Edward L. Alpen

High-LET Radiation Carcinogenesis'

The dose-response curves for the induction of tumors by high-LET radiation are complex and are insufficiently understood. There is no model or formulation to describe the dose-response relationship over a range 0-100 rad. Evidence suggests that at doses below 20 rad the response is linear, at least for life shortening and some tumor systems. Thus limiting values of RBEs for the induction of cancer in various tissues can be determined, but it will require sufficient data obtained at low single doses or with small fractions. The results obtained from experiments with heavy ions indicate an initial linear response with a plateauing of the curve at a tumor incidence level that is dependent on the type of tissue. The RBE values for the heavy ions using 60Co gamma rays as the reference radiation increase with the estimated LET from 4 or 4H to about 27 for 56Fe and 40Ar. The dose responses and RBEs for 56Fe and 40Ar are similar to those for fission neutrons. These findings suggest the possibility that the effectiveness for tumor induction reaches a maximum.

Fluence-based relative biological effectiveness for charged particle carcinogenesis in mouse Harderian gland

Neoplasia in the rodent Harderian gland has been used to determine the carcinogenic potential of irradiation by HZE particles. Ions from protons to lanthanum at energies up to 670 MeV/a have been used to irradiate mice, and prevalence of Harderian gland tumors has been measured 16 months after irradiation. The RBE for tumor induction has been expressed as the RBEmax, which is the ratio of the initial slopes of the dose vs prevalence curve. The RBEmax has been found to be approximately 30 for ions with LET values in excess of 100 keV/micrometer. Analysis on the basis of fluence as a substitute for dose has shown that on a per particle basis all of the ions with LET values in excess of 100 keV/micrometer have equal effectiveness. An analysis of the probabilities of ion traversals of the nucleus has shown that for these high stopping powers that a single hit is effective in producing neoplastic transformation.

The Relative Biological Effect of High-Z, High-LET Charged Particles for Spermatogonial Killing'

ALPEN, E. L., AND POWERS-RIsIus, P., The Relative Biological Effect of High-Z, HighLET Charged Particles for Spermatogonial Killing. Radiat. Res. 88, 132-143 (1981). The testis weight loss at 28 days after irradiation was used to assess the relative effectiveness of high-energy heavy ions for cell killing of mouse spermatogonia. The testis weights are corrected for the radiation-insensitive fraction. With this model the data were found to fit an exponential survival curve with no shoulder. For all ions the extrapolation number is 1.0. For 60Co radiation the extrapolation number is 1.0 and the Do is 107 rad. All ions have killing effectiveness higher than 60Co radiation except nonstopping helium ions. The radiation modalities used were 228 MeV/amu helium ions and 400 to 670 MeV/amu carbon, neon, and argon ions. The RBE for spermatogonial killing was a maximum of 3.0 for plateau argon ions. Both neon and argon ions were shown to have reduced effectiveness if the LET exceeded 100 keV/Mm. One of the most significant observations made in these studies is that LET alone does not predict the effectiveness of the various ions. Large differences in effectiveness are seen as a function of atomic number of the particle, even though LET is held approximately constant.

Current status of clinical particle radiotherapy at Lawrence Berkeley laboratory

Clinical experience with charged particle irradiation of human cancers has been underway at the University of California Lawrence Berkeley Laboratory. Over 150 patients have been irradiated with heavy charged particle beams including helium, carbon, neon, and argon ions. Pilot studies have included such tumor sites as glioma of the brain, carcinoma of the esophagus, carcinoma of the pancreas, carcinoma of the stomach, ocular melanoma, and carcinoma of the uterine cervix. Prospective studies are planned to investigate the improved dose localization potential (helium) and the enhanced biologic and physical dose potential (carbon, neon) in a controlled trial against the best available megavoltage irradiation techniques.

Fluence-related risk coefficients using the Harderian gland data as an example

The risk of radiation-induced cancer to space travelers outside the earths magnetosphere will be of concern on missions to the Moon and beyond to Mars. High energy galactic cosmic rays with high charge (HZE particles) will penetrate the spacecraft and the bodies of the astronauts, sometimes fragmenting into nuclear secondary species of lower charge but always ionizing densely, thus causing cellular damage which may lead to malignant transformation. To quantitate this risk, the concept of dose equivalent (in which a quality factor Q as a function of LET is assumed) may not be adequate, since different particles of the same LET may have different efficiencies for tumor induction. Also, RBE values on which quality factors are based depend on response to low-LET radiation at low doses, a very difficult region for which to obtain reliable experimental data. Thus, we introduce a new concept, a fluence-related risk coefficient (F), which is the risk of a cancer per unit particle fluence and which we call the risk cross section. The total risk is the sum of the risk from each particle type: sigma i integral Fi(Li) phi i(Li) dLi, where Li is the LET and phi i(Li) is the fluence-LET spectrum of the ith particle type. As an example, tumor prevalence data in mice are used to estimate the probability of mouse Harderian gland tumor induction per year on an extra-magnetospheric mission inside an idealized shielding configuration of a spherical aluminum shell 1 g/cm2 thick. The combined shielding code BRYNTRN/GCR is used to generate the LET spectra at the center of the sphere. Results indicate a yearly prevalence at solar minimum conditions of 0.06, with 60% of this arising from charge components with Z between 10 and 28, and two-thirds of the contribution arising from LET components between 10 and 200 keV/micrometers.

Heavy-Ion Radiobiology: Normal Tissue Studies

Publisher Summary This chapter provides an overview of heavy-ion radiobiology. Some of the considerations of the rationale for the use of accelerated heavy charged particles include physical advantages provided to heavy particles by virtue of their depth–dose profiles, where an increased physical dose may be deposited at depth in tissue as compared to the plateau region of ionization, biological advantages provided by the increase in the relative biological effectiveness (RBE) of the heavy-ion beam at depth where a presumptive tumor may lie, significant decreases in the radiation resistance of hypoxic cells, evidenced by a decrease in the oxygen enhancement ratio (OER) for heavy-ion beams, and reduction in the influence of cell age in responses and production of potentially exploitable perturbations in cellular kinetics. A major emphasis of studies of accelerated heavy ions on normal tissues is to determine the nature of the response when multiple exposures are given. The spinal cord has often been used as a model for radiation-induced damage to the central nervous system, as functional impairment denoted by overt myelopathy and paralysis can be easily assessed as a function of total radiation dose.

Recovery from potentially lethal damage and recruitment time of noncycling clonogenic cells in 9L confluent monolayers and spheroids.

Cells that have been grown as multicell tumor spheroids exhibit radioresistance compared to the same cells grown in monolayers. Comparison of potentially lethal damage (PLD) repair and its kinetics was made between 9L cells grown as spheroids and confluent monolayers. Survival curves of cells plated immediately after irradiation showed the typical radioresistance associated with spheroid culture compared to plateau-phase monolayers. The dose-modification factor for spheroid cell survival is 1.44. Postirradiation incubations in normal phosphate-buffered saline (PBS), conditioned media, or 0.5 M NaCl in PBS reduced the differences in radiosensitivity between the two culture conditions. Postirradiation treatment in PBS or conditioned medium promoted repair of potentially lethal damage, and 0.5 M NaCl prevented the removal of PLD and allowed the fixation of damage resulting in lower survival. Survival of spheroid and monolayer cells after hypertonic NaCl treatment was identical. NaCl treatment reduced Do more than it did the shoulder (Dq) of the survival curve. PLD repair kinetics measured after postirradiation incubation in PBS followed by hypertonic NaCl treatment was the same for spheroids and for plateau-phase monolayers. The kinetics of PLD repair indicates a biphasic phenomenon. There is an initial fast component with a repair half-time of 7.9 min and a slow component with a repair half-time of 56.6 min. Most of the damage (59%) is repaired slowly. Since the repair capacity and kinetics are the same for spheroids and monolayers, the radioresistance of spheroids cannot be explained on this basis. Evidence indicates that the time to return from a Go (noncycling G1 cells) state to a proliferative state (recruitment) for cells from confluent monolayers and from spheroids after dissociation by protease treatment may be the most important determinant of the degree of PLD repair that occurs. Growth curves and flow cytometry cell cycle analysis indicate that spheroid cells have a lag period for reentry into a proliferative state. Since plating efficiency remains high and unchanging during this period, one cannot account for the delay on the basis of the existence of a large fraction of Go cells which are not potentially clonogenic. The cell cycle progression begins in 6-8 h for monolayer cells and in 14-15 h for spheroids. It is hypothesized that the slower reentry of spheroid cells into a cycling phase allows more time for repair than for the rapidly proliferating monolayer cells.

High energy beams of radioactive nuclei and their biomedical applications

Abstract It is possible to produce energetic beam of radioactive nuclei, as secondary beams, from the heavy-particle compound accelerator called BEVALAC. These beam can be focused into experimental areas without significant contamination using suitable magnetic filters and proper beam-optics. Properly selected high-energy beam of radioactive nuclei (those which decay by positron emission) can provide a truly unique opportunity to evaluate the effectiveness of these beam in localizing the Bragg peak on a tumor volume—necessary in heavy-particle therapy. Preliminary data are presented here to demonstrate the possible use of these beam in radiotherapy treatment-planning verification.

The RBE-LET relationship for rodent intestinal crypt cell survival, testes weight loss, and multicellular spheroid cell survival after heavy-ion irradiation.

This report presents data for survival of mouse intestinal crypt cells, mouse testes weight loss as an indicator of survival of spermatogonial stem cells, and survival of rat 9L spheroid cells after irradiation in the plateau region of unmodified particle beams ranging in mass from 4He to 139La. The LET values range from 1.6 to 953 keV/microns. These studies examine the RBE-LET relationship for two normal tissues and for an in vitro tissue model, multicellular spheroids. When the RBE values are plotted as a function of LET, the resulting curve is characterized by a region in which RBE increases with LET, a peak RBE at an LET value of 100 keV/microns, and a region of decreasing RBE at LETs greater than 100 keV/microns. Inactivation cross sections (sigma) for these three biological systems have been calculated from the exponential terminal slope of the dose-response relationship for each ion. For this determination the dose is expressed as particle fluence and the parameter sigma indicates effect per particle. A plot of sigma versus LET shows that the curve for testes weight loss is shifted to the left, indicating greater radiosensitivity at lower LETs than for crypt cell and spheroid cell survival. The curves for cross section versus LET for all three model systems show similar characteristics with a relatively linear portion below 100 keV/microns and a region of lessened slope in the LET range above 100 keV/microns for testes and spheroids. The data indicate that the effectiveness per particle increases as a function of LET and, to a limited extent, Z, at LET values greater than 100 keV/microns. Previously published results for spread Bragg peaks are also summarized, and they suggest that RBE is dependent on both the LET and the Z of the particle.

Quiescence in 9L Cells and Correlation with Radiosensitivity and PLD Repair

The onset of quiescence, changes in X-ray sensitivity, and changes in capacity for potentially lethal damage (PLD) repair of unfed plateau-phase 9L44 cell cultures have been systematically investigated. The quiescent plateau phase in 9L cells was the result of nutrient deprivation and was not a cell contact effect. Eighty-five to 90% of the plateau-phase cells had a G1 DNA content and a growth fraction less than or equal to 0.15. The cell kinetic shifts in the population were temporally correlated with a developing radioresistance, which was characterized by a larger shoulder in the survival curve of the quiescent cells (Dq = 5.71 Gy) versus exponentially growing cells (Dq = 4.48 Gy). When the quiescent plateau-phase cells were refed, an increase in radiosensitivity resulted which approached that of exponentially growing 9L cells. Delayed plating experiments after irradiation of exponentially growing cells, quiescent plateau-phase cells, and synchronized early to mid-G1-phase cells indicated that while significant PLD repair was evident in all three populations, the quiescent 9L cells had a higher PLD repair capacity. Although data for immediate plating indicated that 9L cells may enter quiescence in the relatively radioresistant mid-G1 phase, the enhanced PLD repair capacity of quiescent cells cannot be explained by redistribution into G1 phase. When the unfed quiescent plateau-phase 9L cells were stimulated to reenter the cell cycle by replating into fresh medium, the first G1 was extended by 6 h compared with the G1 of exponentially growing or refed plateau-phase 9L cells.(ABSTRACT TRUNCATED AT 250 WORDS)