UNLABELLED: Treatment of head and neck squamous cell carcinomas with radiotherapy and the epidermal growth factor receptor (EGFR) inhibitor cetuximab shows an improved response in a subgroup of patients. The aim of this study was to noninvasively monitor treatment response by visualizing systemically accessible EGFR with (111)In-cetuximab-F(ab')2 while simultaneously evaluating tumor metabolism with (18)F-FDG PET during combined-modality treatment. METHODS: Eighty mice with patient-derived head and neck squamous cell carcinomas xenografts, SCCNij202 or SCCNij185, were imaged with SPECT/CT using (111)In-cetuximab-F(ab')2 (5 μg, 28 ± 6.1 MBq, 24 h after injection), followed by PET imaging with (18)F-FDG (9.4 ± 2.9 MBq, 1 h after injection). Scans were acquired on mice 10 d before treatment with either single-dose irradiation (10 Gy), cetuximab alone, or cetuximab-plus-irradiation combined or on untreated control mice. Scans were repeated 18 d after treatment. Tumor growth was monitored up to 120 d after treatment. EGFR expression was evaluated immunohistochemically. RESULTS: SCCNij202 responded to combined treatment (P < 0.01) and cetuximab treatment alone (P < 0.05) but not to irradiation alone (P = 0.13). SCCNij185 responded to combined treatment (P < 0.05) and irradiation (P < 0.05) but not to cetuximab treatment alone (P = 0.34). (111)In-cetuximab-F(ab')2 uptake (tumor-to-liver ratio, scan 2 - scan 1) predicted response to therapy. A positive response to treatment significantly correlated with a reduced tracer uptake in the tumor in the second SPECT scan, compared with the first scan (P < 0.005 and <0.05 for SCCNij202 and SCCNij185, respectively). Resistance to therapy was characterized by a significantly increased (111)In-cetuximab-F(ab')2 tumor uptake; tumor-to-liver ratio was 2.2 ± 0.6 to 3.5 ± 1.2, P < 0.01, for (irradiated) SCCNij202 and 1.4 ± 0.4 to 2.0 ± 0.3, P < 0.05, for (cetuximab-treated) SCCNij185, respectively. (18)F-FDG PET tumor uptake (maximum standardized uptake value, scan 2 - scan 1) correlated with tumor response for SCCNij202 (P < 0.01) but not for SCCNij185 (P = 0.66). EGFR fractions were significantly different: 0.9 ± 0.1 (SCCNij202) and 0.5 ± 0.1 (SCCNij185) (P < 0.001). The EGFR fraction was significantly lower for irradiated SCCNij202 tumors than for controls (P < 0.005). CONCLUSION: (111)In-cetuximab-F(ab')2 predicted and monitored the effects of EGFR inhibition or irradiation during treatment in both head and neck carcinoma models investigated, whereas (18)F-FDG PET only correlated with tumor response in the SCCNij202 model. Thus, the additional value of the (111)In-cetuximab-F(ab')2 tracer is emphasized and the tracer can aid in evaluating future treatments with EGFR-targeted therapies.