Pulmonary Actinomycosis mimicking lung malignancy

Clinical History

We report a case of pulmonary Actinomycosis who mimicked lung malignancy and showed intense uptake of 18-Flurodeoxyglucose PET/CT scan. The diagnosis was reached by histopathological examination of the surgical specimen, the patient having undergone a right lower and middle lobectomy. We discuss the radiological features of Actinomycosis and the caveats of PET/CT scans.

Imaging Findings

A 62-year-old lady was referred for a chest radiograph by her GP, following an eight month history of productive green phlegm. The initial chest radiograph showed a small dense soft tissue mass projected over the right hilum. She had worked in a dairy farm, subsequently worked in a bacon farm. She smoked 20 cigarettes a day for 40 years (80 pack years). She also drank 42 units of alcohol a week. Physical examination was unremarkable. A contrast enhanced CT of the Thorax was performed a week later which showed a 5cm diameter heterogeneous area (with a cystic core and some cavitation surrounded by soft tissue density material) in a collapsed apical segment of the right lower lobe. This soft tissue area was crossing the oblique fissure into the upper lobe. A bronchoscopy and bronchial washings were unremarkable. The patient was subsequently referred for a CT-guided lung biopsy. The histological examination showed inflammatory changes and failed to show any microorganisms. A repeat CT Thorax 3 months later showed no change. A subsequent FDG-PET scan detected an abnormal peripheral uptake in the 5.4 cm right lower lobe mass with a central area of photopaenia . There was no significant uptake in the hila or mediastinum, and no evidence of distant metastases. The patient was then referred for a right middle and lower lobe lobectomy. The histology of the operated specimen revealed clumps of Actinomycosis organisms with filamentous forms. The patient was put on a 3 month course of antibiotic therapy.

Discussion

Actinomycosis is a rare infection that was historically thought to be due to a fungus but now is considered to be due to branching filamentous, Gram-positive, anaerobic to microphilic, non spore-forming bacteria which are not acid-fast. The commonest species encountered is Actinomycosis Israelii.[1] Actinomyces species are present in normal oral flora frequently found at gingival margins of people with poor oral hygiene and dental caries .Such are believed to be predisposing factors to pulmonary Actinomycosis; aspiration of saliva being the mechanism of spread. Histopathological examination of resected tissue may show the characteristic sulphur granules (yellowish granules of clustered mycelia) surrounded by polymorphonuclear cells and gram positive branching filamentous organisms that stain positively with Gomori’s Methenamine silver. Actinomycosis has a variable radiological picture [2,3]. More commonly on CT, there is a chronic segmental airspace consolidation with central areas of low attenuation, peripheral enhancement, and associated pleural thickening. It behaves in an infiltrative manner not obeying interlobar fissural boundaries or tissue planes by producing proteolytic enzymes. There is often an associated pleural effusion, pleural thickening and also a collection of fluid in the extrapleural space resulting from direct extension through the parietal pleura and the chest wall, called empyema necessitatis [4]. It involves the chest wall in a contiguous way forming a chest wall mass, with sinus tracts and can involve the adjacent ribs causing periosteal proliferation which can have a peculiar wavy configuration [5,6]. Actinomyces species also have a tendency to colonise devitalised lung tissue that may have already been damaged by previous tuberculosis or other bacterial infections. In such cases, CT features include localised bronchiectasis and irregular bronchial wall thickening with irregular peribronchial consolidation,as the Actinomycosis exacerbates further the pre-existing bronchiectasis [7]. Endobronchial Actinomycosis is a rarer entity and tends to occur following secondary colonisation of either a pre-existing proximal broncholith or an aspirated foreign body. On CT there is associated post obstructive pneumonic consolidation [8]. On rarer cases, it has been noted to extend into the abdomen through the diaphragm [9]. Its CT features can mimic other conditions apart from malignancy, including TB and Nocardiosis. In general clinical practice, FDG-PET/CT studies are being used increasingly to try and distinguish between benign and malignant pathology. FDG, however, is by no means a cancer specific agent, but simply an indicator of glycolytic metabolic activity [10]. There are only a few published FDG-PET studies on Actinomycosis [11]. A case report of an FDG-PET study in a patient with a left upper lobe mass invading the chest wall, with mediastinal lympadenopathy and a pleural effusion showed enhanced uptake within the left chest wall. Fortunately, yellow pus spontaneously dripped out following biopsy of the chest wall and this offered the diagnosis of Actinomycosis on microbiological examination. An FDG-PET scan carried out on an asymptomatic patient for a whole body cancer screening revealed focal linear uptake in the left buttock. A malignant tumour was suspected. However, at surgery an anorectal fistula with Actinomycosis and abscess formation was found [12].

Differential Diagnosis List

Final Diagnosis

Pulmonary Actinomycosis

URL:	https://www.eurorad.org/case/5654
DOI:	10.1594/EURORAD/CASE.5654
ISSN:	1563-4086