Idiopathic Granulomatous Mastitis

A 37-year-old woman presented with erythema and swelling on the medial side of the left breast. She had several episodes of similar complaints in both breasts in the last two years. An antibiotic regimen did not ameliorate her symptoms. She had a history of pregnancy and lactation a few years ago.

Mammography demonstrated an asymmetric, irregular, and ill-defined opacity medially in the left breast (Fig. 1, white circle). Several enlarged lymph nodes were present in the left axilla. Ultrasound of the left breast showed an ill-defined, diffuse heterogeneously hypoechoic area (Fig. 2, white arrows).

On MRI, contrast-enhanced subtraction images demonstrated an irregular non-mass enhancement with a segmental distribution on the medial side of the left breast (Fig. 3A, white circle). Diffusion restriction of the segmental non-mass enhanced was observed on b1000 diffusion-weighted images (DWI, Fig. 3B, white circle) and apparent diffusion coefficient maps (ADC, Fig. 3C, white circle).

The core biopsy showed histological features of a granulomatous inflammatory reaction with negative stains for acid-fast bacilli and fungi (Fig. 4). No sarcoid granulomas nor angiitis lesions were seen. The diagnosis of granulomatous mastitis was made.

Idiopathic granulomatous mastitis (IGM) is a rare, benign and inflammatory lesion breast lesion with unknown aetiology [1]. It usually affects women of childbearing age and is associated with elevated hormonal states such as pregnancy, lactation and oral contraceptives. Other risk factors include including non-Caucasian ethnicity, smoking, diabetes mellitus, local trauma, hyperprolactinemia and alpha1-antitrypsin deficiency [2,3].

A painful mass is the most common symptom of IGM. Other symptoms are nipple retraction, erythema, swelling, ulceration,  and axillary lymphadenopathy [4,5]. It may occur in anywhere in the breast, but it is most frequent unilaterally in the retroareolar region [5].

The literature describes multiple theories on the pathophysiology of IGM. The most accepted theory states that an initial insult to the ductal epithelial cells causes luminal secretions to the breast stroma and a local inflammatory and granulomatous response [4].

Imaging features of IGM are often nonspecific. Focal or regional asymmetric density is the most common finding on mammography. Architectural distortion, skin thickening or axillary lymph nodes with a preserved fat hilum may be present [6].

On ultrasound, hypoechoic or heterogeneous masses may be present, with potential tubular extensions and indistinct, angular or lobulated margins [6–8]. A fluid collection is another possible ultrasonographic presentation of IGM [6–8].

On MRI, either a heterogeneous non-mass-like enhancement (NME) may be present, typically with a clustered ring pattern and a segmental distribution or a mass with smooth or ill-defined borders [9].

These masses may show diffusion restriction, ring enhancement and hyperintensity on T2-WI [3]. Most studies report that IGM lesions have benign time-signal intensity curves on DCE-MRI, although malignant type 3 time-signal intensity curves with a washout pattern may be also detected [9–11].

The most important differential diagnosis is inflammatory breast carcinoma (IBC). Clinically, IBC patients may show erythema, oedema and peau d’orange, with or without an underlying mass. Patients with IBC are typically older than those with IGM. Extensive skin thickening, axillary adenopathy and breast enlargement are imaging features more suggestive of IBC [4]. However, biopsy should be performed to reliably differentiate IBC from IGM. Other differential diagnoses include infective mastitis or granulomatous mastitis secondary to systemic disease (e.g. granulomatosis with polyangiitis, sarcoidosis) or granulomatous infection. No imaging characteristics can be used to distinguish infective mastitis or specific granulomatous mastitis from IGM. Therefore, a combination of clinical, biochemical, and histopathological features should be used to differentiate these entities [4].

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