Adenocarcinoma in situ of the lung

The genes mutated in AIS differ based on exposure to tobacco smoke. Non-smokers with AIS commonly have mutations in EGFR (a driver) or HER2 (an important oncogene), or a gene fusion with ALK or ROS1 as one of the elements.[3]

Nonmucinous AIS is thought to derive from a transformed cell in the distal airways and terminal respiratory units, and often shows features of club cell or Type II pneumocyte differentiation.[4] Mucinous AIS, in contrast, probably derives from a transformed glandular cell in distal bronchioles.[5]

A multi-step carcinogenesis hypothesis suggests a progression from pulmonary atypical adenomatous hyperplasia (AAH) through AIS to invasive adenocarcinoma (AC), but to date this has not been formally demonstrated.[6]

Type-I cystic adenomatoid malformation (CAM) has recently been identified as a precursor lesion for the development of mucinous AIS, but these cases are rare.[7][8]

Rarely, AIS may develop a rhabdoid morphology due to the development of dense perinuclear inclusions.[9]

The criteria for diagnosing pulmonary adenocarcinoma have changed considerably over time.[10][11] The 2011 IASLC/ATS recommendations, adopted in the 2015 WHO guidelines, use the following criteria for adenocarcinoma in situ: [12]

• tumor ≤3 cm
• solitary tumor
• pure "lepidic" growth*[13]
• No stromal, vascular, or pleural invasion
• No histologic patterns of invasive adenocarcinoma
• No spread through air spaces
• Cell type mostly nonmucinous
• Minimal/absent nuclear atypia
• ± septal widening with sclerosis/elastosis

* lepidic = (i.e. scaly covering) growth pattern along pre-existing airway structures

A true diagnosis of AIS can only be made once the entire tumor has been surgically removed, because it must be evaluated for evidence of microscopic invasion into the adjacent normal lung. Findings of malignant invasion would mean that the tumor must be reclassified as invasive adenocarcinoma.

By this standard, AIS cannot be diagnosed according to core biopsy or cytology sampling.[2] Recommended practice is to report biopsy findings previously classified as nonmucinous BAC as adenocarcinoma with lepidic pattern, and those previously classified as mucinous BAC as mucinous adenocarcinoma.[14] In other words, these are handled under the assumption that there is invasive disease.

This information is mostly in reference to the now outdated entity of BAC, which included some invasive forms of disease.

The treatment of choice in any patient with BAC is complete surgical resection, typically via lobectomy or pneumonectomy, with concurrent ipsilateral lymphadenectomy.[16]

Non-mucinous BAC are highly associated with classical EGFR mutations, and thus are often responsive to targeted chemotherapy with erlotinib and gefitinib. K-ras mutations are rare in nm-BAC.[18]

Mucinous BAC, in contrast, is much more highly associated with K-ras mutations and wild-type EGFR, and are thus usually insensitive to the EGFR tyrosine kinase inhibitors.[19] In fact, there is some evidence that suggests that the administration of EGFR-pathway inhibitors to patients with K-ras mutated BAC may even be harmful.[20]

This information is mostly in reference to the now outdated entity of BAC, which included some invasive forms of disease.

Taken as a class, long-term survival rates in BAC tend to be higher than those of other forms of NSCLC.[21][22] BAC generally carries a better prognosis than other forms of NSCLC, which can be partially attributed to localized presentation of the disease.[23] Though other factors might play a role. Prognosis of BAC depends upon the histological subtype and extent at presentation but are generally same as other NSCLC.[24]

Recent research has made it clear that nonmucinous and mucinous BAC are very different types of lung cancer.[4][25] Mucinous BACis much more likely to present with multiple unilateral tumors and/or in a unilateral or bilateral pneumonic form than nonmucinous AIS .[4] The overall prognosis for patients with mucinous AIS is significantly worse than patients with nonmucinous AIS .[26][27]

Although data are scarce, some studies suggest that survival rates are even lower in the mixed mucinous/non-mucinous variant than in the monophasic forms.[27]

In non-mucinous BAC, neither club cell nor type II pneumocyte differentiation appears to affect survival or prognosis.[4]

Information about the epidemiology of AIS is limited, due to changes in definition of this disease and separation from BAC category.

Under the new, more restrictive WHO criteria for lung cancer classification, AIS is now diagnosed much less frequently than it was in the past.[28] Recent studies suggest that AIS comprises between 3% and 5% of all lung carcinomas in the U.S.[26][23]

The criteria for classifying lung cancer have changed considerably over time, becoming progressively more restrictive.[12][2]

Beginning with 1999 WHO classification, there was an important change in the definition of BAC. The 1999 definition of BAC was far more restrictive, and excluded cases with stromal, vascular, or pleural invasion. These excluded tumors were reclassified as adenocarcinoma mixed type with predominant bronchioloalveolar pattern. The new definition would exclude most tumors which had previously been categorized as BAC. This marked change was done in order to isolate BAC as a purely non-invasive tumor with better prognosis.

In 2011, the IASLC/ATS/ERS classification recommended discontinuing the BAC classification altogether, as well as the category of mixed subtype adenocarcinoma. This change was made because the term BAC was being broadly applied to small solitary noninvasive tumors, minimally invasive adenocarcinoma, mixed subtype invasive adenocarcinoma, and even widespread disease.[14] In addition to creating the new AIS and minimally-invasive categories, the guidelines recommend new terminology to clearly denote predominantly-noninvasive adenocarcinoma with mild invasion (lepidic predominant adenocarcinoma), as well as invasive mucinous adenocarcinoma in place of mucinous BAC.[14]

Mucinous BAC

Non-mucinous BAC

• Atypical adenomatous hyperplasia of the lung
• Minimally invasive adenocarcinoma of the lung
• Adenocarcinoma of the lung