Abstract:Objectives: To assess the genotoxic effect of a new antitumor ozone-photodynamic therapy using the improved modification of the COMET assay. Methods: Xenograft cancer models on 58 rats were used. The sarcoma RA was transplanted subcutaneously, and after increasing of tumor volume from 0.5 to 4.2 cm 3 , rats were divided into the four groups: "Intact"-healthy,"Control"-with xenografted tumors and no treatment, "PDT"-the rats treated with the photodynamic therapy, "PDT + ozone"-the rats were treated with both photodynamic therapy and injections of ozonated saline solution. The toxicity of treatment was assessed by DNA damage in leukocytes using the new modification of the COMET assay. The analysis of the "COMETs" was performed following the percentage of DNA in the tail of the "COMET" (% TDNA). Results: A combination of PDT and ozone makes the strongest negative impact on tumor growth. The tumor growth inhibition is associated with low genotoxic exposure of ozone-photodynamic therapy on whole blood leukocytes of cancer rats. Conclusions: A new modification of the COMET assay can provide the assessment of the genotoxic effect of the antitumor therapy in experimental neoplasia.