MTT-hybrid assay incorporates the advantages of both clonogenic and MTT assay radiosensitivity testing for fresh tumor samples.

: After devising a 3-(4, 5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT)-Hybrid assay that incorporates the advantages of both the clonogenic and standard MTT assays, we evaluated the validity of this system using a malignant fibrous histiocytoma (MFH) cell and four xenografted tumors. In the MFH cells the surviving fractions were correlated with radiation dose at three assays. In xenografted tumor cells, the surviving fractions determined by the MTT assay did not show a similar dose response, possibly because of contamination by normal cells. Changes in the surviving fraction of the xenografted cells were correlated to the radiation dose as determined by the clonogenic assays, but this assay took longer performance times and was not useful for evaluating tumors with a low planting efficiency. With the MTT-hybrid assay, changes in fractions of the four xenografted tumor cells were correlated to radiation dose. The MTT-hybrid assay required a smaller optimal number seeding cells and shorter assay time than those of the clonogenic assay. Therefore, the MTT-Hybrid assay can be used successfully to assess the radiosensitivity of both established tumor cell lines and fresh tumor samples.