Purpose: Interstitial growth (IG), which is defined as tumor cells continuously growing into the alveolar septa at the tumor periphery, was originally reported as a growth pattern of metastatic sarcoma of the lung. On the other hand, IG in the primary lung cancers has not been well described. This study aimed to examine clinicopathological features of primary lung cancer that harbors IG. Methods: A total of 2558 primary lung cancers which were resected from 2003 to 2012 in our hospital were examined for IG. We compared clinicopathological data and prognoses between patients with IG+ and IG− specimens. Results: Thirty-three cases out of 2558 (1.3 %) had IG components. IG was significantly more associated with positive smoking history, advanced pathological stage, presence of vascular invasion and pleural invasion. Thirty-three IG+ cases include nine pleomorphic carcinoma, nine squamous cell carcinoma and eight adenocarcinoma. Interestingly, nine (24 %) out of 38 pleomorphic carcinoma specimens had IG components, which was a higher rate than any other histological subtypes. The IG+ cancers had significantly shorter overall and recurrence-free survival than did the IG− cancers. Conclusions: We firstly reported on IG in various types of primary lung cancer. IG appears to be a sign of an aggressive lung cancer phenotype, mainly found in pleomorphic carcinoma.
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