Background: The formation and spatial distribution of $\mu$-hotspots in high-energy photon radiotherapy are important factors influencing radiation-induced DNA damage. These hotspots can enhance the local linear energy transfer (LET) component, affecting therapeutic outcomes. Purpose: This study aims to investigate the characteristics of $\mu$-hotspots using measurements of the inclusive $\alpha$-particle spectrum from the ${}^{12}\mathrm{C}(\gamma ,3\alpha )$ breakup reaction, induced by 14.8 MeV bremsstrahlung photons, and to correlate these features with radiobiological effects, such as DNA double-strand breaks (DSBs) and micronuclei formation. Results: Geant4 simulations incorporating TALYS-based differential cross sections and angular distributions calculated via distorted wave Born approximation (DWBA), fitted with experimental measurement, reveal that $\mu$-hotspots have a characteristic root-mean-square diameter of approximately $12\mathrm{\mu }\mathrm{m}$ and a mean spatial separation of $120.73\pm 0.0075\mathrm{\mu }\mathrm{m}$ within a $1{\mathrm{cm}}^3$ volume, assuming uniform distribution. The enhancement in DSBs induced by $\mu$-hotspots shows a strong linear correlation with micronuclei formation, indicating a potential mechanistic link. Conclusion: The microscale clustering effect of $\mu$-hotspots significantly increases the high-LET component of the radiation field, with critical implications for radiobiological effectiveness and therapeutic outcomes in photon-based cancer treatments.

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• Breakup reactions
• Cancer
• Nuclear reactions
• Photonuclear reactions
• Radiation therapy
• DNA
• Tumors
• Gamma-ray techniques