Paragonimiasis is a food-borne parasitic disease caused by the trematode P. wertermani
, which is endemic in East Asia [1
]. Adult P. westermani
worms are bean-shaped, 5-12 mm long, 3-6 mm wide, and 2-5 mm thick, a size that can be seen on radiologic images. Several studies reported that the common radiologic findings of pleuropulmonary paragonimiasis included pleural effusion, hydropneumothorax, pulmonary nodules, or air-space consolidation, and cysts [10
]. A report by Kim et al. [11
] suggested that the main CT feature of pleuropulmonary paragonimiasis in 31 patients was pulmonary nodules, including multiple nodules in 7 patients (23%) as in our patient. In the present case, areas of surrounding GGO were observed around the nodules, which suggest hemorrhage. It led us to consider the findings as hemorrhagic metastasis, though these finding are also shown in a number of benign conditions, such as angioinvasive aspergillosis and paragonimiasis [12
]. Abnormally increased FDG uptake (max SUV: 3.1) in the lung lesions on FDG-PET CT supported a possibility of malignancy further.
FDG-PFT is a highly accurate imaging in evaluating pulmonary lesions, taking advantage of the higher metabolic rate of tumor cells to show increased accumulation of FDG in malignant lesions. However, an increased metabolic rate is also observed in inflammation [13
]. Many reports have demonstrated increased FDG uptake in inflammatory, granulomatous, and infectious lung diseases, including bacterial, mycobacterial, fungal, and parasitic infections.
Several cases of pulmonary paragonimiasis were reported that showed a high uptake suggestive of a malignancy on FDG-PET CT images [5
]. All of them were suspected as primary lung cancers and confirmed as paragonimiasis by detecting eggs in the tissue obtained from biopsy or operation. Although the definite causes of FDG accumulation have not yet been proven in paragonimiasis, it seems likely that inflammatory cells, including eosinophilic infiltration, active inflammatory responses, and viable worms, cause a high FDG uptake [4
The diagnosis of paragonimiasis can be made by detecting eggs in the tissue section, or more commonly, in the stool, sputum, or BAL fluid, or by a positive anti-Paragonimus
antibody test. However, egg detection rates have been reported to be low (28-38%) and eggs are not present in the sputum until 2-3 months after an infection. Serum determination of antibodies to Paragonimus
is more accurate in early infections, and ELISA is highly sensitive (92%) and specific [15
]. In our case, Paragonimus
eggs were not found in the stool, sputum, and BAL fluid. Instead, the patient's serum strongly reacted to P. wertermani
antigen in ELISA.
Considering lung nodular lesions with GGO on chest X-ray and CT, in combination with ELISA test for paragonimiasis, FDG-PET itself was an unnecessary examination to diagnose paragonimiasis in our patient. In addition, nodular lesions were continuing to the visceral pleura, which is rather unusual for metastatic lung cancers. On the other hand, Paragonimus
invades from the pleural cavity to the lung parenchyma so that continuous lesions from the pleura to the nodular lesion in the lung parenchyma are a typical feature of radiological finding in paragonimiasis. Nevertheless, FDG-PET was performed to exclude the possibility of malignancy owing to the elevated serum level of CA 125. The serum CA 125 is often increased in patients with malignancy, such as ovarian cancer, endometrial cancer, and pancreatic cancer. However, it is also increased in a variety of benign conditions, including endometriosis, uterine fibroids, pelvic inflammatory disease, heart failure, liver and renal disease, as well as in approximately 1 percent of healthy women [16
]. There has been no report of paragonimiasis patients showing high level of CA 125.
As a possibility in the differential diagnosis of multiple pulmonary nodules or masses with FDG-PET positive finding, especially in endemic regions with a high prevalence of food-borne parasitic diseases, paragonimiasis should be considered.