A Rare Case of IgG4-Related Disease Causing Inflammatory Breast Mass Successfully Treated with Anti-CD20 Biological Therapy: A Discussion of Clinical Case with Literature Review.

We report a rare yet successful utilisation of anti-CD20 therapy using rituximab for treatment of a case of IgG4-related mastitis proven by clinical, serological, and histopathological evidence. This was affecting a mid-aged female who was referred to the rheumatology clinic by the breast surgeons to help assessing for the possibility of an underlying inflammatory process involving the breast tissue unilaterally. The clinical course was apparently complex with an onset of an induration in the right lateral superior quadrant of the breast with mild discomfort and heaviness sensation. This increased over a course of 2 weeks before presentation to the general surgery clinic. Subsequent investigations confirmed that the case was IgG4-related mastitis and a trial of steroids and disease modifying anti-rheumatic drugs (DMARDs) was partially helpful, but not to a full degree, mandating the utilisation of a more advanced mode of therapy, so rituximab was selected. LEARNING POINTS: Consider inflammatory conditions including IgG4-related disease in the differential diagnosis of lumps including breast masses.IgG4 disease is a rare condition but can have complex clinical course and significant complications that need a low threshold of suspicion in terms of diagnosis.Occasionally, a repeat study and re-reading of histopathological specimen with appropriate immunostaining can open the way for accurate diagnosis in challenging cases with an unreached diagnosis.