Evaluation of hypoxia in an experimental rat tumour model by [18F]Fluoromisonidazole PET and immunohistochemistry

This study aimed to evaluate tumour hypoxia by comparing [F-18] Fluoromisonidazole uptake measured using positron emission tomography ([F-18] FMISO-PET) with immunohistochemical (IHC) staining techniques. Syngeneic rhabdomyosarcoma (R1) tumour pieces were transplanted subcutaneously in the flanks of WAG/Rij rats. Tumours were analysed at volumes between 0.9 and 7.3 cm(3). Hypoxic volumes were defined using a 3D region of interest on 2 h postinjection [F-18] FMISO-PET images, applying different thresholds (1.2 - 3.0). Monoclonal antibodies to pimonidazole (PIMO) and carbonic anhydrase IX ( CA IX), exogenous and endogenous markers of hypoxia, respectively, were used for IHC staining. Marker-positive fractions were microscopically measured for each tumour, and hypoxic volumes were calculated. A heterogeneous distribution of hypoxia was observed both with histology and [F-18] FMISO autoradiography. A statistically significant correlation (P<0.05) was obtained between the hypoxic volumes defined with [F-18] FMISO-PET and the volumes derived from the PIMO-stained tumour sections ( r = 0.9066; P = 0.0001), regardless of the selected threshold between 1.4 and 2.2. A similar observation was made with the CA IX staining ( r = 0.8636; P = 0.0006). The relationship found between [F-18] FMISO-PET and PIMO- and additionally CA IX-derived hypoxic volumes in rat rhabdomyosarcomas indicates the value of the noninvasive imaging method to measure hypoxia in whole tumours.