An interesting article from Japan was published out in the Journal of Thoracic Oncology that asks how long a duration of follow-up imaging of a ground-glass opacity (GGO) is really needed to be confident it’s going to remain stable and not grow. It’s very common to see small lung nodules that are ambiguous in their significance, for which follow-up scans are typically recommended rather than diving into a biopsy, and non-solid, hazy GGOs are another form of lung lesion that might possibly represent a lung cancer but are also the way a little inflammation or small infection would appear. Even when they turn out to be something technically called cancer based on its appearance under the microscope, it’s often a non-invasive adenocarcinoma (sometimes termed bronchioloalveolar carcinoma, or BAC, but shifting in terminology to adenocarcinoma in situ, or AIS) or minimally invasive adenocarcinoma (MIA), in which the invasive component is less than 5 mm in diameter. Even when they grow, it can be at an extremely slow pace.
It’s not uncommon for a question here to be about the a pathologist’s terminology on a report that equivocates about whether a lesion is bronchioloalveolar carcinoma (BAC) or another form of adenocarcinoma, perhaps “well-differentiated adenocarcinoma”, especially if it has a radiographic appearance of a hazy infiltrate or many small ground glass opacities. Meanwhile, there’s a new classification of lung cancer subtypes that obliterates the term BAC, instead favoring a definition of adenocarcinoma in situ, classifying small non-invasive lesions previously called BAC as a pre-malignant condition. How have the changes over the past few years changed how we should approach BAC?
I would have to say that the new reassignment of BAC as adenocarcinoma in situ hasn’t taken the lung cancer world by storm and that I still think of the clinical entity as BAC. For the preceding decade, the definition the pathologist’s used technically excluded a lesion with even 1% or 5% of the lesion being invasive as being called BAC, even if it acted for all the world like BAC. Clinicians learned not to be too hung up on a pathologist’s technical definitions and tended to define BAC more functionally/operationally. General oncologists and expert lung cancer specialists alike managed BAC based on the overall picture of how it behaved if it looked like a BAC pattern.
This month’s Journal of Thoracic Oncology includes a landmark article, written by a multidisciplinary group of lung cancer experts that features several of the leading lung cancer pathologists in the world, that is attempting to do no less than present a new categorization of the pathology of lung cancer, focusing primarily on adenocarcinomas, but also touching on other lung cancer subgroups. Among the concepts included is a proposal that the term bronchioloalveolar carcinoma (BAC) be discontinued and recategorized based on whether patients have any invasive component to their lung cancer or not, and whether is is mucinous or non-mucinous. As significant as it is to redefine an entire disease of BAC, the concepts it introduces have implications that actually lead to a fundamentally different way of thinking about lung cancer, or at least NSCLC.
Bronchioloalveolar carcinoma, or BAC, is a unique subtype of non-small cell lung cancer (NSCLC) that has unique features in terms of the demographics of who gets it, how it appears on scans, how it often behaves, and potentially in how it responds to treatment. It is a subset of lung cancer for which most of what we know emerged in the last 10 years, with our understanding of this entity, and even the definition of BAC, still evolving.
What is BAC?
BAC was first identified and defined as a separate subtype of lung cancer by Dr. Averill Liebow in 1960. At that time, he highlighted it as a form of well differentiated adenocarcinoma of the lung that appeared to not be able to invade the surrounding lung scaffolding and spread within the lung(s), presumably aerogenously and/or through lymphatic channels.
In one of my earliest posts about bronchioloalveolar carcinoma (BAC) (in the dark ages, pre-Twitter), I wrote on the subject of managing small BAC-type lesions, which tend to appear as small hazy areas called “ground glass opacities” (GGOs) and suggested that some of these cancers may be so indolent that they don’t need to be treated, even if they have the word “carcinoma” in the diagnosis.
Now there is a proposal to change BAC to “adenocarcinoma in situ“, a pre-cancerous condition, reflecting the idea that these lesions have such a favorable prognosis that they shouldn’t necessarily be put in the same category as invasive lung cancers (pure BAC is a non-invasive lesion that shouldn’t be able to get into the bloodstream and spread outside of the lungs). And now, there’s a new article out of Japan that describes the experience of patients with BAC and multiple GGOs, some of which were resected and some not very accessible and some just watched. It turned out that just watching seemed to be a pretty good strategy.