BAC pattern

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BAC pattern

Bronchioloalveolar carcinoma pattern. A term of art referring to a so-called lepidic (hobnail-like cells spreading across a surface) growth pattern of a tumour that mimics classic bronchioloalveolar carcinoma.
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These relate to the discontinuation of the terms bronchioloalveolar carcinoma and adenocarcinoma, mixed subtype, as well as the introduction of micropapillary as a new histologic subtype, the term lepidic pattern for the former bronchioloalveolar carcinoma growth pattern, and the specific term invasive mucinous adenocarcinoma for overtly invasive tumors previously classified as mucinous bronchioloalveolar carcinoma.
If a small biopsy shows a totally lepidic pattern of growth in the sample (Figure 9, A and B), the diagnosis should be adenocarcinoma with lepidic pattern, and a comment should be made that this could be from AIS, MIA, or an adenocarcinoma with a lepidic pattern, whether it is lepidic-predominant adenocarcinoma or an overtly invasive adenocarcinoma with a minor lepidic component.
Minimally invasive adenocarcinoma is a small, solitary adenocarcinoma ([less than or equal to] 3 cm), with a predominantly lepidic pattern and invasion of 5 mm or less in greatest dimension in any one focus (Table 2).
The invasive component to be measured in MIA is defined as follows: (1) histologic subtypes other than a lepidic pattern (ie, acinar, papillary, micropapillary, and/or solid) or (2) tumor cells infiltrating myofibroblastic stroma.
Invasion is defined as (1) histologic subtypes other than a lepidic pattern (ie, acinar, papillary, micropapillary, and/or solid) and/or (2) myofibroblastic stroma associated with invasive tumor cells (Figure 6, C).
Tumors with the lepidic pattern predominant do relatively well, whereas those that are predominantly solid or micro-papillary do relatively badly.
6,10,26,27) Averill Liebow, MD, originally coined the term to refer to peripheral adenocarcinomas in which the malignant cells grew in a row along the intact alveolar septa, replacing the healthy epithelial cells that lined the alveolar septa (called the lepidic pattern of cancer growth).
Beginning with the 1999 WHO classification, (16) bronchioloalveolar carcinoma was defined as an adenocarcinoma growing in a lepidic pattern along intact alveolar septa without invasion into the underlying stroma, pleura, or lymphovascular spaces (Figures 3 and 4).
However, even today, it is also used to describe the lepidic pattern often seen at the periphery of mixed-pattern adenocarcinomas.
Following the belief that solitary small BAC is in situ carcinoma, numerous studies with slightly different methodologies announced that a variety of light microscopic findings, including the size of the parenchymal scar, percentage of lepidic growth, percentage of papillary growth, presence of vascular invasion, size of invasive focus within BAC, and location of stromal invasion with respect to the scar and the lepidic pattern, hold prognostic significance and even confirm the current adenocarcinogenesis model.