Lung cancers can be classified into two broad categories: 
1. Small cell lung cancer (20-30%)
2. Non small cell lung cancer (70-80%)
– Squamous cell carcinoma (30-35%)
– Adenocarcinoma ( 35 %)
– Large cell carcinoma (10-15%)

Imaging:

The imaging of lung cancers can be determined on the following points:
A. Shape
B. Calcification and cavitation
C. Ground-glass density, air bronchograms and cyst likely agencies
D. Contrast enhancement
E. Rate of growth
F. Hilar enlargement

A. SHAPE:
Peripheral tumors:
Spherical or ovoid configuration; exceptions pancoast tumors which resembles apical pleural thickening and bronchoalveolar carcinoma and carcinoma arising in the areas of interstitial fibrosis resembling ill-defined area of consolidation.
Lobulated margins have a PPV  80% for a malignant and Spiculated margins have positive predictive value of 33-94 % for malignant potential for a solitary pulmonary nodule.

The term corona radiata indicates multiple strands extending into the surrounding lung because of either tumor extension or fibrotic response to the tumor and is best seen on CT and has a positive predictive value of 33-94 %  for a malignant potential if seen in a solitary pulmonary nodule.
It is not entirely specific for malignant lesions and can be seen in chronic infective process and granulomas.
A single linear band like opacity may be seen in subpleural lesions connecting the lesion to the pleura this is called the pleural tail sign and is nonspecific and can be seen in both benign and malignant pulmonary nodules.

The lung cancers presenting as ill-defined lesions with morphology similar to pneumonia show slow growth over serial CTs and not responding to antibiotics, which in general allows that distinction from infectious pathologies.

The lesions arising in segmental and subsegmental bronchi are seen as soft tissue opacities occluding the affected bronchi with distal dilated bronchi filled with insipiated mucoid secretions. Mucoid impactions result in V or Y – shaped tree in bud densities.

Central tumors:
The main signs of central tumor or collapse and consolidation in the lung parenchyma beyond the tumor and presence of hilar enlargement. Obstruction of a major bronchus may lead to varying amount of collapse-consolidation of the distal lung and thereby making the task of defining the extent of the central tumor lesions within the post obstructive collapse consolidation difficult on non contrast CT. After contrast administration there is differential enhancement, early phases are acquired the neoplastic tissue enhances to a minimal degree whereas distal atelectasis show substantial enhancement.
MRI helps in differentiating the tumor from post obstructive pulmonary changes on T2 and post contrast images. The following features may suggest pneumonia is secondary to an obstructive lesion:
– Alteration in the shape of the collapsed or consolidated lobe directly due to the bulk of the underlying tumor. For eg. Golden S sign.
– Presence of visible mass or irregular stenosis in the main stem or lobar bronchus.
– Presence of an associated central mass.
– A localized pneumonia that remains unchanged for more than 2 weeks or the one that reoccurs in the same lobe.

B. CALCIFICATION AND CAVITATION:

Calcification

Most lung cancers are soft tissue density but some may show partial calcification or cavitation.
CT may show calcification within 6%-10% of bronchogenic carcinomas.
Tumor calcifications are rarely seen on chest radiographs. Mostly the calcification represent pre-existing granulomatous calcifications and golf by the tumor. However, amorphous or cloud like calcification which may be new in onset can be seen in a significant proportion. Calcification can be seen in both small cell and non-small cell carcinomas.

Cavitation:

Cavitation may be seen in lung cancers of any size and are best demonstrated by CT. Thick cavity can be eccentric and the walls of the cavitated tumor are often irregular and the tumor nodules may be visible. Cavitating tumor may also have a smooth inner and outer margin it has been suggested that very thin walled cavity is represent tumor cells lining bullae rather than true cavitation. Fluid levels can also be seen within the necrotic tumor and very rarely it may represent as a mycetoma.
Squamous cell carcinomas are much more likely to cavitate than other cancers in a series of 100 cavitating cancers 82 were squamous cell carcinomas.
Small cell carcinomas for all practical purposes do not cavitate.

C. GROUND-GLASS DENSITY, AIR-BRONCHOGRAMS

The most common type of lung cancer presenting as ground-glass density are lesions of adenocarcinoma with bronchoalveolar cell component or bronchoalveolar carcinoma/ Lepidic growth pattern.
The greater the ground-glass component as compared to a solid component of the lesion better is the prognosis.

6 types of morphological patterns based on histology are classified for Small peripheral the adenocarcinomas:
A: Localized bronchoalveolar carcinoma, LBAC
B: LBAC with foci of structural collapse of alveoli
C: LBAC with foci of active fibroblastic proliferation
D: Poorly differentiated adenocarcinoma
E: Tubular adenocarcinoma
F: Papillary adenocarcinoma with compressive growth pattern

Type A and type B show no lymph node metastasis and have the most favourable prognosis of the 6 types. The follow up recommendations have been laid down by fleischner guidelines 2017.

Air bronchograms :
Adenocarcinomas may contain air bronchograms are cyst like lucencies and hence air bronchogram sign generally associated with infection may also be seen in peripheral cancers presenting as consolidation.

D. CONTRAST ENHANCEMENT

Contrast enhancement exhibits tumoral vascularity. Lung cancers enhance differently from adjacent collapsed and consolidated lung after intravenous contrast injections at both CT and MR imaging and this may be used to differentiate them from adjacent lung collapse on dynamic studies as there is a differential enhancement of the tumor versus the surrounding consolidation.

E. RATE OF GROWTH: 

A solid nodule showing absence of growth for a 2 year period is a relatively reliable indicator for the benign nature of the nodule. However, this criteria is not absolute.
Lesion doubling time for a spherical lesion is usually 25% increase in the diameter and can be used to distinguish benign from malignant lesions; malignant lesions can have doubling time ranging from 30 to 450 days.

F. Hilar enlargement:

Hilar enlargement may be a common feature in radiographs of patients having lung cancer. This may represent the tumor in itself or enlargement of the hilar nodes of metastatic etiology.

Contributed by – Dr. Varun Tyagi, Mumbai

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