Radio-prevention of micrometastases

Impact of time interval and dose rate on cell survival following low-dose fractionated exposures

Enhanced cell lethality, also known as hyper-radiosensitivity, has been reported at low doses of radiation (≤0.5 Gy) in various cell lines, and is expected to be an effective cancer therapy. We conducted this study to examine the impact of time interval and dose rate of low-dose fractionated exposures with a short time interval. We evaluated the cell-survival rates of V79 and A549 cells using clonogenic assays. We performed fractionated exposures in unit doses of 0.25, 0.5, 1.0 and 2.0 Gy. We exposed the cells to 2 Gy of X-rays (i) at dose-rates of 1.0, 1.5 and 2.0 Gy/min at 1-min intervals and (ii) at a dose-rate of 2.0 Gy/min at 10-s, 1-min and 3-min intervals by fractionated exposures. Apoptosis and cell cycle analyses were also evaluated in the fractionated exposures (unit dose 0.25 Gy) and compared with single exposures by using flow cytometry. Both cell-type survival rates with fractionated exposures (unit dose 0.25 Gy) with short time intervals were markedly lower than those for single exposures delivering the same dose. When the dose rates were lower, the cytotoxic effect decreased compared with exposure to a dose-rate of 2.0 Gy/min. On the other hand, levels of apoptosis and cell cycle distribution were not significantly different between low-dose fractionated exposures and single exposures in either cell line. These results indicate that a stronger cytotoxic effect was induced with low-dose fractionated exposures with a short time interval for a given dose due to the hyper-radiosensitivity phenomenon, suggesting that dose rates are important for effective low-dose fractionated exposures.

Impact of dose-rate on the low-dose hyper-radiosensitivity and induced radioresistance (HRS/IRR) response

PURPOSE

To ask whether dose-rate influences low-dose hyper- radiosensitivity and induced radioresistance (HRS/IRR) response in rat colon progressive (PRO) and regressive (REG) cells.

METHODS

Clonogenic survival was applied to tumorigenic PRO and non-tumorigenic REG cells irradiated with (60)Co γ-rays at 0.0025-500 mGy.min(-1). Both clonogenic survival and non-homologous end-joining (NHEJ) pathway involved in DNA double-strand breaks (DSB) repair assays were applied to PRO cells irradiated at 25 mGy.min(-1) with 75 kV X-rays only.

RESULTS

Irrespective of dose-rates, marked HRS/IRR responses were observed in PRO but not in REG cells. For PRO cells, the doses at which HRS and IRR responses are maximal were dependent on dose-rate; conversely exposure times during which HRS and IRR responses are maximal (t(HRSmax) and t(IRRmax)) were independent of dose-rate. The t(HRSmax) and t(IRRmax) values were 23 ± 5 s and 66 ± 7 s (mean ± standard error of the mean [SEM], n = 7), in agreement with literature data. Repair data show that t(HRSmax) may correspond to exposure time during which NHEJ is deficient while t(IRRmax) may correspond to exposure time during which NHEJ is complete.

CONCLUSION

HRS response may be maximal if exposure times are shorter than t(HRSmax) irrespective of dose, dose-rate and cellular model. Potential application of HRS response in radiotherapy is discussed.

Liver transplantation is an alternative treatment of hepatocellular carcinoma beyond the Milan criteria

BACKGROUND

The decision to perform liver transplantation (LT) or liver resection (LR) for patients with hepatocellular carcinoma (HCC) who are beyond the Milan criteria remains controversial.

METHODS

We retrospectively analyzed outcome data for 179 patients with HCC beyond the Milan criteria who were treated with LR (n = 135) or LT (n = 44). Univariate and multivariate Cox proportional hazard models were established. Kaplan-Meier survival curves were generated, and a log-rank test was performed to compare group survival status.

RESULTS

Patients who underwent LR group were significantly older, had a lower TNM stage, and were more likely to have unilateral disease and noncirrhotic liver. Significantly more patients in the LR group had recurrence (53.3% vs 29.5%) or died (61.5% vs 43.2%) than patients in the LT group. Recurrence-free survival rates were 11.9% for the LR group and 61.5% for the LT group. The median overall survival duration showed no statistically difference between the LR group (28.0 months) and the LT group (50.0 months).

CONCLUSIONS

LT may be the better choice for patients with HCC beyond the Milan criteria.

Hyperradiosensibilité aux très faibles doses: impact en radiothérapie des micrométastases

Radiobiologists have pointed out a novel radiobiological phenomenon observed in many tumor and normal cell lines: hyper-radiosensitivity to very low-dose (HRS) followed by induced radioresistance (IRR) after a threshold dose of 0.1-0.3 Gy that depends on the cell line. Radioresistance at high dose (i.e. higher than 0.5 Gy) and metastatic potential of tumor cells are likely major factors of failure in radiotherapy. A careful review of literature suggests that: 1) radiotherapy does not increase the metastatic potential of tumor cells; 2) radioresistance at high dose and metastatic potential are not related. However, inside a given tumor cell line, highly metastatic clones may elicit more cells showing HRS or are more radiosensitive at high dose than poorly metastatic ones. Recent data obtained from molecular techniques (comet and immunofluorescence assays) applied to single cells irradiated at very low radiation doses (1-100 mGy) suggest that DNA single-strand breaks (SSB) and double-strand breaks (DSB) may be the key-lesions responsible for the HRS phenomenon. These data suggest that the HRS phenomenon may find application in radiotherapy for micrometastasis. These early disseminated and probably unvascularised cells may escape the influence of high-dose chemotherapy after excision of the primary tumor. Considering the link between metastatic potential and HRS, we have previously proposed to apply very low-dose total body irradiation (TBI) at M(0) stage that may prevent the development of micrometastases. Literature data suggest that the smallest radiation dose that can produce HRS without increasing the risk of cancer may be in the milliGrays range.

Not all 2 gray radiation prescriptions are equivalent: Cytotoxic effect depends on delivery sequences of partial fractionated doses

PURPOSE

To test whether or not the commonly prescribed daily dose of 2 Gy (whole fraction), when delivered as various partial fraction (PF) dose sequences simulating clinical treatment fields, produces equal biologic effects.

METHODS AND MATERIALS

Eleven actively proliferating cell lines derived from human and animal tissues were used in this study. 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium bromide (MTT) and clonogenic assays were used to determine the radiation effects on cell proliferation and survival, respectively. The 2 Gy dose was divided into 2 or more PFs for delivery to simulate the delivery of clinical treatment fields. Most irradiation sequences contained two parts consisting of at least 1 small PF, denoted by S which was 0.5 Gy or less, and a large PF, denoted by L which was 1 Gy or more. Irradiation schemes were designed to include the following conditions: (a) the 2 Gy dose divided into combinations of an L-dose and one or more S-doses; (b) the L-dose given either before or after the S-doses; and (c) delivery of all partial fractions within a fixed total time.

RESULTS

Significant differences in biologic effect were observed between sequences in which the L-dose was given before or after the S-doses in both the MTT and clonogenic assays. Nearly all the latter schemes, that is S-L, produced greater cytotoxic effects than the L-S schemes.

CONCLUSIONS

These data demonstrate that the biologic effects of 2 Gy may differ in different clinical settings depending on the size and sequence of the partial fractions. The variation between cytotoxic effects is likely a result of the combination of low-dose hyper-radiosensitivity (HRS) and higher-dose increased radioresistance (IRR) effects established recently. We suggest that to ensure the optimal biologic effect of a prescribed dose of 2 Gy clinically, it is critical to consider the sequence in which the treatment fields are delivered when partial fractions of different sizes are used.