<title>Abstract</title><sec>
<title>Background</title>
Intravesical instillation of bacillus Calmette–Guérin (BCG) as a treatment for superficial bladder cancer rarely causes pulmonary complications. While published cases have been pathologically characterized by multiple granulomatous lesions due to disseminated infection,
no case presenting as a solitary pulmonary nodule has been reported.


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<title>Case presentation</title>
A man in his 70 s was treated with intravesical BCG for early-stage bladder cancer. After 1 year, he complained of productive cough with a solitary pulmonary nodule at the left lower lobe of his lung being detected upon chest radiography. His sputum culture result came back positive, with conventional polymerase chain reaction (PCR) identifying <italic>Mycobacterium tuberculosis</italic> complex. However, tuberculosis antigen-specific interferon-gamma release assay came back negative. Considering a history of intravesical BCG treatment, multiplex PCR was conducted, revealing the strain to be <italic>Mycobacterium tuberculosis</italic> var. BCG. The patient was then treated with isoniazid, ethambutol, levofloxacin, and para-aminosalicylic acid following an antibiotic susceptibility test showing pyrazinamide resistance, after which the size of nodule gradually decreased.


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<title>Conclusion</title>
This case highlights the rare albeit potential radiographic presentation of <italic>Mycobacterium tuberculosis</italic> var. BCG, showing a solitary pulmonary nodule but not multiple granulomatous lesions, after intravesical BCG treatment. Differentiating <italic>Mycobacterium tuberculosis</italic> var. BCG from <italic>Mycobacterium tuberculosis</italic> var. <italic>tuberculosis</italic> is crucial to determine whether intravesical BCG treatment could be continued for patients with bladder cancer.


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