Observations and an interpretation of dose-response relationships for cellular transformation in terms of induced (T) repair

Might intrinsic radioresistance of human tumour cells be induced by radiation?

Survival measurements were made on six human tumour cell lines in vitro after irradiation with single doses of X rays. Doses up to 5 Gy were used giving surviving fractions down to 20%, but the majority of the measurements were made at doses < 1 Gy. These six cell lines have very different intrinsic radiosensitivities: HT29, Be11, and RT112 are radioresistant with surviving fractions at 2 Gy (SF2) between 60 and 74%, while MeWo, SW48, and HX142 are radiosensitive (SF2 = 3-29%). For all the cell lines, response over the dose range 2-5 Gy showed a good fit to a Linear-Quadratic (LQ) model. However, HT29, Be11, and RT112 cells showed a significant increase in X-ray radiosensitivity at doses below < 1 Gy compared with the prediction extrapolated from a LQ model fitted to the data at higher doses. The LQ model also slightly underpredicted the effect of low-dose X rays in MeWo cells, but the response of SW48 and HX142 cells was well described by the LQ model at all doses, with no evidence of increased low-dose effectiveness. The most plausible explanation for this phenomenon is that it reflects an induced radioresistance so that low doses of X-rays in vitro are more effective per Gy than higher doses, because only at higher doses is there sufficient damage to trigger repair systems or other radioprotective mechanisms. It follows that variation in the amount of inducible radioresistance might explain, in part, differences in intrinsic radiosensitivity above > 1 Gy between cell lines: cells would be intrinsically radiosensitive because they have a diminished inducible response.

Induction of micronuclei by acute and chronic exposure in vivo to gamma rays in murine polychromatic erythrocytes

The effect on micronuclei (MN) frequency of in vivo exposure to different dose rates of gamma rays in murine polychromatic erythrocytes (PCE) was studied. Groups of animals were irradiated with 1.0 Gy of gamma rays administered in 10, 100, 1000 or 10,000 min; the micronucleated polychromatic-erythrocytes (MN-PCE) frequency was scored in blood samples obtained from the tail of mice at various times before, during or after irradiation. The time-response curves for the 10, 100, and 1000 min exposure were similar; however, the two first curves showed a peak at 1800 min and the last at 2400 min after exposure. The curve obtained from the 10,000 min exposure showed a plateau for a long period during and after the exposure. However, the integration of the area under the curves indicates that the damage caused by the different radiation protocols was the same, suggesting that throughout time, the lesions were not repaired but rather diluted by cell division.

The effect of very low radiation doses on the human bladder carcinoma cell line RT112

Human RT112 cells were exposed to single doses of X-rays (0.05-5 Gy), and cell survival was measured using a Dynamic Microscopic Imaging Processing Scanner (DMIPS) with which individual cells can be located in tissue-culture flasks, their positions recorded, and after an appropriate incubation time the recorded positions revisited to allow the accurate scoring of survivors. The response over the X-ray dose range 1-5 Gy showed a good fit to a Linear-Quadratic (LQ) model. For X-ray doses < 1 Gy, an increased effect of X-rays was observed with cell survival below the prediction from the LQ model extrapolated from higher doses. Several arguments suggest that this phenomenon could reflect an induced radioresistance so that in this cell line, low single doses of X-rays are more effective per Gray than higher doses in reducing cell survival because only at higher doses, above a threshold, is there sufficient damage to trigger radioprotective mechanisms.

A comparison of early effects with two dose rates in brachytherapy of cervix carcinoma in a prospective randomised trial

This phase III randomised trial examined the early effects of two low dose rates (0.38 and 0.73 Gy/h) in brachytherapy of stage I and IIp cervical cancer patients. A total of 204 patients were included between January 1985 and September 1988. Since the main analysis of this paper concerned surgical difficulties, only the 155 patients (76%) on whom surgery was performed at the Institut Gustave-Roussy were retained in this analysis. Treatment consisted of uterovaginal 137Cs irradiation followed by immediate or deferred surgery. The two groups were similar for pretreatment characteristics except for endocervix involvement. Their brachytherapy parameters were also similar (60 Gy pear dimensions, doses to critical organs, total kerma, etc.). The factors with a poor prognosis were, for surgical difficulties, older age, stage II and a small irradiated pear volume; for difficulties with haemostasis, immediate surgery, stage II and previous surgery; and for difficulties in dissection, lymph node involvement. The dose rate significantly influenced surgical difficulties for the stage IIp patients operated on by deferred surgery. Those treated with the higher dose rate showed a 2-fold increase in surgical difficulties compared to those irradiated at the lower dose rate (P = 0.03). The independent prognostic factors for sterilisation of the surgical specimen were small tumour size and absence of lymph node involvement. An inverse dose rate effect was observed for medium size tumours, with significantly more sterilisations observed in stage IIp patients in the lower dose rate group (P < 0.01).

Hypersensitivity of a human tumour cell line to very low radiation doses

Survival of HT29 cells was measured after irradiation with single doses of X-rays (0.05-5 Gy) and neutrons (0.025-1.5 Gy), using a Dynamic Microscopic Imaging Processing Scanner (DMIPS) with which individual cells can be accurately located in tissue culture flasks, their positions recorded, and after an appropriate incubation time the recorded positions revisited to allow the scoring of survivors. The response over the X-ray dose range 2-5 Gy showed a good fit to a Linear-Quadratic (LQ) model. For X-ray doses below 1 Gy, an increased X-ray effectiveness was observed with cell survival below the high-dose LQ prediction. The value of --dose/loge (SF) for each experimental data point, plotted against dose, demonstrated clearly how X-rays are maximally effective at doses approaching zero, becoming less effective as the dose increases and with minimal effectiveness at about 0.6 Gy then becoming more effective again as the dose increases above 1.5 Gy. This phenomenon was not seen with neutrons. Neutron RBE was calculated for each X-ray data point by taking each X-ray survival value and comparing it with the common LQ fit to all the neutron data. Over the X-ray dose range 0.05-0.2 Gy, the RBE is close to 1 indicating that these very low doses of X-rays are of similar effectiveness to neutrons in killing cells. The increase in RBE with increasing dose over the range 0.05-1 Gy, and the slight decrease in RBE above 1 Gy, reflect primarily the changes in X-ray sensitivity over the whole dose range of 0.05-5 Gy. Several arguments suggest that this phenomenon could reflect an induced radioresistance so that in this system low single doses of X-rays are more effective per Gy than higher doses in reducing cell survival because only at higher doses, above a threshold, is there sufficient damage to trigger radioprotective mechanisms.

The number of clonogenic cells in a mouse jejunal crypt

The near linearity of cellular dose-survival curves for neutrons facilities back-extrapolation to their origin at zero dose. This zero dose intercept is the number of clonogenic cells per circumference, from which the average number of clonogenic cells per crypt can be calculated. The average estimate of clonogenic cell number per crypt (k) from back-extrapolation of 11 single dose neutron survival curves to a common intercept was 100. Multifraction experiments provide an even better estimate of k because more complete dose survival curves can be constructed on the assumption of an equal effect per equal dose fraction. The short back-extrapolation of five such curves to a common intercept yields an estimated k value of 123 (108-140, 95% confidence interval) cells per crypt. These k values were higher than those estimated by Hendry's two-dose gamma-ray method.