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Cynthia Keppel
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Journal Articles
International Journal of Particle Therapy (2021) 7 (4): 11–18.
Published: 19 March 2021
Abstract
Purpose Proton therapy precisely delivers radiation to cancers to cause damaging strand breaks to cellular DNA, kill malignant cells, and stop tumor growth. Therapeutic protons also generate short-lived activated nuclei of carbon, oxygen, and nitrogen atoms in patients as a result of atomic transmutations that are imaged by positron emission tomography (PET). We hypothesized that the transition of 18 O to 18 F in an 18 O-substituted nucleoside irradiated with therapeutic protons may result in the potential for combined diagnosis and treatment for cancer with proton therapy. Materials and Methods Reported here is a feasibility study with a therapeutic proton beam used to irradiate H 2 18 O to a dose of 10 Gy produced by an 85 MeV pristine Bragg peak. PET imaging initiated >45 minutes later showed an 18 F decay signal with T 1/2 of ∼111 minutes. Results The 18 O to 18 F transmutation effect on cell survival was tested by exposing SQ20B squamous carcinoma cells to physiologic 18 O-thymidine concentrations of 5 μM for 48 hours followed by 1- to 9-Gy graded doses of proton radiation given 24 hours later. Survival analyses show radiation sensitization with a dose modification factor (DMF) of 1.2. Conclusions These data support the idea of therapeutic transmutation in vitro as a biochemical consequence of proton activation of 18 O to 18 F in substituted thymidine enabling proton radiation enhancement in a cancer cell. 18 O-substituted molecules that incorporate into cancer targets may hold promise for improving the therapeutic window of protons and can be evaluated further for postproton therapy PET imaging.