Introduction 1 2 3 5 6 7 90 131 177 1 8 12 90 177 13 14 15 10 16 19 15 17 20 21 15 22 5 −5 23 24 5 25 26 5 5 5 5 16 17 24 26 27 32  T 1/2 28 29 30 32 33 34 5 Materials and methods Irradiation chambers 35 32 32 32 T 1/2 32 32 32 Dosimetry 35 36 60 60 32 Tumour cells  L 37 38 39 40 177 41 35 35 28 Choice of dose rate and exposure time 32 35 32 21 23 29 42 43 10 16 18 27 44 Four cell dishes were placed in each irradiation chamber when the dose rate was as expected and the cells were then kept there for 24, 72 or 168 h (1, 3 or 7 days). The radiation exposure time was chosen not to be longer than a week, since it is known from numerous articles in the field of radionuclide therapy that the tumour cell retention of radioactivity is generally in the range of some days up to about 1 week (in some cases it is only a few hours). Furthermore, it is well known that the biological half-life of tumour targeting agents (ligands, antibodies and antibody fragments) in the systemic circulation is often shorter than a week (in the case of small ligands, it is only a few hours). Thus, the longest exposure time considered to be realistic was 1 week. Cell counting 2 Cell culture conditions during and after irradiations 2 5 5 2 5 Growth curves 15 1 25 1 5 6  t Cell kill versus regrowth 5 Growth delay 10 Literature survey The Medline-based PubMed database was used to survey effects of targeted radionuclide therapy and of low dose rate therapy. Results Growth curves 1 1 Fig. 1 a b  1 1 The HT-29 cells recovered to the control growth rate after a growth delay. The U-118MG cells recovered after a growth delay but continued to grow at a slower rate than the controls. The A-431 cells continued to grow without delay but at a slower rate than the control The U-373MG and SKBR-3 cells died. Cell kill and regrowth versus dose rate and exposure time 2 Fig. 2 a b c d e  bold solid lines  dashed line  2 Materials and methods 2 2 Cell kill and regrowth versus total dose 2 Growth delay as a function of dose rate 3 3 3 Fig. 3 a b c d e  boxes  3 3 3 3 Growth delay as a function of total dose 3 4 Fig. 4 a b c 3  Discussion The obtained results can hopefully serve as a guideline for the combinations of dose rate and exposure time necessary to kill tumour cells when applying low dose rate beta irradiation. The shift from regrowth to “cure” (no surviving cells) fell, for each cell type, within a narrow range of combinations. The U-118MG cells were more resistant to the treatments than the U-373MG and SKBR-3 cells. The A-431 and HT-29 cells showed intermediate resistance. However, the differences between the cell lines were not dramatic, and it can be generally stated that when applying 7 days of continuous irradiation, initial dose rates of about 0.2–0.3 Gy/h were enough to kill all cells in the cultures. When exposed for 3 days, an initial dose rate in the order of 0.4–0.6 Gy/h was needed. When the cells were exposed for only 24 h it was not possible to kill all the cells (with the exception of SKBR-3 cells), even if the initial dose rate was as high as about 0.8 Gy/h. 10 16 18 16 19 45 44 46 25 5 30 32 5 When considering radionuclide therapy it is, of course, also important to consider unwanted effects on normal tissues. The tolerance doses for most normal tissues are not known when exposure is to low dose rate irradiation. Targeted radionuclide therapy, using, for example, radiolabelled antibodies, fragments of antibodies or various receptor ligands, is expected to result in highly tumour-specific uptake of the therapeutic radionuclides. Thus, for a curative intent it is reasonable to establish the necessary tumour dose rates and exposure times. Another obvious question is which targeting agent should be tried for each type of tumour and, most importantly, whether the required conditions can be achieved without excessively severe side-effects on normal tissues. However, analyses of effects on normal tissues were beyond the scope of this study. 35 2 2 2 2Gy 2Gy 1 28 47 54 Table 1 Survival at 2Gy after exposure to high dose rates Cell line Survival at 2Gy References U-118MG 0.44–0.70 28 47 U-373MG 0.60–0.62 28 48 49 HT-29 0.55–0.78 28 50 51 52 53 54 A-431 ≈0.52 53 2Gy 55 2Gy 1 2Gy 2Gy 2Gy 2Gy 35 28 56 57 58 5 1