In this study, we report an extremely rare disease, PPM, which shows no significant uptake of
18F-FDG on PET/CT scan, a feature that has never been reported before. It is often misdiagnosed as lung metastasis owing to the difficulty of distinguishing the two pathologies on conventional chest CT scan and sometimes due to the high uptake of
18F-FDG on PET/CT scan [
4,
8,
10]. In our case, it was first regarded as lung metastasis of rectal carcinoma owing to the presentation of solitary pulmonary nodular with well-defined boundary on CT images. On contrast enhanced CT scan, mild centripetal enhancement of the nodular was observed and it was difficult to exclude the diagnosis of metastasis. Kim et al. reported a well-defined pulmonary nodular with intense and homogeneous enhancement that was pathologically confirmed to be PPM [
11]. Therefore, the inconsistent enhancement might indicate heterogeneity of blood supply.
The clinical symptoms of PPM vary greatly, mostly asymptomatic [
8,
9,
12], sometimes with atypical chest pain [
11], but rarely with hemoptysis [
13]. In our study, the patient showed asymptomatic and only incidentally observed a solitary pulmonary nodule on abdominal CT scan. From previously published case reports, the clinical symptoms may be related to the location of pulmonary nodules. The distribution area of PPM plays a critical role in clinical symptoms. Most patients present asymptomatic because of the site of PPM that is located in the peripheral pulmonary region, but sometimes it presents with hemoptysis, indicating involvement of venule in the pulmonary parenchyma. It mainly presents as benign tumor, while malignant features, e.g. lymph node, liver or bone metastasis, can also be seen in rare instances [
1,
14]. The prognosis of patients with PPM is favorable although it presents as malignancy, possibly due to the slow progression.
18F-FDG PET/CT has been demonstrated to be more accurate than conventional CT in evaluating solitary pulmonary nodules (SPNs) [
15]. The benign diagnosis of SPN on PET and CT is strongly associated with low uptake of
18F-FDG (SUVmax < 1.5–2.0) and well-defined margin [
15]. According to this criteria, the SPN in the right lower lobe of our case is probably a benign nodule. However, a thoracoscopic wedge resection of the right lower lobe was performed owing to the suspicion of malignancy, albeit with a very small possibility. The final diagnosis of PPM depends on pathological and radiological examinations showing no involvement of the central nervous system. Interestingly, PPM often showed high uptake of
18F-FDG on PET/CT scan in previous case reports [
6,
8,
13]. An asymptomatic SPN with increased uptake of
18F-FDG in the right upper lobe was reported by Cura et al. that demonstrated to be a pulmonary meningioma [
8]. Similarity, Meirelles et al. showed a SPN with a false positive PET scan that proved to be PPM by histopathological examinations [
6]. Both were misdiagnosed as primary lung carcinoma. However, no significant uptake of
18F-FDG was found in our case, which might indicate the existence of heterogeneity of PPM in glucose metabolism. To our knowledge, it was the first case to report a PPM that showed no uptake of
18F-FDG on PET/CT scan in a patient with rectal carcinoma. The absent uptake of
18F-FDG might be explained by the low WHO grade of meningioma (grade I) and Ki-67 index [
16]. Overall, the diagnosis of pulmonary meningioma still depends on histopathologic examinations, because it’s difficult to make this diagnosis by clinical features and imaging modalities, including
18F-FDG PET/CT.