From facial images of different quality to score based LR.

A simple method is proposed to assess the quality of a trace facial image in the context of the facial recognition system used using the similarity scores with low quality different-source facial images, defined as the Confusion Score (CS). Methods are proposed to calculate the probability of finding the correct facial image in a database using low quality images for investigational purposes using the CS, as well as calculation of the Likelihood Ratio (LR) for comparison of low quality trace facial images with good quality reference facial images, based on the assessed CS of the trace image. Improvement of performance of an LR-system using training datasets stratified on CS over the use of pooled data is demonstrated. Examples of using the proposed approach in simulated case examples are presented.

Growth characteristics of glioblastoma spheroids.

Cell spheroids have been proposed as models of early tumor growth from which a better understanding of tumor cell heterogeneity and its effects on treatment response might be gained. Results of experiments performed to understand the underlying dynamics of cell growth within a spheroid formed by SNB19, a high-grade glioblastoma cell line, are presented. We discuss the spatiotemporal distribution of the cells and their cell cycle status based on physical measurements, immunohistochemistry, and flow cytometry analysis. The size of the spheroids and their growth rates were dependent on the initial cell number, the proliferation was mostly limited to the outermost region as the spheroids grew in size, and the number of dead cells increased with age and size as well. Interestingly, though the population of the proliferating cells became localized to the outer rim as spheroids grew, the fraction of proliferating cells did not change drastically. Also, our data reveal that the calculated density varied with respect to age of the spheroid as well as position within the spheroid. We show that a simple exponential model is not adequate for modelling the growth characteristics that have been seen by these experiments. In contradiction to available studies, we report that an acellular (necrotic) center appeared and then disappeared during the period of investigation. Furthermore, after the acellular region disappeared, a few proliferative cells appeared in the center area, raising many questions about the growth-related dynamics of the spheroids formed by this particular cell type.

Quantification of histochemical staining by color deconvolution.

OBJECTIVE: To develop a flexible method of separation and quantification of immunohistochemical staining by means of color image analysis. STUDY DESIGN: An algorithm was developed to deconvolve the color information acquired with red-green-blue (RGB) cameras and to calculate the contribution of each of the applied stains based on stain-specific RGB absorption. The algorithm was tested using different combinations of diaminobenzidine, hematoxylin and eosin at different staining levels. RESULTS: Quantification of the different stains was not significantly influenced by the combination of multiple stains in a single sample. The color deconvolution algorithm resulted in comparable quantification independent of the stain combinations as long as the histochemical procedures did not influence the amount of stain in the sample due to bleaching because of stain solubility and saturation of staining was prevented. CONCLUSION: This image analysis algorithm provides a robust and flexible method for objective immunohistochemical analysis of samples stained with up to three different stains using a laboratory microscope, standard RGB camera setup and the public domain program NIH Image.

Growth factors: biological and clinical aspects.

PURPOSE: The purpose of this meeting summary is to provide an overview of cytokine research and its role in radiation oncology. METHODS AND MATERIALS: The sixth annual Radiation Workshop was held at the International Festival Institute at Round Top, TX. RESULTS: Presentations of seventeen speakers provided the framework for discussions on the biological and clinical aspects of cytokine research. CONCLUSION: Orchestration of coordinated cellular responses over the time course of radiation effects requires the interaction of many growth factors with their receptors as well as cell-cell and cell-matrix interactions. Cytokine networks and integrated systems are important in tumor development, cancer treatment, and normal and tumor response to cancer treatment.

Induction of transforming growth factor alpha in irradiated mouse jejunum.

PURPOSE: To determine the involvement of the mitogenic growth factors transforming growth factor alpha (TGF alpha), epidermal growth factor (EGF), and the EGF receptor (EGF-R) in the proliferative response after irradiation of the mouse jejunum. METHODS AND MATERIALS: C3Hf/Kam mice were whole-body irradiated with 5 and 11 Gy 250 kV X rays. Mice were killed 1-10 days after irradiation, and immunohistochemistry, in situ hybridization (ISH), and RNase protection assays were performed. RESULTS: Damage to the jejunal crypts caused by irradiation resulted in a strong proliferative response 1-5 days after 5 Gy and 3-6 days after 11 Gy. Expression of TGF alpha, EGF, and EGF-R increased at 1-2 days and decreased at 4-8 days after 5- or 11-Gy irradiation. Also, TGF alpha mRNA increased during the early phase of the proliferative response (1-2 days after 5 or 11 Gy) followed by a decrease at 4 days after 5 Gy and 8 days after 11 Gy. CONCLUSION: These data indicate that, at the beginning of the proliferative response after irradiation, the transcription of TGF alpha mRNA is increased, and that it is inhibited just before compensatory proliferation decreases. Thus, active regulation of TGF alpha expression takes place at least at the transcriptional level, resulting in upregulation of TGF alpha production and increased TGF alpha levels in the crypts during the proliferative response.

Diurnal variations in the expression of radiation-induced apoptosis.

Experiments were performed to determine whether diurnal variations in apoptosis in the mouse small intestine after irradiation with 2.5 Gy gamma rays depended on the time of day that the mice were irradiated, the time of day that the mice were sacrificed or the interval between irradiation and sacrifice. Experiments were performed with a 12-h light:dark regimen with the light period from 6:00 to 18:00 h. With fixed intervals of 6 h and 24 h between irradiation and sacrifice, a peak in induced apoptosis (16%) was observed in mice sacrificed at 8:00 h, two times higher than the nadir of response at 23:00 h (8%). When variable intervals were used between irradiation and measurement of apoptosis, i.e. sacrifice, at 8:00 h or 23:00 h, the induced apoptosis was dependent on the interval, with a peak for 18-h intervals. However, the level of apoptosis was always about twofold higher when measured at 8:00 h than at 23:00 h. No correlation was observed between diurnal variations in apoptosis and survival of mouse intestinal crypts. The diurnal variations in apoptosis after irradiation can be interpreted either in terms of expression of apoptosis during the G2/M phase of the cell cycle in partially synchronized cells, or in terms of a systemic mechanism such as diurnal variation in the neurohormone melatonin.

Tolerance of rat spinal cord to continuous interstitial irradiation.

PURPOSE: To study the kinetics of repair in rat spinal cord during continuous interstitial irradiation at different dose rates and to investigate the impact of a rapid dose fall off over the spinal cord thickness. MATERIAL AND METHODS: Two parallel catheters were inserted on each side of the vertebral bodies from the level of T10 to L4. These catheters were afterloaded with two 192Ir- wires of 4 cm length each (activity 1-10 mCi/cm) or connected to the HDR- microSelectron. Experiments have been carried out to obtain complete dose response curves at 7 different dose rates: 0.53, 0.90, 1.64, 2.56, 4.4, 9.9 and 120 Gy/h. Paralysis of the hindlegs after 5 - 6 months and histopathological examination of the spinal cord of each animal were used as experimental endpoints. RESULTS: The distribution of the histological damage was a good reflection of the rapid dose fall - off over the spinal cord, with white matter necrosis or demyelination predominantly seen in the dorsal tracts of the spinal cord or dorsal roots. With each reduction of the dose rate, spinal cord tolerance was significantly increased, with a maximum dose rate factor of 4.3 if the dose rate was reduced from 120 Gy/h to 0.53 Gy/h (ED50 of 17.3 Gy and 75.0 Gy, respectively). Estimates of the repair parameters using different types of analysis are presented. For the direct analysis the best fit of the data was obtained if a biexponential function for repair was used. For the 100% dose prescribed at the ventral side of the spinal cord the alpha/beta ratio is 1.8 Gy (0.8 - 2.8) and two components of repair are observed: a slow component of repair of 2.44 h (1.18 - infinity) and a fast component of 0.15 h (0.02 - infinity). The proportion of the damage repaired with the slow component is 0.59 (0.18 - 1). For the maximum of 150% of the prescribed dose at the dorsal side of the spinal cord the alpha/beta ratio is 2.7 Gy (1.5 - 4.4); the two components for the kinetics of repair remain the same. CONCLUSIONS: Spinal cord radiation tolerance is significantly increased by a reduction in dose rate. Depending on the dose prescription, the alpha/beta ratio is 1.8 or 2.7 Gy for the 100% and 150% of the reference dose (rate), respectively; for the kinetics of repair a biphasic pattern is observed, with a slow component of 2.44 hours and a fast component of 0.15 hours, which is independent of the dose prescription.

Quantification of immunohistochemical staining by color translation and automated thresholding.

OBJECTIVE: To develop a method of quantitating immunohistochemical staining using diaminobenzidine (DAB) and hematoxylin by means of color image analysis. STUDY DESIGN: Red-green-blue (RGB)-filtered grayscale values from images from microscopic slides of mouse jejunum, stained with DAB, hematoxylin or both were analyzed using the public domain program NIH Image. Based on the correlations between the R-G- and B-filtered grayscale values, a simple translation algorithm using the RGB information was developed, providing the option for separation of DAB only- and double-stained areas from hematoxylin only-stained areas by means of automated thresholding. The method was tested by staining mouse jejunum for the growth factors EGF, TGF-alpha and TGF-beta 1-3 using immunohistochemical techniques. RESULTS: A good separation of DAB- and double-stained pixels from hematoxylin-stained pixels was achieved, with misclassification of only 2.4% of the pixels as compared to 34% misclassification using automated thresholding of the blue component of the RGB image, the untransformed grayscale images with the most contrast for DAB- and non-DAB-stained areas. Significant differences in relative areas stained and mean specific optical density for the growth factors in mouse jejunal crypts and villi were observed. CONCLUSION: The image analysis method described offers the possibility of objective determination of stained area in histologic slides with the commonly used DAB and hematoxylin chromophores. It shows that reproducible and objective measurements can be made based on RGB true color images acquired using a simple microscope and video camera setup and the public domain program NIH Image.

The effect of proliferative status and clonogen content on the response of mouse jejunal crypts to split-dose irradiation.

The rate of cell production in hierarchical tissues is affected by the differentiation probability after each clonogen division and the frequency with which divisions take place. We have modified the latter by a high-fiber diet, which caused an increase in the BrdUrd labeling index (LI) in jejunal crypts of mice, and have tested for a change in tolerance using the in vivo colony assay. Clonogens were counted using the in vivo colony assay of crypt regeneration with Poisson correction. The LI was estimated by counting BrdUrd-labeled cells in longitudinal sections of complete crypts. Arrest in mitosis induced by injection of paclitaxel was used to test for a difference in the rate of crypt cell production in mice fed low- and high-fiber diets. Split-dose studies were used to test whether the change in proliferative status of the crypts was accompanied by changes in either the number of clonogens per crypt or their radiosensitivity, or an increased proliferative response to radiation-induced cell killing. We found an increase in the crypt LI induced by the high-fiber diet was 15-25% and was dependent on the time of day. The data on arrest in mitosis did not demonstrate a difference in cell production rates based on diet, possibly because of insufficient resolution of the assay. We conclude that the high-fiber diet had no effect on radiosensitivity, the number of clonogens per crypt (again, within the resolution of the assay) or the "repopulated dose," the dose represented by the shift in the dose-response curve for 2.5 days relative to that for 6 h. When the number of clonogens at the start of rapid proliferation was different (on account of different first doses), the repopulated dose was the same when 5 Gy X rays was given first but was higher for the animals on the high-fiber diet when 12 Gy was given first. The high-fiber diet caused an increase in the LI in the crypts that was not accompanied by any change in radiosensitivity or, within the resolution of the assay, numbers of clonogens per crypt. The increased LI also did not result in an increase in clonogen repopulation between split (and equal) doses. However, in split-dose experiments where the first dose was higher and as a consequence the number of clonogens at the start of the proliferative response was lower, there was evidence of a higher rate of clonogen production with the high-fiber diet than with the low-fiber diet.

Spatial and temporal patterns of expression of epidermal growth factor, transforming growth factor alpha and transforming growth factor beta 1-3 and their receptors in mouse jejunum after radiation treatment.

The goal of the present study was to assess changes in proliferation in the mouse jejunum after irradiation and the role of the growth factors EGF, TGF-alpha and TGF-beta 1-3 in the proliferative response. Our working hypothesis was that feedback signals from the villus to cells in the crypt regulate proliferation, and that the growth factors EGF and TGF-alpha with their common receptor EGF-R are involved in stimulation of proliferation, while the growth factors TGF-beta 1-3 with their receptors TGF-beta RI and TGF-beta RII are involved in inhibition of proliferation during this regulation. Immunohistochemical detection methods and automated image analysis were used for objective quantification of growth factor expression. The data indicate that, after 5 Gy irradiation, growth stimulation in the crypts takes place before major changes in the villi are observed. However, the combination of the reduction in the cell number, the number of cells expressing TGF-beta 1-3 and the reduction in the level of expression of TGF-beta 1-3 in the villi may cause the release of crypt cells from regulatory growth inhibition and initiate a proliferation-stimulating signal by an increase in the production of TGF-alpha and EGF. Regulation of proliferation after initiation of a proliferative response seems to be related more to the growth factors EGF, TGF-alpha and TGF-beta 3 in the crypts than to villus cellularity or growth factor expression, supporting the concept of stem cell autoregulation as a mechanism of cell regeneration in the intestinal crypt.

Strain difference in jejunal crypt cell susceptibility to radiation-induced apoptosis.

Levels of radiation-induced jejunal crypt cell apoptosis were compared in C57BL/6J, C3Hf/Kam and C3H/HeJ mice. Apoptosis levels were consistently lower in the C3H strains than in C57BL/6J. Although other explanations are possible, the strain difference is most likely to have a genetic basis, and in fact a preliminary analysis of the F2 progeny of C3H/HeJ and C57BL/6J mice indicates that more than one gene is involved. Both C3H strains also had lower levels of radiation-induced thymocyte apoptosis than C57BL/6J mice. Jejunal crypt cell apoptosis levels did not co-segregate with thymocyte apoptosis levels in the F2 progeny of C57BL/6J and C3H/HeJ mice. These results imply that the genes responsible for the difference in radiation-induced thymocyte apoptosis levels between these two strains are not the same as those responsible for the strain difference in radiation-induced jejunal crypt cell apoptosis levels. The experiments reported here identify strain-specific differences in levels of radiation-induced crypt cell apoptosis and are a first step towards identifying genetic polymorphisms that influence sensitivity of the small intestine to damage from ionizing radiation.

Accelerated repopulation during fractionated irradiation of a murine ovarian carcinoma: downregulation of apoptosis as a possible mechanism.

PURPOSE: To test whether accelerated tumor clonogen repopulation occurs during continuous fractionated radiotherapy of a slow-growing mouse ovarian tumor, and if so whether the accelerated rate of repopulation is predicted by the pretreatment potential doubling time, and whether changes in apoptotic response are a possible mechanism for this change. METHODS AND MATERIALS: The rate of clonogen production during fractionated radiotherapy was followed using the tumor-control assay, with an independent determination of the sensitivity to repeated dose fractions in vivo in the absence of repopulation. The pretreatment potential doubling time was measured by bromodeoxyuridine (BrdUrd) labeling and fluorescence measurements. The apoptotic and mitotic indices at various times during treatment were scored histologically. RESULTS: The slow-growing (pretreatment volume doubling time 6 days) ovarian tumor OCA responds to daily irradiation with 6 Gy under hypoxia by negligible tumor clonogen production in the first few days, followed by a change at about 9 days to accelerated repopulation, after which the effective clonogen doubling time Tclon was about 2 days, near the pretreatment Tpot of 1.7 days. Alternative interpretations of the data, such as a change in radiosensitivity vs. a change in the repopulation rate or acceleration at 3 days as opposed to 9 days, were shown to be unlikely. This change was accompanied by a reduced apoptotic response (measured morphometrically). CONCLUSIONS: When sensitivity to fractionated doses has been corrected for in vivo, this slow-growing mouse tumor exhibits a change to accelerated clonogen production during a continuous radiotherapy regimen that is accompanied or preceded by a reduced histologic apoptotic response. Tclon during accelerated repopulation was slightly longer than the pretreatment Tpot.

Changes in clonogen number and radiation sensitivity in mouse jejunal crypts after treatment with dimethylsulfoxide and retinoic acid.

Retinoic acid (RA) and the drug carrier dimethylsulfoxide (DMSO) have been shown to reduce cellular radiation sensitivity in vitro because of their hydroxyl radical scavenging properties. Both agents have also been shown to induce differentiation in vitro and in vivo. As intestinal crypts are multicellular systems, crypt survival after irradiation depends not only on the cellular sensitivity of the clonogenic cells, but also on the number of clonogenic cells in each crypt, which may be changed by treatments with agents which induce differentiation. In the present experiments we examined the effects of DMSO and RA on the radiosensitivity of mouse jejunal crypts in vivo using the microcolony assay. Mice were treated with five daily intraperitoneal doses of 0-500 microgram RA in 0.1 ml DMSO per mouse, the last dose applied 4 h before the start of irradiation. The results showed a clear protection by 100 and 500 micrograms/day RA in 0.1 ml DMSO for crypt survival over the dose range of 9-16 Gy. The D0 was increased from 1.30 Gy for untreated controls to 1.59 Gy after treatment with DMSO alone, and to 1.85 Gy after treatment with 100 micrograms/day RA in DMSO. Split-dose experiments showed a reduction in the number of clonogens by a factor of about 2 from DMSO treatment alone, with no additional effect of RA on the number of clonogens. Despite this reduction, the number of BrdUrd-labeled cells per crypt remained roughly the same, as did the count of cells per longitudinal villus section. We conclude that, in addition to the protective effects of RA in DMSO, there is induced differentiation of crypt clonogens which is counteracted by increased proliferative activity of transit cells with the result that villus cellularity is maintained.

Repair capacity and kinetics for human mucosa and epithelial tumors in the head and neck: clinical data on the effect of changing the time interval between multiple fractions per day in radiotherapy.

Early mucosal reactions are a major concern in radiotherapy of tumors in the head and neck. With some of the current strategies for altered fractionation these reactions may become dose limiting, and this has created a renewed interest in their radiobiology. The present paper reviews data on the response of mucosa in the head and neck to a change in dose per fraction (repair capacity) and time interval between dose fractions (repair kinetics). A review of clinical data on repair capacity shows that the alpha/beta ratio of the linear-quadratic model is high, around 10 Gy. Also, the steepness of the dose-response curve for mucositis is high as quantified by the maximum value of the normalized dose-response gradient, gamma 50 = 6.9 +/- 2.1. Computer simulations illustrate how this high value may be explained by the structural and proliferative organization of the mucosa. Finally, data from the following four recent studies are analyzed: the Radiation Therapy Oncology Group studies 7913 and 8313, a study from the M.D. Anderson Cancer Center, and a study from the Center of Oncology in Warsaw. All of these studies showed a decrease in the incidence of mucosal reactions when the interval between fractions given in the same day was increased, typically from around 4 h to around 6 h. It is shown that the maximum dose-equivalent of repair in the time interval between 4 h and 6 h occurs for a repair halftime of about 3.2 h. A comparison between the steepness of dose-response curves from studies not involving incomplete repair and those derived from the above four studies shows that the repair halftime for human mucosa must be relatively long, probably in the range 2 to 4 h. The statistical resolution of the available data as well as a number of caveats in the design of the studies preclude a more rigorous estimation of the repair halftime for mucosa. Although the tumor data are less conclusive, a similar repair halftime cannot be excluded for epithelial tumors in the head and neck.

Effect of intra-peritoneal fludarabine on rat spinal cord tolerance to fractionated irradiation.

The effect of fludarabine (9-beta-D-arabinosyl-2-fluoroadenine-5'- monophosphate), an adenine nucleoside analogue, on the tolerance of the spinal cord to fractionated irradiation was studied in a rat model. Anesthetized female Fisher 344 rats received irradiation to 2 cm of the cervical spine with a telecobalt unit (dose rate 1.14 Gy/min). Radiation was administered in two, four or eight fractions spread over a 48-h period with or without fludarabine. Animals assigned to combined therapy received two daily intraperitoneal injections of fludarabine (150 mg/kg) given 3 h prior to the first daily radiation fraction. It was found that fludarabine reduced the iso-effect dose required to induce leg paresis at 9 months after irradiation for all fractionation schedules. Dose modification factors of 1.23, 1.29 and greater than 1.27 were obtained for two, four and eight fractions, respectively. Fitting the data with the direct analysis method of Thames et al. with an incomplete repair model [18] showed that the potentiating effect of fludarabine may be mediated through reduction in the number of 'tissue-rescuing units' (InK). Alpha and beta values were slightly but not significantly decreased, whereas the alpha/beta ratio was unchanged. These features suggest that fludarabine did not significantly inhibit cellular repair processes but rather reduced the spinal cord tolerance by a fixed additive toxic effect on the same target cells. In rodent models, the combination of fludarabine and fractionated radiation has previously been found to yield a therapeutic gain, i.e., the drug enhanced tumor response to a greater extent than it reduced normal tissue tolerance.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the radiation sensitivity of mouse skin during fractionated and prolonged treatments.

Reactions of the skin of the right thigh of mice were used as an experimental model to test possible changes in the radiosensitivity of mouse skin, as represented by changes in the linear-quadratic (LQ) model parameters alpha and beta, as a function of fractionation interval and overall treatment time. In the first series of experiments, variable numbers of 3-Gy fractions with intervals of 6, 24 or 48 h were applied, followed by top-up doses to increase the skin damage to a level that could be scored. The results showed that mouse skin is more sensitive to 3-Gy fractions applied with 48-h intervals than to 3-Gy fractions applied with 6- or 24-h intervals. In the second series of experiments we used single-dose or fractionated test treatments for previously unirradiated mice and mice treated with priming doses of 10, 20 or 30 Gy given 1-18 days before the test treatment. The sensitivity appeared to be higher after intervals of 14-18 days than after 1-10 days after priming treatments of 20 and 30 Gy. The increased sensitivity 18 days after 20 Gy was mainly the result of an increase in the beta component of the LQ model; higher values of alpha were also determined. We conclude that the radiosensitivity of mouse skin is higher during a radiation-induced proliferative response.

Radiation response of the rat cervical spinal cord after irradiation at different ages: tolerance, latency and pathology.

PURPOSE: The investigation of the age dependent single-dose radiation tolerance, latency to radiation myelopathy, and the histopathological changes after irradiation of the rat cervical spinal cord. METHODS AND MATERIALS: Rats, ages 1-18 weeks, were irradiated with graded single doses of 4 MV photons to the cervical spinal cord. When the rats showed definite signs of paresis of the forelegs, they were killed and processed for histological examination. RESULTS: The radiation dose in paresis due to white matter damage in 50% of the animals (ED50) after single dose irradiation was about 21.5 Gy at all ages > or = 2 weeks (mean 21.4 (mean 21.4 Gy; 95% CI 21.0, 21.7 Gy). Only the ED50 at 1 week was significantly lower (19.5 Gy; 18.7, 20.3 Gy). The latency to the development of paresis clearly changed with the age at irradiation, from about 2 weeks after irradiation at 1 week to 6-8 months after irradiation at age > or = 8 weeks. The white matter damage was similar in all symptomatic animals studied. The most prominent were areas with diffuse demyelination and swollen axons, often with focal necrosis, accompanied by glial reaction. This was observed in all symptomatic animals, irrespective of the age at irradiation. Expression of vascular damage appeared to depend on the age at irradiation. No vascular damage was observed in the rats irradiated at 1 week, clearly altered blood vessels were seen in animals symptomatic 10 weeks after irradiation at > or = 3 weeks, and vascular necrosis occurred after > or = 6 months in some rats irradiated at > or = 8 weeks. CONCLUSION: Although the latency to myelopathy is clearly age dependent, single dose tolerance is not age dependent at age > or = 2 weeks in the rat cervical spinal cord. The white matter damage is similar in all symptomatic animals studied, but the vasculopathies appear to be influenced by the age at irradiation. It is concluded that white matter damage and vascular damage are separate phenomena contributing to the development of radiation myelopathy, expression of which may depend on the radiation dose applied and the age at irradiation.

Comparison of continuous and pulsed low dose rate brachytherapy: biological equivalence in vivo.

PURPOSE: Recent studies of human cell lines cultured in vitro and mathematical modeling of the response of acute and late responding tissues have predicted conditions for the equivalence in terms of cell killing of continuous and pulsed dose rate brachytherapy. The aim of this study was to test these predictions in vivo using an acutely responding normal tissue. METHODS AND MATERIALS: The microcolony assay was used to quantify the survival of jejunal stem cells in vivo. Mice were exposed to graded doses of 60Co delivered continuously or as 1- or 10-min pulses given once-per-hour at an average dose rate of 0.7 Gy/hr. In both cases the total dose-per-hour was 0.7 Gy. Overall exposure times ranged between about 30 and 60 h. Mice were sacrificed 3.5 days after exposure, the bowel removed for routine histological preparation, and number of surviving crypts quantified microscopically. RESULTS: An average dose-per-hour of 0.7 Gy, a pulse width of 10 min, and a pulse frequency of 1 h resulted in biological equivalence of pulsed to continuous treatment. Delivering the pulse in a period of 1 min at a dose rate 10-fold higher resulted in a modest 3-4% shift in the survival curve to lower isoeffective doses. The slopes of the survival curves as described by D(o) values were similar for all treatment regimens tested. CONCLUSION: This in vivo study validates the prediction of biological equivalence between pulsed and continuous brachytherapy at a clinically relevant average dose rate and may generate further interest in this new treatment modality because of its advantages in radiation protection, dose optimization, and cost relative to standard low dose rate brachytherapy techniques.

Repair kinetics of radiation damage in the developing rat cervical spinal cord.

The kinetics of repair of sublethal radiation damage was examined in the cervical spinal cord of developing, 1-week-old rats. Split-dose irradiation treatments were given with time intervals of 0.5-96 h. The data, supplemented with fractionation data from previous experiments, were analysed using direct analysis based on the incomplete repair (IR) model. The best fit to the monoexponential repair model resulted in an estimated half-time of repair (T1/2) of 1.5 (1.3-1.8) h. No indications of a slow or second component of repair could be detected in the 1-week-old cervical spinal cord. This is in contrast with literature reports of experiments with the adult rat cervical spinal cord, suggesting bi-exponential repair, with 65% of the damage repaired with a slow repair T1/2 of about 4 h. Two-step methods, although statistically inferior to direct analysis, are still in use for analysis of repair experiments. A number of two-step methods used for data analysis in previous reports concerning sublethal damage repair, are dose (un)repaired, proportion of dose unrepaired, and proportion of damage unrepaired. It is argued that of the methods discussed, only analysis of the data expressing the results as proportion unrepaired damage (delta Eu) and using split-dose experiments, does not result in introduction of an artificial second repair T1/2 in tissues with a high fractionation sensitivity. Two-step analysis of the present data using delta Eu suggested monoexponential repair with a T1/2 value of 1.5 (SE 0.2) h.