Magnetic resonance imaging correlated with the histopathological effect of Pd‐bacteriopheophorbide (Tookad) photodynamic therapy on the normal canine prostate gland

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Abstract
Background and Objective To determine the optimal magnetic resonance imaging (MRI) methodology to assess photodynamic therapy (PDT)‐induced histopathological responses in the prostate. Study Design/Materials and Methods Laparotomy was performed in five healthy dogs. Cylindrical diffuser was placed in the prostates to deliver light of 50–300 J/cm at 150 mW/cm and 763 nm to activate IV‐injected Tookad (1 mg/kg b.w.). Fast spin echo (FSE) T2‐weighted, post‐contrast‐enhanced T1‐(CE‐T1) and diffusion weighted images (DWI) were obtained pre‐ and 2 days, 7 days, and 1 month post‐PDT. Radiological‐histopathological correlation was performed at 7 days (n = 4) and 1 month (n= 1) after PDT. A qualitative assessment of signal changes and apparent diffusion coefficient (ADC) mapping was performed. Results At 2 or 7 days post‐PDT, there was good spatial correlation between PDT‐induced hemorrhagic necrosis and unenhanced regions on CE‐T1 images. There was a rapidly and persistently enhancing rim corresponding to edema and inflammation. FSE T2 and DWI showed altered signal but did not clearly define necrosis in all cases. At 1 month, it was hard to correlate MR images to histopathologic changes as they represented a mixture of necrosis and developing fibrosis, which led to a mixed signal intensity and less demarcated contrast enhancement. Conclusions At 7 days after PDT, gadolinium DTPA contrast‐enhanced MRI is superior to DWI and T2 imaging in assessing the boundary of Tookad PDT‐induced tissue necrosis in the normal canine prostate. Lasers Surg. Med.