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Positronium (Ps) is an exotic atom consisting of a positron and an electron, and around 1011 Ps atoms form in the human body during a PET scan. We have discussed little Ps in PET because its formation does not change the spatial information obtained by PET; Most Ps annihilates into back-to-back gamma-ray photons. However, Ps can provide other unique information due to the delay of the gamma-ray emission as long as its lifetime. The lifetime varies according to the chemical and physical environment for Ps. For example, the lower dissolved oxygen concentration (pO2), the longer a Ps atom survives in solutions. This is because of the unpaired electrons in the O2 molecule that enhance Ps annihilation via the electron exchange interaction. Knowing pO2 distribution is important for cancer patients because hypoxic cells are often resistant to radiotherapy as well as chemotherapy.
Moskal et al. named the new concept of PET as “Ps imaging [1],” and they also found that the Ps lifetime differs between healthy and cancer cells [2]. The difference may come from a combination of several chemical and physical conditions, but how each factor changes the Ps lifetime is a challenging matter to be understood. Some efforts have revealed how O2 molecules reduce Ps lifetime. Stepanov [3] found a positive correlation between the pO2 and Ps annihilation rate (the inverse of the lifetime) in water. Furthermore, we found good linearity between them by accumulating 50 times larger number of counts, as shown in Fig. 1 [4]. This line is namely calibration line for Ps as an oxygen sensor, and this result indicates a possibility of Ps as a hypoxia biomarker candidate. In other words, during a PET scan, 1011 nanosized sensors for O2 are spontaneously created in vivo, and it is worth trying to read the indicator for improving cancer treatments.
Fig. 1 pO2 vs Ps decay rate: squares (Lee [5]), triangles (Stepanov [3]), and circles (Shibuya [4]). (see at https://www.nature.com/articles/s42005-020-00440-z/figures/3)
References
[1] P. Moskal, et al., Nature Rev. Phys., 1, 527‒9 (2019).
[2] P. Moskal, et al., bioRxiv (2021). (doi: 10.1101/2021.08.05.455285)
[3] P. S. Stepanov, et al., Phys. Chem. Chem. Phys. 22, 5123 (2020).
[4] K. Shibuya, et al., Commun. Phys., 3, 173 (2020).
[5] J. Lee, et al., J. Chem. Phys. 44. 2506 (1966).
