Abstract
Lipoid pneumonia presents with a variety of lung abnormalities, particularly mass forming lesions that mimic lung cancers. While 18F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) is expected to discriminate both diseases, some previous reports showed pseudo-positive FDG uptake in lipoid pneumonia. Here, we report a case of pathologically proven chronic lipoid pneumonia in a 78-year-old Japanese man. Computed tomography (CT) showed multi-lobar mass-forming lesions with a fat-density. PET confirmed the spotty accumulation of FDG in the corresponding fat-density area on CT, suggesting lipoid pneumonia. We reviewed the literature and discussed the FDG uptake patterns in lipoid pneumonia.
Keywords: Computed tomography, Lipoid pneumonia, Positron emission tomography
1. Introduction
Lipoid pneumonia is an uncommon condition and is pathologically characterized by abnormal lipid deposition in the lungs, which initiates inflammatory processes. Different sources of oily substances of mineral, vegetal, or animal origin have been described in association with exogenous lipoid pneumonia. Chronic lipoid pneumonia typically manifests as mixed-density consolidation and mimics primary lung cancers on computed tomography (CT). While 18F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) is expected to discriminate both diseases, previous studies reported a pseudo-positive FDG uptake in lipoid pneumonia, illustrating a limitation of this approach. Here, we report a case of pathologically-proven chronic lipoid pneumonia showing a spotty FDG uptake in the corresponding fat-density area on CT. We focused on the FDG uptake patterns in lipoid pneumonia and added a literature review.
2. Material and methods
2.1. Case presentation
A 78-year-old man presented with abnormal shadows on routine postoperative thoracic CT. He had a surgical history of colonic and gastric cancers, which were operated upon 8 and 13 years before, respectively. Physical examination revealed no abnormalities. The CT revealed mixed-dense consolidation in the middle and right lower lobes with a fat density of less than −30 Hounsfield units (Fig. 1A–D). On PET, the FDG uptake was distributed in spots in the consolidation of both lobes (standardized uptake value (SUV): 5.7) (Fig. 2). Areas of high FDG uptake partly matched with the fat-density areas on corresponding CT (Fig. 2, arrow marks). Pathological diagnosis was made using biopsy specimens obtained via bronchoscopy from the right lower lobe. Pathological examination of hematoxylin and eosin-stained sections revealed aggregation of rounded, empty vacuoles corresponding to lipid deposition (Fig. 3, asterisk marks). Small lipid droplets were phagocytosed by alveolar macrophages (Fig. 3, arrow marks), which are characteristic findings of lipoid pneumonia. In an additional interview, the patient reported none of the specific conditions predisposing to lipid aspiration or inhalation.
Fig. 1.
Computed tomography images Computed
tomography shows mixed-dense consolidations that include fat density in the middle and lower lobes.
Fig. 2.
Positron emission tomography images The
positron emission tomography scan shows spotty accumulation of 18F-fluorodeoxyglucose in the matched low density area on computed tomography (arrow marks).
Fig. 3.
Pathological examination of a lung biopsy specimen Aggregation
of lipid deposition is observed as rounded, empty vacuoles stained with hematoxylin and eosin (asterisk marks). Small lipid droplets are phagocytosed by alveolar macrophages (arrow marks).
3. Results and discussion
Lipoid pneumonia is a rare condition that is characterized by the deposition of exogenous or endogenous lipids in lung tissues. Previous reports showed diverse CT findings in lipoid pneumonia, including masses, nodules, dense or ground-glass infiltration, crazy-paving appearance, pleural effusion, pneumothorax, calcification, and cavitation [1,2]. Among these, mass-forming lesions, the most common subtype in chronic lipoid pneumonia, are particularly important in differentiating from lung cancers. While presence of fat density in mass lesions is a key finding in lipoid pneumonia, necrotic lesions and mucous retention in lung cancer also appear similarly low density on CT. Recently, FDG-PET plays a crucial role on diagnosing lung cancers; however, pseudo-positive FDG uptake has been reported in lipoid pneumonia, resulting in the simulation of lung cancers [[3], [4], [5], [6]]. We comprehensively searched English literature and summarized 9 cases of lipoid pneumonia conducting FDG-PET as presented in Table 1 [[3], [4], [5], [6], [7], [8], [9], [10], [11]].
Table 1.
A summary of lipoid pneumonia cases conducting18F-fluorodeoxyglucose-positron emission tomography.
| Case (Ref) | Age, sex | Lipid source | CT scan |
PET scan |
||||
|---|---|---|---|---|---|---|---|---|
| Form | Distribution | Size (mm) | Fat density | SUV | Pattern | |||
| 1 (3) | 72M | vegetable oil | multiple | RUL, RLL | ND | spotty | ND | peripheral |
| 2 (7) | 82M | mineral oil | solitary | RML | 33 | ND | 4.2 | nodular |
| 3 (4) | 65F | mineral oil | solitary | RML | 30 | – | 3.2 | nodular |
| 4 (5) | 65M | ND | solitary | RML | ND | ND | ND | spotty |
| 5 (8) | 76M | animal fat | solitary | RUL | 30 | ND | 11.6 | nodular |
| 6 (6) | 54M | ND | solitary | RLL | ND | spotty | 4.4 | spotty |
| 7 (9) | 72F | mineral oil | multiple | RLL, LLL | ND | spotty | ND | peripheral |
| 8 (10) | 73F | mineral oil | multiple | RLL, LUL | 60 | + | 5.4 | nodular |
| 9 (11) | 83F | vegetable oil | solitary | LUL | 20 | ND | normal | – |
| Present | 78M | unknown | multiple | RLL, LUL | 45 | spotty | 5.7 | spotty |
ND: no data, RUL: right upper lobe, RLL: right lower lobe, RML: right middle lobe.
Based on the chest imaging and description that appeared in literature, two pulmonologists independently characterized CT and PET findings into categories in a blinded manner and finally gained identical results in all cases. On CT, the lesions were solitary (n = 6) or multiple (n = 4) with predilection for the right middle and lower lobes, and measured 20–60 mm in diameter. CT-fat density was evaluated in 5 cases, in which 4 cases manifested spotty distribution. On FDG-PET, SUVs ranged between 3.2 and 11.6 [[3], [4], [5], [6], [7], [8], [9], [10]] with an exception of negative uptake in one case [11]. In all cases, SUVs exceeded a threshold of 2.5 that was previously proposed for the best discrimination between benign and malignant solitary lung nodules [12], illustrating a limitation of SUV alone for discriminating from lung cancers. Therefore, we focused on the pattern recognition of FDG uptake in lipoid pneumonia and tried to categorize the cases into three groups, single nodular, spotty, and peripheral (ring-shaped) patterns. In a total of 9 cases assessed, FDG uptake was shown as having a nodular (n = 4), spotty (n = 3), or peripheral (n = 2) pattern. The patterns were not associated with other factors including a location or multiplicity of the lesions, SUVs, or lipid sources. We believe a spotty pattern of FDG uptake, observed in 2 previous and the present cases, might be a key finding in lipoid pneumonia, because FDG uptake is usually positive in entire lesions of lung cancers. Nevertheless, lung cancers frequently have intratumoral necrosis showing low FDG area, and a variant of adenocarcinoma characterized by a rich mucous production has shown to manifest spotty FDG uptake [13]. Regarding different etiologies of CT-low density in lipoid pneumonia and lung cancer, we were interested in matched FDG hotspot and CT-low density in the present case as well as in the study by Chardin et al. [8]. Active inflammation was ongoing in the site of lipid deposition (CT-low density area) where activated inflammatory cells are recruited and glucose metabolism was promoted, leading to positive FDG uptake in lipoid pneumonia [8]. Thus, we concluded that this finding could be a hallmark for suspecting lipoid pneumonia.
Our study has several limitations. First, the snapshot of the imaging in each case report was not sufficient to evaluate the entire CT and PET patterns. Second, the resolution of PET was lower than that of CT and insufficient to evaluate the small heterogeneity in the lesion. Moreover, there should be unreported cases showing positive or negative FDG uptake in lipoid pneumonia. It is necessary to accumulate more cases to determine the specific recognition pattern on PET in lipoid pneumonia cases.
In summary, we reported a case of lipoid pneumonia and discussed the CT and PET findings along with a literature review. A spotty FDG uptake in corresponding to CT-fat density could be a key finding to suspect lipoid pneumonia.
4. Conclusion
We reported a case of lipoid pneumonia demonstrating spotty FDG uptake on PET and analyzed the literature of FDG-PET studies on lipoid pneumonias simulating lung cancers.
Funding
No funding was received for this work.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Author's contribution
All of the authors ensure to task force for preparing the manuscript and approved the final version of the manuscript. All authors contributed to the clinical care for the patient in specialized settings.
Consent for publication
Written consent was obtained from the patient for publication of this case report and for use of accompanying images.
Declaration of interest
None of the authors has any conflicts of interest or any financial ties to disclose.
Submission declaration and verification
The present study was not published or is not currently submitted to any other journal.
Acknowledgments
We would like to thank Editage (www.editage.jp) for English language editing.
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