We report herein a patient with pulmonary actinomycosis mimicking lung cancer on FDG-PET. To obtain a definitive diagnosis of pulmonary actinomycetes, isolation, identification, and culture of microorganisms are required. In this case, the culture results were negative because of the anaerobic nature of
Actinomyces, prior antibiotic therapy, and use of inadequate culture techniques. In case of post-bronchoscopy fever [
4,
5], use of prophylactic antibiotics is not recommended according to the guidelines of the British Thoracic Society [
6]. Moreover, tracheobronchial stenosis was considered an independent risk factor for post-bronchoscopy pneumonia via a multivariate analysis in a case–control study [
7]. Inappropriate use of antibiotics may contribute to false-negative culture results.
The incidence of actinomycosis has significantly decreased within the last three to four decades, and the clinical features are less aggressive compared with those in the preantibiotic era [
8]. Proper dental hygiene and early antimicrobial treatment of infections are contributing factors. However, recently, only a few studies have assessed the clinical characteristics of pulmonary actinomycosis. In a case series of 94 Asian patients with pulmonary actinomycosis during the first decade of the twenty-first century [
9], the median age was 57.7 (range 31–83) years, and the male:female ratio was 66:28. In total, 50 patients had history of smoking, and 45 patients presented with underlying pulmonary comorbidities such as infections caused by mycobacteria (
n = 21), bronchiectasis (
n = 18), and chronic obstructive pulmonary disease (
n = 10). The nonpulmonary comorbidities were diabetes mellitus (
n = 18), hypertension (
n = 18), and alcohol abuse (
n = 16). The presenting symptoms were cough (77.7%), hemoptysis (64.9%), and sputum secretion (61.7%). The chest CT scan findings were consolidation (74.5%), mediastinal or hilar lymph node enlargement (29.8%), atelectasis (28.7%), cavitation (23.4%), ground-glass opacity (14.9%), and pleural effusion (9.6%). Based on the clinical and radiological findings, lung cancer (35.1%) was the initial diagnosis, followed by pneumonia (19.1%) and mycobacterium infection (17.0%). The confirmatory diagnosis was obtained via surgical biopsy (
n = 47), bronchoscopic biopsy (
n = 24), and percutaneous transthoracic needle biopsy/aspiration (
n = 23). Intravenous, followed by oral, antibiotic treatment was initiated after the diagnosis of actinomycosis. The mean durations of treatment with intravenous and oral antibiotics were 14.7 (range: 1–56) days and 153.2 (range: 5–672) days, respectively. Intravenous antibiotics, mostly comprising penicillin G (34.1%), cephalosporin (27.3%), ampicillin/sulbactam (7.9%), and amoxicillin (5.7%), were administered. The indications for surgical resection (
n = 49) were persistent hemoptysis (
n = 22), differential diagnosis of pulmonary malignancy (
n = 17), and absence of radiologic response despite medical treatment (
n = 4). In total, 92 patients completely recovered, and only 2 died from complications.
Actinomycosis is a rare condition, and physicians have limited experience in assessing its clinical manifestations. Moreover, laboratory cultivation and identification are challenging. Therefore, this condition is still difficult to diagnose. With consideration of the risk of actinomycosis, this infection can be diagnosed using less invasive methods, and unnecessary surgeries can be prevented. Only a few case reports have documented pulmonary actinomycosis assessed via FDG-PET [
10,
11]. In 11 patients with pathologically confirmed pulmonary actinomycosis, the median maximal SUV on PET-CT scan increased to 5.5 (interquartile range: 4.2–8.8). However, this was higher than the threshold value of 2.5 that indicates malignancy [
12]. The diagnostic value of FDG-PET in ruling out malignancy is limited.
We report herein the case of a patient with pulmonary actinomycosis mimicking lung cancer. This infection is rarely considered by pathologists when establishing a diagnosis. Because there is no difference in PET/CT findings between actinomycosis and lung cancer, the diagnostic value of FDG-PET is limited. If patients have fever after bronchoscopy, careful observation is generally recommended, and inappropriate use of antibiotics may lead to false-negative culture results.