Abstract
Photon hormesis refers to the phenomenon where the biological effect of ionizing radiation with a high linear energy transfer (LET) value is diminished by photons with a low LET value. The present paper studied the effect of photon hormesis from X-rays on dose responses to alpha particles using embryos of the zebrafish (Danio rerio) as the in vivo vertebrate model. The toxicity of these ionizing radiations in the zebrafish embryos was assessed using the apoptotic counts at 20, 24, or 30 h post fertilization (hpf) revealed through acridine orange (AO) staining. For alpha-particle doses ≥ 4.4 mGy, the additional X-ray dose of 10 mGy significantly reduced the number of apoptotic cells at 24 hpf, which proved the presence of photon hormesis. Smaller alpha-particle doses might not have inflicted sufficient aggregate damages to trigger photon hormesis. The time gap T between the X-ray (10 mGy) and alpha-particle (4.4 mGy) exposures was also studied. Photon hormesis was present when T ≤ 30 min, but was absent when T = 60 min, at which time repair of damage induced by alpha particles would have completed to prevent their interactions with those induced by X-rays. Finally, the drop in the apoptotic counts at 24 hpf due to photon hormesis was explained by bringing the apoptotic events earlier to 20 hpf, which strongly supported the removal of aberrant cells through apoptosis as an underlying mechanism for photon hormesis.
Highlights
Linear energy transfer (LET) describes the quality of an ionizing radiation
To further study the effect of photon hormesis on the dose response of alpha-particle irradiated zebrafish embryos, an additional X-ray dose of 10 mGy was delivered to the embryos immediately after their alpha-particle irradiation
Together with existing results in the literature, the present findings suggested that at least three conditions would be needed for photon hormesis to operate successfully: (1) the additional dosage of X-ray or gamma-ray photons should be sufficiently large, for example from 1–2 mGy and beyond on Wistar rats to act against alpha-particle irradiation [28,29], and between 7–10 mGy on zebrafish embryos to act against neutron irradiation [27]; (2) the damages inflicted by the primary ionizing radiation should be above the threshold; and (3) the photon dose should be applied within a certain period of time after the primary dose
Summary
Linear energy transfer (LET) describes the quality of an ionizing radiation. In general, high-LET radiation, such as alpha particles and heavy ions, generate clusters of damage along their trajectories in a medium, while low-LET radiation, such as X-rays and gamma rays, mainly induce dispersed damage, so high-LET radiation can induce more severe biological effects in cells [1]. It was established that high- and low-LET radiation damage had different kinetics of induction and repair [2,3]. The general public is exposed to background gamma radiation together with alpha-particle exposures from indoor radon levels [4,5,6,7,8,9,10]. Cancer patients could be exposed to mixed beams of high- and low-LET radiation during radiotherapy. In boron neutron capture therapy, patients are exposed to radiation fields consisting of a mixture of radiation with different LETs, including the high-LET products generated during the 10B(n,α)7Li reaction and the low-LET gamma rays released during radioactive capture (n,γ) reactions [13]
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