Introduction

According to the World Health Organization, more than 450,000 cases of breast cancer were diagnosed in 2012, which accounted for more than 28% of all female malignancies⁽¹⁾. The etiopathogenesis of breast cancer remains unclear in most cases. The most important risk factors include gender, age, genetic predisposition (family history of breast cancer, BRCA1 and BRCA2 mutation carriers), long-term exposure to estrogens (both exogenous and endogenous) and ionizing radiation. The impact of obesity, improper diet and excessive alcohol consumption is also emphasized⁽²⁾. Currently, a reduction in breast cancer mortality has been observed, particularly among young women⁽³⁾. This is explained by an increased detection of earlystage neoplastic lesions as well as enhanced treatment efficacy. Mammography screening programs aimed to reduce breast cancer mortality and increase the detectability of subclinical lesions were implemented in numerous European countries⁽⁴⁾. The therapeutic management in pre-invasive ductal carcinoma and early-stage invasive breast cancer (T1NOMO, T2NOMO, T1N1MO, T2N1MO) involves primary surgical treatment, frequently combined with radiation therapy and/or systemic treatment. In the case of patients with locally advanced tumor, surgical treatment and/or radiation therapy is preceded by systemic treatment (chemotherapy and/or hormone therapy; in the case of HER2+ patients – combined with monoclonal antibody [trastuzumab] treatment)⁽⁵⁾.

Two types of surgical treatment are used:?

In both types of surgery, the extent of the axillary procedure depends on sentinel lymph node assessment. Breast cancer patients in whom palpation did not reveal the presence of enlarged lymph nodes are qualified for sentinel lymph node biopsy. If no sentinel lymph node metastases are shown in biopsy, axillary dissection can be safely avoided⁽⁷⁾. It is worth noting that in more than 60% of cases of breast cancers initially intended for operative treatment no lymph node metastases are found in histopathological evaluation^{(8, 9)}. Axillary lymphadenectomy with an accurate assessment of metastatic lymph nodes is performed in patients with clinical suspicion of axillary lymph node metastasis who, during the preoperative diagnostics, underwent ultrasound-guided biopsy confirming the presence of tumor cells.

Developing an appropriate algorithm for the management at the stage of preoperative diagnostics due to, e.g. limited sensitivity and specificity of physical examination in patient qualification for sentinel lymph node biopsy, is also a challenge⁽¹⁰⁾. The use of ultrasound for axillary lymph node assessment before a decision to perform lymphadenectomy or sentinel lymph node biopsy has increased. There has been also interest in the comparison of the diagnostic possibilities of different lymphoscintigraphy protocols as well as an assessment of novel radiopharmaceuticals and image acquisition methods for optimal sentinel lymph node imaging.

Axillary lymph node ultrasound

There are about 20–30 axillary lymph nodes in each armpit, and they are located in one of three anatomical levels. Level I is represented by lymph nodes located below and laterally to the lateral edge of the pectoralis minor muscle. Level II lymph nodes are located behind the pectoralis minor (Rotter nodes are also located at this level, between the pectoralis major and the pectoralis minor). Level III is formed by lymph nodes located upward and medially to the medial edge of the pectoralis minor. The lymph from medial quadrants of the breast as well as the chest wall, the upper abdomen and upper extremities drains into the axillary lymph nodes (Fig. 1). The lymph reaches the lymph node via the afferent lymphatic vessels, it flows through marginal, cortical and medullary sinuses and leaves it via the efferent lymphatic vessels in its hilus ⁽¹¹⁾ (Fig. 2).

Fig. 1

Three anatomical levels of axillary lymph nodes: level I (blue) – lymph nodes located below and laterally from the lateral edge of the pectoralis minor; level II (yellow) – lymph nodes located behind the pectoralis minor; level III (pink) – lymph nodes located upward and medially to the medial edge of the pectoralis minor

Fig. 2

Schematic structure of a lymph node. The lymph reaches the lymph node via the afferent lymphatic vessels, it flows through marginal, cortical and medullary sinuses and leaves it via the efferent lymphatic vessels in its hilus

On ultrasound, normal lymph nodes are bean shaped, oval structures, usually not larger than 1 cm in size. Lymph nodes usually have hypoechogenic cortex and hyperechogenic core, which is more deeply located. The long/short axis ratio should not be less than 2:1. Color and Power Doppler allow to assess lymph node vascularity, with the visualization of normal, typical vessel arrangement⁽¹¹⁾.

The usefulness of numerous ultrasonographic signs, including lymph node size and morphological criteria, was analyzed to identify lymph nodes highly suspected of metastasis. Lymph node enlargement is often seen as a reactive response, which may be caused by e.g. armpit depilation, physical work and the use of antiperspirants. Therefore, this sign, as an isolated symptom, is considered as sensitive, yet poorly specific indicator of the presence of metastasis. Different cut-off values for the size of enlarged lymph nodes have been adopted. Assuming that the diameter of an enlarged node is greater than 5 mm, Bonnema et al. showed 87% sensitivity and 56% specificity in metastatic lymph node detection^{(12, 13)}.

It was observed that breast cancer metastases initially occur in the subcapsular or cortical sinuses, leading to cortical thickening⁽¹⁴⁾. Therefore, metastatic lymph nodes can often show eccentric thickening of the cortex. Fine needle aspiration biopsy (FNAB) should be performed in these thickened sites⁽¹¹⁾. In some patients, the metastatic thickening of the cortex may be very severe and cause obliteration of the lymph node hilum – blurring or absence of the hilum may be seen on ultrasound. Although highly specific for metastatic lesions, it occurs in advanced stages of cancer^{(12, 13)}. A similar mechanism underlies the rounding of the node, i.e. long/short axis disproportion, which is also a sign of metastatic lymph nodes. The presence of tumor cells can also decrease cortical echogenicity and result in blurred outlines of the capsule. In some patients, calcifications may occur in the involved lymph nodes, especially if they were present in the primary malignancy^{(11, 15)} (Fig. 3).

Fig. 3

B-mode ultrasound image of axillary lymph node suspected of breast cancer metastasis. Thickened lymph node cortex as well as its reduced echogenicity and hilar blurring are noticeable

Bedi et al. used the discussed criteria (echogenicity, cortex thickness) for in vitro ultrasonographic evaluation of lymph nodes. They achieved 77% sensitivity and 80% specificity⁽¹⁴⁾. Other authors pointed to high, even up to 95%, specificity of in vivo morphological criteria (echogenicity, homogeneity), however, with poor sensitivity (36%)⁽¹³⁾.

It should be emphasized that the assessment of all visualized lymph nodes is important during axillary ultrasound. If they show slight symmetrical thickening of the cortex accompanied by the clinical signs of inflammation in the breast or upper limb, reactive lymph nodes should be considered first of all in the differential diagnosis. Enlarged lymph nodes presenting the above mentioned malformations without clinical signs of inflammation raise a greater oncological concern. At the same time, it should be noted that normal ultrasound image of a lymph node cannot exclude the presence of metastasis, micrometastases in particular.

A few studies evaluating the use of axillary ultrasound in lymph node selection for fine-needle aspiration biopsy as well as the usefulness of ultrasound combined with fine-needle aspiration biopsy in patient qualification for sentinel lymph node biopsy, have been published in recent years. Saffar et al. found that the following aspects should be taken into account when identifying lymph nodes for fine-needle aspiration biopsy: cortical thickness of more than 3 mm, the presence of compressed, dislocated hilus, rounded lymph node and reduced cortical echogenicity as well as blurred outlines of nodal capsule⁽¹⁶⁾. Usmani et al. analyzed the use of ultrasound combined with targeted fine-needle aspiration biopsy for qualifying patients with clinically palpable axillary lymph nodes for sentinel lymph node biopsy or axillary lymphadenectomy. Their observations show that the combined use of ultrasound and FNAB allows to extend the indications for sentinel lymph node biopsy by classifying some of the patients with palpable lymph nodes. It may potentially allow to avoid unnecessary procedures of complete axillary lymphadenectomy in some patients⁽¹⁷⁾. Alvarez et al. performed a systematic review of studies on the role of ultrasonography and ultrasound-guided biopsy for axillary lymph node diagnosis in breast cancer patients. The presented data show that the sensitivity of ultrasound-guided FNAB ranged between 30.6% and 62.9%, while the specificity reached 100%. For comparison, the same study assessed sensitivity and specificity of axillary lymph node ultrasound using the lymph node size criteria (69.2% and 75.2%, respectively) and morphological criteria (71% and 86.2%, respectively)⁽¹²⁾.

Furthermore, a number of authors assessed Doppler ultrasound, elastography and sonographic contrast media in terms of their usefulness in the differential evaluation of axillary lymph nodes. Doppler methods are a valuable complementation of morphological criteria during selection of lymph nodes for FNAB. Color and power Doppler allow to identify randomly arranged vessels in the peripheral part of the node. As shown by Yang et al., increased peripheral flow was significantly more common in palpable, cancerous axillary lymph nodes compared to non-metastatic patients. Importantly, the authors did not observe a similar relationship in the group of patients who had impalpable lymph nodes during physical examination⁽¹⁸⁾. Choi et al. assessed the diagnostic usefulness of elastography in the differentiation between reactive and metastatic axillary lymph nodes. It was shown that elastography can increase B-mode sensitivity in the diagnosis of this type of lesions⁽¹⁹⁾. Different results were obtained by Park et al., who also assessed the usefulness of elastography (as an isolated criterion as well as combined with grey-scale ultrasound) in the search for metastatic lymph nodes. The authors did not confirm the usefulness of sonoelastography in the differential analysis of lymph nodes⁽²⁰⁾. Dellaportas et al. assessed ultrasound using contrast media as well as color Doppler in the preoperative evaluation of 50 patients. Sensitivity was estimated at 83.33%, and specificity at 84.38%; the negative predictive value was 90%, and the positive predictive value – 75%⁽²¹⁾. It is worth noting, however, that this method is not recommended by the EFSUMB (European Federation of Societies for Ultrasound in Medicine and Biology).

The use of the protocol combining ultrasound with nuclear medicine seems an interesting proposition for the preoperative assessment of breast cancer patients. Ahna et al. compared, among other things, the sensitivity and specificity of fluorodeoxyglucose positron emission tomography (FDG-PET) (42.5% and 90.4%, respectively) and ultrasound (79.5% and 78.5%, respectively) as well as a protocol combining these two methods in the detection of metastases present in the axillary lymph nodes in a group of 250 breast cancer patients. The protocol involving the use of both techniques showed higher sensitivity (83.6%) and specificity (94.9%) in the detection of metastatic lymph nodes⁽²²⁾.

Lymphoscintigraphy

Sentinel lymph node is the first regional lymph node that receives lymph fluid from the primary tumor and, potentially, the first site to which cancer cells are likely to spread via the lymphatic vessels. Elective breast cancer surgery is preceded by lymphoscintigraphy, which allows both, to specify the flow direction of lymph from the tumor, and to perform an intraoperative identification of sentinel lymph node. In the scintigraphic image, sentinel lymph node is visualized as a focus of increased tracer accumulation in the route of lymph flow. There is usually one sentinel lymph node. However, an increased tracer uptake in several lymph nodes is not rare. Tracer accumulation in the parasternal lymph nodes may also occur and its detection is important for planning the extent of adjuvant radiotherapy. It should be remembered that lymphoscintigraphy findings do not provide information on the presence of metastases in the lymph nodes, but it only locates sentinel lymph node(s). During a surgery, the surgeon uses a scintillation detector to locate the lymph node with higher radioactivity compared to background radiation. This allows to sample material for histopathological testing and assessment for the presence of cancer cells (Fig. 4).

Fig. 4

Microscopic image of axillary lymph node cells with the presence of cancer cells – breast cancer metastases. The material for histopathological evaluation was collected during sentinel lymph node biopsy

Lymphoscintigraphy uses several types of radiopharmaceuticals, usually colloid compounds differing in particle size. Sodium pertechnetate (Na^99mTcO₄) is usually used for labeling. The size of tracer particles determines its velocity of distribution in the lymphatic system as well as the duration of node labeling. Their optimal size is 100-200 nm⁽²³⁾. Usually a sentinel lymph node is visualized within 2 hours, and its sampling should be performed within 16-20 hours after radiopharmaceutical administration⁽²⁴⁾. Both, planar images, usually in two projections: A-P and lateral^{(23, 24)} (Fig. 5), as well as methods of three-dimensional image acquisition, i.e. single-photon emission computed tomography (SPECT) and single-photon emission computed tomography/computed tomography (SPECT/CT) may be used (Fig. 6). Bluemel et al. assessed the usefulness of a novel tool for intraoperative three-dimensional image projection, i.e. freehand SPECT. The authors achieved more than 92% detectability of the sentinel lymph nodes, which had an influence on altered surgical management in about 10% of patients. For comparison – detectability using conventional techniques utilizing typical scintillation probe was 69.1% in this study⁽²⁵⁾.

Fig. 5

Lymphoscintigraphy in a patient with bilateral breast cancer – planar images: A-P, P-A and lateral projections. Foci of increased tracer accumulation were visualized backward from the site of radiopharmaceutical administration, indicating the presence of axillary sentinel lymph nodes

Fig. 6

Lymphoscintigraphy in a patient with left breast cancer performed the day before planned surgical treatment. The SPECT/CT revealed a focus of increased tracer accumulation in the projection of axillary lymph node

The manner of tracer administration is still disputable. It may involve a subcutaneous injection of radiopharmaceutical above the tumor. In some centers, the tracer is administered into the skin at the edge of the areola, at the intersection of the line that connects the nipple and breast tumor. In the case of deep-seated tumors, injection of radionuclide into the vicinity of the tumor is recommended. In the case of impalpable lesions, the injection should be ultrasoundguided⁽²⁵⁾. Injections into the scars after resected tumors are also possible – in this case lymph node labeling may not be effective due to the postoperative lymphatic vessel injury.

Sentinel node and occult lesion localization (SNOLL) was one of the innovative nuclear medicine methods described in 1998 by researchers from the European Institute of Oncology in Milan. The method is used in patients with impalpable breast cancer (about 30% of cases) for an intraoperative location of primary tumor and sentinel lymph node. It involves administration of two types of tracers: human albumin macroaggregates (MAA) with a particle size of 10–150 µm and albumin-nanocolloid (NC) with a particle size of 10–80 nm. Macroaggregates are administered into the tumor under ultrasound guidance, whereas the nanocolloid is injected intradermally over the tumor. The pharmacokinetics of both radiotracers depends on particle size. Macroaggregates remain at the site of intratumoral injection, whereas nanocolloid spreads through the lymphatic vessels to sentinel lymph node, where it accumulates. Both, primary tumor and sentinel lymph node are intraoperatively localized using a gamma camera. It allows for a precise location of the primary tumor in order to perform resection with a margin of healthy tissue as well as to perform sentinel lymph node biopsy⁽²⁶⁾.

There are some published studies on the use of innovative radiotracers to visualize sentinel lymph nodes in patients with breast cancer. Baker et al. assessed the usefulness of a novel radiopharmaceutical – ^99mTc-tilmanocept. This tracer has very small particles (7.1 nm), which bind to mannose receptors (CD206) present on the reticuloendothelial cells in the lymph nodes. The study compared procedures for sentinel lymph node labeling using sulfide colloid and ^99mTc-tilmanocept; both techniques were combined with the staining method. The use of this novel radiopharmaceutical allowed for a removal of significantly limited number of lymph nodes compared to sulfide colloid⁽²⁷⁾.

Despite the use of increasingly effective radiopharmaceuticals and diagnostic tools, there are still some limitations in the radioisotope labeling of sentinel lymph nodes. In individual cases, the presence of tumor cells in lymphatic vessels which drain lymph from the tumor may lead to their complete obstruction and failure in the labeling of sentinel lymph nodes (false negative results) or result in the labeling of lymph nodes other than sentinel lymph nodes (false positive results). Furthermore, cases of patients with previous breast surgeries as well as patients with large focal lesions, in whom lower sensitivity of sentinel lymph node labeling is observed, cause difficulties.

The place of axillary lymph node ultrasound and sentinel lymph node biopsy (SLNB) in the light of the current guidelines

In 2014, the Journal of Clinical Oncology released updated American Society of Clinical Oncology guidelines on sentinel lymph node biopsy in patients with early-stage breast cancer. The recommendations were developed based on 9 randomized clinical trials and 13 cohort studies⁽²⁸⁾.

They aimed at, among other things, answering questions whether axillary lymphadenectomy can be avoided in patients without metastases in the biopsy of sentinel lymph node and whether axillary lymphadenectomy is necessary in all cases of metastatic findings on sentinel lymph node biopsy. It was concluded that axillary lymphadenectomy should not be performed in patients without metastases in sentinel lymph nodes. Furthermore, axillary lymphadenectomy is not recommended in most patients with metastases in one or two sentinel lymph nodes, undergoing breast-conserving surgery with subsequent adjuvant conventionally fractionated radiotherapy of the whole breast ⁽²⁹⁾. Axillary lymph node dissection may be offered for women with the presence of metastases in the sentinel lymph node, who will receive mastectomy. The authors of the guidelines concluded, based on the findings from the cohort studies, that SLNB can be proposed to patients with operable breast cancer who have the following circumstances: multifocal tumor if the cancer stage allows for mastectomy; patients with ductal carcinoma in situ (DCIS) who are scheduled for mastectomy; as well as in females with a history of breast/axillary surgeries or who received preoperative/neoadjuvant systemic treatment. SLNB should not be performed in patients with large or locally advanced invasive breast cancer (T3/T4), inflammatory breast cancer, DCIS (in cases of planned breast-conserving surgery) or in pregnant women.

In 2015, guidelines for the management of breast cancer of the European Society for Medical Oncology were published in the Annals of Oncology⁽⁴⁾. The authors of the recommendations emphasize that regional lymph nodes status remains one of the most important prognostic factors in this patient population. Initial evaluation of lymph nodes should be assessed by clinical examination and ultrasound, supplemented by ultrasound-guided fine needle aspiration or core biopsy of suspicious lymph nodes (IIIA). Furthermore, the high efficacy of lymphoscintigraphy in sentinel lymph node imaging, particularly when combined with intraoperative staining, was emphasized in the recommendations. SLNB was considered to be standard management in patients with early-stage breast cancer and clinically unenlarged axillary lymph nodes, excluding patients with preoperative ultrasound-guided lymph node biopsy findings indicating the presence of metastases (IIA). At the same time, it was pointed out that SLNB is associated with lower risk of complications, such as upper limb lymphoedema, and reduces the duration of hospitalization (IA) compared to axillary lymphadenectomy. The authors of these guidelines also referred to SENTINA and ACOSOG Z1071 trials showing that SLNB has a lower detection rate and a higher false-negative rate in patients after systemic therapy compared to those before neoadjuvant chemotherapy^{(30, 31)}. At the same time, SLNB should be considered after systemic therapy, if no axillary lymph node metastases are visualized in ultrasound and/or PET/CT prior to systemic treatment initiation (VB).

Summary

Precise classification of breast cancer patients for axillary surgery is of great therapeutic and prognostic importance. Both, ultrasound and nuclear medicine techniques play an important role in decision-making regarding the extent of surgery. This article presents the latest reports and recommendations of different scientific societies regarding axillary lymph node diagnostics as well as suggests different methods of management depending on the diagnostic findings. It is worth noting that the departure from lymphadenectomy in patients with no nodal metastases (N0), with the presence of metastases in 1–2 sentinel lymph nodes, with a history of breast-conserving surgery and adjuvant radiotherapy using tangential fields of the whole breast and armpit, is increasingly accepted in the medical society. It should be noted, however, that the decisions on the extent of axillary surgery are usually made in cancer centers, taking into account the opinion of the panel of experts in the fields of surgical oncology, radiotherapy and clinical oncology.