7187 Background: The aim of this study was to investigate the pattern of mediastinal nodal involvement in NSCLC in relation to primary tumour location using PET imaging. METHODS: A database of 800 patients scanned for suspected lung cancer was studied. The locations of the primary tumour and of any nodal metastases were noted. Nodes were considered positive if the uptake was significantly raised relative to the surrounding region. The study was intended to generate hypotheses for potential areas of research. RESULTS: [Figure: see text] UZ = upper zone, MZ = mid-zone, LZ = lower zone, IH = ipsilateral hilar, SC = subcarinal, TB = tracheobronchial, PT = paratracheal, CH = contralateral hilar, AP/SM = Aortopulmonary /Superior Mediastinal, N+ = node positive. In IHN positive patients 14%, 6% and 23% respectively of UZ, MZ and LZ tumours showed increased activity in the SCN, 11%, 6% and 5% in the TBN and 17%, 7% and 14% in the CHN. In IHN negative patients the corresponding figures were 18%, 27% and 70% for SCN, 14%, 18% and 0% for TBN and 21%, 18% and 10% for CHN. Increased activity was seen in the TBN and SCN of 24 and 55 patients respectively, only 5 of these in the same patients. CONCLUSIONS: Hypotheses consistent with these results are: 1. IHN involvement is similar regardless of primary tumour location. 2. SC nodal involvement is more common with LZ tumours. 3. TB nodal involvement is more common with UZ tumours. 4. The CH nodes are more commonly involved than other nodal areas more frequently sampled or treated in routine clinical practice. This could mean that some NSCLC patients are significantly undertreated. 5. The TB and SC node groups might represent 2(nd) station nodes of distinct lymphatic pathways from IH nodes. These results give valuable information for further research into patterns of nodal spread in NSCLC aimed at improving radiotherapy and surgical planning. No significant financial relationships to disclose.