Lung cancer with invasion of the interlobar pleura and minimal invasion of the adjacent lobe: which is the correct T determinant for a correct TNM staging?

Objective: Lung tumours invading the interlobar pleura with minimal invasion of the adjacent lobe (IAL) have been classified as T2 until the most recent TNM edition. Some authors questioned whether these tumors should be reclassified as T3. We analysed our experience of operated lung cancer patients with IAL in order to assess prevalence, correlations and prognostic significance in view of the forthcoming new TNM staging system. Methods: From January 1998 to October 2008 1521 lung cancer patients received resection at our Institution. Of these, 46 patients (3%) had IAL at the time of surgery requiring limited resection of the adjacent lobe in addition to resection of the primary lobe. There were 38 men and 8 women with a mean age of 65 years (range 46-82). Histologically, there were 24 adenocarcinomas, 13 squamous cell carcinomas, 5 bronchiolo-alveolar carcinomas, an 4 other types. N status was: N0 in 32 patients, N1 in 6 and N2 in 8 patients. Mean tumour dimension was 4 cm. (range 2-11). Prevalence, correlations and survival analysis either univariate and multivariate were performed in the population of patients with and without IAL at the time of surgery. For survival analysis, patients with IAL were compared with patients with operated T2 (visceral pleural) and T3 (parietal pleura) M0 stages in the same period. Patients with tumour involvement beyond parietal pleura were excluded. Results: Histology and N status were similar among the IAL patients as compared to the general population of operated NSCLC patients (p=0.1 and p=0.38 respectively). Logistic regression analysis using the presence/absence of IAL as dependent variable and several clinico-pathologic variables including grading, microscopic vascular invasion, tumour-infiltrating lymphocytes, tumour dimension and N factor revealed that the presence of IAL was significantly correlated with microscopic vascular invasion (OR 0.45, 95%CI 0.23-0.89, p=0.002). Five-year survival of patients with IAL, T1, T2 (visceral pleura) and T3 (parietal pleura) disease were 42%, 60%, 48% and 28% overall (p=0.00001), and 53%, 66%, 60% and 35% in N0 disease (p=0.00001). Survival of IAL was similar to that of T2 (visceral pleura) (p=0.40 overall and p=0.61 in N0 disease) and significantly higher than that of T3 (parietal pleura) disease (p=0.04 overall and p=0.05 in N0 disease). In multivariate analysis, the presence of IAL was associated with a nonsignificant increased risk of death (HR 1.08, 95%CI 0.66-1.76, p=0.75) as compared to visceral pleura involvement (HR 1.25, 95%CI 1.06-1.47, p=0.007) and parietal pleura involvement (HR 1.78, 95%CI 1.40-2.25, p=0.00002). Conclusions: In lung cancer, invasion of the interlobar pleura and minimal invasion of the adjacent lobe is observed far less frequently than involvement of visceral or parietal pleura. IAL is associated with microscopic vascular invasion. Patients with IAL have a prognosis more similar to T2-visceral pleura disease, and significantly better than T3 parietal pleura disease. Maintenance of the present TNM classification for these patients seems justifiable.