(A) Evaluation of relative tumor volume in untreated (control), untreated and 2 Gy irradiated (control 2 Gy) and histamine-treated and 2 Gy irradiated animals (Histamine 2 Gy). Tumor volumes were measured by day and non-linear regression fit was performed to evaluate the exponential growth, (Repeated Measures ANOVA and Dunnet post-test, *P < 0.01 Histamine 2 Gy vs. control; Two-way ANOVA and Bonferroni post-test #P < 0.05, ###P < 0.001 Histamine 2 Gy vs. control). (B) Histological and immunohistochemical analysis. Formalin-fixed paraffin embedded tissue sections of the different groups were stained to evaluate histopathological characteristics (H&E), proliferation (PCNA) and apoptosis (TUNEL). 630X original magnification. Scale bar = 20 μm.