Breast imaging of infectious disease

Abstract

Infectious diseases of the breast can demonstrate a wide variety of clinical presentations and imaging appearances. Breast abscesses are often a complication of infectious mastitis of the breast. Puerperal mastitis is the most common cause of breast abscess, typically affecting postpartum females. Often diagnosed clinically, it is usually treated with antibiotics without need for imaging. Non-puerperal mastitis is relatively uncommon and typically subareolar in location. Patients can present with asymmetric breast thickening, a palpable lump, nipple discharge, or axillary adenopathy. These presentations can mimic malignancy. Herein, this pictorial review demonstrates imaging findings of common and uncommon infectious processes of the breast and clinically important mimickers of breast infection.

Introduction

Mastitis refers to inflammation of the breast parenchyma and can be infectious or non-infectious. Mastitis and abscess can be further subdivided into puerperal and non-puerperal etiologies. Puerperal mastitis or lactational mastitis is most commonly due to Staphylococcus aureus. Sonographically, this may demonstrate inflammatory changes with hyperechogenicity of the breast parenchyma and subcutaneous fat. Discrete complex fluid collections can be seen with abscess formation. It is estimated that breast abscesses develop in 3–11% of females with mastitis, with an overall reported incidence rate of 0.1–3% in breast feeding women.¹

Non-puerperal mastitis occurs in non-pregnant, non-lactating females and is typically subareolar in location.^2,3 Risk factors include African American race, obesity, smoking, and being under the age of 40.⁴ The patient may present with symptoms mimicking breast malignancy, such as asymmetric breast thickening, a palpable lump, nipple discharge, or axillary adenopathy.³

Imaging and clinical history can suggest the diagnosis of chronic mastitis, however, aspiration and/or biopsy is often required to confirm the diagnosis. A targeted breast ultrasound can help delineate the extent of infection and need for percutaneous intervention. Clinical or imaging follow-up is important in the treatment of mastitis to exclude other causes of inflammation such as inflammatory carcinoma, invasive carcinoma, or idiopathic granulomatous mastitis. The radiologist should be aware of these conditions as to avoid misdiagnosis and inappropriate treatment.

The purpose of our pictorial review is to illustrate the varied multimodality imaging appearance of different types of breast infections, describe imaging findings, and discuss the role of imaging in directing treatment. We also describe important pitfalls to be aware of when evaluating imaging of mimickers of breast infection.

Acute infection

Puerperal mastitis

Puerperal mastitis, or lactational mastitis, is the most common form of mastitis. Continued breastfeeding despite discomfort and pain for the breastfeeding mother during treatment with broad-spectrum antibiotics leads to better overall clinical outcomes.⁴ Puerperal mastitis is generally not evaluated with imaging unless increasingly symptomatic or refractory to treatment with targeted breast ultrasound being the imaging test of choice (Figure 1).

Figure 1.

Puerperal mastitis in a lactating female. A 37-year-old currently breastfeeding female who is 8.5 months post-partum develops fever with increased erythema of the left breast. Initial sonographic evaluation of the left breast (a) shows no drainable fluid collection. She was already on broad-spectrum antibiotics at the time of evaluation. The patient presents 1 month later after a 10 day course of antibiotics. Targeted sonographic evaluation of the same region (b, c, and d) demonstrated several channels of fluid collections (white arrow) which were new in comparison to the initial evaluation. Findings were consistent with recurrent puerperal mastitis with infectious/inflammatory fluid collections. Attempted drainage yielded no fluid. The patient was continued on broad-spectrum antibiotics for 10 days at which point she experienced resolution of her symptoms. 3 month follow-up diagnostic ultrasound (e) showed normal appearing breast parenchyma at the site of previously imaged mastitis.

Galactocele complicated by iatrogenic breast abscess

Galactoceles are common lesions in breastfeeding females, mostly occurring upon cessation of lactation.^5,6 Features that can mammographically suggest a galactocele include a fat-fluid level best appreciated on a mediolateral or mediolateral oblique view (Figure 2). Sonographic imaging features can closely parallel breast-abscess or malignancy. Diagnosis can often be made based on history and clinical picture, however, percutaneous aspiration or biopsy may be necessary to make a definitive diagnosis when malignancy cannot be excluded.

Figure 2.

Galactocele complicated by iatrogenic breast abscess. A 32-year-old female, 7 weeks post-partum, currently breastfeeding develops a palpable left breast lump. Initial mammographic craniocaudal (a) and mediolateral oblique (b) views of the left breast demonstrate a mass in the left upper outer breast (white arrow) corresponding to the galactocele, with (b) demonstrating a fat-fluid level (yellow arrow: fat; white arrow: fluid). Sonographic evaluation of the left breast (c) shows a 2.9 cm hypoechoic mass with indistinct margins corresponding to the palpable lump (white arrow). This underwent ultrasound-guided biopsy (d) with pathology demonstrating galactocele. The patient develops fever, pain, and erythema at the biopsy site 2 days after the procedure. Repeat ultrasound (e) demonstrates increased skin thickening, subcutaneous edema, and hypervascularity of the subcutaneous tissue consistent with cellulitis (yellow arrow). A heterogeneous fluid collection at the site of previously imaged biopsy-proven galactocele (white arrow) raises the suspicion for possible abscess. This underwent aspiration (f, g) with cultures growing Staphylococcus aureus. The patient underwent a 10-day course of TMP-SMX with follow-up ultrasound (h) demonstrating resolution of the previously imaged breast abscess. TMP-SMX, trimethoprim-sulfamethoxazole.

Acute abscess secondary to inflamed retroareolar cyst

The retroareolar cyst, or cyst of Montgomery, is a benign cyst of the subareolar breast rarely occurring in adolescent females.⁷ They are thought to arise from obstruction of the ducts which drain the Montgomery areolar tubercles.⁷ Prolonged obstruction of the tubercles leads to stasis of the cystic fluid which results in acute inflammation and possibly an asymptomatic mass. The radiologist should consider this diagnosis when evaluating breast abscesses in an adolescent patient as inappropriate percutaneous aspiration may result in defects or destruction of the developing breast bud. Sonographic evaluation will confirm the diagnosis demonstrating retroareolar breast cysts of varying sizes and complexity. Infected retroareolar cysts respond well to antibiotics providing broad-based coverage against Staphylococcus, with serial imaging follow-up to ensure resolution⁸ (Figure 3).

Figure 3.

Acute abscess secondary to inflamed retroareolar cyst. A 13-year-old female presents with a right subareolar breast lump, pain, and erythema. She was started on oral antibiotics prior to imaging. Initial targeted ultrasound (a) demonstrates three adjacent cystic masses, two of which contain debris. These findings were thought to represent organizing abscess in the setting of inflamed cysts of Montgomery in an adolescent patient. Repeat targeted ultrasound 3 weeks after initial evaluation (b) shows decreased size of two of the three cystic masses consistent with resolving organized abscess. Targeted ultrasound 1 week later (c, d) demonstrates that the dominant cystic mass continued to decrease in size. Targeted ultrasound 3 months later (e) shows resolution of all fluid collections.

Acute non-puerperal mastitis

Imaging evaluation in patients with acute non-puerperal mastitis will include a targeted ultrasound at the site of clinical concern. Depending on patient age and clinical context, a mammogram may be indicated to exclude malignancy, especially inflammatory cancer.^3,5 Interventions and treatment of non-puerperal abscesses will usually center around broad-spectrum antibiotics and/or percutaneous aspiration (Figure 4). Depending on the clinical context, several aspirations and/or surgical debridement may be necessary to alleviate symptoms (Figure 5).

Figure 4.

Infected post-operative seroma in the setting of breast reconstruction. A 40-year-old female with history of left breast invasive ductal carcinoma status post bilateral mastectomy with breast reconstruction utilizing bilateral latissimus dorsi harvest sites and bilateral single lumen breast implants presents with clinical concern for infection 5 months after reconstruction. CT of the chest was performed (a) demonstrating inflammatory changes surrounding the implant (white arrow). The implant was subsequently removed (b) which showed a small area of post-operative gas (white arrow). The patient developed sepsis at that time and was hospitalized with a course of antibiotics for broad coverage for the breast abscess. CT of the chest performed 4 months later (c) demonstrates accumulation of an infected fluid collection in the region of the left latissimus dorsi harvest site (white arrow) which underwent CT-guided drainage and placement of a pigtail catheter (d, white arrow). Targeted ultrasound of the left latissimus dorsi harvest site 2 months later (e) shows no reaccumulating fluid (white arrow).

Figure 5.

Acute non-puerperal breast abscess. A 40-year-old female presents with increased redness, warmth, and swelling primarily involving the peripheral right breast. Clinical photograph at the time of presentation (a) demonstrates cellulitis (white arrow) with a developing ulcer (yellow arrow). Photograph of the same area 1 week later (b) demonstrates increased conspicuity of the ulcer in the upper outer breast, now draining purulent fluid. The patient underwent operative incision and drainage with broad-spectrum antibiotics. She clinically presents 1 week later with necrotic tissue and a fibrinous exudate in the wound bed at the site of ulcer (c). Cultures grew Staphylococcus epidermidis, Pseudomonas aeruginosa, and Staphylococcus aureus. Right diagnostic mammogram craniocaudal (d) and mediolateral oblique (e) views demonstrate a large region of increased density with associated soft tissue gas in the upper outer right breast spanning middle to posterior depth (red circle). Evaluation of the right axilla on mediolateral oblique view demonstrates prominent lymph nodes. Targeted ultrasound of the right breast at the site of mammographic abnormality (f) demonstrates extensive subcutaneous gas (white arrow) which obscured further evaluation of the breast tissue. Targeted ultrasound of the right axilla (g) demonstrates symmetrically thickened right axillary lymph nodes (red circle), reactive in etiology. The patient made a full recovery after undergoing multiple surgical explorations with operative debridement, irrigation, placement of a wound vacuum, and broad-spectrum antibiotics.

Recurrent non-puerperal subareolar mastitis

Recurrent non-puerperal subareolar mastitis and abscess can lead to Zuska-Adkins disease which is a rare benign inflammatory breast condition associated with prolonged morbidity (Figure 6). Initial infections are usually due to Staphylococcus bacteria, with recurring infections secondary to mixed bacterial flora requiring broad-spectrum antibiotics for treatment. Patients will present with difficult to treat and recurring breast abscesses often with fistulous tracts that extend to the skin surface.^5,9 Mammographically, Zuska-Adkins disease presents with non-specific findings which can be indistinguishable from other inflammatory conditions of the breast. Sonographic findings can include complex cystic masses containing viscous fluid or heterogeneous masses. Biopsy may be necessary to exclude malignancy.

Figure 6.

Non-puerperal subareolar mastitis (Zuska-Adkins disease). A 41-year-old African-American female with history of smoking and bilateral nipple piercings complicated by breast abscess status post-incision and drainage presents with left breast discharging skin fistula, swelling, and erythema. She was empirically started on amoxicillin/clavulanic acid prior to radiologic evaluation. Left diagnostic mammogram craniocaudal (a) and mediolateral oblique (b) views demonstrates increased density and skin thickening centered within the subareolar left breast (red circle). Targeted ultrasound of the area of increased skin thickening demonstrates a subareolar fluid collection with debris and a fistulous connection to the dermis at the site of patient reported drainage (c, white arrow). Attempted drainage of the fluid collection (d, white arrow) yielded no aspirate. The patient was subsequently lost to follow-up.

Chronic infection

Cutaneous blastomycosis

Breast and cutaneous blastomycosis presents with skin ulcerations and underlying subcutaneous abscesses, demonstrating considerable imaging overlap with other inflammatory conditions of the breast (Figure 7). Most cases of breast and cutaneous blastomycosis occur following lymphohematogenous dissemination of pulmonary blastomycosis. Diagnosis of breast and cutaneous blastomycosis may be challenging as laboratory tests may be non-contributory. Tissue culture is more specific for diagnosis but can take up to 5 weeks, which can lead to a clinically significant delay in treatment.¹⁰ Most cases of cutaneous blastomycosis can be managed with Itraconazole for 6-–12 months. Long-term antibiotics may be necessary for more refractory cases.

Figure 7.

Cutaneous blastomycosis. A 41-year-old female presents with 1-month history of palpable lump in the left breast above the nipple areolar region. Left diagnostic mammogram (a) at the site of triangular marker demonstrates an obscured mass in left slightly upper, anterior depth breast (white arrow). Targeted ultrasound at the site of mammographic finding (b) demonstrates an irregular hypoechoic mass suspicious for malignancy (white arrow). Ultrasound-guided biopsy of the mass revealed necrotizing granulomatous infection with fungal organisms consistent with Blastomycosis. No additional foci of infection were found. The patient was started on a 6–12 month course of itraconazole with full recovery.

Mycobacterium fortuitum

Mycobacterium fortuitum, a group of mycobacteria not encompassing Mycobacterium tuberculosis, has been implicated in cases of pulmonary, soft tissue, and systemic disease. Often seen in the immunocompromised, imaging features are non-specific and can include formation of breast masses, adenopathy, and subcutaneous drainage to the skin surface¹¹ (Figure 8). As Mycobacterium fortuitum is a relatively rare clinical manifestation of breast abscess, the diagnosis may only be considered in cases where initial treatment with broad-spectrum antibiotics fails to alleviate the breast abscess. Treatment of Mycobacterium fortuitum includes broad-spectrum antibiotics, which may include amikacin, imipenem, fluoroquinolones, cefoxitin, sulfonamides, and linezolid.^11,12 In cases refractory to treatment, surgical debridement may be the only viable treatment option.

Figure 8.

Mycobacterium fortuitum. A 32-year-old female presents with a new tender, palpable mass in the left breast. Left diagnostic mammogram (a) demonstrates multiple masses (white arrow) with axillary adenopathy (red arrow). Targeted ultrasound of the left breast demonstrates a heterogenous fluid collection (b, white arrow) with sinus tract extending to the skin surface (red arrow, c). Biopsy of one of the masses demonstrated granulomas with purulent drainage from the needle tract positive for M. fortuitum. Treatment with antibiotics based on sensitivity of the organism led to resolution of infection. Case from: Illman JE, Terra SB, Clapp AJ, et al. Granulomatous diseases of the breast and axilla: radiological findings with pathological correlation. Insights Imaging. 2018;9 (1):59–71. Reproduced with copyright permission.

Bartonella henselae

Breast or axillary manifestations of Bartonella henselae (cat-scratch disease) are an uncommon clinical presentation. Imaging features may show unilateral axillary adenopathy as the only clinical feature in many cases (Figure 9). Pathology of a sampled lymph node or breast lesion can show granulomatous disease, which includes a large differential diagnosis of other infectious etiologies. A positive serological titer confirms the diagnosis of Bartonella henselae, however, negative serological titer does not entirely rule out the disease especially in cases of chronic infection.^11,13 Prompt multidisciplinary clinical management can help if the diagnosis of Bartonella henselae is clinically suspected.

Figure 9.

Bartonella henselae (cat scratch disease). A 59-year-old Turkish female presents with 1-month history of left axillary mass, chills, and fever. Left diagnostic mammogram (a) demonstrates multiple enlarged lymph nodes (white arrow). Targeted ultrasound of the left axilla at the site of enlarged left axillary lymph nodes (b) demonstrates multiple enlarged left axillary lymph nodes (white arrows) one of which underwent biopsy (c). Photomicrograph (d) (original magnification x200; hematoxylin-eosin [H-E] stain) revealed non-necrotizing granulomatous inflammation (white arrow) composed of histiocytes and giant cells. Differential includes infectious (e.g. mycobacteriosis, mycosis, herpes, bacterial infection) and non-infectious (e.g. histiocytic necrotizing lymphadenitis, sarcoid) etiologies. Subsequent CT chest (e) shows an enlarged left axillary lymph node (red circle). Titer for Bartonella henselae (cat scratch disease) was >1:1024, though IgM was negative suggestive of a chronic infection. The patient was sent for dermatology consult as there were at least four areas of red rash—inferior right breast, inframammary fold left breast, posterior thighs which were present for 1 year. Punch biopsy of these areas showed granuloma annulare. Case from: Illman JE, Terra SB, Clapp AJ, et al. Granulomatous diseases of the breast and axilla: radiological findings with pathological correlation. Insights Imaging. 2018;9 (1):59–71. Reproduced with copyright permission.

Mimickers

Post-operative seromas

Post-operative seromas in the setting of surgical excision, lumpectomy, mastectomy, breast augmentation, or breast reconstruction are a common finding which can show considerable overlap with infectious mastitis (Figures 10 and 11). Percutaneous aspiration of a sterile post-operative fluid collection is usually necessary given the potential of infection and delayed treatment. Continued recurrence of serous fluid within a post-operative cavity may require a drainage catheter for resolution.

Figure 10.

Post-operative seroma in the setting of lumpectomy. A 60-year-old female with history of right breast invasive ductal carcinoma with ductal carcinoma in situ, status post-lumpectomy 2 weeks prior, presents with increased erythema at the lumpectomy site. Clinical photograph of the right breast (a) shows an erythematous, tender, and swollen right breast. Targeted right breast ultrasound was performed at the lumpectomy bed and demonstrated multiple interconnected pockets of fluid (b, white arrow) concerning for abscess vs seroma. Aspiration of the fluid pocket (c, white arrow) yielded 60 cc of serosanguinous fluid, consistent with a post-operative seroma. Findings did not reaccumulate in the coming months.

Figure 11.

Implant seroma. A 67-year-old female with history of smooth (non-textured) subpectoral implant augmentation with new onset asymmetry of the left breast 11 years following implant placement. Clinical photograph of the bilateral breasts (a) shows marked deformity of the left breast in relation to the right. MRI of the breast with contrast (b) demonstrates a large left periimplant effusion with nodularity within the posterior and anteroinferior aspects of the implant (white arrows). Targeted left breast ultrasound (c) demonstrates a large complex fluid collection surrounding the implant with mobile debris. This underwent aspiration which was negative for malignancy or organisms. Aspirate was sent for CD30 and ALK which were negative. PET-CT (d) was performed to assess for BIA-ALCL. No areas of focal increased uptake were seen to suggest BIA-ALCL. She underwent removal of the breast implants with full recovery. No evidence for BIA-ALCL on histology. BIA-ALCL, breast-implant associated anaplastic large cell lymphoma. PET-CT, positron emission tomography-CT

Idiopathic granulomatous mastitis

Idiopathic granulomatous mastitis (IGM) is a rare benign inflammatory chronic mastitis that occurs primarily in females of childbearing age, most often in post-partum or breastfeeding mothers.¹⁴ A strong association with Corynebacterium species is postulated though uncertain (Figure 12). Imaging features show a strong degree of overlap with other inflammatory breast conditions and may mimic malignancy (Figure 13). Pathology shows non-caseating granulomas with infiltrates of multinucleated giant cells, plasma cells, epithelioid histocytes and lymphocytes.¹⁴ Other possible causes of mastitis should be excluded before considering IGM as a possibility. Treatment of IGM is often difficult with high rates of recurrence and a prolonged clinical course.

Figure 12.

Recurrent IGM secondary to IGM Corynebacterium. A 32-year-old female with history of left breast infection treated with incision and drainage 6 months prior. Clinical photograph (a) of the left breast shows a large area of inflammation involving the left medial breast with a small open wound draining serosanguinous fluid (white arrow). Left diagnostic mammogram (b) shows diffuse global asymmetry, skin thickening primarily involving the subareolar left breast (red circle), and axillary adenopathy (white arrow). Targeted left breast ultrasound (c) shows diffuse inflammatory changes with multiple discrete hypoechoic fluid collections (white arrow). Targeted left axillary ultrasound (d) demonstrated multiple enlarged axillary lymph nodes (white arrows) with symmetric cortical thickening. The patient underwent aspiration and biopsy of the left breast abscess with pathology yielding non-necrotizing granulomatous inflammation. Special stains for microorganisms were negative. A presumed diagnosis of IGM was made. The patient was treated with antibiotics and steroids for pain relief which lead to full recovery. She presents 1 year later to the emergency department with right breast pain. Ultrasound of the right breast demonstrates diffuse inflammatory changes in the subareolar right breast (e, f) concerning for abscess (white arrows). Cultures grew Corynebacterium kroppenstedtii, in keeping with the clinical diagnosis of IGM. This led to serial aspirations with intralesional steroid injections (g, white arrow) and antibiotics. She experienced relief of her symptoms. IGM, Idiopathic granulomatous mastitis.

Figure 13.

IGM secondary to IGM Corynebacterium. A 36-year-old female with history of prior outside facility right breast abscess status post-percutaneous aspirations, and incision and drainage. Clinical history was suggestive of IGM with recurrence. Right breast craniocaudal (a) and mediolateral oblique (b) views demonstrate a focal asymmetry in the right upper, middle depth breast with periareolar focal skin thickening (white arrow). Targeted right breast ultrasound at the site of focal asymmetry (c) demonstrates a complex fluid collection with mobile internal echoes (white arrow) which underwent aspiration (d). The patient was treated with oral doxycycline for 14 days with a trial of topical steroids which demonstrated clinical improvement of her symptoms. IGM, Idiopathic granulomatous mastitis.

Malignancy

Imaging features of inflammatory breast cancer can be non-specific, with mammogram demonstrating diffuse skin thickening, density, nipple retraction, and trabeculation. Sonographic findings may show fluid pockets, mixed solid and cystic masses, and skin thickening (Figure 14). MR imaging can help delineate extent of disease but may demonstrate non-mass enhancement with rim enhancing lesions which may be confused with mastitis and abscess. Enhancement kinetics can be useful in distinguishing between the two entities, as complex masses in inflammatory breast cancer shows greater initial post-contrast enhancement with washout kinetics.⁵ MRI may additionally identify a target for biopsy in cases where initial ultrasound-guided biopsy is unsuccessful in yielding a diagnosis.³

Figure 14.

Inflammatory breast cancer. A 58-year-old female with history of left breast DCIS, status post- bilateral nipple sparing mastectomy with implant placement, fat grafting, and liposuction of the left breast 1 year prior presents with increased redness of the skin. She was started on antibiotics prior to imaging. Initial evaluation of the reconstructed left breast (a) demonstrates increased erythema and swelling of the left breast most prominently over the lateral breast scar. Targeted ultrasound at the site of erythema in the lateral left breast (b) demonstrates innumerable hypoechoic masses or cystic spaces throughout the subcutaneous fat tracking medially along the left breast implant (white arrow). The patient was re-evaluated 5 days later with mild improvement of the redness and erythema. However, imaging at the same site now revealed a 1.4 cm irregular, hypoechoic mass with internal vascularity (c, white arrow) which underwent ultrasound-guided biopsy. Pathology showed invasive ductal carcinoma, grade II–III. FDG PET-CT revealed multiple additional hypermetabolic masses (d, white arrow). MRI of the breast (e) demonstrates the biopsy-proven malignancy (red circle) in addition to subcutaneous tissue and skin involvement (white arrow). Kinetic mapping (f) demonstrates diffuse mixed enhancement kinetics seen throughout the affected left breast. The biopsy-proven malignancy (red circle) demonstrates mostly washout enhancement kinetics. Skin punch biopsies of a representative area demonstrated metastatic inflammatory cancer with lymphovascular invasion. She underwent neoadjuvant chemotherapy with follow-up MRI breast performed 6 months later demonstrating no evidence of residual disease. DCIS, ductal carcinoma in situ; FDG, fludeoxyglucose; PET, positron emission tomography.

Conclusion

Breast infections can have a wide variety of imaging appearances, depending on clinical picture, time course within the disease, and patient demographics. Most cases of infectious mastitis occur in the post-partum period and require no further work-up. The diagnostic work-up of nonpuerperal mastitis commonly utilizes both mammography and ultrasound, with MRI reserved for unique problem-solving cases. Aspiration with tissue culture or percutaneous biopsy may be required to establish the diagnosis.