Imaging features of common and uncommon lesions of the axilla: The breast radiologist's perspective

Accessory axillary breast tissue

Ectopic (accessory) breast tissue results from incomplete regression of the embryologic milk line; while the most common location is the axilla, breast tissue can be present in any location along the embryologic milk line. Patients may present with a palpable fullness, swelling, or mass that fluctuates with hormonal changes induced by menstruation, pregnancy, or lactation.^[5,6] New axillary masses merit diagnostic evaluation, as pathologic lesions that occur in the breast, including breast cancers, may develop within accessory axillary breast tissue.

Accessory breast tissue can be detected on multiple imaging modalities, and its density can range from predominantly fatty to extremely dense. On ultrasound (US), it appears as fibroglandular breast tissue within subcutaneous adipose tissue [Figure 6]. Mammographic imaging may reveal accessory axillary tissue as either a direct extension of the primary breast parenchyma or as an isolated, distinct island of tissue.^[7] On MRI, accessory axillary breast tissue demonstrates signal characteristics similar to normal breast parenchyma on T1 [Figure 7a] and T2 [Figure 7b] weighted imaging.^[5]

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Figure 6: Accessory axillary breast tissue on ultrasound. Grayscale longitudinal US of the axilla demonstrates heterogeneous appearing fibroglandular tissue within subcutaneous fat (arrow), indistinguishable from normal breast tissue, suggestive of benign axillary breast tissue. US: Ultrasound

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Figure 7: Accessory breast tissue on MRI. (a) Axial T1-weighted non-contrast MRI demonstrates heterogeneously dense breast tissue with extension of breast tissue into the right axilla (arrow), inseparable from right mammary tissue. (b) Axial T2-weighted non-contrast MRI demonstrates hyperintense tissue in the right axilla (arrow). MRI: Magnetic resonance imaging.

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While most patients are asymptomatic or experience minor, transient symptoms, some patients with a large amount of accessory axillary breast tissue may desire surgical excision for cosmesis or symptom management.

Fibroadenoma

Fibroadenomas are one of the most common benign breast masses in women of reproductive age. Fibroadenomas may be solitary or multiple, unilateral or bilateral, and can occur in any location within breast tissue, including accessory axillary breast tissue. Fibroadenomas typically involve menopause and may develop coarse calcifications.^[8]

Typical sonographic features include an oval, circumscribed, hypoechoic mass with parallel orientation within fibroglandular tissue; gently lobulated margins may be present [Figure 8a]. On mammography, fibroadenomas manifest as dense, round, or oval masses with circumscribed margins [Figure 8b and c].^[8] MRI typically demonstrates a T2 hyperintense lesion with homogenous enhancement and persistent kinetics, often featuring internal non-enhancing, T2-dark septations.^[9]

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Figure 8: Fibroadenoma. 40-year-old female with a palpable right axillary mass. (a) Grayscale longitudinal US of the right axillashows an oval hypoechoic mass with a microlobulated margin (arrow). (b) Diagnostic mammogram right MLO view and (c) spot compression view (yellow circle) show an oval, hyperdense mass with an indistinct margin (yellow circle) in the right axilla. US guided was performed with pathology demonstrating a benign fibroadenoma. US: Ultrasound, MLO: Mediolateral oblique

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If imaging features suggest fibroadenoma at initial detection, clinical and sonographic follow-up in six months is recommended. A core needle biopsy is recommended if the lesion has increased in size >20% at the time of follow-up. If the lesion has remained stable at the time of the six-month follow-up, further sonographic follow-up exams may not be necessary in women under age 35. Surgical excision based on a specific size for a biopsy-proven fibroadenoma is generally not warranted, unless the patient experiences pain or has cosmetic concerns. Giant fibroadenomas, which are defined as lesions > 5 cm in size, typically occurring in young women (mean age 21), are recommended for excision, as these may continue to grow, causing pain and breast asymmetry.^[9,10]

When occurring in the axilla, fibroadenomas may mimic lymphadenopathy or primary malignancy. While histopathologic confirmation is often required to exclude malignancy, sonographic characteristics suggest the correct diagnosis. Fibroadenomas typically present as uniformly hypoechoic masses situated superficially within accessory fibroglandular breast tissue. In contrast, axillary lymph nodes reside deeper within the axilla, are surrounded by fat, and exhibit an echogenic hilum with demonstrable hilar vascularity on Doppler imaging.

Primary breast cancer (invasive ductal carcinoma)

Invasive ductal carcinoma (IDC) is the most common histological subtype of breast cancer, accounting for approximately 80%–85% of all invasive breast malignancies.^[11,12] Compared to invasive lobular carcinoma, IDC more commonly forms a discrete mass and is more readily detected clinically and on imaging.^[13] Although rare, primary breast cancer may arise within axillary accessory breast tissue and present as an isolated axillary mass. Due to its low incidence, diagnosis may be delayed, and patients are more likely to present with nodal involvement.

On ultrasound, IDC commonly appears as an irregular, hypoechoic mass with angular or spiculated margins, nonparallel orientation, and posterior acoustic shadowing [Figure 9a and b]. On mammography, IDC typically presents as an irregular, hyperdense mass and may have associated architectural distortion and calcifications [Figure 9c]. MRI demonstrates an irregular enhancing mass with heterogeneous internal enhancement and washout kinetics.^[11]

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Figure 9: Invasive ductal carcinoma. A 47-year-old female with a palpable high left axillary mass. (a) Grayscale radial US of the axilla demonstrates an irregular, hypoechoic mass with angular margins extending to the skin surface (arrow). (b) Color Doppler radial US demonstrates internal flow (arrow) within the mass. (c) Diagnostic investigational axilla mammographic view with skin marker over palpable abnormality demonstrates an irregular, hyperdense mass (arrow) underlying the triangular skin marker. A US-guided biopsy was performed; the pathology results yielded a moderately differentiated invasive ductal carcinoma. US: Ultrasound

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Management includes surgical excision, radiation therapy, endocrine therapy, and consideration of systemic therapy, depending on tumor biology and stage.^[12,13] Given the high propensity for axillary nodal metastases, careful imaging evaluation of the axilla is essential.

Primary breast cancer (invasive lobular carcinoma)

Invasive lobular carcinoma (ILC) is the second most common histological subtype of breast cancer, accounting for 10%– 15% of all invasive breast malignancies. ILC is characterized by loss of the cell adhesion molecule E-cadherin, resulting in a discohesive growth pattern that infiltrates the stroma without forming a discrete mass.^[12,14] Patients may present with subtle fullness or thickening rather than a palpable mass, and clinical detection can be challenging. ILC may rarely arise in accessory axillary breast tissue and should be considered in the differential diagnosis of axillary lesions.

On ultrasound, ILC may appear as an ill-defined hypoechoic mass with posterior acoustic shadowing or focal shadowing without a discrete mass [Figure 10]. Mammographic presentation is diverse, varying from irregular, spiculated mass to a subtle focal asymmetry or architectural distortion, while microcalcifications are rarely observed. MRI remains the most sensitive diagnostic tool, typically revealing irregular masses with heterogeneous non-mass enhancement.^[12,14]

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Figure 10: Invasive lobular carcinoma. 58-year-old female with left axillary discomfort. Grayscale radial US of the left axilla (a) demonstrates a taller than wide, round, predominantly circumscribed, hypoechoic mass (arrow) within accessory breast tissue. Color Doppler radial US (b) demonstrates internal flow (arrow) within the mass. A US-guided biopsy was performed, with pathology demonstrating invasive lobular carcinoma, moderately differentiated. US: Ultrasound

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Management follows standard breast cancer treatment protocols. Given its subtle imaging features and higher likelihood of multifocal or bilateral disease, careful evaluation of both breasts and axilla is essential. MRI may help detect mammographically/sonographically occult extent of disease in patients with invasive lobular carcinoma, particularly in those who have dense breast tissue.^[15]

Metastatic lymph nodes due to breast primary

Axillary lymph node metastases most commonly arise from an ipsilateral primary breast malignancy. Diagnostic evaluation of the axilla is a key component in staging and prognosis of breast cancer. In some cases, axillary nodal metastasis may be the initial presentation of an otherwise occult breast cancer.^[1,16]

On ultrasound, suspicious features include cortical thickening (>3 mm), loss of the fatty hilum, eccentric cortical bulge, rounded morphology, and non-hilar cortical vascularity [Figure 11]. On mammography, abnormal nodes appear enlarged, round, dense, and may lack a radiolucent hilum. MRI demonstrates enlarged nodes with cortical thickening, loss of the fatty hilum, and abnormal enhancement, often with diffusion restriction.^[16-18]

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Figure 11: Metastatic lymph node due to breast primary. 48-year-old female with known invasive lobular carcinoma and a new axillary palpable lump. (a) Grayscale radial and (b) anti-radial US of the left axilla demonstrates an enlarged lymph node with loss of the central fatty hilum (arrows). Color Doppler anti-radial US (c) shows increased cortical flow (arrow). A US-guided biopsy was performed, with pathology demonstrating metastatic high-grade invasive lobular carcinoma. US: Ultrasound

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Image-guided biopsy is indicated for sonographically suspicious axillary lymph nodes. In cases of biopsy-proven metastatic disease that is suspicious for breast origin and where clinical examination, mammography, and ultrasound fail to identify a primary lesion, breast MRI is recommended to evaluate for an occult malignancy.^[19,20]

Unilateral breast cancer rarely results in bilateral axillary lymphadenopathy.^[21]

Surgical management of the axilla has shifted toward de-escalation in early-stage breast cancer, replacing aggressive intervention with a more targeted approach. Traditionally, axillary lymph node dissection (ALND), involving the resection of all levels I and level II axillary lymph nodes, was always performed when a nodal metastasis was diagnosed. ALND carries a risk of significant life-long morbidity, including lymphedema and neurovascular injury.

Advancements in imaging techniques and pivotal clinical trial data have facilitated a reduction in axillary surgery without compromising oncologic outcomes. Notably, the American College of Surgeons Oncology Group (ACOSOG) Z0011 trial demonstrated that in patients with T1-T2 disease and 1-2 positive sentinel lymph nodes undergoing breast-conserving therapy (BCT) with adjuvant systemic therapy and whole-breast irradiation (WBI), ALND can be safely omitted. At a median follow-up of 6.3 years, no significant difference was found in locoregional recurrence, overall survival, or recurrence-free survival between the ALND and sentinel lymph node biopsy (SLNB)-only cohorts.^[22,23] Consequently, axillary ultrasound has become highly valuable for both identifying patients with clinically occult axillary adenopathy, as well as excluding the presence of lymph nodes with sonographic features of malignant involvement, yielding information that dramatically affects patient management. For patients with biopsy-proven axillary nodal disease, NAC is often employed to downstage breast and axillary disease prior to surgery, with 40-75% of patients achieving complete pathologic response in the axilla and avoiding ALND.^[24] Initially, sentinel lymph node biopsy (SLND) was not recommended for patients who received NAC and demonstrated a response by imaging, due to concern for false-negative results.^[25] Subsequently, the ACOSOG Z1071 trial^[26] evaluated the effectiveness of SLNB after NAC in patients with nodal disease at the time of cancer diagnosis. Initially, the false negative rate (FNR) of SLNB was found to be 12.6%, which did not meet the threshold of 10% that was set at the start of the trial. However, the FNR was reduced to 10.8% when the dual-tracer technique (radiolabeled colloid and blue dye) was utilized with the retrieval of at least 3 sentinel nodes. In addition, a subset of patients who had marker clips placed within biopsy-proven positive lymph nodes at the time of USCNB and US-FNA prior to NAC were also evaluated.^[27] When the node containing the clip was excised along with at least two other sentinel nodes, the FNR was reduced to 6.8%.^[28] Since the clipped lymph node was known to be involved by malignancy before NAC, its pathologic assessment is paramount for determining pathologic response to chemotherapy. Therefore, the radiologist must be able to accurately localize the previously clipped lymph node to ensure its inclusion in the surgical specimen, enabling the surgeon to perform targeted axillary dissection (TAD). This can be accomplished with ultrasound guidance, utilizing a localization wire, a radioactive seed, a magnetic seed, or radar reflector devices, such as SAVI SCOUT (Merit Medical Systems, Inc, South Jordan, UT), or radiofrequency identification devices, such as the Hologic LOCalizer (Hologic, Inc, Marlborough, MA), which is currently used at our institution. Communication with the operating surgeon is essential to establish their preference for a localization device, as some surgeons may prefer one device over another.

Axillary ultrasound has become a critical triage tool for identifying candidates for axillary surgery omission, as several new clinical trials demonstrate that SNLB can be avoided in certain patient populations with early-stage breast cancer and a sonographically negative axilla. The SOUND randomized clinical trial (Sentinel node vs. Observation after Ultrasound) evaluated 1405 women (median age of 60, median tumor size 1.1 cm, 88% had Human Epidermal Growth Factor Receptor 2 (HER2) negative tumors) with no sonographic evidence of nodal involvement. Half of the patients were randomized to undergo SLNB at the time of definitive surgical treatment, while the other half were randomized to no axillary surgery. At five-year follow-up, distant disease-free survival (DDFS) was essentially equal in both groups (97.7% in SLNB vs 98% observation), and locoregional relapse rates were non-inferior (1.7% vs 1.6%, respectively). The recommended adjuvant systemic therapy and radiotherapy were similar in both groups. These findings therefore suggest that surgical axillary staging provides no additional survival benefit in this select, ultrasound-negative population of breast cancer patients.^[29] The Intergroup-Sentinel-Mamma (INSEMA) trial evaluated the impact of omitting SLNB in patients with clinically node-negative early breast cancer. Of the 4858 patients, 962 were randomized to undergo no axillary surgery, while 3896 were randomized to SLNB. No significant differences in the recommended adjuvant systemic therapy or radiotherapy were detected between the two groups. The cohort of patients had a median age of 62, T1 or T2 disease, tumor size of ≤ 5 cm (median size of 1.5 cm), and were predominantly hormone receptor (HR) positive, HER2 negative. The 5-year disease-free survival was nearly identical between the omission group (91.9%) and the SLNB group (91.7%). Patients in the surgery-omission group had a lower incidence of lymphedema, better arm mobility with less pain related to arm and shoulder movement when compared to the patients who underwent SNLB.^[30]

Having analyzed the results of multiple clinical trials, the American Society of Clinical Oncology (ASCO) no longer recommends routine SLNB in post-menopausal patients who are ≥ 50 years of age with negative pre-operative axillary ultrasound findings and have grade 1-2 breast tumors that are ≤ 2 cm in size, and who elect to have breast conserving therapy. Furthermore, ASCO no longer recommends ALND in patients with early-stage breast cancer who have one or two sentinel lymph node metastases and are undergoing breast conserving surgery and whole breast radiation therapy (RT) with or without regional nodal irradiation (RNI). Patients with clinically node-negative invasive breast cancer ≤ 5 cm who receive mastectomy and are found to have one or two positive sentinel nodes may be offered post-mastectomy radiation with RNI instead of undergoing ALND.^[31]

Metastatic lymph nodes due to non-breast primary

While breast carcinoma is the primary cause of unilateral malignant axillary lymphadenopathy, axillary metastatic disease from non-mammary primaries may present with similar imaging findings. Melanoma and lung cancer are the most frequent extramammary sources, followed by gynecologic, gastrointestinal, and head and neck malignancies.^[32] Although most patients have an established oncologic history, axillary lymphadenopathy may occasionally represent the initial clinical presentation of a systemic malignancy.

Imaging features often overlap with those of breast metastases. On ultrasound, findings include enlarged nodes with cortical thickening, loss of fatty hilum, and cortical vascularity [Figure 12]. On mammography, nodes appear rounded, dense, and enlarged.^[16] An ultrasound-guided lymph node biopsy is key to yielding a diagnosis and guiding future imaging to establish the primary source of neoplastic disease.

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Figure 12: Metastatic lymph node due to non-breast primary. 56-year-old female with known ovarian cancer presenting with right axillary fullness. Grayscale radial US of the right axilla (a) demonstrates an irregular, lobulated hypoechoic mass (arrow). Color Doppler radial US (b) demonstrates significant peripheral and internal flow (arrow). No other suspicious findings within the breasts were seen on ultrasound or mammogram. A US-guided biopsy was performed, with pathology demonstrating metastatic high-grade carcinoma involving a lymph node (high-grade serous carcinoma of Müllerian origin). US: Ultrasound

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Lymphoma

Lymphoma is an important consideration in the differential diagnosis of axillary masses, particularly when bilateral axillary adenopathy is present. Patients may present with painless, rubbery lymphadenopathy and systemic symptoms such as fever, night sweats, and weight loss (B symptoms). Lymphoma may also rarely present as a primary breast mass with associated axillary adenopathy.^[17]

On ultrasound, lymphomatous nodes are characterized by marked enlargement, rounded morphology, loss of the fatty hilum [Figure 13a], and increased vascularity on color Doppler imaging [Figure 13b]. Lymphomatous nodes may appear homogeneously hypoechoic, while lymph nodes involved by carcinoma have a more heterogeneous appearance. On mammography, lymphomatous nodes appear dense, noncalcified, round or oval [Figure 13c]. In cases of breast parenchymal involvement, lymphoma manifests as a discrete, noncalcified mass or infiltrates the breast parenchyma diffusely, causing increased density of the breast. On MRI, lymphomatous nodes are typically T1 hypointense and T2 hyperintense, with intense homogeneous or heterogeneous enhancement, and demonstrate rapid enhancement with washout kinetics.^[17]

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Figure 13: Lymphoma. 51-year-old male with bilateral palpable axillary masses. Grayscale US of the left axilla (a) shows multiple enlarged lymph nodes (arrows) with loss of the normal fatty hilum. Color Doppler US (b) shows significantly increased peripheral and internal flow (arrow). Diagnostic mammogram bilateral MLO views (c) show hyperdense masses underlying skin markers (arrows) in the bilateral axilla. A US-guided biopsy yielded diffuse large B-cell lymphoma. US: Ultrasound, MLO: Mediolateral oblique.

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It is important to remember that if lymphoma is suspected, fresh lymph node specimens should be placed immediately into sterile transport media (such as RPMI 1640, DMEM, or sterile saline) and transported to the laboratory without delay on cold packs (2-8°C) to ensure optimal cellular preservation for flow cytometry and specialized pathologic processing.^[33]

Reactive axillary lymphadenopathy

Enlarged lymph nodes in the axilla pose a diagnostic problem, as the etiology could be benign or neoplastic. Clinical history is of utmost importance in the evaluation of axillary lymph nodes (i.e., recent vaccination: up to 6 weeks).^[18,34] Individuals may present with tender and enlarged lymph nodes that regress over time. Imaging can be non-specific with a broad differential for lymphadenopathy.^[13] Infectious and inflammatory etiologies can also result in both uni- and bilateral reactive axillary lymphadenopathy. Ipsilateral upper extremity cellulitis, mastitis, cat scratch disease, toxoplasmosis, and tuberculosis may result in unilateral axillary adenopathy.^[6] Bilateral axillary adenopathy in the absence of suspicious breast findings has a broad differential diagnosis. Autoimmune disease (inflammatory arthritides, lupus, scleroderma), infectious disease (cat scratch disease, mononucleosis, HIV), and granulomatous lymphadenitis due to sarcoidosis and tuberculosis may cause bilateral axillary adenopathy.

In the US, a normal/benign axillary lymph node is oval with a preserved fatty hilum, cortical thickness <3 mm, and hilar vascularity. A reactive lymph node will be enlarged (cortical thickness >3mm) with loss of the normal fatty hilum [Figure 14a], though fatty hilum may be preserved in some cases, and increased cortical vascularity [Figure 14b].^[7] On mammography, a benign normal lymph node has an oval shape with a central radiolucent hilum; however, a reactive node may appear as an enlarged, oval, hyperdense mass [Figure 14c]. On MRI, reactive lymph nodes demonstrate early arterial enhancement and washout, with loss of central T1 hyperintense signal.^[16]

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Figure 14: Reactive lymph node. 48-year-old female with a history of rheumatoid arthritis and a new palpable left axillary mass. Grayscale longitudinal US of the left axilla (a) shows a lymph node with diffuse cortical thickening (arrow). Color Doppler longitudinal US (b) shows significant cortical flow (arrow). Diagnostic mammogram left MLO view (c) shows a hyperdense lymph node in the left axilla (arrow). MLO: Mediolateral oblique, US: Ultrasound

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Unless a specific etiology, such as ipsilateral infection, inflammatory arthritis, or recent vaccination, is identified, reactive adenopathy may necessitate a needle biopsy to exclude a neoplastic process. Alternatively, when a plausible benign cause is present, an ultrasound follow-up can be obtained to document regression to normal size.^[35]

Axillary lymph node calcifications

Calcifications involving axillary lymph nodes may occur due to benign and malignant processes [Figure 15]. Benign causes include granulomatous disease, while malignant causes are metastases due to breast, ovarian, and thyroid cancer. It is important to distinguish lymph node calcifications from mimickers, such as silicone deposits from ruptured silicone breast implants, metallic deposits from tattoo ink, and gold therapy in patients with rheumatoid arthritis and collagen vascular disease.^[3]

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Figure 15: (a) Metastatic lymph node with calcification. 66-year-old female with known peritoneal carcinoma with metastasis to the axilla. MLO view shows a lymph node with peripheral microcalcification (arrow). (b) Corresponding US shows a round lymph node with an internal punctate hyperechoic signal along the periphery (arrow), consistent with calcification. MLO: Mediolateral oblique, US: Ultrasound.

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Epidermoid inclusion cyst

Epidermoid inclusion cysts (EIC) are cutaneous cysts that arise from obstructed hair follicles, due to skin displaced into the breast secondary to trauma or surgery, or due to chronic skin conditions. EICs are lined with stratified squamous epithelium that produces keratin, filling the cyst with flaky debris. EICs may occur in women and men and may arise in the scalp, neck, and back, parasternal region, inframammary fold, near the areola, and the axilla. Individuals with certain genetic syndromes, including Gardner or Gorlin syndrome, are at increased risk for EICs.^[36] EICs may be asymptomatic but can become palpable and bothersome as they grow due to keratin accumulation. EICs may spontaneously rupture and cause chemical cellulitis in the surrounding tissue due to spillage of the noxious contents.^[37]

In the US, epidermoid inclusion cysts appear as superficial, oval, circumscribed, hypoechoic, and heterogeneous masses that may demonstrate whorled internal echoes due to the sloughed keratin. Skin may be seen wrapping around the edges of the mass. Posterior acoustic enhancement may also be present [Figure 16a]. No internal vascularity should be demonstrated on color Doppler imaging [Figure 16b], but surrounding vascularity may be present in the setting of concomitant cellulitis. Assessment of a 2D mammogram may be difficult; however, on tomosynthesis, EIC appears as a hyperdense circumscribed mass, subjacent to the skin [Figure 16c]. MRI appearance varies depending on internal contents; the lesion may be T1 hyperintense or mixed signal due to proteinaceous contents and debris. On T2, it is usually hyperintense. While EIC does not enhance following contrast administration, rim enhancement can be seen in the setting of concomitant cellulitis.^[36]

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Figure 16: Epidermoid inclusion cyst. 47-year-old female with a palpable lump in the left axilla. Grayscale anti-radial US of the left axilla (a) demonstrates an oval, circumscribed heterogeneous hypoechoic mass within the cutaneous layer, displacing axillary subcutaneous tissue inferiorly. Color Doppler interrogation (b) shows no internal vascularity. Diagnostic mammogram, left MLO view (c), shows a hyperdense, circumscribed mass (arrow) in the left axilla. US: Ultrasound, MLO: Mediolateral oblique.

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During periods of active infection, antibiotics are recommended. Surgical excision can be considered if these lesions become repeatedly infected or are otherwise bothersome to the patient. A needle biopsy is generally not recommended, as this can cause leakage of noxious contents into the surrounding tissue, resulting in difficult-to-treat chemical cellulitis.^[36,38]

Although extremely rare, malignant transformation into a squamous cell carcinoma has occurred.^[37]

Hidradenitis suppurativa

Hidradenitis Suppurativa (HS) is a chronic inflammatory condition involving apocrine glands, typically affecting intertriginous regions such as the axilla and groin. Follicular occlusion causes retention of secretions, resulting in painful and erythematous nodules. Men and women with obesity, tobacco use, and family history are at higher risk of HS. Patients present with recurrent, painful nodules, abscesses, and sinus tracts. Over time, extensive scarring and fibrosis may occur.^[39]

US findings include oval to irregularly shaped superficial complex fluid collections with debris and internal echoes, with sinus tracts extending to the skin. Peripheral vascularity is frequently demonstrated on Doppler imaging [Figure 17a and b]. HS may wax and wane, involving both axillae. While the diagnosis of HS is usually made clinically, mammography may demonstrate superficial, hyperdense-to-isodense masses with irregular borders, often with overlying cutaneous thickening [Figure 17c and d] in the axillary region(s). MRI may be useful to assess the extent of the disease.^[6,39]

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Figure 17: Hidradenitis Suppurativa. 38-year-old female with a painful left axillary lump. Grayscale longitudinal US of the left axilla (a) shows a hypoechoic dermal-based phlegmonous collection with an echogenic rim (arrow). Color Doppler longitudinal US (b) shows no flow within the collection. Diagnostic mammogram left ML view (c) and spot MLO view of the left axilla (d) show hyperdense asymmetry in the left axilla with associated overlying skin thickening (arrow). US: Ultrasound, MLO: Mediolateral oblique.

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Management includes local antiseptic wound care, topical antibiotics, anti-inflammatory medications, and lifestyle changes, such as weight loss and smoking cessation. Surgical drainage and excision of fistulous tracts may be needed if conservative management is unsuccessful.^[39]

Lymphatic malformation

A lymphatic malformation is a congenital anomaly composed of dilated lymphatic channels that do not communicate with the lymphatic drainage system. Commonly occurring in the neck, axilla, and groin, these may be associated with Turner syndrome and trisomy 21. Lymphatic malformations may be asymptomatic when small or present as palpable masses.

A lymphatic malformation may be detected incidentally on various modalities; for example, on CT imaging, it appears as multiloculated cystic masses without internal enhancement [Figure 18a]. US demonstrates a cluster of anechoic cysts with intertwining septations [Figure 18b], occasionally with detectable color Doppler flow within the septations. [Figure 18c]. Lymphatic malformations may be mammographically occult.^[4,5] MRI demonstrates a T2 hyperintense, multiloculated lesion with thin T2 dark septations and absence of enhancement.^[6]

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Figure 18: Lymphatic malformation. 30-year-old female with a known congenital left axillary lymphatic malformation, concern for increasing size. Contrast-enhanced coronal CT chest (a) demonstrates a complex, multiloculated cystic mass without enhancement in the left axilla (arrow). Grayscale US of the left axilla (b) shows a cluster of anechoic cysts with intertwining septations (arrow). Color Doppler US (c) shows mild vascularity through the septations (arrows). US: Ultrasound

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If symptomatic, lymphatic malformations can be treated with minimally invasive sclerotherapy or cryotherapy, as well as surgical excision.^[40]

Capillary hemangioma

A capillary hemangioma is a benign vascular tumor typically seen in infancy but may persist into adulthood and is more common in women. It may present clinically as a superficial, tender, soft, palpable, and compressible mass with variable overlying skin changes or discoloration.^[41] This entity can rarely present in the parenchyma of the breast and the axilla^[42], but a capillary hemangioma within an axillary lymph node is extremely rare.^[43,44] Capillary hemangiomas may be difficult to differentiate from a malignant process such as angiosarcoma utilizing imaging features alone; radiologic-pathologic correlation is essential in diagnosis.^[6,41]

In the US, a capillary hemangioma usually presents as a circumscribed, hypoechoic mass [Figure 19a] with avascularity on color Doppler imaging. The presence of internal vascularity raises the possibility of malignancy [Figure 19b]. The mammogram typically demonstrates a nonspecific, round or oval hyperdense mass [Figure 19c]. On MRI, capillary hemangiomas appear as T1 intermediate, T2 high intensity lesions [Figure 19d] with avid, homogenous enhancement [Figure 19e], but a slow filling pattern can also be observed.^[6,7,45,46]

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Figure 19: Capillary hemangioma involving a lymph node. 39-year-old female with a new palpable left axillary mass. Grayscale US of the left axilla (a) shows an axillary lymph node with cortical thickening (arrow). Color Doppler US (b) shows markedly increased vascularity within the area of cortical thickening (arrow). Diagnostic mammogram left MLO view (c) shows a round, hyperdense lymph node with a compressed fatty hilum (arrow). Pre-contrast T2 axial MLO (d) shows a T2 intermediate intensity lesion with a focus of T2 hyperintensity (arrow). Post contrast T1 sagittal MRI (e) shows an avidly enhancing lymph node (arrow), corresponding with ultrasound and mammogram findings. A US-guided biopsy was performed, yielding a capillary hemangioma involving a lymph node. US: Ultrasound, MLO: Mediolateral oblique, MRI: Magnetic resonance imaging.

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Since imaging findings may overlap with those of malignancy, an imaging-guided biopsy may be necessary. If biopsy findings are benign and correlate with radiologic findings, then capillary hemangiomas are amenable to follow-up. Occasionally, histopathologic findings may overlap with those of a low-grade angiosarcoma, and surgical excision may be necessary for a definitive diagnosis.^[45] Surgical excision is also recommended if biopsy findings are discordant or if imaging demonstrates suspicious characteristics (size greater than 2 cm, non-parallel orientation, irregular or angular margins, vascularity on color Doppler imaging).^[45,46]

Nerve sheath tumor

Nerve sheath tumors, including schwannomas and neurofibromas, are benign peripheral nerve sheath tumors (PNSTs) that may present as axillary masses arising from the brachial plexus or its branches. Schwannomas are the most common benign PNST and are typically solitary, well-encapsulated, and arise eccentrically from the nerve of origin. In contrast, neurofibromas grow within the nerve fascicles and may be associated with neurofibromatosis type 1. Patients may present with a slowly growing, palpable mass, and may report pain, paresthesias, or a positive Tinel sign (pain along the distal course of the nerve with percussion of the mass).^[47]

On ultrasound, nerve sheath tumors appear as well-defined, oval or fusiform, hypoechoic masses with posterior acoustic enhancement, which may simulate a cyst [Figure 20].

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Figure 20: Neurofibroma. 35-year-old male with a palpable mass in the right axilla. Grayscale US of the right axilla (a) demonstrates a round, hypoechoic mass with circumscribed margins (arrow). Color Doppler US (b) shows no internal vascularity. A US-guided biopsy was performed, with pathology demonstrating a neurofibroma. US: Ultrasound.

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Cystic degeneration may be present [Figure 21a and b], particularly in schwannomas. On a mammogram, a nerve sheath tumor may appear as a circumscribed, hyperdense mass within the axilla, which is difficult to characterize on mammography alone. MRI is the preferred modality for characterization, demonstrating a well-defined, fusiform mass that is isointense to muscle on T1-weighted images [Figure 21c] and hyperintense on T2-weighted images [Figure 21d].^[47] Image-guided biopsy of these lesions should be performed to confirm the diagnosis.

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Figure 21: Schwannoma. A 56-year-old female with biopsy-proven schwannoma in the left axilla presents with worsening pain. (a) Grayscale US of the left axilla shows an oval, hypoechoic mass with well-circumscribed margins and cystic spaces (arrows). Color Doppler US (b) shows mild peripheral vascularity. T1 sagittal MRI (c) shows an oval mass with intermediate T1 signal isointense to muscle and a peripheral dark rim (arrow). T2 sagittal MRI (d) shows T2 hyperintensity with cystic components (arrow). The mass displaces the axillary neurovascular bundle posteriorly. US: Ultrasound, MRI: Magnetic resonance imaging.

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Conservative management with observation is appropriate for asymptomatic lesions; however, surgical excision is recommended for symptomatic tumors or when there is concern for malignant transformation, particularly in the setting of rapid growth or large size.^[47]

Post-operative complications in the axilla

Axillary lymph node dissection and sentinel lymph node biopsy can lead to axillary seroma formation, which is one of the most common complications of breast cancer surgery, occurring in up to 85% of patients. Risk factors for seroma formation include obesity, older age, and a greater number of lymph nodes removed. While managed with suction drains in the immediate post-operative period, seromas can recur after drain removal. When large, the seromas can lead to discomfort, as well as cause delayed wound healing and infection. Percutaneous aspiration with ultrasound guidance can be utilized to manage persistent axillary seromas.^[48,49] On ultrasound and MRI, axillary seromas appear as loculated fluid collections [Figure 22], with or without fluid-debris levels.^[50]

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Figure 22: Seroma. A 70-year-old female status post left breast lumpectomy and sentinel lymph node biopsy one year ago, presenting with an axillary lump. (a) MLO view demonstrates surgical clips in the left axilla with a hyperdense oval circumscribed mass (arrow) at the surgical site. (b) Corresponding US demonstrates hypoechoic fluid collection with posterior acoustic enhancement, consistent with postoperative seroma. MLO: Mediolateral oblique.

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Axillary hematomas after ALND and SLNB are less common than seromas, occurring in about 2-10% of cases, and resulting from uncauterized vessels. Risk factors include pre-operative use of NSAIDs and fish oil or intense arm activity shortly after surgery. While small hematomas resolve spontaneously within 4-6 weeks, large hematomas may require drainage to prevent infection or significant discomfort. On ultrasound, hematomas show variable internal echogenicity depending on the evolution of blood products [Figure 23].^[50] Acute hematomas may resist percutaneous aspiration due to fibrin clot formation, while chronic hematomas have liquefied and can be evacuated percutaneously.

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Figure 23: Patient underwent a sentinel lymph node biopsy. The US demonstrates irregular complex fluid collection with internal echoes consistent with a hematoma. US: Ultrasound

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Fat necrosis is a sterile inflammatory process in the adipose tissue [Figure 24], resulting from vascular insult to fat cells. The imaging findings vary depending on the stage and course of evolution of fat necrosis. On imaging, fat necrosis can appear as an oil cyst, sometimes surrounded by coarse peripheral calcifications in a late stage, or as a spiculated mass [Figure 25] that is impossible to distinguish from a malignancy without a needle biopsy.^[50]

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Figure 24: Fat necrosis. 63-year-old female with bilateral mastectomy and fat grafting in the left axillary region. (a) Cleopatra’s view of the mammogram demonstrates a fat-containing oval asymmetry at the site of the palpable marker (triangle). (b) Corresponding US demonstrates a hypoechoic region surrounded by hyperechoic fat (arrows), suggesting fat necrosis. US: Ultrasound.

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Figure 25: Fat necrosis companion case, 62-year-old female status post mastectomy with DIEP flap reconstruction, presenting with axillary lump. (a) MLO view shows a region of fat necrosis in the axilla (circle), (b) MRI was performed, which shows T1 hyperintensity (arrow) corresponding to the area of concern on the T1 non-fat suppressed axial image and and (c) the corresponding US demonstrates an oval hypoechoic lesion, (d) thin rim enhancement (arrow) on the post-contrast axial subtracted image. US: Ultrasound, MLO: Mediolateral oblique, MRI: Magnetic resonance imaging, DIEP: Deep inferior epigastric perforator.

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