Intramammary lymph nodes: normal and abnormal multimodality imaging features

Abstract

Intramammary lymph nodes (IMLN) are one of the most common benign findings at screening mammography. However, abnormal IMLN features, such as diminished or absent hilum, thickened cortex, not circumscribed margins, increased size or interval change, warrants additional follow-up or pathologic analysis to exclude malignancy. Some benign inflammatory conditions may be associated with imaging-detected suspected abnormal IMLN, such as reactive hyperplasia and silicone-induced lymphadenopathy. In patients with known breast cancer, IMLN are a potential site of locoregional spread, which can change the prognosis and management. In some cases, initial breast carcinomas can also mimic IMLN. Breast radiologists must also be aware of the typical and atypical characteristics of IMLN to suggest further investigation when it is necessary.

Introduction

The expansion of breast cancer screening exams led to the increase of breast lesions detection. Intramammary lymph nodes (IMLN) are one of the most common benign findings at screening mammography.^1,2 They are defined as lymph nodes that should be surrounded by breast tissue in all sides, which differentiate them from those in the lower axillary region. The prevalence of IMLN ranges between 0.7 and 48% on current studies, depending on the method used for identification, radiological or pathologic examination.¹

IMLN with typical features identified in imaging exams are considered a benign finding. However, breast radiologists must also be aware of the atypical characteristics of IMLN to suggest further investigation when it is necessary. The aim of this review is to present normal and abnormal features of IMLN in the most common imaging modalities (mammography, ultrasound and MRI).

Normal and abnormal imaging features

Normal IMLN are typically described in all imaging exams as a circumscribed mass, smaller than 10 mm, with oval or reniform shape and hilar fat, usually at a peripheral location, adjacent to a vein (Figure 1). The most common location (about 70%) is the upper outer quadrants, however, it may be located anywhere in the breast.³ They are stable over time in comparison to previous exams.³ Besides morphological features, a normal IMLN may show hilar vascularization at Doppler ultrasound. At MRI, the IMLN cortex shows high signal intensity at T₂ weighted images, homogeneous enhancement after contrast administration and may show suspicious findings on kinetic curve assessment, such as washout type in the delayed phase.²

Figure 1. .

Normal IMLN imaging features. At mammography (A), typical IMLNs (arrow) present as a circumscribed oval mass, with hilar fat, usually at a peripheral location, adjacent to a vein. Ultrasound (B) shows a circumscribed oval mass, with hyperechoic hilum and hypoechoic cortex. MRI demonstrates a circumscribed reniform mass with hilar fat signal in T₁ weighted images (C). The IMLN cortex shows homogeneous enhancement (D) and high signal intensity at T₂ weighted images (E). IMLN, intramammary lymph nodes.

Breast masses lacking these features cannot be reported as normal IMLN. Abnormal IMLN features, such as diminished or absent hilum, thickened cortex, not circumscribed margins, increased size or interval change, warrants additional follow-up or pathologic analysis to exclude malignancy. Table 1 describes the main characteristics of normal and abnormal IMLN.

Table 1. .

Normal and abnormal IMLN imaging features

IMLN FEATURES	NORMAL	ABNORMAL
SHAPE	Ovoid or reniform	Round or lobulated
MARGINS	Circumscribed	Not circumscribed
SIZE	Less than 1 cm	More than 1 cm
CALCIFICATIONS	Absent	Present
HILAR FAT	Present, may be pronounced	Absent or eccentric
PARTICULARITIES	Usually adjacent to a vein	Significant alteration in size or morphology at follow-up

IMLN, intramammary lymph nodes.

Abnormal benign IMLN

Some benign inflammatory conditions may be associated with imaging-detected suspected abnormal IMLN. One cause of false-positive IMLN is reactive hyperplasia, which is just the normal lymph node response to any antigenic stimulus, such as localized or systemic inflammation. For example, patients submitted to breast cancer treatment, including mastectomy or breast conservative surgery and radiation therapy, may present atypical reactive IMLN with enlargement and cortical thickening (Figure 2).⁴ However, sometimes it is difficult to exclude the possibility of recurrence based solely on imaging findings.⁵ Sometimes, reactive IMLN may show spontaneous regression on follow-up studies (Figure 3), which confirm its benign aetiology.

Figure 2. .

Patient with previous right breast cancer submitted to nipple-sparing mastectomy presented an atypical IMLN in the upper outer quadrant of the same breast at ultrasound (A) and MRI (B), with round shape, eccentric hilum and thickened heterogeneous cortex. Percutaneous ultrasound-guided fine needle aspiration was negative for malignancy and surgical resection confirmed a reactive inflammatory IMLN. IMLN, intramammary lymph nodes.

Figure 3. .

Patient with a typical IMLN in the upper outer quadrant of the right breast at screening mammography (A). After 1 year, the new screening exam (B) showed an increase in the IMLN and a biopsy was suggested. However, after 2 months, when the patient returned to perform the biopsy, the IMLN showed spontaneous regression to normal size (C), suggesting a reactive inflammatory IMLN and the procedure was suspended. IMLN, intramammary lymph nodes.

Another cause of benign atypical IMLN is silicone-induced lymphadenopathy related to implant ruptures and silicone leakage, which can also mimic malignancy, especially at mammography (Figure 4). At ultrasound, these lymph nodes present a characteristic snowstorm appearance, which is even much more sensitive for silicone lymphadenopathy than the silicone signal at MRI.⁶

Figure 4. .

Patient submitted to left mastectomy and breast reconstruction with silicone implant that complicated with extracapsular rupture. Mammography (A) showed a circumscribed, dense, oval mass in the upper outer quadrant of the right breast, at typical IMLN location, that showed a “snowstorm” appearance at ultrasound (B), suggesting an enlarged IMLN with intranodal silicone (silicone adenopathy). IMLN, intramammary lymph nodes.

Other rare benign causes of abnormal IMLN reported in the literature include Castleman’s disease, HIV-associated lymphadenopathy, tuberculosis and toxoplasma lymphadenitis.

Metastatic IMLN

IMLN are a potential site of locoregional spread for ipsilateral breast carcinoma. The incidence of metastases in these nodes ranges between 1 and 34% and they are found in the same quadrant as the primary tumour in almost 50% of cases.¹

The evaluation of IMLN should be meticulous in patients with breast cancer. “Low suspicion” imaging features, such as eccentric/diffuse cortical thickness equal to or greater than 3 mm, should be used for threshold to biopsy in these patients. In contrast, due to the low prevalence of malignancy in imaging-detected suspected abnormal IMLN in females without concurrent breast cancer, only “high suspicion” features, such as eccentric/diffuse cortical thickness equal to or greater than 5 mm or loss of fatty hilum, should be used for threshold to biopsy in this population.⁷

Metastatic IMLN may mimic a synchronous benign mass in a patient with breast cancer; however, its location and proximity of an artery or vein should alert the radiologist to the possibility of a metastatic IMLN (Figures 5 and 6).⁸

Figure 5. .

Mammography (CC view) showing a no special type invasive breast carcinoma in the inner quadrants of the right breast (yellow arrow) associated to an atypical IMLN in the outer quadrants (red arrow), which was confirmed as metastatic. CC, craniocaudal, IMLN, intramammary lymph nodes.

Figure 6. .

Pre-operative MRI in a patient with a no special type invasive carcinoma in the right breast (arrow in A) showed an atypical IMLN with round shape and no fat hilum in the lower quadrants of the same breast, located near the chest wall (circle in B). Second-look ultrasound identified the IMLN (circle in C). After pre-operative localization, the suspected IMLN was resected and confirmed as metastatic (D). IMLN, intramammary lymph nodes.

There is a lack of good quality data on the management of IMLN metastasis and their clinical significance remains uncertain. Metastatic IMLN are usually associated to axillary lymph nodes metastasis. However, many papers have shown that metastasis to IMLN is an independent factor of poor prognosis for breast cancer patients, regardless of the axillary status.^1,9 According to the eighth edition of the American Joint Committee on Cancer staging system, IMLN are designated as axillary lymph nodes for staging purposes.¹⁰ Thus, presence of IMLN metastasis changes the pathologic and clinical staging even in the absence of positive axillary lymph nodes.

On the other hand, there is a prognostic advantage in the presence of an isolated metastatic IMLN when compared to a solitary metastatic axillary lymph node.¹¹ Based on these findings, some authors raised questions about how to treat patients who have a positive IMLN and negative axilla, whether it would be more appropriate to manage the involved IMLN as a separate focus of cancer within the breast parenchyma or as a metastatic lymph node, as suggested by the American Joint Committee on Cancer.¹²

Because IMLN metastasis alone does not represent multicentric disease, breast conservation therapy can be contemplated in such cases. However, an axillary sentinel lymph node biopsy should always be performed to accurately assess the disease status of the axilla. IMLN metastases in the presence of a negative axillary sentinel lymph node biopsy should not imply the radiation or surgical treatment of the axillary lymph nodes.¹³ IMLN are detected as the sentinel node in 0.2–14% of breast cancer patients and in these cases, the decision to perform axillary dissection should be determined by the status of the intramammary sentinel node itself.

In the presence of a histologically confirmed metastatic IMLN without other known suspected lesions at mammography or ultrasound, breast MRI should be performed. Rarely, an abnormal IMLN can represent metastasis from an occult breast carcinoma or may result from lymphoproliferative disease or from metastasis from extramammary cancers (i.e. melanoma and ovarian cancer).^14–16

Invasive breast carcinoma mimicking IMLN

In rare cases, small breast carcinomas can mimic IMLN at mammography (Figures 7 and 8).¹⁷ Comparison to prior exams can help to identify these cases and complementary ultrasound or short-term follow-up may be useful. Any new or enlarging masses should be carefully assessed for minimally suspicious signs, such as slight blurring of margins, to avoid missing early cancers.

Figure 7. .

Patient with a typical IMLN (green arrow) in the upper outer quadrant of the left breast at screening mammography (A). After 1 year, the new screening exam (B) showed stability of the typical IMLN (green arrow), however, there was also a new small circumscribed mass simulating another IMLN in the same location (red arrow). At 6 month follow-up (C) this mass increased and showed irregular shape, being confirmed as a no special type invasive carcinoma. IMLN, intramammary lymph nodes.

Figure 8. .

Patient with previous left breast cancer submitted to conservative surgery presented a small circumscribed mass on the left upper outer quadrant at follow-up mammography, mimicking an IMLN (A). Prior exams were not available. At 6 month follow-up, there was an increase in the mass (B), which corresponded to an irregular hypoechoic mass at ultrasound (C) and was confirmed as a recurrent invasive carcinoma after biopsy. IMLN, intramammary lymph nodes.

Conclusion

Normal IMLN is a common finding at screening mammography. Both benign and malignant conditions can cause abnormal IMLN imaging features. It is important for the breast radiologist to be aware of these conditions in order to avoid delay in the diagnosis and provide proper treatment of malignancies. In patients with breast cancer, further studies are necessary to standardize the management of patients with IMLN metastasis, however, its prognostic value have been already demonstrated.