Identification of the hypoxia regions within the primary foci and imaging time selection in nasopharyngeal carcinoma 18F-fluoromisonidazole imaging

Objective To explore the identification of the hypoxia regions within the primary foci and imaging time selection in 18F-fluoromisonidazole (FMISO) imaging on patients with nasopharyngeal carcinoma (NPC). Methods From July 2013 to July 2014, 44 NPC patients (33 males, 11 females, age range: 18-74(53.45±12.88) years) underwent 18F-FMISO PET/CT imaging, including 3 cases with twice imaging (totally 47 case times). Imaging data were acquired and reconstructed 2 and 4 h after the injection of 18F-FMISO. 18F-FMISO PET/CT images were merged with MRI images obtained 1 week before to construct fusion images. The boundary of primary tumor was determined based on MRI. Visual analysis was performed and SUVmax of posterior cervical muscles, NPC primary foci was measured by 2 observers respectively. The uptake ratio of primary tumor to muscle (TMR) was calculated. The identify consistency of hypoxic region between two observers were evaluated by Kappa test and intraclass correlation coefficient (ICC). The image contrast was evaluated by Wilcoxon paired rank sum test of TMR. Results PET images and MRI images of NPC primary foci were successfully fused. Positive non-NPC tissues were identified by MRI. The visual recognition of hypoxic regions of the two observers for 2 and 4 h imaging were highly consistent (Kappa=0.931 and 0.965, both P<0.001). There was a high degree of consistency between the SUVmax of posterior cervical muscles and that of primary tumors. ICCs of posterior cervical muscles in 2 and 4 h were 0.896 (95% CI: 0.814-0.942) and 0.924 (95% CI: 0.865-0.958), respectively. ICCs of primary tumors in 2 and 4 h were 0.991 (95% CI: 0.985-0.995) and 0.998 (95% CI: 0.996-0.999), respectively. TMRs (M(P25, P75)) in 2 and 4 h were 1.560 (1.341, 3.015) and 1.675 (1.387, 3.001) respectively in 24 positive case times, and the difference was statistically significant (z=-2.557, P<0.05). Conclusions Using fusion images of 18F-FMISO PET and MRI, hypoxic tissues within NPC primary foci can be accurately identified. There is a high degree of consistency within the visual and quantitative analysis of two observers. The image contrast of 18F-FMISO PET at 4 h is superior to that at 2 h. Key words: Nasopharyngeal neoplasms; Cell hypoxia; Tomography, emission-computed; Magnetic resonance imaging; Nitromidazoles