors from various long-term mouse studies performed at the Casaccia laboratories over several years were reconsidered, limiting the analysis to the results available for doses equal to or less than 17 cGy of neutrons and 32 cGy of X rays since these dose limits are reasonably close to the generally accepted low-dose levels for high- and low-LET radiation (i.e. D(high-LET) < 5 cGy and D(low-LET) < 20 cGy, respectively). The following long-term experiments with BC3F1 mice were reviewed: (a) females treated with single doses of 1.5 MeV neutrons or 250 kVp X rays, (b) males treated with fractionated doses of fission neutrons, and (c) mice of both sexes irradiated in utero 17.5 days post coitus with single doses of fission neutrons or X rays. An experiment with CBA mice of both sexes treated with single doses of fission neutrons was also included in this study. Analysis was done on animals at risk; thus all incidences of tumor-bearing animals were expressed as the percentage excess incidence with respect to the controls. Ovarian tumors and other solid neoplasms were considered. The percentage frequencies and mean survival times of tumor-free mice were also recalculated. The results indicate the existence of a region at low doses where the final incidence of solid neoplasms is indistinguishable from the background incidence. These data reinforce the idea that at low doses the effectiveness of ionizing radiation in inducing solid neoplasms in laboratory mice is very low.     

Effects of aqueous cinnamon extract on chemically-induced carcinoma of hamster cheek pouch mucosa

This study aimed to investigate the effects of aqueous cinnamon extract (ACE) on 7, 12-Dimethylbenz[a]anthracene (DMBA)-induced oral carcinogenesis in hamster cheek pouch (HCP) mucosa. Sixty male Syrian hamsters were randomly divided into six equal groups. The hamsters of groups I, II and III received no treatment, DMBA and ACE respectively, for 16 weeks. Groups IV and V were handled as group II and concomitantly treated with ACE for the same period and additionally group V received ACE for other 16 weeks after the stoppage of DMBA application. Group VI hamsters were handled as group III and additionally received DMBA for other 16 weeks after the stoppage of ACE supplementation. Hamsters of each group were euthanized according to the experimental schedule. The buccal pouches were and prepared for H&E stain, PAS reagent, CD3 and PDGF immunohistochemical reactivity. All groups showed dysplastic changes with varying degrees except groups I and III. Deep invasive carcinomas were recorded in 90% of the samples of group II, 60% of group IV, 50% of group V and 40% of group VI. From the previous results, it can be concluded that ACE has the potentiality preventing oral cancer initiation better than inhibiting oral cancer progression.     

Recovery of murine lens epithelial cells from single and fractionated doses of X rays and neutrons

Subpopulations of mouse lens epithelial cells, differing in proliferative status, were irradiated with either X rays or fission spectrum neutrons given singly or in four weekly fractions. After various times, epithelia were mitogenically stimulated by wounding and DNA synthesis responses were determined by incorporation of [3H]thymidine. At 1 h following both X and neutron irradiations, significant suppression of the wound response after single doses and a sparing effect of fractionation were evident in both the mitotically quiescent and the slowly proliferating subpopulations. At 1 week following single or fractionated doses of both radiations, recovery was evident in both subpopulations. By 4 weeks, the quiescent subpopulation showed significant recovery after both single and fractionated doses of X rays or neutrons. In contrast, a marked decreased ability to respond after neutron irradiation and, in addition, a significant enhancement effect of neutron fractionation were observed for the slowly proliferating subpopulation. Per gray, neutrons were about 7.5 times more effective than X rays as a single dose and 25 times more effective in four equal fractions. The shift from an initial sparing to a final enhancing effect of neutron fractionation for the slowly proliferating subpopulation has importance for understanding divergent early and late radiation responses following dose fractionation.     

Lung cancer risk in mice: analysis of fractionation effects and neutron RBE with a biologically motivated model

Data from Argonne National Laboratory on lung cancer in 15,975 mice with acute and fractionated exposures to gamma rays and neutrons are analyzed with a biologically motivated model with two rate-limiting steps and clonal expansion. Fractionation effects and effects of radiation quality can be explained well by the estimated kinetic parameters. Both an initiating and a promoting action of neutrons and gamma rays are suggested. While for gamma rays the initiating event is described well with a linear dose-rate dependence, for neutrons a nonlinear term is needed, with less effectiveness at higher dose rates. For the initiating event, the neutron RBE compared to gamma rays is about 10 when the dose rate during each fraction is low. For higher dose rates this RBE decreases strongly. The estimated lifetime relative risk for radiation-induced lung cancers from 1 Gy of acute gamma-ray exposure at an age of 110 days is 1.27 for male mice and 1.53 for female mice. For doses less than 1 Gy, the effectiveness of fractionated exposure to gamma rays compared to acute exposure is between 0.4 and 0.7 in both sexes. For lifetime relative risk, the RBE from acute neutrons at low doses is estimated at about 10 relative to acute gamma-ray exposure. It decreases strongly with dose. For fractionated neutrons, it is lower, down to about 4 for male mice.     

Single low doses of X rays inhibit the development of experimental tumor metastases and trigger the activities of NK cells in mice

There is evidence indicating that low-level exposures to low- LET radiation may inhibit the development of tumors, but the mechanism of this effect is virtually unknown. In the present study, BALB/c mice were irradiated with single doses of 0.1 or 0.2 Gy X rays and injected intravenously 2 h later with syngeneic L1 sarcoma cells. Compared to the values obtained for sham-irradiated control mice, the numbers of pulmonary tumor colonies were significantly reduced in the animals exposed to either 0.1 or 0.2 Gy X rays. Concurrently, a significant stimulation of NK cell-mediated cytotoxic activity was detected in splenocyte suspensions obtained from irradiated mice compared to sham-exposed mice. Intraperitoneal injection of the NK-suppressive anti-asialo GM1 antibody totally abrogated the tumor inhibitory effect of the exposures to 0.1 and 0.2 Gy X rays. These results indicate that single irradiations of mice with either 0.1 or 0.2 Gy X rays suppress the development of experimental tumor metastases primarily due to the stimulation of the cytolytic function of NK cells by radiation.     

Low-dose total-body γ irradiation modulates immune response to acute proton radiation

Health risks due to exposure to low-dose/low-dose-rate radiation alone or when combined with acute irradiation are not yet clearly defined. This study quantified the effects of protracted exposure to low-dose/low-dose-rate γ rays with and without acute exposure to protons on the response of immune and other cell populations. C57BL/6 mice were irradiated with ??Co (0.05 Gy at 0.025 cGy/h); subsets were subsequently exposed to high-dose/high-dose-rate proton radiation (250 MeV; 2 or 3 Gy at 0.5 Gy/min). Analyses were performed at 4 and 17 days postexposure. Spleen and thymus masses relative to body mass were decreased on day 4 after proton irradiation with or without pre-exposure to γ rays; by day 17, however, the decrease was attenuated by the priming dose. Proton dose-dependent decreases, either with or without pre-exposure to γ rays, occurred in white blood cell, lymphocyte and granulocyte counts in blood but not in spleen. A similar pattern was found for lymphocyte subpopulations, including CD3+ T, CD19+ B, CD4+ T, CD8+ T and NK1.1+ natural killer (NK) cells. Spontaneous DNA synthesis by leukocytes after proton irradiation was high in blood on day 4 and high in spleen on day 17; priming with γ radiation attenuated the effect of 3 Gy in both body compartments. Some differences were also noted among groups in erythrocyte and thrombocyte characteristics. Analysis of splenocytes activated with anti-CD3/anti-CD28 antibodies showed changes in T-helper 1 (Th1) and Th2 cytokines. Overall, the data demonstrate that pre-exposure of an intact mammal to low-dose/low-dose-rate γ rays can attenuate the response to acute exposure to proton radiation with respect to at least some cell populations.     

The action of caffeine on X-irradiated HeLa cells. X. Depressed recovery from potentially lethal damage in cells containing 5-bromodeoxyuridine

HeLa S3 cells were sensitized to the lethal action of 220-kV X rays by partially replacing the thymidine in their DNA with 5-bromodeoxyuridine (BrdU). To examine the expression of and recovery from potentially lethal radiation damage (PLD), both BrdU-grown and control cells were treated with 4 mM caffeine for increasing times up to 2 days, either immediately after irradiation or after increasing delays up to 28 h. When the same dose of X rays (3 Gy) was applied to BrdU-grown and control cells, the difference in survival that is found in the absence of caffeine disappeared after about 30 h of incubation in its presence; when isosurvival doses were applied (BrdU-grown cells, 2.5 Gy; control cells, 4 Gy), the control cells suffered more killing. When treatment with caffeine was delayed for progressively longer times after both groups of cells received 3 Gy, the control cells achieved a higher level of survival. These results indicate that the increased radiation sensitivity of cells containing BrdU derives from a decreased ability to repair PLD.     

Two types of X-ray-induced radioresistance in mice: Presence of 4 dose ranges with distinct biological effects

Preirradiation with 0.05 Gy of X rays 2 months before a second exposure to a mid-lethal dose significantly enhanced the survival rate in both female and male ICR strain mice. The radioresistance was observed between 2–2.5 months after exposure to 0.05 Gy. It did not appear within 1.5 months, and disappeared after 3 months. This radioresistance was induced only by whole-body preirradiation (not by partial irradiation of the head or the trunk). On the other hand, preirradiation with 0.30 Gy as well as 0.50 Gy resulted in radioresistance 2 weeks later, but not 2 months later. The radioresistance was induced by whole-body preirradiation or partial preirradiation of the trunk. No radioresistance was evident after exposure of intermediate preirradiation doses of 0.15 and 0.20 Gy administered before 2 months and 2–5 weeks, respectively. The present and previous results show that the biological effects of ionizing radiation may be distinguished with the following four radiation dose ranges; (1) below 0.025 Gy: no radioresistance after 2 months; (2) 0.05–0.10 Gy: significant radioresistance after 2–2.5 months; (3) 0.20 Gy: no radioresistance after 2–5 weeks; and (4) 0.30–0.50 Gy or more: significant radioresistance after 2 weeks. These results conflict with previous findings of the biological effects of ionizing radiation in which the radiation hazard increases in relation to increasing accumulated doses. Some stimulation, in addition to adaptation, by low dose irradiation may have occurred.     

Fractionated X-radiation treatment can elicit an inducible-like radioprotective response that is not dependent on the intrinsic cellular X-radiation resistance/sensitivity

Inducible responses are well documented to play a role in the radiation response of cells. However, it is not known whether clinically relevant fractionated X-radiation treatment could elicit an inducible-like radioprotective response and whether there is a direct correlation between the inducible radiation response phenomenon and the intrinsic radiation response of the cell. Therefore, the purpose of this study was to determine whether closely related human colorectal tumor (HCT116) clones treated with fractionated X rays could elicit an inducible-like radiation response to a subsequent acute (i.e. single) X-ray challenge, and whether the magnitude of the inducible-like response correlates with the intrinsic X-ray resistance of the responding clones. After fractionated X irradiation, only the radiosensitive clone showed enhanced clonogenic survival with a subsequent acute X-ray exposure. Cell cycle changes or the selection of subclones with increased intrinsic radiation resistance induced by the fractionated X rays were excluded as the basis of this enhanced tolerance, suggesting the presence of an inducible-like radioprotective response. Using the comet assay, we found similar amounts of intrinsic DNA damage among the clones after acute X irradiation. Our findings demonstrate that fractionated X-ray treatment can elicit an inducible-like radioprotective response and represent the first evidence that this response is independent of the intrinsic radiation resistance/sensitivity of the responding cells.     

Bystander and delayed effects after fractionated radiation exposure

Human immortalized keratinocytes were exposed to a range of single or fractionated doses of gamma rays from (60)Co, to medium harvested from donor cells exposed to these protocols, or to a combination of radiation and irradiated cell conditioned medium (ICCM). The surviving fractions after direct irradiation or exposure to ICCM were determined using a clonogenic assay. The results show that medium harvested from cultures receiving fractionated irradiation gave lower "recovery factors" than direct fractionated irradiation, where normal split-dose recovery occurred. The recovery factor is defined here as the surviving fraction of the cells receiving two doses (direct or ICCM) separated by an interval of 2 h divided by the surviving fraction of cells receiving the same dose in one exposure. After treatment with ICCM, the recovery factors were less than 1 over a range of total doses from 5 mGy-5 Gy. Varying the time between doses from 10 min to 180 min did not alter the effect of ICCM, suggesting that two exposures to ICCM are more toxic than one irrespective of the dose used to generate the response. In certain protocols using mixtures of direct irradiation and ICCM, it was possible to eliminate the bystander effect. If bystander factors are produced in vivo, then they may reduce the sparing effect of the dose fractionation.     

The effects of a fractionated gamma irradiation on life shortening and disease incidence in BALB/c mice

BALB/c male mice (12 weeks old) were exposed to a single or fractionated exposure of 137Cs gamma rays. The fractionated dose was split into 10 equal doses delivered at an interval of 1 day. The causes and possible causes of spontaneous death were ascertained by autopsy and histological examination, and the data were treated by competing risk analysis. Life shortening followed a linear dose dependency and was about the same for fractionated (38.1 +/- 3.1 days/Gy) as for single (46.2 +/- 4.3 days/Gy) exposure. Death from tumor disease was enhanced and that from nonstochastic lung and kidney diseases was reduced after fractionated compared to single exposure.     

Relative cataractogenic effects of X rays, fission-spectrum neutrons, and 56Fe particles: a comparison with mitotic effects

The eyes of Sprague-Dawley rats were irradiated with doses of 2.5-10 Gy 250-kVp X rays, 1.25-2.25 Gy fission-spectrum neutrons (approximately 0.85 MeV), or 0.1-2.0 Gy 600-MeV/A 56Fe particles. Lens opacifications were evaluated for 51-61 weeks following X and neutron irradiations and for 87 weeks following X and 56Fe-particle irradiations. Average stage of opacification was determined relative to time after irradiation, and the time required for 50% of the irradiated lenses to achieve various stages (T50) was determined as a function of radiation dose. Data from two experiments were combined in dose-effect curves as T50 experimental values taken as percentages of the respective T50 control values (T50-% control). Simple exponential curves best describe dose responsiveness for both high-LET radiations. For X rays, a shallow dose-effect relationship (shoulder) up to 4.5 Gy was followed at higher doses by a steeper exponential dose-effect relationship. As a consequence, RBE values for the high-LET radiations are dose dependent. Dose-effect curves for cataracts were compared to those for mitotic abnormalities observed when quiescent lens epithelial cells were stimulated mechanically to proliferate at various intervals after irradiation. Neutrons were about 1.6-1.8 times more effective than 56Fe particles for inducing both cataracts and mitotic abnormalities. For stage 1 and 2 cataracts, the X-ray Dq was 10-fold greater and the D0 was similar to those for mitotic abnormalities initially expressed after irradiation.     

Changes in osteoclasts after irradiation with carbon ion particles

We examined the effects of radiation on decreases in osteoclast numbers after regional irradiation of rats with carbon ions and gamma rays. Male Wistar rats were subjected to hind-leg irradiation with carbon ions (290 MeV/u) or gamma rays at doses of 15, 22.5, or 30 Gy. The effects of carbon ions and gamma rays on osteoclasts were studied using histologic and morphometric methods. At doses of 15 Gy and 22.5 Gy, osteoclast numbers increased transiently until day 5 after irradiation and then decreased rapidly in both the carbon ion and gamma ray irradiation groups. The carbon ion group showed reduced osteoclast size compared with the gamma ray group. Carbon ion irradiation had a more marked effect on osteoclast activity, and suppressed maturation to a greater extent than gamma irradiation. These observations suggest that carbon ion irradiation induces differential modulation of osteoclast growth factor expression.