Adenocarcinoma in situ and minimally invasive adenocarcinoma in lung

Adenocarcinoma in situ minimally invasive adenocarcinoma in lung is a relatively new classification entity and was formally known as bronchoalveolar carcinoma . 

In 2011 the International Association for the Study of Lung Cancer and other societies jointly revised the classification for adenocaricnoma 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. 

New terminology and concepts

Before a general discussion of the topic it is worth highlighting some of the new 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 formerly BAC) 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 BAC.

Invasive mucinous adenocarcinoma is a variant invasive adenocarinoma previously known as mucinous BAC.

This article will refer to the non-invasive subtypes of adenocarcinoma: AIS and MIA.

Epidemiology

AIS an MIA are an uncommon type of bronchial carcinoma which occurs most frequently among non smokers, women and Asians. It is a sub type 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, 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 sub types are recognised ³

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

Radiographic features

There are 3 recognised radiographic patterns ¹

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

Plain film

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,  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 are 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 sub types have worse porgnosis ⁴ - probably because or aerogenous spread to form infiltrating, multifocal, or satellite tumours ¹¹.

Etymology

The entity which was formally 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 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