Background: Understanding the basis of clinical radiosensitivity is a key goal of radiation research. In this study, we used the limiting dilution assay (LDA) to analyze in vitro radiosensitivity of cell lines from individuals with breast and other cancers, who had been treated with ionizing radiation, and who either had a non-radiosensitive (RS) radiation response or who were clinically RS.Methods: Lymphoblastoid cell lines (LCLs) were created from 29 cancer patients including 19 RS patients, 10 controls who had not developed severe normal tissue reactions, and 1 ataxia telangiectasia RS control cell line. The clinically RS patients had grade 3 or grade 4 reactions; one had a grade 2 reaction. All cells were exposed to graded doses of gamma-radiation in vitro and cell survival assessed via LDA. Cell survival was expressed on non-linear regression analysis-fitted survival curves and also as the surviving fraction at 2 Gray (Gy) (SF 2 ).Results: Our LDA analysis yielded two notable positive results. Firstly, it could distinguish control cells from cells from pooled breast cancer cases with severe reactions of all types (acute reactors, consequential late reactors and late reactors). Secondly, two radiosensitivity outliers were detected on the fitted curves, corresponding clinically to grade 3 and 4 late radiation reactions in breast and head and neck cancer cases respectively. The assay showed considerable cell survival heterogeneity. Conclusions:The LDA as used here may provide unique clinical utility in detecting potential RS breast cancer patients prior to radiotherapy (RT), a form of personalized medicine. The assay may be especially useful in situations where its results can be temporally available prior to therapy initiation (e.g., those patients not undergoing RT until some months after surgery, typically those having adjuvant chemotherapy prior to RT). Two LCLs from RS outliers could potentially yield insight into the cellular and/or genetic basis of radiosensitivity, for example by undertaking genomic analyses on these cell lines.