Cell Survival and DNA Damage in Normal Prostate Cells Irradiated Out-of-Field

Abstract

Interest in out-of-field radiation dose has been increasing with the introduction of new techniques, such as volumetric modulated arc therapy (VMAT). These new techniques offer superior conformity of high-dose regions to the target compared to conventional techniques, however more normal tissue is exposed to low-dose radiation with VMAT. There is a potential increase in radiobiological effectiveness associated with lower energy photons delivered during VMAT as normal cells are exposed to a temporal change in incident photon energy spectrum. During VMAT deliveries, normal cells can be exposed to the primary radiation beam, as well as to transmission and scatter radiation. The impact of low-dose radiation, radiation-induced bystander effect and change in energy spectrum on normal cells is not well understood. The current study examined cell survival and DNA damage in normal prostate cells after exposure to out-of-field radiation both with and without the transfer of bystander factors. The effect of a change in energy spectrum out-of-field compared to in-field was also investigated. Prostate cancer (LNCaP) and normal prostate (PNT1A) cells were placed in-field and out-of-field, respectively, with the PNT1A cells being located 1 cm from the field edge when in-field cells were being irradiated with 2 Gy. Clonogenic and γ-H2AX assays were performed postirradiation to examine cell survival and DNA damage. The assays were repeated when bystander factors from the LNCaP cells were transferred to the PNT1A cells and also when the PNT1A cells were irradiated in-field to a different energy spectrum. An average out-of-field dose of 10.8 ± 4.2 cGy produced a significant reduction in colony volume and increase in the number of γ-H2AX foci/cell in the PNT1A cells compared to the sham-irradiated control cells. An adaptive response was observed in the PNT1A cells having first received a low out-of-field dose and then the bystander factors. The PNT1A cells showed a significant increase in γ-H2AX foci formation when irradiated to 20 cGy in-field in comparison to out-of-field. However, no significant difference in cell survival or colony volume was observed whether the PNT1A cells were irradiated in-field or out-of-field. Out-of-field radiation dose alone can have a damaging effect on the proliferation of PNT1A cells when a clinically relevant dose of 2 Gy is delivered in in-field. Out-of-field radiation with the transfer of bystander factors induces an adaptive response in the PNT1A cells.

Radiation induced bystander effects in the spleen of cranially-irradiated rats

“…This indicates that low doses of ionizing can have measurable molecular effects on the cells and tissue. Our findings are in agreement with the previous data showing that out-of-field radiation dose alone can have a damaging effect on the proliferation of PNT1A cells when a clinically relevant dose of 2 Gy is delivered in in-field 58 and the induction of the bystander effect existed when the cells were irradiated with γ-ray at a dose as low as 3 mGy. 59,60 In conclusion, an abscopal effect in the form of DNA damage and induction of apoptosis in spleen of cranially irradiated rats that was demonstrated.…”

Targeted and Non-Targeted Mechanisms for Killing Hypoxic Tumour Cells—Are There New Avenues for Treatment?

“…Such signals induce a range of responses, including death of cells [168], induction of genomic instability [169][170][171], mitochondrial changes and other effects [172,173], which appear to result from increased ROS in the responding cell [174,175]. Most of these effects make the normal unirradiated cells more sensitive to death and may account for out-of-field and memory or legacy effects [176][177][178]. However, there have been reports of survival enhancing bystander effects [179], and induction of adaptive responses by bystander signals have also been reported [180][181][182].…”

Relevance of Non-Targeted Effects for Radiotherapy and Diagnostic Radiology; A Historical and Conceptual Analysis of Key Players

“…Also, given that NTE are more prevalent in normal cells than in tumour cells, the adverse effects of NTE in radiotherapy could be considerable. This area has been considered in detail by those studying “out of field” effects [180,181]. Reducing NTE in normal tissue is considered to be a potential novel target for improving radiotherapy outcomes.…”