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.

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.

Intrinsic radiosensitivity of normal human fibroblasts and lymphocytes after high- and low-dose-rate irradiation.

The existence of heritable radiosensitivity syndromes and clinical observations in radiotherapy patients suggests that human cellular radiosensitivity differs among individuals. We report here an in vitro study of radiosensitivity in 30 fibroblast and 29 lymphocyte cultures obtained from cancer patients and controls. In 25 cases, both fibroblasts and lymphocytes were obtained from the same donors. Fibroblasts were cultured from skin biopsy samples, and peripheral T-cell lymphocytes were cultured from blood. Clonogenic survival assays were performed by using high- and low-dose-rate irradiation; lymphocytes were in G0 phase and fibroblasts in confluent plateau phase. Various end points were calculated and compared (i.e., surviving fraction at 2 Gy, initial slope of the survival curve, and doses resulting in 10 and 1% survival, respectively). Depending on the end point, the coefficient of variation of the survival parameters ranged from 31 to 68% for lymphocytes and 21 to 41% for fibroblasts following high-dose-rate irradiation. Similar ranges were obtained after low-dose-rate irradiation. Variance analysis performed on replicate assays in cultures derived from the same patient showed that variation due to technical or sampling errors was significantly lower than variation between individuals (P = 0.00034 and 0.014 for fibroblasts and lymphocytes, respectively). No correlation was observed between the radiosensitivity of lymphocyte and fibroblast cultures derived from the same donors. We conclude that there is significant variation in normal cell radiosensitivity among individuals. On the other hand, comparisons of lymphocyte and fibroblast radiosensitivities suggest that tissue-specific characteristics, such as differentiation status, may variably modulate radiosensitivity.

Radiosensitivity measurement of keratinocytes and fibroblasts from radiotherapy patients.

Genetic diversity is believed to influence cellular radiosensitivity and individual variability in normal tissue reactions to radiotherapy. To measure normal cell radiosensitivity in vitro, we investigated a culture technique that yields keratinocyte and fibroblast cell cultures from small skin biopsy samples (average weight 32 mg). This technique uses 3T3 NIH cells as feeder cells, culture medium containing dialyzed fetal calf serum, low calcium, and various growth factors for keratinocyte growth. A calcium concentration of 4 x 10(-3) M and the use of lethally irradiated NIH 3T3 feeder cells were critical to the success of this method. Primary keratinocyte cultures were successfully obtained from nine biopsy specimens, and radiosensitivity measurements were obtained in six of the resulting strains. Keratinocytes were, in general, more radioresistant than fibroblasts derived from the same specimen. We conclude that radiosensitivity assessment of keratinocyte and fibroblast cultures derived from small punch biopsy specimens is feasible. Further studies can now be carried out to determine the degree of variability between individuals and the relationship between in vitro keratinocyte and fibroblast radiosensitivity and their value in predicting normal tissue responses to radiotherapy.

Cellular radiosensitivity of primary head and neck squamous cell carcinomas and local tumor control.

The low dose survival parameters of human tumor cell lines have been shown to correlate with the perceived clinical radiosensitivity of different tumor histologic types. This conclusion has been generated from the analysis of a large number of cell lines and has, therefore, served as the basis for attempts to develop predictive assays of tumor radiocurability. In this study, the tumors from 72 patients with head and neck squamous cell carcinoma have been grown in an adhesive tumor cell assay system and their sensitivity to radiation has been measured. All patients in this study were treated with post-operative radiotherapy, the surgical margins were negative, and any patient that had received chemotherapy was excluded. The average S2 (survival at 2.0 Gy) value of the 72 cultures was 0.33, with the values ranging from 0.11 to 0.91. All patients were evaluated for local tumor control. They have been followed for about 1 year and continued follow-up is still in progress. The average survival at 2.0 Gy of cultures derived from the 12 patients that have had recurrences so far is slightly higher (0.40) than that from those who appear so far to have local tumor control (0.30). Although the general trend is that recurrent tumors yield primary cultures that are slightly more resistant, the difference is not statistically significant.