Lung adenocarcinoma


The pulmonary neoplasms including the benign and malignant tumors were subjected to changes in the 2015 World Health Organiztion classification. The use of immunohistochemistry and genetics was emphasized to be used in classification. Some of the tumors were reclassified, renamed or grouping was changed. Adenocarcinoma was subjected to majority of changes as explained under.

Changes in adenocarcinoma:

  • The term Bronchioalveolar carcinoma and mixed type carcinoma were obseleted.
  • New entities adenocarcinoma in situ and minimally invasive adenocarcinoma were introduced
  • The invasive adenocarcinoma is classified according to the predominant pattern and the percentage of various patterns is to be reported. The mentioning of the percentages is important like if micropapillary pattern is present even in small quantities it is associated with poor prognosis
  • Adenocarcinoma in situ: diagnostic criteria is;
    • A small tumor less than or equal to 3cm
    • A solitary adenocarcinoma
    • Pure lepidic groeth
    • No stromal, vascular or pleural invasion
    • No patterns of invasive adenocarcinoma (such as acinar, papillary, micropapillary, solid, colloid, enteric, fetal or invasive mucinous adenocarcinoma
    • No spread through air spaces
    • Cell type mostly nonmucinous (type II pneumocytes or Clara cells), rarely may be mucinous (tall columnar cells with basal nuclei and abundant cytoplasmic mucin)
    • Nuclear atypia is absent or inconspicuous
    • Septal widening with sclerosis is common, particularly in nonmucinous adenocarcinoma in situ
  • Minimally invasive adenocarcinoma: diagnostic criteria is as under
    • A small tumor less than or equal to 3cm
    • A solitary adenocarcinoma
    • Predominantly lepidic pattern
    • Less than or equal to 0.5 cm invasive component in greatest dimension in any focus
    • Invasive component to e measured includes
      • Any histologic subtype other than a lepidic pattern (such as acinar, papillary, micropapillary, solid, colloid, fetal or invasive mucinous adenocarcinoma
      • Tumor cells infiltrating myofibroblastic stroma
    • Minimally invasive adenocarcinoma diagnosis is excluded if the tumor
      • Invades lymphatics,blood vessels, air spaces or pleura
      • Contains tumor necrosis
      • Spreads through air spaces
    • The cell type mostly nonmucinous (type II pnumocytes or Clara cells), but rarely may be mucinous
  • The subtypes of clear cell and signet ring adenocarcinoma are discontinued and recognizing these as a feature when any amount is present
  • Discontinuing the term mucinous cystadenocarcinoma and including it into the category of colloid adenocarcinoma
  • It was decided that the tumors formerly classified as large cell adenocarcinomas with pneumocyte marker expression (TTF1 and/or Napsin A) as solid adenocarcinoma.
  • Solid Adenocarcinoma: it should be distinguished from squamous cell carcinoma and large cell carcinomas. Solid cell carcinoma should show at least two high power fields with five or more cells showing intracytoplasmic mucin. Expression of TTF1 and/or Napsin –A is sufficient for diagnosing solid carcinoma and distinguishing it from squamous cell carcinoma
  • The diagnosis of AIS and MIA has to be made only in resected tumor that has been submitted entirely for histologic evaluation so that the invasive foci should be ruled out
  • If a small biopsy shows a nonmucinous lepidic pattern the diagnosis should be, adenocarcinoma with lepidic pattern with a comment that it could be from a lesion epresening AIS, MIA or invasive adenocarcinoma with a lepidic component


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