Radiosensitivity Of Fibroblasts Research Articles

Radiation therapy tolerance limits. For one or for all?--Janeway Lecture.

The optimal use of radiation therapy for cancer treatment is hampered by the application of tolerance limits of normal tissues derived empirically from population averages. Such limits do not reflect the considerable differences from patient to patient in susceptibility to late radiation sequelae. Assays that accurately predict normal tissue tolerance in individual patients would permit real application of the concept of treatment to tolerance thereby increasing the probability of an uncomplicated cure for the population as a whole. A summary of laboratory research is presented to test the hypothesis that the cellular radiosensitivity of normal skin fibroblasts can predict the severity of late connective tissue damage that develops following radiotherapy. The pathogenesis of radiation reactions and the possible role of radiation induced cellular senescence in the development of clinical late effects are briefly reviewed. Although the pathogenesis of radiation injury is highly complex, several clinical studies have demonstrated a significant correlation between fibroblast radiosensitivity and the severity of late sequelae from treatment. However, the precision and reproducibility of fibroblast cell survival assays are inadequate for routine clinical use. Newer assays incorporating insights into the effects of radiation on cellular senescence and cytokine production are being developed. Such assays may, in the future, be complemented or replaced by molecular and/or cytogenetic probes to derive robust estimates of individual tolerance. The principle of prediction of tolerance to radiotherapy has been established. Although current assays lack the precision required for clinical use, the goal of individualized treatment to tolerance ultimately should be achieved.

How much could the radiotherapy dose be altered for individual patients based on a predictive assay of normal-tissue radiosensitivity?

Predictive assays are presently being developed to identify the patients at highest risk for developing severe late normal-tissue complications. If such patients could be identified prior to treatment, then the doses to those patients could be reduced to lower their complication rate. In addition, patients identified as being relatively radioresistant could receive higher doses without an increase in complications. The aim of the present study was to estimate the magnitude of the dose adjustments that could potentially be made if radiotherapy doses were tailored to the individual patient using a predictive assay of normal-tissue radiosensitivity. The dose adjustments were estimated by re-analyzing data from an earlier study [13] to determine the influence of dose and in vitro fibroblast radiosensitivity on the incidence of severe late normal-tissue complications. Although the dose estimates are preliminary and based on limited data, the results of this study support the concept that a significant therapeutic gain could be achieved for a subset of patients from the use of a predictive assay of normal-tissue radiosensitivity.

How much could the radiotherapy dose be altered for individual patients based on a predictive assay of normal-tissue radiosensitivity?

Predictive assays are presently being developed to identify the patients at highest risk for developing severe late normal-tissue complications. If such patients could be identified prior to treatment, then the doses to those patients could be reduced to lower their complication rate. In addition, patients identified as being relatively radioresistant could receive higher doses without an increase in complications. The aim of the present study was to estimate the magnitude of the dose adjustments that could potentially be made if radiotherapy doses were tailored to the individual patient using a predictive assay of normal-tissue radiosensitivity. The dose adjustments were estimated by re-analyzing data from an earlier study [13] to determine the influence of dose and in vitro fibroblast radiosensitivity on the incidence of severe late normal-tissue complications. Although the dose estimates are preliminary and based on limited data, the results of this study support the concept that a significant therapeutic gain could be achieved for a subset of patients from the use of a predictive assay of normal-tissue radiosensitivity.

67 ASTRO research fellow presentation — A comparison of the comet assay and pulsed-field gel electrophoresis as a predictive assay for radiosensitivity in human fibroblasts

67 ASTRO research fellow presentation — A comparison of the comet assay and pulsed-field gel electrophoresis as a predictive assay for radiosensitivity in human fibroblasts

Low dose-rate fibroblast radiosensitivity and the prediction of patient response to radiotherapy

The relationship between cellular radiosensitivity and normal-tissue response to radiotherapy in individual cancer patients has attracted increasing attention over the last few years. Recent work has suggested that a correlation exists between fibroblast sensitivity and normal-tissue reactions. We have examined the radiosensitivity of fibroblasts grown from skin biopsies of four normal individuals and three patients identified as having suffered unexpectedly severe reactions to clinical radiotherapy, called here 'over-reactor' (OR) patients. Clonogenic survival was measured after high (HDR) and low dose-rate (LDR) irradiation. By comparing the two, and LDR Recovery Factor was derived. Potentially-lethal damage repair was examined in 4 cell strains. After HDR the OR strains were indistinguishable from the normals. At LDR the range of sensitivity was expanded. The OR strains fell at the sensitive end of the range and were characterized by a lack of LDR recovery, which clearly distinguished them from the normal strains. Experimental errors were estimated by considering all the data sets together rather than viewing each experiment individually. Duplicate strains from several patients were tested, and the differences between them were found to be within the estimated experimental errors, suggesting that these differences were not biologically significant. The data are consistent with the hypothesis that normal-tissue response is linked to individual cellular radiosensitivity. Our data confirm the importance of using LDR irradiation in clinical investigations of cellular sensitivity.

254Predictive assays in radiotherapy The Aarhus experience fibroblast radiosensitivity versus early and late normal tissue radiation response in patients with breast cancer

254Predictive assays in radiotherapy The Aarhus experience fibroblast radiosensitivity versus early and late normal tissue radiation response in patients with breast cancer

Selective assessment of in vitro radiosensitivity of tumour cells and fibroblasts from single tumour biopsies using immunocytochemical identification of colonies in the soft agar clonogenic assay

The assumed selective growth of tumour cells has formed the basis for the use of the soft agar clonogenic assay to test in vitro radio- and chemosensitivity of tumours. However, recent studies have demonstrated that fibroblasts proliferate in soft agar in addition to tumour cells. The present study was initiated to quantify the contaminating growth of non-malignant cells in the modified form of the Courtenay-Mills soft agar assay, in order to establish a reliable assay for estimating tumour cell radiosensitivity in squamous cell carcinomas of the head and neck. DNA flow cytometry analysis confirmed that ‘tumour fibroblasts’ (fibroblasts obtained from tumour biopsies) grow in soft agar. In contrast, white blood cells did not form colonies. Different media were tested with soft agar, but a selective medium for tumour cells was not found. Therefore, a colony filter-technique combined with an immunocytochemical analysis was developed to quantify the number of tumour cell and fibroblast colonies. In 12 tumour biopsies, 2–33% of the colonies were Cytokeratin AE1–3 positive, whereas 83–100% of the colonies were 5B5 fibroblast antibody positive. The parameter normally reported, the overall SF 2 (surviving cell fraction at 2 Gy) based on colonies in agar, was found to be statistically significantly correlated to the fibroblast SF 2, but not to the tumour cell SF 2. The overall SF 2 was significantly different from the tumour cell SF 2 in half of the tumours. Furthermore, the tumour cell SF 2 was not correlated to fibroblast SF 2. In consequence of our findings, correcting for fibroblast contamination is a necessity, when studying in vitro sensitivity of tumour cells. Combining the soft agar clonogenic assay with the new colony filter-technique and the immunocytochemical analysis appear to be useful for making this routine correction and for measuring the in vitro radiosensitivity of both tumour cells and fibroblasts from single tumour biopsies, which is of interest in future clinical studies on predictive assays.

480 In vitro radiosensitivity of head and neck tumour cells and fibroblasts assessed from single biopsies

The assumed selective growth of tumour cells in the soft agar clonogenic assay has formed the basis for its use to test in vitro sensitivity of tumours. However, studies in our laboratory have shown that also normal skin fibroblasts grow in soft agar. The present study was initiated to quantify the contaminating growth of fibroblasts in the enriched modified Courtenay-Mills soft agar assay, and to investigate ways to overcome this problem. A new colony filter-technique including immunohistochemical identification was developed, and 88–100% of the colonies obtained from 13 head and neck tumours were 5B5 Fibroblast Antibody positive, whereas 0–25% of the colonies were Cytokeratin AE1-3 positive. Thus, the present technique identifies tumour and fibroblast colonies, respectively, and it can measure the in vitro radiosensitivity of both tumour cells and fibroblasts. The parameter normally reported, the overall Surviving Fraction at 2 Gy (SF 2 ), based on the number of colonies in agar, was significantly correlated to SF 2 of fibroblasts (SF 2 F), but not to SF 2 of tumour cells (SF 2 T). The overall SF 2 was significantly different from SF 2 T in half of the examined patients. The SF 2 T was not correlated to SF 2 F. Thus, it is necessary to make a routine correction for fibro blast contamination, when studying tumour cell radiosensitivity. If this is done, the possibility of estimating both tumour cell and fibroblast radiosensitivities from a single biopsy is of interest in future clinical studies on predictive assays.

822 Assessment of human fibroblast radiosensitivity by the micronucleus and colony-forming assay

The relationship between radiosensitivity of normal human skin fibroblasts determined by the micronucleus (Mn) assay and the colony-forming assay was investigated Prediction of radiation-induced normal-tissue response in individuals necessitates a rapid and reproducible in vitro in vitro assay that correlates well with cell survival after irradiation. The Mn-assay is a quick test that was applied to primary skin biopsies from 10 unselected breast cancer patients in whom a significant correlation previously was found between the expression of subcutaneous fibrosis after radiotherapy and the cell surviving fraction at SF 3.5 (Spearman's rho = –0.81, P 3.5 as determined by the colony-forming assay. A possible reason for lack of correlation include variable numbers of scorable binucleated cells after irradiation with 3.5 Gy (4%-21%).

Fibroblast radiosensitivity versus acute and late normal skin responses in patients treated for breast cancer

Purpose/Objective : To determine if the radiosensitivity of normal human skin fibroblasts, measured in early passage cultures, is significantly correlated with the degree of acute or late normal skin damage in patients treated for breast cancer with radiotherapy. Methods and Materials : In the 1970s, a series of breast cancer patients was treated at the Department of Oncology in Gothenburg, Sweden with postoperative irradiation to the parasternal region. Patients were treated bilaterally using different fractionation schedules and doses to the right and left fields. Peak acute reactions were scored on a six-point scale, and skin erythema was measured by reflectance spectrophotometry. Telangiectasia was graded over time on a six-point scale. In April 1992, two small skin biopsies were obtained from 22 patients in two treatment groups (i.e., four dose-fractionation schedules) and, using either delayed or immediate plating, fibroblast radiosensitivity was measured in early passage cultures by clonogenic survival, after high and low dose-rate irradiations. Survival at 2.0 Gy (SF2) was calculated from complete survival curves. Results : To test assay reproducibility, SF2 values derived from paired biopsies of the same patient (12 cases) were compared. A reasonably good correlation ( p = 0.075) was obtained for SF2s determined by high dose-rate irradiations with immediate plating, but not for delayed plating or low dose-rate treatments. The median coefficient of variation in the replicate SF2s after high dose-rate treatment and immediate plating was 13%m suggesting that the poor correlation in paired SF2 values is due to the magnitude of the uncertainty in SF2 relative to the overall spread in SF2 values between patients (CV = 28%). Individual SF2 values and averaged values from patients with data from two biopsies were compared with the acute and late clinical reactions. A significant negative correlation was found between SF2 and relative clinical response, but only when averaged high dose-rate SF2 values and telagiectasia scores were compared. There was no significant correlation between average SF2 values and acute responses or between individual SF2 measurements and either the acute or late clinical response. Conclusion : The results of this study suggest that the degree of late telangiectasia is at least partially dependent upon the intrinsic cellular radiosensitivity of normal fibroblast, but the relationship is not clear cut. Multiple replicate assays are necessary to obtain reliable estimates of fibroblast SF2 values using current techniques.

Bowman-Birk proteinase inhibitor (BBI) modulates radiosensitivity and radiation-induced differentiation of human fibroblasts in culture

The radiosensitivity and differentiation pattern of cultured normal human fibroblasts was analysed as a function of treatment of the cells with the Bowman-Birk proteinase inhibitor (BBI). Upon irradiation with doses from 0 to 8 Gy normal human fibroblasts are induced to a premature terminal differentiation within 14–21 days of postirradiation incubation. Treatment of the cells with 10 μM BBI for 2 h prior to the irradiation procedure resulted in a significant shift of the radiation survival curve, increased SF 2 values 0.63 vs. 0.84 and the cell type composition of the test fibroblast cultures. Upon pretreatment with BBI the radiation-induced premature terminal differentiation of progenitor fibroblasts to postmitotic fibrocytes could significantly be inhibited. Based on this data, it can be postulated that BBI may serve as a radioprotector of normal fibroblasts which are involved in radiation-induced tissue injuries like radiation fibrosis.

Normal tissue radiosensitivity in breast cancer patients

To investigate whether in vitro radiosensitivity of lymphocytes derived from a blood sample will predict late effects from radiotherapy in breast cancer patients. Blood samples were collected from consenting patients who had received radiotherapy for breast cancer. Lymphocytes were extracted and transformed by the Epstein-Barr virus. The resulting lymphoblastoid cell lines (LCLs) were assessed for in vitro radiosensitivity using a tetrazolium-based colorimetric assay (MTT). Survival curves were generated using doses of 0.5 to 2 Gy. For each analysis an LCL derived from an individual with the radiosensitive condition ataxia-telangiectasia (AT) was run as control. Some patients also consented to give skin biopsies from which fibroblast cultures were derived. Clonogenic assays were performed to generate survival curves using doses in the range of 0.5 to 4 Gy. Comparison was made with the data obtained from LCLs. Late effects of radiotherapy were assessed using the Radiation Therapy Oncology Group (RTOG) scoring scheme and compared with the in vitro radiosensitivity data. LCLs from 56 breast cancer patients were assessed for in vitro radiosensitivity. Surviving fraction to 2 Gy (SF2) generated from survival curves ranged from 0.04-0.35 with coefficient of variation for the whole group of 41%. None of the LCLs equalled the sensitivity of the AT line, but 16% showed equal or greater sensitivity to a line derived from an AT heterozygote. Comparison of LCL and fibroblast radiosensitivity showed reasonable correlation for 12 paired samples (r = 0.64). The majority of patients showed no or minimal effects after radiotherapy (Grade 0, 1 effects) but seven developed a Grade 2 reaction and four a Grade 3 or 4 reaction. Patients with a Grade 2-4 reaction were found to be more sensitive in vitro than those with a Grade 0-1 reaction (p < 0.02). The use of the MTT assay to assess LCL radiosensitivity has demonstrated considerable heterogeneity amongst the breast cancer population. The presence of a proportion of patients showing in vitro sensitivity but normal clinical response to radiotherapy would limit the usefulness of the assay for predictive purposes.

Increased radiosensitivity and radioresistant DNA synthesis in cultured fibroblasts from patients with coronary atherosclerosis.

Cultured skin fibroblasts from five patients with atherosclerosis who underwent coronary artery bypass graft surgery were compared with those from one ataxia telangiectasia (AT) homozygote, three AT heterozygotes, and five healthy subjects to determine their sensitivity to gamma radiation as determined by a colony survival assay. Fibroblasts from four of these patients were also compared with those from two AT homozygotes, two AT heterozygotes, and three healthy subjects to determine postirradiation [3H]thymidine incorporation, indicating the levels of radioresistant DNA synthesis (RDS). On the basis of colony survival assay, after long-term irradiation (at low dose rate, ie, 0.007 Gy/min), fibroblasts from all five patients with atherosclerosis exhibited radiosensitivity that was intermediate between that of the healthy subjects and that of patients with the known radiosensitive syndrome AT. However, there was a considerable interstrain difference in the radiosensitivity of fibroblasts from patients with atherosclerosis, with their mean D10 values (radiation dose resulting in 10% cell survival) varying between 2.3 and 6.2 Gy, whereas the mean D10 values for the cells from the AT homozygote, AT heterozygotes, and healthy subjects were 2.0, 3.8, and 9.0 Gy, respectively. One of the patients with atherosclerosis showed cellular radiosensitivity quite similar to that of the AT homozygote, up to 2% to 10% of survival levels after short- (at a dose rate of 8 Gy/min) and long-term irradiation, respectively. The results of [3H]thymidine incorporation showed an AT heterozygote-like RDS in fibroblasts from patients with atherosclerosis that appeared to be intermediate between that of AT homozygotes and that of healthy subjects, suggesting a partial deregulation of cell cycle in the patients with atherosclerosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular radiosensitivity and dna damage in primary human fibroblasts

Purpose : To evaluate the relationship between radiation-induced cell survival and DNA damage in primary human fibroblasts to decide whether the initial or residual DNA damage levels are the more predictive of normal tissue cellular radiosensitivity. Methods and Materials : Five primary human nonsyndromic and two primary ataxia telangiectasia fibroblast strains grown in monolayer were studied. Cell survival was assessed by clonogenic assay. Irradiation was given at high dose rate (HDR) 1–2 Gy/min. DNA damage was measured in stationary phase cells and expressed as fraction released from the well by pulsed-field gel electrophoresis (PFGE). For initial damage, cells were embedded in agarose and irradiated at HDR on ice. Residual DNA damage was measured in monolayer by allowing a 4-h repair period after HDR irradiation. Results : Following HDR irradiation, cell survival varied between SFZ 0.025 to 0.23. Measurement of initial DNA damage demonstrated linear induction up to 30 Gy, with small differences in the slope of the dose-response curve between strains. No correlation between cell survival and initial damage was found. Residual damage increased linearly up to 80 Gy with a variation in slope by a factor of 3.2. Cell survival correlated with the slope of the dose-response curves for residual damage of the different strains ( p = 0.003). Conclusion : The relationship between radiation-induced cell survival and DNA damage in primary human fibroblasts of differing radiosensitivity is closest with the amount of DNA damage remaining after repair. If assays of DNA damage are to be used as predictors of normal tissue response to radiation, residual DNA damage provides the most likely correlation with cell survival.

Cellular sensitivity and low dose-rate recovery in Fanconi anaemia fibroblasts.

Fanconi anaemia (FA) is a rare inherited condition characterized by developmental abnormalities and progressive bone marrow failure, which requires bone marrow transplantation for successful treatment. This involves the use of alkylating agents and total body or thoraco-abdominal irradiation. Both chemical clastogens and irradiation cause increased chromosome damage in FA cells compared with controls. In some studies FA fibroblasts have been found to be more radiosensitive than normal. From these data it has been inferred that patients with FA might be more sensitive than normal to radiotherapy. However, increased radiosensitivity of FA fibroblasts has not been a uniform finding. The radiosensitivity of fibroblasts from two FA patients was studied at high and low dose-rate (LDR), and their sensitivity compared with normal strains. Both FA strains fell at the sensitive end of the range, but both demonstrated marked dose-rate sparing, with D0.01 recovery factors of 1.23 and 1.27, similar to the normal strains. These recovery factors are inconsistent with the suggestion that FA patients are recovery deficient. The data indicate that at least some FA strains are capable of LDR recovery, and imply that these patients would probably have a clinical benefit from fractionated or low dose-rate total body irradiation.

Elevated chromosome aberration frequency after X-ray exposure of cultured fibroblasts derived from patients with porokeratosis

Porokeratosis (PK) is a rare genetic skin disorder inherited as an autosomal dominant trait and regarded as a disease predisposing to cancer. To evaluate chromosomal radiosensitivity of PK cells, we examined chromosome aberration frequency after X-irradiation of cultured skin fibroblasts derived from PK patients and controls. Without X-ray exposure, frequencies of chromosome-type aberrations (exchanges or deletions) were not different between the patients and controls. Following X-ray irradiation, frequencies of deletions in the patient group were significantly increased, whereas those of exchanges were not elevated. No differences in chromatid-type aberration frequency were found between the patients and controls with or without exposure to X-ray. The observed radiosensitivity, though not as high as in ataxia telangiectasia (AT) cells, agrees well with the previously reported higher radiosensitivity of PK fibroblasts in survival analysis.

Radiation hypersensitivity of LEC strain rats controlled by a single autosomal recessive gene

LEC strain rats (LEC rats), which are known to develop hereditarily spontaneous fulminant hepatitis 4–5 months after birth, were highly sensitive to whole-body X-irradiation when compared to WKAH strain rats. The radiosensitivity of F1 hybrids of LEC and WKAH rats was similar to that of WKAH rats and significantly lower than of LEC rats. Segregation data of backcross hydrids (F1 × LEC and LEC × F1) suggested than the hypersensitivity of LEC rats to whole-body irradiation is controlled by a single autosomal recessive gene. The radiosensitivity of fibroblasts from LEC rats was higher than that of fibroblasts from WKAH rats. The repair process of DNA double-strand breaks in LEC cells was slower than that in WKAH cells. LEC rats could provide a useful animal model to assist in understanding the mechanism of radiation-induced DNA damage and repair.

Prospective comparison of in vitro normal cell radiosensitivity and normal tissue reactions in radiotherapy patients

Purpose: This pilot study was undertaken to assess the relationship between in vitro radiosensitivity of different normal cell types and the type and severity of normal tissue reactions in individual patients after radiotherapy. Methods and Materials: Twenty-one patients with head and neck cancer were studied prospectively; four with head and neck and two with breast cancer were studied retrospectively. The retrospective cases were chosen because they exhibited unusual (severe or minimal) normal tissue reactions after radiotherapy. Small skin biopsies and blood samples were obtained and used to generate in vitro fibroblast and lymphocyte cultures, respectively. Clonogenic assays were used to measure in vitro fibroblast and lymphocyte radiosensitivity after high- and low-dose rate irradiation. Head and neck patients were treated by conventional, hyperfractionated, or concomitant boost regimens, which have been found to yield an equal probability of late normal tissue reactions. The highest dose received by each normal tissue in the target volume was estimated using computed tomography treatment plans. The median patient follow-up time was 19 months (range: 13–25). Results: The distributions of in vitro radiosensitivity parameters and the grade of tissue reaction scores in the patients showed a broad range between individuals. When in vitro parameters were compared to the acute and late tissue reactions, the radiosensitivity of fibroblasts, measured as surviving fraction at 2 Gy after high-dose rate irradiation, showed a highly significant correlation with the maximum grade of late effects ( p < 0.0001 for the whole group and p = 0.0013 for the group of patients studied prospectively). No significant correlation was found between fibroblast radiosensitivity and maximum grade of acute effects or between lymphocyte radiosensitivity and either acute or late effects. Conclusion: We conclude that individuals vary in normal cell radiosensitivity, and that in vitro measurements of fibroblast radiosensitivity may predict the magnitude of late normal tissue reactions after radiotherapy. These preliminary results, however, need to be validated in a larger group of patients.

Increased radiosensitivity of granulocyte macrophage colony-forming units and skin fibroblasts in human autosomal recessive severe combined immunodeficiency.

We studied the radiosensitivity of granulocyte macrophage colony-forming units (GM-CFU) in patients with a severe combined immunodeficiency (SCID). Three patients lacking both mature T and B cells showed a twofold higher GM-CFU radiosensitivity calculated as the DO value (dose required to reduce survival to 37%), and an identical observation was made with fibroblasts from one of these patients. A patient with an SCID with hypereosinophilia, i.e., Omenn's syndrome characterized by extremely restricted T cell heterogeneity and a lack of B cells, also showed abnormal GM-CFU radiosensitivity. In contrast, GM-CFU from a patient lacking only T cells (X-linked form of SCID) showed normal GM-CFU radiosensitivity. These data further support the similarity between human T(-) B(-) SCID and the murine acid mutation characterized by a defect in T cell receptor and immunoglobulin gene rearrangement, and by an abnormal double-strand DNA break repair function. In addition, they strongly suggest that the Omenn's immunodeficiency syndrome may be a leaky T(-)B(-) SCID phenotype as previously indicated by the coexistence of the two phenotypes in siblings.

Lack of correlation of human fibroblast radiosensitivity in vitro with early skin reactions in patients undergoing radiotherapy.

Fibroblasts from breast cancer patients were obtained as outgrowths in vitro from punch biopsies and their radiosensitivity tested in early passages. Skin erythema reactions in the same patients were also measured, as degree of redness using reflectance spectrophotometry. Measurements were taken before and during a 4-week radiotherapy treatment with electrons to the thoracic wall. Of 59 biopsies studied, radiosensitivity and erythema were concurrently studied in 32. In 24, evaluable data from both clinic and laboratory were obtained. A population growth assay in 96-well plates, using absorption of sulphur rhodamine B as the stain for cell numbers, showed good agreement with the colony-formation assay. Plating efficiencies and growth rates in the colony assay were higher using human serum in place of foetal calf serum. Cell survival curves with human serum were mostly exponential with little shoulder. The parameters of survival at 2 Gy (SF2) and the dose required to give 10% survival (D10) were used in the correlations with clinical data; these were 0.25 +/- 0.09 and 3.03 +/- 0.50 Gy, respectively. There was a strong correlation between these two survival curve parameters (r = 0.98). Skin redness was found to linearly increase with time during radiotherapy. The slope of the increase differed markedly from patient to patient, with a range of a factor approx. 10. No correlation was found between SF2 and erythema response in the 24 evaluable patients (r = 0.13, p > 0.5). A similar lack of correlation was found using D10 as the radiosensitivity parameter (r = 0.12, p > 0.5). These data indicate that fibroblast radiosensitivity measured in vitro cannot be used to predict erythema reactions to radiotherapy in breast cancer patients.