Cystic neutrophilic granulomatous mastitis: A clinicopathologic study of a distinct entity with supporting evidence of a role for Corynebacterium-targeted therapy

Cystic neutrophilic granulomatous mastitis (CNGM) is a distinct histopathologic entity characterized by neutrophilic and granulomatous inflammation surrounding clear cystic spaces. Rare gram-positive bacilli are sometimes identified within these cystic spaces. Studies in the literature have identified these gram-positive bacilli to be Corynebacterium species. We describe the clinicopathologic features of 7 cases of CNGM, including a case with evidence of Corynebacterium amycolatum. Patients were young to middle aged parous women ranging in age from 28 to 53 years (median age: 41 years). Gram-positive bacilli were identified in 4 cases, all within cystic spaces. Microbial culture from a 41-year old Hispanic woman grew Corynebacterium species on multiple occasions and Corynebacterium amycolatum was identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) on two separate occasions. Antibiotic susceptibility testing performed both times showed resistance to multiple antibiotics and susceptibility to vancomycin. Follow-up of all patients (range 3-12 months, median 6 months) showed a widely variable clinical course and varying response to a variety of treatment modalities. Five of the seven CNGM patients were parous, reproductive-aged Hispanic women who were born outside of the United States. Our findings further support the association of CNGM with corynebacteria and gram-positive bacilli. Furthermore, this study shows that Corynebacterium amycolatum, a nonlipophilic and multidrug-resistant corynebacterium can be associated with CNGM, hence the need for targeted antibiotic therapy. We propose identifying corynebacteria to the species level and performing antibiotic susceptibility testing in patients with CNGM because of the varied susceptibility testing profile that has been reported among different species of corynebacteria.