Can FDG-PET assist in radiotherapy target volume definition of metastatic lymph nodes in head-and-neck cancer?

D. Schinagl, A. Hoffmann, W. Vogel, J. van Dalen, S. Verstappen, W. Oyen and J. Kaanders

Department of Radiation Oncology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands.
Apr, 2009



The role of FDG-PET in radiotherapy target volume definition of the neck was evaluated by comparing eight methods of FDG-PET segmentation to the current CT-based practice of lymph node assessment in head-and-neck cancer patients.Seventy-eight head-and-neck cancer patients underwent coregistered CT- and FDG-PET scans. Lymph nodes were classified as "enlarged" if the shortest axial diameter on CT was 10mm, and as "marginally enlarged" if it was 7-10mm. Subsequently, lymph nodes were assessed on FDG-PET applying eight segmentation methods: visual interpretation (PET(VIS)), applying fixed thresholds at a standardized uptake value (SUV) of 2.5 and at 40\% and 50\% of the maximum signal intensity of the primary tumor (PET(SUV), PET(40\%), PET(50\%)) and applying a variable threshold based on the signal-to-background ratio (PET(SBR)). Finally, PET(40\%N), PET(50\%N) and PET(SBRN) were acquired using the signal of the lymph node as the threshold reference.Of 108 nodes classified as "enlarged" on CT, 75\% were also identified by PET(VIS), 59\% by PET(40\%), 43\% by PET(50\%) and 43\% by PET(SBR). Of 100 nodes classified as "marginally enlarged", only a minority were visualized by FDG-PET. The respective numbers were 26\%, 10\%, 7\% and 8\% for PET(VIS), PET(40\%), PET(50\%) and PET(SBR). PET(40\%N), PET(50\%N) and PET(SBRN), respectively, identified 66\%, 82\% and 96\% of the PET(VIS)-positive nodes.Many lymph nodes that are enlarged and considered metastatic by standard CT-based criteria appear to be negative on FDG-PET scan. Alternately, a small proportion of marginally enlarged nodes are positive on FDG-PET scan. However, the results are largely dependent on the PET segmentation tool used, and until proper validation FDG-PET is not recommended for target volume definition of metastatic lymph nodes in routine practice.