Purpose: Evaluation of the theoretical and practical value of using low-dose rate (LDR) irradiation to increase the resolution of radiosensitivity testing of primary human tumors using clonogenic assays.
Methods and materials: Fourteen human tumor cell lines were assessed for surviving fraction at 2-8 Gy (SF2-SF8) using low-dose rate irradiation and a clonogenic assay. Further data were collected from the literature for 64 low-dose rate irradiation survival curves from human tumor cell lines. The data were grouped into five different radioresponsiveness categories (A-E). An analysis was made of the ability of the graded survival levels to discriminate between the different radioresponse groups and compared with previous analyses for high-dose rate SF2. Fifteen human cervical carcinoma specimens were analysed for SF2 and SF3.5 following high- and low-dose rate irradiation.
Results: Low-dose rate irradiation increased the spread of tumor cell line radiosensitivity data and the ability to discriminate between radioresponse groups was greater at low than at high-dose rates. Using low-dose rate irradiation on primary tumor specimens and a soft agar clonogenic assay decreased the success rate in obtaining data. The latter dropped from 70% for high-dose rate SF2 to 51% for low-dose rate SF3.5.
Conclusions: The work on cell lines illustrates that low-dose rate irradiation does improve the ability of clonogenic radiosensitivity measurements to discriminate between tumors of different radioresponsiveness groups. However, using low-dose rate irradiation on primary human tumors with a soft agar clonogenic assay was not practical because of reducing the success rate for obtaining data for radiosensitivity measurements.