A Dosimetric Reappraisal of the Photobiomodulation Literature Assessing Animal Safety and Phototoxicity Evidence for Photobiomodulation in Oncology: Phantom Dangers

Abstract

Five studies are frequently cited as experimental evidence that photobiomodulation (PBM) may stimulate tumour growth or produce phototoxicity, contributing to a documented barrier to clinical adoption in oncology. We reappraise these studies against their own reported dosimetry. None characterised the spatial power distribution of its beam. Each was assessed for arithmetic consistency, beam profile, device category, and biological-model appropriateness, and re-examined against the Arrhenius framework for thermal damage. In four of the five, the exposures delivered lay outside the therapeutic photobiological window on the authors' own stated parameters: surface temperatures of 55 °C and, in places, 71 °C, with injury the original authors describe as coagulation and necrosis extending to paralysis and death; irradiances approximately 25 times those used clinically; and, in one case, parameters the authors themselves describe as high. The fifth, a xenograft study, used a dose within the therapeutic range, but its immunodeficient host cannot express the immune-mediated response at issue. A sixth study, from the same group and using the same device as one of the five but operated at a therapeutic irradiance, is included as an internal control and reported therapeutic benefit. Injury in these regimes is best understood as oxygen-dependent, with temperature acting as a sensitising variable and as a marker of the regime rather than as the proximate cause, a reading supported by the original authors' own helium-substitution, cooling and heated-probe controls. We outline a two-tier mechanistic framework, in which correctly dosed PBM may engage systemic anti-tumour immunity, as a hypothesis for prospective testing rather than as a finding of this analysis. None of these studies provides dosimetrically secure evidence of a hazard from therapeutic-range PBM; equally, the absence of such evidence is not a demonstration of safety. Beam-profile reporting should become standard in PBM oncology research.