Bronchoalveolar carcinoma

In 1999, the World Health Organization (WHO) redefined bronchioloalveolar carcinoma (BAC) to include only those lesions with a so-called pure lepidic (classic) growth pattern with no evidence of invasion by a tumor. If stromal, vascular, or pleural invasion are present, the neoplasm is classified as an adenocarcinoma, mixed type, with bronchioloalveolar features. This narrow definition was applied because lesions with these features that measure less than 2 cm are often curable. Those with an invasive component are much more likely to recur and metastasize. BAC is the only subtype of adenocarcinoma with a substantially better prognosis than other forms of lung cancer. BAC is an uncommon type of pulmonary adenocarcinoma: the new WHO classification makes it even more uncommon.

Immunohistochemistry

reference 1:

non-mucinous tumours: mean staining score (1=mild, 2=moderate, 3=strong staining)
proportion of lepidic non-invasive component	CK7		CK20		TTF-1
proportion of lepidic non-invasive component	lepidic component	invasive component	lepidic component	invasive component	lepidic component	invasive component
100%	2.8	-	0	-	2.5	-
75%-99%	3.0	2.6	0	0.1	2.9	2.2
50%-74%	3.0	2.6	0	0	3.0	2.6
25%-49%	3.0	2.8	0	0.2	2.8	1.8
0%-24%	3.0	2.5	0	0.3	3.0	2.3

mixed mucinous/non-mucinous tumours: mean staining score (1=mild, 2=moderate, 3=strong staining)
proportion of lepidic non-invasive component	CK7		CK20		TTF-1
proportion of lepidic non-invasive component	lepidic component	invasive component	lepidic component	invasive component	lepidic component	invasive component
75%-99%	2.7	2.7	0	0.1	2.3	2.0
50%-74%	3.0	2.5	0	0	2.0	1.5
0%-24%	3.0	3.0	0	0.3	3.0	2.0

mucinous tumours: mean staining score (1=mild, 2=moderate, 3=strong staining)
proportion of lepidic non-invasive component	CK7		CK20		TTF-1
proportion of lepidic non-invasive component	lepidic component	invasive component	lepidic component	invasive component	lepidic component	invasive component
100%	3.0	3.0	1.0	1.0	1.0	0.7
75%-99%	3.0	2.5	1.0	1.0	1.0	0.5

Nonmucinous and mixed BACs show identical staining patterns for CK7 and TTF-1. The mucinous tumours show disparate staining for CK7, CK20, and TTF-1. Because the staining pattern of mucinous BAC differs from that of all other lung neoplasms, immunohistochemistry is less helpful in distinguishing primary tumours from metastatic lung tumours.

Differential diagnosis

There is a case report of a mesothelioma masquerading as a bronchioloalveolar carcinoma with multiple discrete nodules in the pleura and lung, the latter consisting of tall columnar malignant cells arranged on alveolar surfaces in a lepidic growth pattern2.

References

1 Sarantopoulos GP, Gui D, Shintaku P, et al. Immunohistochemical analysis of lung carcinomas with pure or partial bronchioloalveolar differentiation. Arch Pathol Lab Med 2004; 128:406-14

2 Felner KJ, Wieczorek R, Kline M, et al. Malignant mesothelioma masquerading as a multinodular bronchioloalveolar cell adenocarcinoma with widespread pulmonary nodules. Int J Surg Pathol 2006; 14:229-33