The multifocal form is the most typical presentation of OP and accounts for 40–70 % of all cases [22, 23, 26, 32]. It is characterized by multiple bilateral alveolar opacities predominating in the subpleural regions and the lower lung zones, often containing an air bronchogram (Fig. 24.2) [14, 18, 19, 32–34]. A chest computed tomography (CT) is a useful non invasive procedure if OP is suspected, as it often shows more opacities than the chest X-ray, and this multifocal pattern provides an important diagnostic clue for OP. Spontaneous disappearance of some opacities over time and appearance of new infiltrates in other sites occurs in 25–50 % of cases of OP [23, 31], either before treatment or when a relapse occurs (Fig. 24.3). This phenomenon called “migratory opacities” provides another important diagnostic clue for OP, as the differential diagnosis is relatively narrow (Table 24.2). Positron emission tomography has shown a significant fluorodeoxyglucose uptake in OP presenting with parenchymal consolidation [35], but this procedure is not part of the routine assessment of OP. Pleural effusion has usually been reported as uncommon in OP [19, 23], although a small effusion has been found in up to 35 % of cases in one series [28]. A moderate enlargement of mediastinal lymph nodes may be found in about 14 % of cases [36].

This form has been termed “localized”, “solitary”, “nodular”, or “focal” OP, and represents 5–20 % of cases [14, 22, 26]. It appears as a solitary nodule or mass with smooth or irregular margins [14, 37–41] (Fig. 24.4a). In around half of patients, the lesion is found incidentally [39–42].

In pooled data from five series of nodular OP totalizing 105 cases [39–43], 69 % were men (range across series 56–100 %) and 74 % were smokers or ex-smokers (range 57–72 %, n = 87). Only 47 % were symptomatic (range 17–77 %). A history of recent infection was found in 29 % (range 12–57 %). The upper lobes were affected in 45 % of cases (range 29–58 %, n = 47). The mean size of the nodular opacity was 21 mm (range 6–68 mm). Irregular, lobulated or spiculated margins were present in 72 % (range 54–94 %). An air bronchogram was found in 18 % of cases (n = 47). Satellite nodules were found in 40 % (range 29–56 %, n = 65) and mediastinal lymphadenopathy in 7 % (range 0–19 %, n = 73).

Isolated nodular OP presents with contrast enhancement on CT and positive tracer uptake on positron emission tomography [40, 41], and cannot be confidently distinguished from primary or metastatic malignancy at imaging. This tumor-like appearance frequently leads to surgical resection, and the diagnosis of OP is made retrospectively at pathological examination. In one report, lung resections for isolated nodular OP represented 0.8 % of 1,612 thoracic surgical procedures performed in a 3-year period at one institution [40]. In 105 patients with nodular OP from five series, preoperative transthoracic or transbronchial biopsy was performed in only 23 % of patients (range 0–83 %), whereas 70 % underwent a wedge resection or a segmentectomy (range 17–100), and 7 % had a lobectomy (range 0–24 %). The surgical procedure was curative in most cases without the need for subsequent corticosteroid therapy [40, 41]. Of note, in all of 17 non-operated cases, a spontaneous improvement of the opacity was observed [39, 43]. One practical difficulty in the management of nodular OP is thus to avoid unnecessary lobectomy in this benign disorder mimicking lung cancer. The causes of nodular OP are discussed later in this chapter.

A diffuse infiltrative imaging pattern has been reported to occur in 10–40 % of cases in several series of OP [2, 10, 22, 26, 33, 44], some presenting with severe, rapidly progressive disease and respiratory failure [23, 45–51]. Some cases were associated with drugs, connective tissue diseases, or toxic exposure [50–52], whereas other appeared cryptogenic [23, 46, 47, 52].

Diffuse infiltrative OP probably represents a heterogeneous group. It has mainly been reported in early series of OP, suggesting misclassification or overlap with other entities which were unknown at that time. Some early descriptions of diffuse infiltrative OP would probably be now better classified as nonspecific interstitial pneumonia (NSIP), an idiopathic interstitial pneumonia described in the 1990s and characterized histologically by homogeneous chronic interstitial inflammation and/or fibrosis with preserved lung architecture, in which intra-alveolar buds of granulation tissue are a common ancillary finding. Hence, OP pattern representing usually less than 10 % (but sometimes up to 20 %) of the total abnormalities is found in half of cases of NSIP at surgical lung biopsy [53, 54]. Sampling of such focal OP lesions by transbronchial biopsies might thus have led to misdiagnose NSIP as diffuse infiltrative OP. It has also been suggested that a continuum exists between OP and NSIP [54], and that OP/NSIP overlap might explain part of the diffuse infiltrative cases of OP [55]. Hence, patients presenting at imaging with both interstitial changes (histologically corresponding to NSIP) and consolidations (histologically corresponding to OP) have been reported [56]. In a large series of NSIP, the distinction between OP and NSIP has been based upon whether OP pattern represents more or less than 10 or 20 % of the total abnormalities at surgical lung biopsy, an arbitrary criterion [54]. In support of the concept of overlap between OP and NSIP, one study of 22 patients with OP proven by surgical lung biopsy and prolonged CT follow-up reported the evolution of OP consolidations into reticular changes resembling NSIP pattern in a subset of patients [57]. The coexistence of OP and NSIP histological patterns at surgical lung biopsy has been especially observed in idiopathic inflammatory myopathies, in contrast with other autoimmune diseases [58], but more histological data are needed to support the concept of OP/NSIP overlap as a distinct entity.

Other cases diagnosed as diffuse infiltrative OP may actually have had acute interstitial pneumonia, with OP being only a minor histopathological feature or overlapping with DAD at the organizing stage. Other cases could correspond to “acute fibrinous and organizing pneumonia” (AFOP), a recently described entity combining clinical and pathological features of DAD and OP [59] (see below).

Finally, other cases initially reported as diffuse OP may have had acute exacerbation of interstitial lung disease, a recently described acute event occurring in the natural history of idiopathic pulmonary fibrosis, NSIP and other fibrotic interstitial disorders [60]. Acute exacerbations of interstitial lung disease have been associated with histological patterns of either OP or DAD at lung biopsy, the former being associated with a much better short term prognosis [61].

Although genuine diffuse infiltrative OP probably exists, it still awaits better characterization and distinction from similarly appearing entities. Meanwhile, the above-mentioned disorders need to be considered in the differential diagnosis.

Rarely, OP may present as multiple, sometimes cavitary nodules [62–65], a micronodular pattern, with multiple small well- or poorly- defined nodules, or nodules with an air bronchogram [66]. Other variants include a bronchocentric pattern, a perilobular pattern resembling thickened interlobular septas, circumferential subpleural linear opacities, and radial opacities [32, 62, 66–69]. A “ring-like”, “reversed halo” or “atoll” pattern has rarely been reported in OP, consisting of a focal round area of ground glass surrounded by a crescent or ring of consolidation (Fig. 24.4b) [66]. Contrary to early beliefs, this sign is not specific to OP and may also be found in Churg-Strauss syndrome, granulomatosis with polyangiitis (GPA, formerly Wegener’s granulomatosis), chronic eosinophilic pneumonia, lymphomatoid granulomatosis, tuberculosis, and various fungal infections [70].