Dose calculations for [(131)i] meta-iodobenzylguanidine-induced bystander effects

Targeted radiotherapy is a potentially useful treatment for some cancers and may be potentiated by bystander effects. However, without estimation of absorbed dose, it is difficult to compare the effects with conventional external radiation treatment.

METHODS

Using the Vynckier - Wambersie dose point kernel, a model for dose rate evaluation was created allowing for calculation of absorbed dose values to two cell lines transfected with the noradrenaline transporter (NAT) gene and treated with [(131)I]MIBG.

RESULTS

The mean doses required to decrease surviving fractions of UVW/NAT and EJ138/NAT cells, which received medium from [(131)I]MIBG-treated cells, to 25 - 30% were 1.6 and 1.7 Gy respectively. The maximum mean dose rates achieved during [(131)I]MIBG treatment were 0.09 - 0.75 Gy/h for UVW/NAT and 0.07 - 0.78 Gy/h for EJ138/NAT. These were significantly lower than the external beam gamma radiation dose rate of 15 Gy/h. In the case of control lines which were incapable of [(131)I]MIBG uptake the mean absorbed doses following radiopharmaceutical were 0.03 - 0.23 Gy for UVW and 0.03 - 0.32 Gy for EJ138.

CONCLUSION

[(131)I]MIBG treatment for ICCM production elicited a bystander dose-response profile similar to that generated by external beam gamma irradiation but with significantly greater cell death.

Induction of bystander response in human glioma cells using high-energy electrons: a role for TGF-beta1

We examined bystander cell death produced in T98G cells by exposure to irradiated cell conditioned medium (ICCM) produced by high-energy 20 MeV electrons at a dose rate of 10 Gy min(-1) and doses up to 20 Gy. ICCM induced a bystander response in T98G glioma cells, reducing recipient cell survival by more than 25% below controls at 5 and 10 Gy. Higher doses increased survival to near control levels. ICCM was analyzed for the presence of transforming growth factor alpha (TGF-alpha) and transforming growth factor beta1 (TGF-beta1). Monoclonal antibodies for TGF-alpha (mAb TGF-alpha) and TGF-beta1 (mAb TGF-beta1) were added to the ICCM to neutralize any potential effect of the cytokines. The results indicate that TGF-alpha was not present in the ICCM and addition of mAb TGF-alpha to the ICCM had no effect on bystander cell survival. No active TGF-beta1 was present in the ICCM; however, addition of mAb TGF-beta1 completely abolished bystander death of reporter cells at all doses. These results indicate that bystander cell death can be induced in T98G glioma if a large enough radiation stress is applied and that TGF-beta1 plays a downstream role in this response.

Effect of dose rate on the radiation-induced bystander response

Radiation-induced biological bystander effects have become a well-established phenomenon associated with the interaction of radiation with cells. These so-called bystander effects have been seen across a variety of end points for both high and low linear energy transfer (LET) radiations, utilizing a variety of dose rates and radiation sources. In this study, the effect of dose rate and different low LET sources on the bystander cell survival fraction (SF) was examined. The cell line investigated was the human keratinocyte HPV-G. The bystander response was measured via clonogenic assay after medium transfer protocol. Cells were irradiated using (60)Co gamma-rays and 20 MeV electrons at doses of 0.5, 5 and 10 Gy with varying dose rates. Both gamma and electron irradiation decreased recipient SF at 0.5 Gy and 5 Gy, respectively. Subsequent recovery of the SF to control levels for 10 Gy was observed. There was no dose rate dependence for (60)Co irradiation. A significant difference in the survival fraction was observed for electron irradiation at 10 Gy and a high dose rate. Furthermore, survival fractions were compared between (60)Co and 20 MeV electron irradiations. This showed a significant increase in the survival fraction 'recovery' at 10 Gy for a (60)Co dose rate of 1.1 Gy min(-1) compared to 20 MeV electrons at 1.0 Gy min(-1). No such difference was observed when comparing at higher dose rates. Lastly, increases in survival fraction at 10 Gy were abolished and the SF decreased by the plating of increased numbers of recipient cells. Such evidence may help gain insight into the nature and mechanism(s) surrounding bystander signal production, how these phenomena are tested and their eventual application in a clinical setting.

A role for bioelectric effects in the induction of bystander signals by ionizing radiation?

The induction of "bystander effects" i.e. effects in cells which have not received an ionizing radiation track, is now accepted but the mechanisms are not completely clear. Bystander effects following high and low LET radiation exposure are accepted but mechanisms are still not understood. There is some evidence for a physical component to the signal. This paper tests the hypothesis that bioelectric or biomagnetic phenomena are involved. Human immortalized skin keratinocytes and primary explants of mouse bladder and fish skin, were exposed directly to ionizing radiation or treated in a variety of bystander protocols. Exposure of cells was conducted by shielding one group of flasks using lead, to reduce the dose below the threshold of 2mGy (60)Cobalt gamma rays established for the bystander effect. The endpoint for the bystander effect in the reporter system used was reduction in cloning efficiency (RCE). The magnitude of the RCE was similar in shielded and unshielded flasks. When cells were placed in a Faraday cage the magnitude of the RCE was less but not eliminated. The results suggest that liquid media or cell-cell contact transmission of bystander factors may be only part of the bystander mechanism. Bioelectric or bio magnetic fields may have a role to play. To test this further, cells were placed in a Magnetic Resonance Imaging (MRI) machine for 10 min using a typical head scan protocol. This treatment also induced a bystander response. Apart from the obvious clinical relevance, the MRI results further suggest that bystander effects may be produced by non-ionizing exposures. It is concluded that bioelectric or magnetic effects may be involved in producing bystander signaling cascades commonly seen following ionizing radiation exposure.