Rare pulmonary tumours: a histological and radiological overview

The primary lung carcinoid is a neuroendocrine tumour that has an endobronchial localisation primarily, and it represents 0.5–2.5% of lung tumours. There are two types of carcinoid tumours, namely, typical (80–90%) and atypical (10–20%). The average age of presentation is 45 years. In terms of location, this tumour has a predilection for subsegmental bronchi. Most carcinoid tumours have a sizeable extraluminal component, which is why they have been called “iceberg lesions” (4,5).

Imagistically, it presents as a solitary nodule or mass, with a round or oval shape and a slightly lobed margin; rarely, it may present irregular edges (Figure 1). Calcifications can be distinguished in up to 30% of the patients (5). In some cases, carcinoid tumours can cause consolidations or atelectasis by obstructing a segmental bronchus (4). The atypical form of the carcinoid tumour was more associated with the occurrence of hilar and mediastinal adenopathies (6,7).

Figure 1

Histologically, the tumour contains multiple interconnected trabeculae separated by vascular stroma with numerous blood vessels with thin walls. The differences between the two forms, typical and atypical, are (a) mitotic activity, (b) cytological pleomorphism, (c) necrosis zones, (d) cell distribution, and (e) the presence of distortions (Figure 2).

Figure 2

Airway papillomatosis is a benign tumour of the tracheobronchial tree of squamous-epithelial origin. In many cases, glandular elements can also be found in its structure. It has been associated with the human papilloma virus infection, most likely by vertical transmission, and it occurs more frequently in the proximal airways (2).

Lung parenchymal involvement occurs as a major complication of airway papillomatosis. This tumour presents the risk of relapse after many years of latency. The most severe complication is the malignant transformation. Patients with pulmonary papillomatosis have an increased risk of developing squamous carcinoma (histologically, it presents a well-differentiated cytological structure without anaplastic transformation characteristics despite the tendency for accelerated growth).

Computer tomography detects a tumour with a pedunculated or sessile aspect that presents a tendency to form cystic opacities, located predominantly in the posterior lung segments (6).

Among tumours with tracheal localisation, tracheobronchial gland tumours, in particular, cystic adenoid carcinoma, are the second most frequent after squamous carcinoma.

The tumour is usually situated intraluminally; thus, it has clinical expression. The patients can present symptoms and signs of obstruction as well as haemoptysis (6).

The histological structure is composed of small cells with a hypochromic nucleus and few mitoses.

The CT image may reveal a solitary nodule, sometimes with punctual calcifications. More often, because the tumour has an endoluminal localisation, post-obstructive pneumonia occurs, and consolidations become visible on the CT scans. In these tumours, the most important challenge is to classify the degree of malignancy. However, even in advanced stages, it shows better evolution and prognosis than bronchogenic carcinoma (4).

Pulmonary sclerosing haemangioma is a low-incidence pulmonary tumour with uncertain histology; the latest research suggests that it has an epithelial origin. It occurs more often in young and middle-aged women.

Histologically, it presents a network of multiple thin-walled vessels and poorly represented stroma (2,4). The nucleus has numerous atypia with rare mitoses. The tumour contains two cell types: surface cells and round cells. Based on cell arrangements, there are four types of pulmonary haemangioma, namely, papillary, sclerotic, solid, and haemorrhagic.

Radiologically, they are described as round or ovoid formations located in the peripheral regions of the pulmonary parenchyma, most frequently in the subpleural area, preferentially in the right lung (6).

The differential diagnosis between “true” haemangioma and arteriovenous malformations is essential. The latter is not considered a tumour formation, but rather a congenital vascular malformation.

Most commonly, they are asymptomatic because their vessels are infused into the pulmonary circulation. If they become large, they may lead to the appearance of a right-left shunt, associating symptoms such as cyanosis, polycythaemia, and clubbing (2).

Despite being one of the most common benign lung tumours, pulmonary lipoma is a rare tumour but also among the first pulmonary tumours reported in the literature by Rokitansky in 1854. They appear more commonly in middle-aged men (8,9).

Histologically, it consists of well-known nodules that are composed of mature adipocytes. CT scans will reveal well-defined nodular lesions or pulmonary masses with either endobronchial disposition, central disposition, or a peripheral one. Endobronchial lipomas are most often pedunculated (2,4,6), (Figure 3).

Figure 3

Primary pulmonary liposarcoma is one of the rarest forms of sarcoma with pulmonary localisation. Histologically, malignant lipoblasts are present with the ability of neoangiogenesis and nuclear polymorphism.

Imagistically, the liposarcoma is frequently described as a large heterogeneous mass with lipomatous structure and well-differentiated edges (Figure 4). This type of lesion should be distinguished from other lipomatous lesions such as pulmonary lipoma, pulmonary hamartoma, and lipoid pneumonia (6).

Figure 4

It is the most common benign pulmonary tumour, and it represents between 5 and 8% of the solitary pulmonary nodules. It occurs predominantly in male patients in the fifth and sixth decades of life (10).

Imagistically, the hamartoma is a well-defined lung nodule. The characteristic “popcorn” aspect is described when the tumour presents central calcifications (Figure 5). The hamartoma is considered to be a true neoplasm of bronchial mesenchymal tissue, having a variety of differentiation capabilities (11). Histologically, it consists of a combination of mesenchymal elements (mixoid and fibromixoid elements) and epithelial elements, with the absence of necrosis (Figure 6).

Figure 5

Figure 6

The primary pulmonary angiosarcoma is rarer than metastasic pulmonary lesions of heart angiosarcomas. Primary pulmonary angiosarcoma usually occurs in middle-aged adults, and in most cases, patients present haemoptysis, which is unresponsive to treatment.

Histological analysis shows positive keratin cells with rare atypia (Figure 7). The CT exam reveals the presence of pulmonary masses or nodules that have similar densities to the muscle tissue (Figure 8). Most commonly, these tumours can invade the mediastinum, including the trachea, the thoracic wall, and both the pleura and the pericardium, causing fluid accumulation. The most common radiological sign of the lung angiosarcoma is the “halo sign” describing a central soft tissue area surrounded by a glass-glazing halo.

Figure 7

Figure 8

Differential diagnosis is necessary to rule out other pathologies that may present a halo sign (granulomatosis with polyangiitis or Kaposi's sarcoma) (6).

Primary lung lymphomas are a rare form of lymphoma, accounting for about 1% of lung malignancies. The definition of primary pulmonary lymphoma over time has undergone multiple changes; presently, it is defined as the proliferation of lymphoid clones without another localisation, besides the lung, at least 3 months after diagnosis.

There are three phenotypes, each with its features and particular radiological description (6).

Histological analysis reveals the presence of interstitial lymphoid infiltration with an angiocentric pattern, central necrosis, and a cavitation tendency. Sometimes, a variable-sized vessel is present in the centre of the lesion, justifying its name—angiocentric lymphoma. Immunohistochemically, most lymphoid cells are CD4-positive.

CT scans reveal soft-tissue nodules situated in both lungs, predominantly in the upper lung segments. Pleural involvement occurs in about 30% of cases (12) (Figure 9).

Figure 9

The mean age incidence is between 25 and 85 years, both sexes being equally affected. Approximately half of the cases are asymptomatic at the time of diagnosis. Cell type and disease status at diagnosis are the main prognostic factors. Histologically, it presents areas of non-necrotising lymphoproliferative infiltrations with the preservation of the pulmonary architecture. Most commonly, BALT lymphoma has a typical lymphocyte pattern (13).

Imagistically, the typical aspect is that of a solitary pulmonary mass with variable dimensions (2–8 cm), but it can take various forms. Rarely, multiple or bilateral nodular lesions, infiltration of the bronchial vascular structures or the interlobar septa, or infiltrative appearance similar to a pneumonic area may be present. Frequently, the nodular lesions associate ground-glass areas. The involvement of lymph nodules occurs in approximately 30% of the cases (4,13).