Bone marrow fibrosis and caseating granulomas associated with intravesicular BCG treatment.

Intravesicular instillation of bacille Calmette-Guérin (BCG), an attenuated variant of the virulent strain of Mycobacterium bovis, has been extensively used as the treatment of choice for bladder cancer since first approved in 1976. Adverse effects of treatment are rare and mostly local, with systemic reactions complicating up to 5% of patients. The majority of serious side effects have occurred in immunocompromised patients.