18F-FLT PET does not discriminate between reactive and metastatic lymph nodes in primary head and neck cancer patients.

Unlabelled: Repopulation of clonogenic tumor cells is inversely correlated with radiation treatment outcome in head and neck squamous cell carcinomas. A functional imaging tool to assess the proliferative activity of tumors could improve patient selection for treatment modifications and could be used for evaluation of early treatment response. The PET tracer 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) can image tumor cell proliferation before and during radiotherapy, and it may provide biologic tumor information useful in radiotherapy planning. In the present study, the value of (18)F-FLT PET in determining the lymph node status in squamous cell carcinoma of the head and neck was assessed, with pathology as the gold standard.

Methods: Ten patients with newly diagnosed stage II-IV squamous cell carcinoma of the head and neck underwent (18)F-FLT PET before surgical tumor resection with lymph node dissection. Emission (18)F-FLT PET and CT images of the head and neck were recorded and fused, and standardized uptake values (SUVs) were calculated. From all 18 (18)F-FLT PET-positive lymph node levels and from 8 (18)F-FLT PET-negative controls, paraffin-embedded lymph node sections were stained and analyzed for the endogenous proliferation marker Ki-67 and for the preoperatively administered proliferation marker iododeoxyuridine. The sensitivity, specificity, positive predictive value, and negative predictive value were calculated for (18)F-FLT PET.

Results: Primary tumor sites were oral cavity (n=7), larynx (n=2), and maxillary sinus (n=1). Nine of the 10 patients examined had (18)F-FLT PET-positive lymph nodes (SUV(mean): median, 1.2; range, 0.8-2.9), but only 3 of these patients had histologically proven metastases. All metastatic lymph nodes showed Ki-67 and iododeoxyuridine staining in tumor cells. In the remaining 7 patients, there was abundant Ki-67 and iododeoxyuridine staining of B-lymphocytes in germinal centers in PET-positive lymph nodes, explaining the high rate of false-positive findings. The sensitivity, specificity, positive predictive value, and negative predictive value of (18)F-FLT PET were 100%, 16.7%, 37.5%, and 100%, respectively.

Conclusion: In head and neck cancer patients, (18)F-FLT PET showed uptake in metastatic as well as in nonmetastatic reactive lymph nodes, the latter due to reactive B-lymphocyte proliferation. Because of the low specificity, (18)F-FLT PET is not suitable for assessment of pretreatment lymph node status. This observation may also negatively influence the utility of (18)F-FLT PET for early treatment response evaluation of small metastatic nodes.