Pulmonary actinomycosis: Before the pathology specimen was obtained, tuberculin skin testing, sputum analysis for acid-fast bacilli, and routine cultures were also ordered; all findings were negative. Additional laboratory studies, including a serum creatinine determination, yield normal levels. HIV serology also returned a negative result. The stain shown in Image 3 demonstrates aggregates of filamentous organisms highlighted with silver stain. This pattern is consistent with actinomyces, and pulmonary actinomycosis is diagnosed.

The differential diagnosis of a cavitary lung mass consists of a wide variety of pulmonary diseases. Some, such as bronchogenic carcinoma, tuberculosis (TB), and suppurative lung disease, are common, and others, such as actinomycosis, are relatively infrequent.

On clinical evaluation, pulmonary actinomycosis is commonly confused with TB, suppurative lung disease, and malignancy. Patients with pulmonary actinomycosis usually present with a pulmonary consolidation. However, as in this case, some patients present with cavitation and spread to the adjacent tissues. Bronchoscopic findings are usually nondiagnostic, and most patients require open lung biopsy. The organism involved in this condition is Actinomyces israelii. This microorganism is slender, branching gram-positive bacillus embedded in the matrix of the sulfur granules. The hallmark of actinomycosis is the formation of the yellow sulfur granules.

The optimal duration of treatment is not clearly established, and the traditional recommendation of intravenous antibiotic therapy (beta-lactam) for 2-6 weeks followed by oral antibiotic therapy for 6-12 months is not always necessary. The condition is best treated with individualized therapeutic protocols depending on factors such as the initial burden of disease, the patient's response to and the success of surgical resection, and the patient's progressive clinical and radiologic response to continuing antibiotic regimens. The most common indications for surgery are hemoptysis and empyema. As a complication, chronic sinus drainage has become decreasingly frequent, presumably because of the widespread use of the antibiotics.

The patient in this case responded well to antibiotic therapy continued postoperatively. She completed a 6-month course of amoxicillin with no clinically significant adverse effects. During follow-up, a repeat chest CT scan showed no evidence of active disease.