Or lesions that were smaller than 0.15 cm3 and tumor lesions that were between 0.15 cm3 and 0.5 cm3. Similar to the Pemafibrate site PyV-mT mammary carcinomas, we did not observe any significant differences in [18F] FAZA uptake between the small and medium sized CT26 colon carcinomas. However, in the CT26 tumor model, there was a tendency toward a higher tumor uptake with increasing tumor size, which is in contrast to the PyV-mT model in which the opposite effect was observed (Table 1). Investigations of the [18F]FAZA clearance revealed a significant relative clearance of 40 ?15 in the endogenous PyV-mT mammary carcinomas (mice: n = 9, tumors: n = 20) and a similar significant relative clearance of 39 ?12 in the exogenous CT26 colon carcinomas (n = 10) 3 h after tracer injection (p < 0.01 compared to ID/cc 1 h after injection, Figure 4). The [ 18 F]FAZA clearance was significantly higher in the muscle tissue of the PyV-mT mammary carcinoma (73 ?6 ; C57BL/6) and the CT26 colon carcinoma-bearing mice (72 ?5 ; BALB/c) compared to the mammary and colon carcinomas (Figure 4, p < 0.01). Importantly, bothMaier et al. Radiation Oncology 2011, 6:165 http://www.ro-journal.com/content/6/1/Page 6 of1h p.i.2h p.i. time3h p.i.Figure 4 [18F]FAZA tumor and muscle clearance in both tumor models. [18F]FAZA clearance is displayed in both animal models (relative to 1 h after injection) with a higher muscle clearance (** p < 0.01; PyV-mT C57/BL6, n = 9; CT26 colon carcinoma-bearing BALB/c, n = 10).Figure 2 [18F]FAZA tumor and muscle uptake in both tumor models. The [18F]FAZA muscle clearance is clearly more pronounced compared to tumor clearance (** p < 0.01; PyV-mT C57/BL6, n = 9; CT26 colon carcinoma-bearing BALB/c, n = 10). Note that there is no difference in between the two animal models in terms of tracer clearance from both tumor and muscle tissue (breathing protocol P0, air).tumor types showed a similar tracer clearance (p = 0.90). The rapid clearance from the non-hypoxic muscle tissue compared to the hypoxic tumor tissue explains the improved detectability of hypoxic tumor tissue at later scan time points (Figure 2, Figure 4). To exclude necrotic areas in the CT26 colon carcinomas, we also performed [18F]FDG scans on some of themice. Importantly, we did not identify any impaired [18F]FDG uptake in the CT26 colon carcinomas, which is a sign of necrosis (Figure 5A). This was further confirmed by H E staining of the slices of the CT26 colon carcinomas, which displayed no signs of necrotic tumor tissue (Figure 5B). The correlation between in vivo [18F] FAZA scans (Figure 5C) and ex vivo pimonidazole immunohistochemistry staining of the hypoxic areas in slices of CT26 colon carcinomas (Figure 5D) further confirmed the specificity of [18F]FAZA to hypoxic tumor tissue.Oxygen incubation time-dependent [18F]FAZA tumor uptakeWe analyzed the role of oxygen breathing during the preincubation time (2 min vs. 1 h) and during the first hour after [ 18 F]FAZA-injection using three different breathing protocols with different air or 100 oxygen incubation times (Figure 1A) in the CT26 colon carcinoma-bearing mice. Moreover, we analyzed the role of air breathing between the 1 h, 2 h, and 3 h PET scans (P1, P2). During the scans, the animals were placed in a custom-made chamber on the scanner bed (Figure 1B). Tumor-to-muscle ratios of [18F]FAZA 2 h and 3 h p.i. were significantly higher (p < 0.01) when the mice PubMed ID:https://www.ncbi.nlm.nih.gov/pubmed/26104484 breathed air (3.56 ?0.55; P0) compared to the oxygen breathing protocol.