IntroductionLow-dose computed tomography (CT) scans can induce DNA damage, including chromosomal aberrations (CAs) and DNA double-strand breaks, which may contribute to long-term cancer risk. D-limonene, a bioactive component present in several Kampo formulas, has shown radioprotective effects in preclinical studies; however, clinical evidence remains limited, and the gastrointestinal side effects associated with isolated D-limonene may restrict its clinical application. This single-arm observational pilot study aimed to evaluate whether Kampo formulas containing D-limonene could mitigate CT-induced cytogenetic damage and to generate preliminary data for designing a future definitive trial.MethodsThirty-nine patients with functional dyspepsia received one of four D-limonene-containing Kampo formulas: bukuryoin, rikkunshito, heiisan, or nichinto. Peripheral blood lymphocytes were collected before and after Kampo treatment and exposed in vitro to CT radiation. Chromosomal aberrations were assessed using peptide nucleic acid fluorescence in situ hybridization (PNA-FISH), while DNA double-strand breaks were evaluated by γ-H2AX immunofluorescence. Wilcoxon signed-rank tests and linear mixedeffects models were used to assess radiation effects and Kampo-related interactions, and intraclass correlation coefficients (ICCs) were calculated to compare assay reliability.ResultsCT exposure significantly increased CAs before Kampo treatment (p = 0.024), whereas this increase was attenuated after treatment (p = 0.417). However, neither the pre–post difference-in-differences analysis (p = 0.428) nor the CT × Kampo interaction in the mixed-effects model (β = –0.72; 95% CI, –2.19 to 0.75; p = 0.343) reached statistical significance. γ-H2AX showed a similar trend. PNA-FISH demonstrated higher measurement reliability than γ-H2AX immunofluorescence (ICC = 0.89 vs. 0.65). Sample size estimation based on PNA-FISH indicated that 275–295 evaluable participants per arm would be required for a future definitive trial.DiscussionOverall, Kampo formulas containing D-limonene showed a trend toward mitigating CT-induced cytogenetic injury, and PNA-FISH appeared to provide a more reliable endpoint than γ-H2AX. These exploratory findings support further investigation in a randomized controlled trial to determine whether D-limonene-containing Kampo formulas can meaningfully mitigate CT-associated genotoxicity in humans.