Adenocarcinoma in situ, minimally invasive adenocarcinoma and invasive adenocarincoma of lung are relatively new classification entities which now replaces the now defunct term bronchoalveolar carcinoma (BAC).

In 2011 the International Association for the Study of Lung Cancer and other societies jointly revised the classification for adenocarcinoma of lung ¹³. The terms BAC and mucinous and non-mucinous BAC are no longer used. The new classification strategy is based on a multidisciplinary approach to diagnosis of lung adenocarcinoma.

Terminology

Before a general discussion of the topic it is worth highlighting some of the updated terminology and concepts, as for many who were taught the term bronchoalveolar carcinoma, some adjustment will be necessary ⁵.

• adenocarcinoma in situ of lung (AIS) (≤3 cm) has a number of subtypes
  • the most common subtype is non-mucinous and rarely mucinous or mixed subtypes
  • histology pattern: no growth patterns other than lepidic and no feature of necrosis or invasion
• minimally invasive adenocarcinoma of lung (MIA) ≤3 cm
  • describes small solitary adenocarcinomas with either pure lepidic growth or predominant lepidic growth with ≤5 mm of stromal invasion

Two invasive adenocarcinomas previous termed non-mucinous and mucinous BAC are no longer used.

• lepidic predominant adenocarcinoma describes invasive adenocarcinoma with a predominant lepidic pattern with >5 mm invasion; formerly known as non-mucinous bronchoalveolar carcinoma
• invasive mucinous adenocarcinoma is a variant invasive adenocarcinoma previously known as mucinous bronchoalveolar carcinoma

Epidemiology

AIS anand MIA are an uncommon type of bronchial carcinoma which occurs most frequently among non-smokers, women and Asians. It is a subtype of adenocarcinoma, but has a significantly different presentation, treatment and prognosis. AIS and MIA represents between 2-14% of all primary pulmonary malignancies ¹¹. There is no significant gender predilection unlike other lung cancer types which are more prevalent in men.

Risk factors

Focus of pulmonary fibrosis, e.g. tuberculosis scar, infarct, scleroderma.

Clinical presentation

Presentation is often insidious, and a large proportion (50%) of patients may be asymptomatic at the time of detection ¹.  Alternatively, as these tumours can produce large quantities of mucus, patients may present with bronchorrhea.

Persistent consolidation for weeks despite appropriate antimicrobial therapy should raise the suspicion of a neoplastic process. CT or guided biopsy may be planned in such cases.

Pathology

AIS: ≤3 cm, demonstrates a lepidic growth pattern, spreading along the walls of the lung without destroying the underlying architecture. In addition they are characterised by absence of stromal, vascular or pleural invasion.

MIA: ≤3 cm, describes small solitary adenocarcinomas with either pure lepidic growth or predominant lepidic growth with ≤5 mm of stromal invasion.

Three pathological subtypes are recognised ³:

• non-mucinous
• mucinous: goblet cell (mucus secreting), often multi-centric
• mixed

Radiographic features

There are three recognised radiographic patterns ¹

• single mass or nodular form (commonest): ~45 %
• consolidative form: ~30 %
• multinodular form: ~25 %

Plain radiograph

May show segmental or lobar consolidation with chronic unilateral airspace opacification and air bronchograms. Can also present as a pulmonary nodule, mass or a cluster of diffuse nodules ^1,2. The nodular form (commonest) can be indistinguishable from another adenocarcinoma subtype or inflammatory granuloma on plain film ¹.

CT

The appearance of bronchoalveolar carcinoma on CT depends on its pattern of growth, and thus it may appear as a:

• peripheral nodule
  • commonest appearance
  • typically solitary and well circumscribed ⁵
  • the nodule may be surrounded by a halo of ground glass opacity, the so called fried egg sign
  • cavitation
    • pseudocavitation (presence of bubble like lucencies) is recognised ^1,5
    • overt cavitatory changes rarely occur (~7%)
    • cavitating pulmonary metastases may occur (Cheerios sign ⁹)
• focal area of ground glass (early sign)
  • heterogeneous attenuation
• region of ground glass +/- consolidation
• hilar and mediastinal adenopathy and pleural effusion are uncommon
• • air bronchograms may be seen (also known as open bronchus sign)
  • CT angiogram sign

Nuclear medicine

¹⁸F-FDG-PET is often negative ^6,7.

Treatment and prognosis

Surgical resection is required with a lobectomy or pneumonectomy.

Overall tumours that demonstrate only lepidic growth tend to be indolent, with high 5 year survival. Frequently, however, solid, invasive components are present and despite radical treatment, recurrence rates are high. Mucinous subtypes have worse prognosis ⁴, probably because of aerogenous spread to form infiltrating, multifocal, or satellite tumours ¹¹.

History and etymology

The entity which was formerly known as bronchoalveolar carcinoma was first described by Malassez in 1876, as a bilateral, multinodular form of malignant lung tumour ^11-12.

Differential diagnosis

General imaging differential considerations are broad and its useful to consider differentials for specific patterns which include:

• differential of chronic alveolar opacity
• differential of a solitary pulmonary nodule
• differential of ground glass opacity

See also

• non small cell lung cancer
• adenocarcinoma of lung
  • invasive adenocarcinoma of lung