Pattern and predictors of occult mediastinal lymph node involvement in non-small cell lung cancer patients with negative mediastinal uptake on positron emission tomography

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We sought to assess the incidence, pattern and predictors of occult mediastinal lymph node involvement (N2) in non-small cell lung cancer patients with negative mediastinal uptake of 2-deoxy-2-[(18)F]-fluoro-d-glucose ((18)FDG) on integrated positron emission tomography-computerised tomography (PET-CT). All patients who underwent surgical resection in our unit over a 30-month period were reviewed (n=215). All patients had preoperative PET-CT prior to lung resection as an adjunct to a dedicated chest CT. Diabetic patients, patients who received neoadjuvant chemotherapy and those with positive mediastinal nodes on PET-CT (N2/N3) were excluded from this study. The population of interest was 153 non-small cell cancer patients (NSCLC), all of which had no FDG uptake in the mediastinum. No preoperative mediastinoscopy was carried out in this group and all underwent curative intent surgical resection. The pathological results were retrospectively reviewed and correlated with CT and integrated PET-CT findings. The incidence of occult N2 disease in NSCLC patients with negative mediastinal uptake of (18)FDG on PET-CT was 16% (25 of 153). The highest incidence of occult N2 involvement was in American thoracic society (ATS) 7 (16 of 25 patients, 64%) followed by ATS 4 (seven patients of 25, 28%). In univariate analysis, the following were significant predictors of occult N2 disease: centrally located tumours (P=0.049), right upper lobe tumours (P=0.04), enlarged lymph nodes (>1cm) on CT (P=0.048) and PET positive uptake in N1 nodes (P=0.006). In multivariate analysis, the following were independent predictors of occult N2 disease: centrally located tumours, right upper lobe tumours and PET positive uptake in N1 nodes (P<0.05). In NSCLC patients who are clinically staged as N2/N3 negative in the mediastinum by integrated PET-CT, 16% will have occult N2 disease following resection. Patients with the following: centrally located tumours, right upper lobe tumours and positive N1 nodes on PET should have preoperative cervical mediastinoscopy to rule out N2 nodal involvement, especially in ATS stations 7 and 4 as the incidence of occult nodal metastasis in these nodes is high. This study has potential implications in decision-making and planning best treatment approach.