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.

Cell density dependent plating efficiency affects outcome and interpretation of colony forming assays

BACKGROUND AND PURPOSE

The usefulness of colony forming assays (CFA) has been established for almost 40 years (Puck and Marcus, J.Exp.Med. 103: 653-666, 1956). Although time-consuming and not successful for all cell lines, it is generally considered to be the gold standard of assays for testing the sensitivity of cell lines to ionizing radiation or other cytotoxic agents in vitro. We recently found for several cell lines that the plating efficiencies of both control and irradiated cells is dependent upon the density of cells seeded for colony formation; that is, increasing cell inoculum levels resulted in a non-linear relationship with colony formation, even at relatively low colony numbers.

MATERIAL AND METHODS

All data from a human melanoma cell line, transfected with c-myc or N-ras, as well as from normal human diploid fibroblasts, were taken to see how this phenomenon influenced outcome and interpretation of clonogenic assays. Survival was recalculated using all data, or only data with a linear relationship between inoculum level and colony formation.

RESULTS

It is found that when data with a non-linear relationship between inoculum level and colony formation are included, survival can be underestimated due to inhibition of colony formation in treated cultures.

CONCLUSION

For validity, colony forming assays must be standardized to assure a constant relationship between the cell density and colony forming efficiency. This usually requires a much lower density of colonies than has been typically published for many cell survival studies.

Tumor volume and local control probability: clinical data and radiobiological interpretations

PURPOSE

It is well established for certain human tumor histologies that increasing tumor volume leads to a decreasing probability of tumor control. The simplest explanation for these findings is that the number of tumor clonogens that must be sterilized to control a tumor increases with tumor volume. In this investigation we consider whether clinical evidence favors a further hypothesis, namely, that clonogen number increases in direct proportion to tumor volume.

METHODS AND MATERIALS

Previously published data on the volume-cure relationship for breast tumors, neck nodes, malignant melanoma, and squamous cell carcinomas of the oropharynx and the uterine cervix were analyzed.

RESULTS

We found in all these data sets evidence that the effect of tumor volume on tumor control probability was less than what would be expected under the assumption of proportionality between number of clonogens and volume. We describe good reasons to believe that this is the result of patient-to-patient variability in radiocurability, and possibly other factors as well.

CONCLUSIONS

Clinical data do provide evidence for a highly significant reduction of tumor control probability with increasing tumor volume. However, because of heterogeneity in patient and tumor characteristics, the volume effect is less pronounced than would be expected from a simple proportionality between number of clonogens and volume. In principle this simple proportionality does hold in individual patients, so that standard approaches for treatment plan optimization in individuals may still be valid.

The clinical significance of ratios of radiobiological parameters

PURPOSE

Interindividual heterogeneity of the radiobiological characteristics of malignant and normal tissues hampers the derivation of radiobiological parameters from clinical data. Focusing on the ratio Dprolif, i.e., the dose to compensate 1 day of treatment interruption, this article investigates the hypothesis that ratios of parameters might be less sensitive to interpatient heterogeneity and may constitute a more reliable description of the radiobiological properties of tissues than the parameters themselves.

METHODS AND MATERIALS

Analytic calculations were performed in an idealized example in which the only source of heterogeneity was the number of clonogenic cells. Computer simulations were used to assess the effects of heterogeneity in radiosensitivity and in proliferative capacity. Treatment outcome was simulated in pseudopatients with increasing dose-time correlation.

RESULTS

Interindividual heterogeneity in clonogenic cell number, radiosensitivity, or proliferative ability results in a marked underestimation of the response parameters describing these processes. In contrast, the estimates of the ratio Dprolif were more stable. The coefficients of variation increased with increasing heterogeneity. However, this only became unacceptable when heterogeneity in radiosensitivity was marked, or when total dose and treatment time were closely correlated.

CONCLUSION

Parameter ratios may provide more robust radiobiological information than single parameters estimated from clinical data except when interindividual heterogeneity is very large or when the treatment modalities are too highly correlated. As usual, caution is advised in the presence of patient selection, a correlation between treatment prescription and expected outcome, or limited ranges of dose-time treatment patterns.

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.

The tumor volume and clonogen number relationship: tumor control predictions based upon tumor volume estimates derived from computed tomography

PURPOSE

While tumor volume is an important parameter predicting clinical outcome, its relationship to clonogen number remains uncertain. This uncertainty is related to many factors, among them treatment response heterogeneity, which obscures the influence of patients and treatment-related parameters. In this study, we analyze the effect of tumor volume on local and regional recurrence in a setting tightly controlled for dose, treatment time, and patient selection. The hypothesis that changes in clonogen number scale directly with changes in tumor volume is tested.

METHODS AND MATERIALS

Using digital reconstruction of diagnostic computed tomography (CT) scans, primary and total tumor volumes were estimated in 51 cases of advanced squamous cell carcinoma of the head and neck. All patients were managed with a concomitant boost accelerated superfractionated schedule to a median dose of 70.2 Gy. Clinical data were fitted to a mixture model to relate tumor volume parameters to control probability where volume and clonogen number were related by the relationship m = a.Vb, where m is initial clonogen number, a is a proportionality constant, V is tumor volume, and b is the volume exponent.

RESULTS

Tumor volume estimates for primary tumor ranged from 3-196 cm3 and for total tumor volume 5-196 cm3. Actuarial local-regional control is 63%. The estimated volume exponent b is 0.85 (95%, confidence interval (c.i.): 0.40-1.29) for primary tumor volume and 1.1 (95%, c.i.: 0.33-1.85) for total tumor volume.

CONCLUSION

This study quantifies the adverse influence of tumor volume on local-regional disease control in advanced head and neck cancer. The derived volume exponent approximates to one, the theoretical expectation if the growth fraction is roughly constant and clonogen number increases linearly with volume. Finally, these results suggest that radiobiological parameters are more reliably estimated from clinical data with narrowly defined strata.

Radiation-induced lung damage after thoracic irradiation for Hodgkin's disease: the role of fractionation

PURPOSE

to estimate the alpha/beta ratio for damage to human lung after thoracic irradiation for Hodgkin's disease.

PATIENTS AND METHODS

The criterion for lung injury was the presence of radiological changes in the vicinity of the mediastinum as assessed on regular follow-up chest X-ray examinations. Patients with supradiaphragmatic stage I-II Hodgkin's disease received mantle field irradiation as part of their treatment between 1964 and 1981 (E.O.R.T.C. protocols H1, H2, and H5). The total mediastinal doses fixed by the protocols were 35-40 Gy. The fractional doses were left to the decision of the physicians in charge: the most frequent regimens were 5 x 1.8, 5 x 2.0, 4 x 2.5 and 3 x 3.3 Gy per week. The data were fit to the linear-quadratic (L.Q.) model using time-to-injury as endpoint.

RESULTS

1048 (97%) of 1082 patients were evaluable. The mean follow-up duration was 8 years. One hundred and ninety-five cases of radiologically-visible lung damage were observed after a median interval of 6 months (range: 0-101). The 3-year actuarial probability of lung damage was 19% (95% confidence limits: 17, 21). Multivariate analysis (Cox model, stratified by protocol) showed an increased risk of damage with dose per fraction (relative risk, R.R. = 2.22 per Gy (1.75, 2.82)), the presence of systemic symptoms (R.R. = 1.53 (1.09, 2.15)), and total mediastinal dose (R.R. = 1.06 per Gy (1.01, 1.12)). Age, sex, histological type, number of involved nodal sites and radiotherapy duration did not significantly modify the risk of lung damage. The L.Q. model parameters were: alpha = 0.031 Gy-1 (0.003, 0.059), beta = 0.010 Gy-2 (0.007, 0.013), alpha/beta = 3.07 Gy (-0.23, 8.46).

CONCLUSION

this low alpha/beta ratio is consistent with late effects values from animals and humans, and illustrates the influence of large fraction sizes on the occurrence of late pulmonary complications.

Variations in radiation sensitivity and repair among different hematopoietic stem cell subsets following fractionated irradiation

The radiation dose-survival of various hematopoietic cell subsets in murine bone marrow (BM) was determined in the cobblestone area forming cell (CAFC) assay under conditions of single-, split-, and multiple-dose irradiation. A greater recovery in cell survival with decreasing dose per fraction, or increasing fraction number, was observed for primitive CAFC day-28 and day-35 than for CAFC day-6 and day-12 (colony-forming unit (CFU)-granulocyte macrophage and CFU-spleen day-12 equivalents). Linear quadratic (LQ) model analysis of CAFC survival data provided an estimate of the alpha/beta ratio that is an inverse index of the fractionation effect and is known to be lower for late than for acutely responding tissues. This analysis gave decreasing alpha/beta ratios with increasing primitiveness of the CAFC subset. These values were found to be comparatively low (about 4 Gy) for CAFC day-28 and day-35 and are in general agreement with previous studies on long-term repopulation in vivo. In contrast, alpha/beta ratios of CAFC day-6 and day-12 were relatively high (above 6 Gy) and are consistent with values obtained from acute marrow failure. Delayed harvesting of BM after a single dose of 6 Gy showed little evidence of proliferative repopulation over 1 week and hence the differential dose-sparing effect of fractionation among the CAFC subsets appears to be mostly attributable to the influence of sublethal damage repair. These results require a reevaluation of previous notions of marrow stem cell radiosensitivity and repair based on acute marrow lethality (LD50/30) or spleen colony (CFU-S) data, especially when applied to fractionated total body irradiation effects on long-term repopulating stem cells in a BM transplant setting.

Radiation nephropathy in the rhesus monkey: morphometric analysis of glomerular and tubular alterations

PURPOSE

The morphologic responses of the monkey kidney glomeruli and tubules to fractionated irradiation were assessed.

METHODS AND MATERIALS

Both kidneys of adult female rhesus monkeys were irradiated with doses of gamma-rays ranging from 24 Gy in 12 fractions up to 36 Gy in 18 fractions. Serial renal biopsies were taken between 1 and 12 weeks after irradiation. The kidneys were removed at necropsy 16-23 weeks after irradiation. Glomeruli were assessed for the presence of pathologic features, including intercapillary eosinophilic material, ectatic capillaries, thrombi, hemorrhage, adhesions, and sclerosis. The relative proportion of renal cortex occupied by glomeruli, interstitium, or tubules was determined using a Chalkley point grid. Tubules were further scored as being either normal or abnormal in appearance.

RESULTS

Examination of the renal biopsies revealed that progressive glomerular lesions were evident within 4-12 weeks after irradiation. Tubular changes were mild and focal. Morphometric analysis of whole kidneys removed at necropsy demonstrated that numbers of glomeruli with ectatic capillaries, thrombi, and hemorrhage were significantly different from controls at 16-23 weeks after irradiation by all of the doses in the range of 24 to 36 Gy. A significant (p < 0.05) increase in the relative proportion of renal cortex occupied by glomeruli and interstitium was indicative of tubule loss. Further analysis of these tubular changes revealed a highly significant (p < 0.001) dose-dependent increase in the proportion of abnormal to normal tubules. Thus following a dose of 24 Gy in 12 fractions, the ratio of abnormal: normal tubules was approximately 1:2; after 36 Gy in 18 fractions the ratio was 3:1.

CONCLUSIONS

Glomeruli appeared to be very radiosensitive because after the clinically relevant dose of 24 Gy in 12 fractions essentially all glomeruli were altered in the irradiated kidneys as compared to controls. Thus, efforts aimed at increasing the threshold dose for development of radiation nephropathy should be directed primarily at preventing the glomerular lesions.

A 100-year Nordic perspective on the dose-time problem in radiobiology

The therapeutic use of x-rays began almost immediately after their discovery by Röntgen, and within a few years two Swedish physicians could report the first successful treatment of human skin cancer by radiotherapy. Almost from the start it was clear that the biological effect of ionizing radiation depended critically on the exact distribution of the dose in time. The present paper reviews the historical development of dose-time concepts in radiotherapy as seen from a Nordic perspective. Among the topics reviewed are the discussion of single versus fractionated doses, Strandqvist's thesis and the development of power-law biological dose formulas, the effect of dose per fraction and of overall treatment time. It is only within the last 10-15 years that biologically and clinically important dissociation between the radiobiology of early- and late-responding human normal tissues has been appreciated. Biological developments have led to the proposal of altered fractionation schedules, hyperfractionation and accelerated fractionation, that are currently undergoing clinical trial.

Repair rate in mouse lung after clinically relevant radiation doses per fraction

Data published previously have shown that repair of sublethal damage in mouse lung proceeds with two significantly different repair half-times of 0.4 h and 4.0 h and that the fast component has approximately four times more weight than the slow component. None of these data, however, were obtained after small dose fractions similar to those used in clinical radiotherapy. The purpose of the experiments presented here was to determine the half-time of the fast component only of repair in mouse lung after doses per fraction of 2.0 Gy. We irradiated the whole thoraces of mice with six equal doses of 2.09 Gy given at intervals ranging from 0 to 45 min. The dose was topped up 24 h later by a range of single doses designed to bring the response, i.e. breathing rate and death from pneumonitis, into the observable range. Data on breathing rate were converted into quantal response data. All data were analyzed by the linear-quadratic model that contains two rates of repair (H.D. Thames et al., Radiother, Oncol. 15, 49-53, 1989). The data showed that the repair rate is very rapid, giving a t1/2 ranging from 0.25 to 0.75 h for breathing rate and mortality, in agreement with our data published previously for higher dose fractions. There were no differences between the t1/2's obtained from the two assays of damage. These data indicate that the half-time of the fast component of repair in mouse lung is approximately 0.4 h after clinically relevant dose fractions.

Chronic radiation damage in the rat rectum: an analysis of the influences of fractionation, time and volume

PURPOSE

Analysis of four different sets of experiments performed by the G.S.F. group in Munich investigating the late tolerance of the rat rectum to external or intracavitary irradiation.

MATERIAL AND METHODS

The endpoint was late rectal stenosis in female Wistar rats. The raw data were fitted to the linear-quadratic model by means of a likelihood maximization method (Direct Analysis). The model was altered to allow for repopulation, incomplete repair, and varying irradiated lengths of the rectum.

RESULTS

Fractionation sensitivity was high or intermediate (alpha/beta ratio values [95% confidence limits] ranging from 2.67 [0.86, 4.80] to 6.65 [2.21, 11.73] Gy). Significant repopulation occurred when treatments were longer than 5 days (Dprolif equal to 0.61 [0.20, 1.47] and 1.08 [0.58, 1.90] Gy/day, in fractions of 4 Gy). Another interpretation is that radiosensitivity changed during treatment. Repair half-time estimates ranged between 1.84 [1.52, 2.34] and 5.02 [2.83, 21.7] h. Finally, the present analysis indicated that the smallest surviving compartment capable of tissue rescue was about 1/50 to 1/100 of a 1 cm high cylinder of the rectum wall.

CONCLUSIONS

The radiobiological features of late stenosis in the rats are consistent with combined injuries of early and late responding components of the rectal wall. This raises some concerns about the possible danger of hyperfractionated treatments, where the beneficial impact of fraction size reduction may be obviated for interfraction intervals that are too short. Also, accelerated irradiation may result in more late complications because of increased early reactions.

Radiation response of the monkey kidney following contralateral nephrectomy

PURPOSE

The long-term functional and morphologic responses of the hypertrophied monkey kidney after unilateral nephrectomy to fractionated irradiation were assessed.

METHODS AND MATERIALS

The right kidney of 13 adult female rhesus monkeys was removed. Twelve weeks after unilateral nephrectomy (UN) the remaining kidney received fractionated doses of gamma-rays ranging from 35.2 Gy/16 fractions (F) up to 44 Gy/20 F. Glomerular filtration rate, effective renal plasma flow, blood urea nitrogen, serum creatinine, and hematocrit values were measured up to 107 weeks postirradiation (PI). The monkeys were killed and the remaining kidneys were removed 107 weeks PI or earlier when end-stage renal failure was exhibited. Glomeruli were scored for the presence/absence of several pathologic features including increased intercapillary eosinophilic material (ICE), ectatic capillaries, and thrombi. The relative proportion of renal cortex occupied by glomeruli, interstitium, normal tubules or abnormal tubules was determined using a Chalkley point grid. These quantal dose response data were analyzed using a logistic regression model.

RESULTS

Irradiation of the remaining kidney in UN monkeys resulted in a dose-dependent reduction in renal function and anemia. Glomerular dysfunction preceded tubular dysfunction. Animals receiving 44 Gy all manifested progressive clinical renal failure. Conversely, those receiving < or = 39.6 Gy showed stable, albeit impaired, renal function for the duration of the observation period of 107 weeks. Morphologically, the incidence of ICE, ectatic glomerular capillaries, thrombi, and periglomerular fibrosis was significantly dose-related (p < 0.005). A significant (p < 0.001) dose-related increase in the relative proportion of renal cortex occupied by abnormal tubules was indicative of tubular injury. A highly significant (p < 0.001) dose-dependent increase in the proportion of abnormal to normal tubules was also seen.

CONCLUSION

The pathogenesis of radiation nephropathy is difficult to fully understand because of the complex and dynamic interactions among all components of the nephron that make discrimination between primary radiation effects and secondary pathophysiological consequences very difficult. Notwithstanding, the current experiment shows that the functional and morphological expressions of radiation injury in the kidney are dose dependent. Renal failure occurs when both the glomeruli and tubules are dysfunctional. In monkeys following UN, a total dose of 44 Gy to the remaining kidney damages all components of the nephron and causes renal failure in less than 45 weeks. With lower doses, changes to the glomeruli predominate and the animals survive. Kidney doses of up to 39.6 Gy/18 fractions of 2.2 Gy are compatible with survival for at least 2 years in primates.

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.

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.