Recently, a new classification of lung adenocarcinomas has been proposed for tumors with lepidic spread. The greatest diameter of the invasive component determines minimally invasive cancers, and the term bronchioloalveolar carcinoma is no longer used.
We retrospectively reviewed 87 resected adenocarcinomas of the lung; 30 tumors with lepidic growth and without high-grade invasive areas were identified, and the invasive component was measured morphometrically and categorized. A dimension of 5 mm was the cutoff for invasion. Regional lymph node involvement and short-term follow-up were compared among subtypes of these well-differentiated and moderately differentiated adenocarcinomas.
There were 11 well-differentiated adenocarcinomas with lepidic growth: 3 adenocarcinomas in situ (nonmucinous) and 8 minimally invasive adenocarcinomas (MIAs) (4 mucinous and 4 nonmucinous). There were 19 invasive moderately differentiated adenocarcinomas with a prominent lepidic growth pattern (LPAs). The mean size of the 3 adenocarcinomas in situ cases was 0.9±0.7 mm; the total size of the 8 MIA cases was 1.4±1.8 cm and that of the 19 LPA cases was 3.2±2.1 cm. The invasive size of the MIA was 0.3±0.6 and that of the LPA was 2.2±0.3. The invasive pattern of the LPAs was papillary and acinar without desmoplasia (n=3) and acinar with desmoplasia (n=16). Seven of the invasive desmoplastic tumors showed complex single-cell invasion or lymphatic invasion. Identification of the transition from lepidic to invasive acinar was straightforward because of the presence of elastotic desmoplasia. The transition between complex acinar papillary invasion and lepidic growth was often difficult to discern. Lymph node metastases were present in 5 cases (26%), all in tumors with an acinar, desmoplastic invasive component of >1 cm, with areas of single-cell invasion. With follow-up, progressive nodal involvement or distant metastases occurred in 4 patients, all with complex invasive patterns; 3 with invasion >1 cm and 1 with lymphatic invasion in smaller invasive tumors. Recurrent lung nodules occurred in 5 patients, including 1 patient with MIA, 1 with nondesmoplastic invasion, 2 with desmoplastic invasion, and 1 with complex desmoplastic invasion.
Approximately one third of lung adenocarcinomas have significant lepidic spread, and of these nearly one third are minimally invasive. Measurement of the invasive component may be difficult without elastotic desmoplasia. In this small series, lymph node and distant metastases occurred only in those with complex invasive patterns, but lung recurrence occurred in all subtypes, including MIAs.