Background and Aim: To review the characteristics, radiological findings and diagnostic steps of patients with pulmonary actinomycosis (PA) and to further assess 18F-FDG PET-CT scan findings in the diagnosis of PA.

Methods: The study was designed retrospectively in a tertiarry referral hospital and patients diagnosed with PA between January 2012 and January 2022 were investigated. Demographics, clinical and radiological findings at presentation, diagnostic steps, 18F-FDG PET-CT findings, and time interval of diagnosis were analyzed.

Results: Of all the 34 patients, the mean age at the diagnosis was 49.2 (23-77) and 56% male. The most common symptom was cough in 23 (67,8). The most common underlying diseases were chronic obstructive pulmonary disease and bronchiectasis. Common chest tomography manifestations were nodular lesions, mass lesion, consolidation, bronchiectasis, and atelectasis. The initial pre-diagnoses were lung cancer in 16 (47%), tuberculosis in 9 (27%), and late resolving pneumonia in 6 (18%). None of the patients received a correct initial diagnosis of PA. All diagnoses were yielded histopathologically via specimen obtained in sputum analysis in 1 (2,94%), flexible bronchoscopy in 18 (52,94%), endobronchial ultrasonography in 2 (5,88%), transthoracic needle aspiration in 6 (17,65%), and surgical resection in 7 (20,58%). Mean duration from the symptoms to the definite diagnosis was 53.2 ± 44.1 (9-175) days. In the diagnostic workout 16 (47%) patients underwent 18F-FDG PET-CT while 10 (29%) patients underwent cranial magnetic resonance imaging. From the re-assessment of 13 18F-FDG PET-CT scans, the following values were derived: SUV Max value was 6.98 ± 2.74 (min 0.9- max 9.92), SUV Mean value was 3.95 ± 1.51 (min 0.51- max 5.30), SUV Peak value was 5.68 ± 2.24 (min 0.64- max 7.89), tumor lesion glycolysis value was 138.58; ± 151.86 (min 3- max 440.5), and metabolic tumor volume was 27.85 ±37.97 (min 0- max 131.00)

Conclusion: The diagnosis of PA is challenging and takes time. often misdiagnosed as lung cancer or pulmonary tuberculosis and diagnosis is challenging and takes time. As it demonstrates moderate metabolic uptake on 18F-FDG PET-CT, differentiating lung malignancy from PA is not sufficient by itself for accurate and timely diagnosis.