Inokuti, M. and Seltzer, S. M. Physics as an Element of Radiation Research. Radiat. Res. 158, 3–12 (2002).

Since its inception in 1954, Radiation Research has published an estimated total of about 8700 scientific articles up to August 2001, about 520, or roughly 6%, of which are primarily related to physics. This average of about 11 articles per year indicates steadily continuing contributions by physicists, though there are appreciable fluctuations from year to year. These works of physicists concern radiation sources, dosimetry, instrumentation for measurements of radiation effects, fundamentals of radiation physics, mechanisms of radiation actions, and applications. In this review, we have selected some notable accomplishments for discussion and present an outlook for the future.

Kellerer, A. M. Electron Spectra and the RBE of X Rays. Radiat. Res. 158, 13–22 (2002).

For an assessment of the possible difference in effectiveness between mammography X rays and conventional X rays, the energy and LET spectra of the released electrons are examined. At photon energies below 20 keV and above 100 keV, the energy of the electrons increases with increasing photon energy, which implies that higher-energy photons produce less densely ionizing radiation and are therefore somewhat less effective per unit dose. However, in the intermediate energy range from 20 keV to 100 keV—the range that is relevant to medical diagnostics—the change from the photoelectric effect to the Compton effect causes a transient decrease of electron energies. The ionization density is therefore similar for 200 kVp X rays and 30 kVp mammography X rays, and the distributions of dose in LET suggest an RBE of 30 kVp mammography X rays compared to 200 kVp X rays of up to 1.3. This is in line with an earlier assessment by Brenner and Amols in terms of microdosimetric data, but it is strongly at variance with a recent claim that X rays for mammography are about four times more effective at small doses than conventional X rays and that they cause a correspondingly greater risk for breast cancer. Since LET need not be the only relevant factor, general response functions are examined here that specify—at low dose—the effect per electron of initial energy E and account, for example, for a particular role of the electron range. It is shown that, with any response per electron track that is a nondecreasing function of its starting energy, the low-dose RBE of the mammography X rays relative to the 200 kVp X rays must be substantially less than 2. The Auger electron that accompanies most photoelectrons, but only a minority of the Compton electrons, may increase the effectiveness of the mammography X rays somewhat, but it cannot explain the reported high values of the RBE.

Abdoul-Carime, H., Cecchini, S. and Sanche, L. Alteration of Protein Structure Induced by Low-Energy (<18 eV) Electrons. I. The Peptide and Disulfide Bridges. Radiat. Res. 158, 23–31 (2002).

We present measurements of low-energy (<18 eV) electron-stimulated desorption of anions from acetamide (CH₃CONH₂) and dimethyl disulfide [DMDS: (CH₃S)₂] films. Electron irradiation of physisorbed CH₃CONH₂ produces H⁻, CH₃⁻ and O⁻ anions, whereas the H⁻, CH₂⁻, CH₃⁻, S⁻, SH⁻ and SCH₃⁻ anions are observed to desorb from the DMDS film. Below 12 eV, the dependence of the anion yields on the incident electron energy exhibits structures that indicate that a resonant process (i.e. dissociative electron attachment) is responsible for molecular fragmentation. Within the range of 1–18 eV, it is found that (1.7 and 1.4) × 10⁷ H⁻ ions/incident electron and (7.8 × 10⁻¹¹ and 4.3 × 10⁻⁸) of the other ions/incident electron are desorbed from acetamide and DMDS films, respectively. These results suggest that, within proteins, the disulfide bond is more sensitive to low-energy electron attack than the peptide bond. In biological cells, some proteins interact closely with nucleic acid. Therefore, the observed fragments, when produced from secondary low-energy electrons generated by high-energy radiation, not only may denature proteins, but may also induce reactions with the nearby nucleic acid and damage DNA.

Rydberg, B., Heilbronn, L., Holley, W. R., Löbrich, M., Zeitlin, C., Chatterjee, A. and Cooper, P. K. Spatial Distribution and Yield of DNA Double-Strand Breaks Induced by 3–7 MeV Helium Ions in Human Fibroblasts. Radiat. Res. 158, 32–42 (2002).

Accelerated helium ions with mean energies at the target location of 3–7 MeV were used to simulate α-particle radiation from radon daughters. The experimental setup and calibration procedure allowed determination of the helium-ion energy distribution and dose in the nuclei of irradiated cells. Using this system, the induction of DNA double-strand breaks and their spatial distributions along DNA were studied in irradiated human fibroblasts. It was found that the apparent number of double-strand breaks as measured by a standard pulsed-field gel assay (FAR assay) decreased with increasing LET in the range 67–120 keV/μm (corresponding to the energy of 7–3 MeV). On the other hand, the generation of small and intermediate-size DNA fragments (0.1–100 kbp) increased with LET, indicating an increased intratrack long-range clustering of breaks. The fragment size distribution was measured in several size classes down to the smallest class of 0.1–2 kbp. When the clustering was taken into account, the actual number of DNA double-strand breaks (separated by at least 0.1 kbp) could be calculated and was found to be in the range 0.010–0.012 breaks/Mbp Gy^–1. This is two- to threefold higher than the apparent yield obtained by the FAR assay. The measured yield of double-strand breaks as a function of LET is compared with theoretical Monte Carlo calculations that simulate the track structure of energy depositions from helium ions as they interact with the 30-nm chromatin fiber. When the calculation is performed to include fragments larger than 0.1 kbp (to correspond to the experimental measurements), there is good agreement between experiment and theory.

Cornforth, M. N., Bailey, S. M. and Goodwin, E. H. Dose Responses for Chromosome Aberrations Produced in Noncycling Primary Human Fibroblasts by Alpha Particles, and by Gamma Rays Delivered at Sublimiting Low Dose Rates. Radiat. Res. 158, 43–53 (2002).

As the total dose of X or γ rays is delivered at lower and lower rates, the yield of chromosome aberrations progressively diminishes. Simultaneously, the shape of the dose response changes from one exhibiting pronounced upward curvature at high dose rates to one approaching linearity at low dose rates. Although the maximum sparing effect caused by lowering the dose rate can be predicted from classical cytogenetic theory, it has yet to be verified experimentally. Here, noncycling normal human fibroblasts were exposed to graded doses of ¹³⁷Cs γ rays at chronic dose rates of 6.3 and 2.8 cGy h⁻¹, dose rates that we reasoned should be lower than those required to achieve maximal sparing. This was indeed shown to be the case, after it was determined that the two chronic dose rates produced identical linear dose responses of 0.05 total aberrations per cell Gy⁻¹. Consistent with cytogenetic theory, this value was statistically indistinguishable from the linear coefficient derived from a fit to aberration frequencies produced by high-dose-rate exposure. Exposure to ²³⁸Pu α particles also produced a linear dose response for total aberrations, whose slope—with respect to ¹³⁷Cs γ rays as a reference radiation—implied a maximum RBE of 35 ± 2.

Goto, A., Takebayashi, Y., Liu, D., Li, L., Saiga, T., Ishikawa, Y., Mori, T., Yamadera, A. and Fukumoto, M. Microdistribution of Alpha Particles in Pathological Sections of Tissues from Thorotrast Patients Detected by Imaging Plate Autoradiography. Radiat. Res. 158, 54–60 (2002).

Thorotrast is a colloidal suspension of radioactive ²³²ThO₂ that naturally emits α particles (90%), β particles and γ rays (10%). Thorotrast was used as a radiographic contrast agent in the 1930s–1950s; it caused liver cancer several decades after injection because of its life-long deposition and exposure. Determination of the amount and the distribution of radioactive thorium are essential for assessment of radiation risks. We visualized α particles on ordinary archival tissue sections using an imaging plate and a BAS5000 image analyzer. Furthermore, we confirmed that the imaging system is sensitive enough to detect α particles and accurate in measuring the total amount of thorium deposited in the organ from a single tissue section. This method revealed that the amount of thorium deposited in tumor tissue is correlated to that in non-tumor tissue. Thorotrast deposition was not associated with DNA damage determined by histochemistry. In combination with histological findings, it is suggested that radioactive thorium always migrates within the deposited organs by macrophages, and that the organs are evenly exposed to α particles.

Kellerer, A. M. and Walsh, L. Solid Cancer Risk Coefficient for Fast Neutrons in Terms of Effective Dose. Radiat. Res. 158, 61–68 (2002).

Cancer mortality risk coefficients for neutrons have recently been assessed by a procedure that postulates for the neutrons a linear dose dependence, invokes the excess risk of the A-bomb survivors at a γ-ray dose D₁ of 1 Gy, and assumes a neutron RBE as a function of D₁ between 20 and 50. The excess relative risk (ERR) of 0.008/mGy has been obtained for R₁ = 20 and 0.016/mGy for R₁ = 50. To compare these results to the current ICRP nominal risk coefficient for solid cancer mortality (0.045/Sv for a population of all ages; 0.036/Sv for a working population), the ERR is translated into lifetime attributable risk and is then related to effective dose. The conversion is not trivial, because the neutron effective dose has been defined by ICRP not as a weighted genuine neutron dose (neutron kerma), but as a weighted dose that includes the dose from γ rays that are induced by neutrons in the body. If this is accounted for, the solid cancer mortality risk for a working population is found to agree with the ICRP nominal risk coefficient for neutrons in their most effective energy range, 0.2 MeV to 0.5 MeV. In radiation protection practice, there is an added level of safety, because the effective dose, E, is—for monitoring purposes—assessed in terms of the operational quantity H*, which overestimates E substantially for neutrons between 0.01 MeV and 2 MeV.

Ban, N., Yoshida, K., Aizawa, S., Wada, S. and Kai, M. Cytogenetic Analysis of Radiation-Induced Leukemia in Trp53-Deficient C3H/He Mice. Radiat. Res. 158, 69–77 (2002).

C3H/He mice develop acute myeloid leukemia (AML) after whole-body irradiation, but the strain becomes highly susceptible to stem cell leukemia (SCL) when a null mutation is introduced into the Trp53 gene. To examine the etiology of SCL and the influence of chromosomal instability on leukemogenesis, 12 SCLs and two AMLs arising from Trp53-deficient C3H/He mice were investigated cytogenetically. Each SCL demonstrated cell-to-cell variation in the number and structural integrity of their chromosomes, indicating chromosomal instability. Typical deletion of chromosome 2 was observed in the two AML cases, while most SCL cells did not display this aberration. Deletions and rearrangements of chromosome 11 were noticeable in SCLs from Trp53 heterozygotes but not in AMLs. Analysis of loss of heterozygosity revealed that aberrations involving chromosome 11 in SCLs resulted in loss of the wild-type Trp53 allele. These results suggest that loss of Trp53 function triggers the tumorigenic process leading toward SCL through the induction of chromosomal instability, and that SCL and AML are distinct varieties of leukemia.

Pazzaglia, S., Mancuso, M., Rebessi, S., Di Majo, V., Tanori, M., Biozzi, G., Covelli, V. and Saran, A. The Genetic Control of Chemically and Radiation-Induced Skin Tumorigenesis: A Study with Carcinogenesis-Susceptible and Carcinogenesis-Resistant Mice. Radiat. Res. 158, 78–83 (2002).

Outbred carcinogenesis-resistant (Car-R) and carcinogenesis-susceptible (Car-S) mouse lines were generated by phenotypic selection for resistance or susceptibility to two-stage skin carcinogenesis. These two Car mouse lines differ by >100-fold in susceptibility. In the present study, we tested the hypothesis that a subset of genetic loci responsible for susceptibility or resistance to chemical skin tumorigenesis may also be involved in radiation-induced skin tumorigenesis. Skin tumorigenesis was tested in groups of Car-S/R mice after X-ray initiation and 12-O-tetradecanoylphorbol-13-acetate (TPA) promotion. We found that ionizing radiation can initiate skin tumors in Car-S mice but not in Car-R mice. In Car-S mice, the most effective radiation doses (6 and 10 Gy given in four fractions) gave a threefold increase in tumor multiplicity and a twofold increase in tumor incidence compared to a TPA-only control group. We performed a molecular analysis of Hras gene mutations in skin tumors of Car-S mice induced by X-ray initiation/TPA promotion or by TPA promotion alone. The most notable difference emerging from the comparison of these mutation patterns is the high incidence (∼50%) of papillomas lacking Hras gene mutations in X-ray-initiated/TPA-promoted papillomas compared to 13% in papillomas induced by TPA alone, suggesting that lack of Hras gene mutations is a consistent feature of radiation-induced papillomas.

Xiao, H. H., Makeyev, Y., Butler, J., Vikram, B. and Franklin, W. A. 7-Hydroxystaurosporine (UCN-01) Preferentially Sensitizes Cells with a Disrupted TP53 to Gamma Radiation in Lung Cancer Cell Lines. Radiat. Res. 158, 84–93 (2002).

Mutations in TP53 occur in more than 50% of the lung cancer patients and are associated with an increased resistance to chemotherapy and radiotherapy. The human lung adenocarcinoma cell lines A549 and LXSN contain a wild-type TP53 and were growth arrested at both the G₁- and G₂-phase checkpoints after irradiation. However, a TP53-disrupted cell line, E6, was arrested only at the G₂-phase checkpoint. UCN-01 (7-hydroxystaurosporine), a CHEK1 inhibitor that abrogates the G₂ block, has been reported to enhance radiation toxicity in human lymphoma and colon cancer cell lines. In this study, UCN-01 preferentially enhanced the radiosensitivity of the TP53-disrupted E6 cells compared to the TP53 wild-type cells. This effect was more pronounced in cells synchronized in early G₁ phase, where the E6 cells showed a higher resistance to radiation in the absence of drug. These results indicate that the combination of UCN-01 and radiation can more specifically target resistant TP53 mutated cancer cells and spare TP53 wild-type normal cells.

Minchinton, A. I., Tonn, D. A., Sutherland, D. P. and Kyle, A. H. Carbogen Breathing after Irradiation Enhances the Effectiveness of Tirapazamine in SiHa Tumors but not SCCVII Tumors in Mice. Radiat. Res. 158, 94–100 (2002).

The penetration of anticancer agents into tumor tissue has recently attracted considerable attention. This study examines the effect of carbogen breathing on the antitumor activity of tirapazamine combined with radiation. Our hypothesis is based on the observation that the diffusion of tirapazamine through tissue is dependent on oxygen tension. We postulated that carbogen breathing might enhance the ability of tirapazamine to diffuse to hypoxic cells located distal to functional blood vessels in tumors. We first determined that carbogen breathing caused no significant change in the pharmacokinetics of tirapazamine, suggesting that any effect of carbogen breathing on the activity of tirapazamine is not attributable to modulation of pharmacokinetics. Cell survival in SCCVII and SiHa tumors after 10 Gy X rays alone was similar. However, when tirapazamine was administered 30 min after radiation treatment under air-breathing conditions, cell killing was greater in SCCVII tumors compared to SiHa tumors. Carbogen breathing during the exposure to tirapazamine did not change the cell survival in SCCVII tumors, but it enhanced cell killing in the SiHa tumors. Interestingly, carbogen breathing during radiation treatment produced greater cell killing in the SiHa tumors than in the SCCVII tumors. The vascular architecture and type of hypoxia in the two tumors probably underlie the differences in the responses of the two tumors. These findings suggest that the effectiveness of tirapazamine and other hypoxic cytotoxins may be dependent on tumor type.

Murley, J. S., Kataoka, Y., Weydert, C. J., Oberley, L. W. and Grdina, D. J. Delayed Cytoprotection after Enhancement of Sod2 (MnSOD) Gene Expression in SA-NH Mouse Sarcoma Cells Exposed to WR-1065, the Active Metabolite of Amifostine. Radiat. Res. 158, 101–109 (2002).

SA-NH mouse sarcoma cells were grown to confluence and then exposed to either 40 μM or 4 mM of WR-1065, i.e. the active thiol form of amifostine, for 30 min and then washed. Total RNA and protein were isolated at various times up to 24 h after exposure. Both concentrations of WR-1065 were equally effective in affecting Sod2 (also known as MnSOD) gene expression and protein levels. Northern blot analysis using a mouse cDNA probe revealed three Sod2 transcripts of 1, 4 and 6 kb. Expression of both the 4- and 6-kb transcripts increased by 20 and 60%, respectively, and remained elevated over a period of 4 to 20 h. Sod2 protein levels, as determined by Western blot analysis, increased 15-fold over background control levels over the same interval. Sod2 protein was evaluated using activity gels and was found to be active. SA-NH cells were irradiated with X rays either in the presence of 40 μM or 4 mM WR-1065 or 24 h later after its removal, when Sod2 protein levels were most elevated. No protection was observed for cells irradiated in the presence of 40 μM WR-1065. In contrast, survival after a dose of 2 Gy was elevated 1.27-, 1.14- and 1.20-fold in SA-NH cells irradiated in the presence of 4 mM WR-1065 or 24 h after exposure of the cells to 40 μM and 4 mM WR-1065, respectively. The increased survival levels observed 24 h after exposure to WR-1065 represents a delayed radioprotective effect of WR-1065 and corresponds to the time at which Sod2 protein levels are most elevated. These data demonstrate a novel mechanism for radioprotection by WR-1065 and suggest a new potential concern regarding the issue of tumor protection.

Evans, H. H., Evans, T. E. and Horng, M. F. Antimutagenicity of WR-1065 in L5178Y Cells Exposed to Accelerated ⁵⁶Fe Ions. Radiat. Res. 158, 110–114 (2002).

The ability of the aminothiol WR-1065 [N-(2-mercaptoethyl)-1,3-diaminopropane] to protect L5178Y (LY) cells against the cytotoxic and mutagenic effects of exposure to accelerated ⁵⁶Fe ions (1.08 GeV/nucleon) was determined. It was found that while WR-1065 reduced the mutagenicity in both cell lines when it was present during the irradiation, the addition of WR-1065 after the exposure had no effect on the mutagenicity of the radiation in either cell line. No marked protection against the cytotoxic effects of exposure to ⁵⁶Fe ions was provided by WR-1065 when added either during or after irradiation in either cell line. We reported previously that WR-1065 protected the LY-S1 and LY-SR1 cell lines against both the cytotoxicity and mutagenicity of X radiation when present during exposure, but that its protection when administered after exposure was limited to the mutagenic effects in the radiation-hypersensitive cell line, LY-S1. The results indicate that the mechanisms involved differ in the protection against cytotoxic compared to mutagenic effects and in the protection against damage caused by accelerated ⁵⁶Fe ions compared to X radiation.

Goffman, T. E. and Glatstein, E., Intensity-Modulated Radiation Therapy. Radiat. Res. 158, 115–117 (2002).

Intensity-modulated radiation therapy (IMRT) is an increasingly popular technical means of tightly focusing the radiation dose around a cancer. As with stereotactic radiotherapy, IMRT uses multiple fields and angles to converge on the target. The potential for total dose escalation and for escalation of daily fraction size to the gross cancer is exciting. The excitement, however, has greatly overshadowed a range of radiobiological and clinical concerns.

Moulder, J. E. Report on an Interagency Workshop on the Radiobiology of Nuclear Terrorism. Molecular and Cellular Biology of Moderate Dose (1–10 Sv) Radiation and Potential Mechanisms of Radiation Protection (Bethesda, Maryland, December 17–18, 2001) Radiat. Res. 158, 118–124 (2002).

The events of September 11, 2001 have focused attention on the possibility of nuclear terrorism, and 1–10 Sv is arguably the dose range of biological interest, since doses in this range both pose a risk of acute effects and are potentially survivable. Because of this interest, a coalition of U.S. government agencies (NCI, DOD, DOE) and the Radiation Research Society convened a workshop in December 2001 “to focus on molecular, cellular and tissue changes that occur [at doses of 1–10 Sv] and potential mechanisms of radioprotection”. A draft report of this workshop was posted on the NCI website in February 2002. According to the draft, the workshop was also intended to “determine the research opportunities and resources required [and] develop a research-action plan for further discussion and implementation.” Injuries after exposure to ionizing radiation are important to patients with cancer and to populations potentially subject to accidental or intentional exposure. In these populations, partial- or whole-body exposures in the range of 1–10 Sv are possible. The consequences of exposure of limited tissue volumes to doses above 10 Sv have been researched because of their applicability to cancer therapy, while exposure to doses below 1 Sv has been researched because of nuclear fallout and space exploration issues. Except for research aimed at protection of members of the armed forces, the intervening dose range has received relatively little attention. The workshop participants concluded that although we currently have only a limited ability to deal with the consequences of radiation exposures in this range, focused research would have the potential of rapidly expanding such capabilities.