Sonographic localisation and description of inguinofemoral lymph nodes in patients with vulvar squamous cell carcinoma

Sonographic imaging of the inguinofemoral lymph nodes can be performed in patients with vulvar cancer to discriminate between normal (benign and reactive) and malignant appearing lymph nodes. A systematic sonographic protocol for imaging inguinofemoral lymph nodes is not commonly used, but could aid in homogenising the reporting of lymph node location and status. This paper unpacks the relative anatomy of the inguinofemoral lymph nodes. A protocol outlining the sonographic assessment of inguinofemoral lymph nodes and the criteria to use to guide the reporting of their size, shape, echogenicity and vascular architecture is discussed.


| INTRODUCTION
Vulvar cancer (also known as vulval cancer) is rare and accounts for 3%-5% of gynaecological cancers globally. 1,2 It occurs in any part of the vulva, and most commonly in the labia minora, labia majora and perineum and is more common in postmenopausal women. 3 Patients will often present with a painful vulvar mass or ulcer which may be associated with pruritis, bleeding and discharge. 4 Squamous cell carcinoma (SCC) accounts for 80%-95% of malignant vulvar cancer cases. 5,6 There is a limited role for diagnostic imaging in the primary detection of vulvar cancer but sonographic imaging, if requested, may demonstrate a soft tissue mass with internal vascularity. 7 Vulvar cancer spreads by local invasion directly into local organs (vagina, urethra and anus), lymphatic metastasis to regional lymph nodes 2 and less commonly via haematogenous spread to distal sites (liver, lungs, and bones). 8 Upon diagnosis, the groin is clinically assessed to determine if there is any inguinofemoral lymph node involvement. 4 The risk of nodal metastasis of vulvar cancer increases with the stage of disease, size of the lesion and depth of the invasion. 2 Vulvar cancer is staged using the American Joint Committee on Cancer TNM staging system (T = tumour, N = spread to lymph nodes, M = spread to distant sites) and the International Federation of Gynaecology and Obstetrics (FIGO) staging system. 6 Inguinofemoral lymph nodes are the predominant site of initial (sentinel) lymph node (LN) spread. 6,7 LN metastasis into the superficial inguinofemoral LNs has been shown to represent the most significant prognostic factor for recurrence and survival for vulvar cancer patients. 8,9 Although ultrasound is not included in current clinical staging algorithms of vulvar cancer by FIGO, it is being increasingly requested at the pre-operative stages of histologically proven vulvar cancer to evaluate inguinofemoral LNs to identify any malignant appearing LNs. 10 Sonographic findings can be used to guide and tailor surgical planning to allow for minimally invasive surgery techniques of inguinofemoral lymphadenectomy. 4,10 Radical groin LN dissection can be performed, however, has high rates of post-operative morbidity due to postoperative complications such as wound infection, wound breakdown and lymphoedema. 4,11 Sonography can be additionally requested in the post-operative stages of vulvar cancer management to monitor inguinofemoral LN status over time.
The sonographic criteria to differentiate normal appearing LNs from abnormal (those suggestive of malignant spread) must be well appreciated. 'Normal' LNs in this context will include non-malignant/ non-neoplastic nodes, which encapsulate both benign and reactive LNs. Reactive LNs in the inguinofemoral region swell and react due to infection in areas of the lower body from bacteria or viruses. 12 This can include sexually transmitted infections such as chlamydia, syphilis and chancroid, but also infections of the lower limbs. 13 The invasion of LNs by metastatic tumour cells usually changes the sonographic architecture of a LN. 10 Other causes of metastatic spread to inguinofemoral LNs (in addition to vulvar cancer) include but are not limited to lymphoma, malignant melanoma, SCC of the skin of lower trunk and extremities or the anal canal and penis. 7 Sonography of inguinofemoral LNs can be challenging due to operator dependence and variations in, or lack of sonographic protocols to image and assess these LNs. There is often a lack of appreciation of where to extend scanning to and from, how to image and measure LNs, and terminology to use to describe the location and ultrasound appearance of LNs. 10,14 Hence a sonographic protocol to image, assess and report the status of inguinofemoral LNs is group's work. 10 The sonographic descriptions of LN architecture can be applied to vulvar and non-vulvar cancer patients and applied to peripheral LN assessment in other sites.

| ANATOMY OF INGUINOFEMORAL LYMPH NODES WITHIN THE FEMORAL TRIANGLE
A deep understanding of the anatomy of the inguinofemoral and pelvic LNs and their surrounds is required to adequately perform a sonographic assessment. The inguinal ligament and groin crease are often used as sonographic landmarks. The inguinal ligament runs between the anterior superior iliac spine (ASIS) and the pubic tubercle and is used to divide the pelvis from the groin. 15 The groin crease runs generally parallel to the inguinal ligament, but about two centimetres inferior. The LNs of the pelvic region can be divided into zones relative to the local positioning of vessels: the external iliac, internal iliac, common iliac and paraaortic zones 16 ( Figure 1A).
Inguinofemoral LNs are located within the region outlined by the femoral triangle, also known as the Scarpa's triangle. 16 The boundaries of the femoral triangle include the inguinal ligament superiorly, the medial border of the sartorius muscle laterally and the medial border of the adductor longus muscle medially. 17 The roof of the femoral triangle is formed by the fascia lata. 17 The floor of the femoral triangle is formed by the iliopsoas and pectineus muscles. 18 The neurovascular structures that run through the region of the femoral triangle from lateral to medial include the femoral nerve, artery, vein, and lymphatic channel, allowing for the acronym NAVL 15 ( Figure 1B).
Although the femoral triangle defines structures deep to the fascia lata, the same boundaries can be used as a guide for the extent of a sonographic assessment of inguinofemoral LNs. 16,19 There is a paucity of studies reporting the anatomic distribution of inguinofemoral nodes. 17 There is however consensus that the inguinofemoral LNs, are categorised into superficial and deep groups, which are separated by the fascia lata. 19,20 The deep inguinofemoral LNs (one to five in number) are also be called the deep femoral or deep sub-inguinal nodes and are located deep to the fascia lata, within the femoral sheath, medial to the common femoral and femoral veins. 19 They receive efferent flow from the deep lymphatics of the lower extremity, from the superficial inguinofemoral LNs, and lymphatic drainage from the glans penis or clitoris. 15 The most proximal deep inguinofemoral LN sits just inferior to the inguinal ligament and is termed 'Cloquet's or Rosenmuller's node' 15,18 ( Figure 1C).
The superficial inguinofemoral LNs range in number from 12 to 17. 7,12,21 They are divided into the inguinal and femoral (sub-inguinal) groups 17 ( Figure 1D). Superficial inguinal LNs run more transversely across the groin, parallel and inferior to the course of the inguinal ligament. 22 Superficial femoral LNs are distributed more vertically along the proximal part of the great saphenous vein (GSV) and its tributaries of the thigh. 15,18 The most common GSV tributaries encountered, variably include the superficial external pudendal, superficial epigastric and superficial circumflex iliac veins. 10,22,23 Superficial femoral LNs predominantly drain the superficial lymphatic vessels of the lower leg. 21 Collectively, superficial inguinofemoral LNs can be subdivided into five anatomical zones outlined by Daseler. 24 These zones can be used to sonographically describe the location of suspicious appearing LNs. Four quadrants are defined by drawing a line along the path of the common femoral and femoral veins. 24 A line is then drawn perpendicular to this, over the saphenofemoral junction (usually at the groin crease or 1-1.5 cm inferior to this). 16 The quadrants are termed as zone 1: Supero-medial (SM), zone 2: supero-lateral (SL), zone 3: infero-lateral (IL) and 4: infero-medial (IM) zone. 16 A fifth zone sits superficial to the sapheno-femoral junction and is termed the central zone (C) 10 ( Figure 2). The superficial and deep inguinofemoral LNs both drain into the external iliac LNs of the pelvis. 15,19 The expected areas of lymphatic drainage from the perineum and genitals are firstly the superficial inguinofemoral LNs in the superomedial quadrant/zone and subsequently the external iliac LNs. 16 Hence, the superomedial zone (zone 1) has the highest risk of involvement of nodal metastases from vulval cancer. 17  The location of LNs should be described according to their relationship relative to the saphenofemoral junction, and whether LNs are superficial or deep in location. Sonographic imaging of LNs should be appropriately magnified to allow adequate qualitative assessment (B-mode and colour and/or power Doppler) and measurements. 10

| SONOGRAPHIC APPEARANCES OF LYMPH NODE ARCHITECTURE
The components of a LN which can be identified sonographically consist of the capsule, cortex, paracortex, medulla, hilum, and hilar vessels. 10 The deeper portion of the cortex is called the paracortex. The cortex and paracortex surrounds a centrally placed homogenous medulla (often referred to as the hilum) which is hyperechoic relative to the cortex. 7 The LN hilum is the depression on the side of a LNs where hilar vessels enter and exit the LN. 10 The vessels enter the hilum and then the medulla and radiate out peripherally at the paracortex and symmetrically through the cortex (but may not be seen with colour Doppler in all LNs) 4 ( Figure 3).
Sonographic features used to distinguish a 'normal' from a 'malignant' appearing LN include the size (or morphometry), shape, (or morphology, including capsular integrity and distinction from adjacent LNs), echogenicity (cortex, medulla, corticomedullary differentiation, perinodal tissues and nodal-core sign) and the vascular architecture and are outlined below. Variations from normal sonographic appearances of any LN which make it suspicious of being infiltrated with tumour must be documented. 10

| Sonographic assessment of the size (morphometry) of lymph nodes
The LN size can be sonographically quantitatively assessed. The length and depth (antero-posterior (AP) or short axis dimension) of a LN is measured from an image demonstrating the longest LN axis to gain a long-to-short-axis (L/S) ratio. 7,10 Benign LNs generally have an L/S ration >2 (length two times greater than depth). 10 A L/S ratio <2 and usually closer to 1 is suggestive of metastatic LNs 7 ( Figure 4A).
Normal LN cortical thickness should be homogenous throughout. 10 Cortical thickening occurs when the cortex is thicker than the medulla, 25 and this can be focal or diffuse. 19 Focal cortical thickening involves less than 50% of the LN cortex and diffuse involves more than 50%. 10 Quantitative measures of cortical thickness can be used to differentiate between benign and malignant appearing LNs. The AP superficial (C1) and deep (C2) cortical (C) and medullary (M) thickness can be measured perpendicular to the long axis of a LN where the LN is thickest to determine the cortical-to-medulla-thickness (C/M) ratio. 7,10,25 Cortical thickening is defined when (C1 + C2)/M (superficial cortex (C1) + deep cortex (C2) thickness is divided by medulla (M) thickness) is greater than 1 10 ( Figure 4B).
LN cortical thickness can also be measured at its separate poles 7 ( Figure 4C). The thickest cortical diameter (C max) is divided by the thinnest cortical diameter (C min) to provide a maximum-to-minimalcortical-thickness (C max/C min) ratio which should be less than 2 at both poles of the node. 4,7 If the C max/C min ratio ≥2, non-uniform cortical thickening can be reported. 4 4.2 | Sonographic assessment of lymph node shape and outline (morphology) The morphology of a LN includes the qualitative assessment of its shape, capsular integrity, and distinction from other surrounding nodes.

| Lymph node shape
The shape of a LN must be defined as either oval, round, or irregular. 26 Normal LNs should appear oval. 27 Round or irregular shaped LNs can indicate malignant involvement. 26 The term 'irregular' is used when the outer margin of the node is either lobulated or spiculated. 10 Spiculated is defined as the contour having stellate distortions or spikes on its surface, and a blurred margin. 26 Spiculated LNs may demonstrate disruption of the capsule 27 ( Figure 5).

| Lymph node capsule integrity
The capsule of the normal LN should sonographically appear smooth and well-defined. 4 If the capsule separating the LN from the perinodal tissue is blurred or interrupted either focally or diffusely, this can indicate extracapsular tumour spread 28 ( Figure 6).
However, it must be noted, malignant LNs may maintain smooth borders. 10

| Lymph node grouping
An individual LN should appear sonographically separate to adjacent LNs. 7 Nodal grouping occurs when LNs with metastases group together either partially or completely (matting) with other nodes. 10 Partial grouping is defined when two LNs are in contact with each other but can be still recognised as individual nodes. Complete grouping or matting occurs when it is not possible to sonographically distinguish one LN from another (Figure 7). F I G U R E 5 Shapes of lymph nodes. Lymph nodes are defined as oval (A) which is a normal shape, or round (B), or irregular (C). The round or irregular shaped lymph nodes are indicative of malignancy. Irregular nodes can be subdivided into either irregular lobulated or irregulate spiculated shaped nodes.

| Sonographic assessment of lymph node and perinodal echogenicity
Sonographic assessment of the echogenicity of the LN cortex, medulla, corticomedullary differentiation, and presence of a core nodal sign should be conducted. 10 In addition, the surrounding tissue (perinodal) echogenicity should also be assessed. 4

| Echogenicity of the cortex and medulla, corticomedullary interface and perinodal tissues
The sonographic echotexture and echogenicity of the cortex and medulla of a LN should appear homogenous. 10 The cortex should appear hypoechoic or isoechoic with surrounding tissues 25 and the medulla hyperechoic relative to the cortex. 7 The interface between the LN cortex and medulla should be well defined. 4 Non-homogeneity of the cortical or medullary echogenicity, such as focal or diffuse change, can indicate LN abnormalities. 4,7 Focal cortical echogenicity changes may be seen as hyperechoic deposits or cystic areas. 10 Diffuse cortical changes may be described as cystic or microcystic changes throughout described as reticulation 7 (Figure 8).
Reticulation is identified by thin hyperechoic lines throughout the cortex or uneven tiny multiple echogenic areas throughout described as 'sand pattern' change. 10

| Sonographic presence of the nodal-core sign
The nodal core of a LN is formed by the medulla and the hilum. 10 A complete nodal-core sign denotes when both an echogenic hilum and the medulla are seen. 10 A partial nodal-core sign denotes when an echogenic medulla is seen, but the hilum (where vessels enter and exit LN) is not sonographically appreciated (hilum not echogenic). 4 Both complete and partial nodal core signs can be seen in normal LNs. 4,7 The partial nodal-core sign may be seen when the LN is reactive, and cortical thickening compresses the hilum which obscures its visualisation sonographically. 10 Absence of a nodal-core sign (neither echogenic medulla nor hilum is present) is suggestive of a metastatic LN 10,25,27 (Figure 9).  identified to branch radially and perpendicular to central flow. 10 Hilar vascular flow when the nodal-core sign is absent may be seen in reactive LNs. 4,10 Transcapsular flow is identified by vessels penetrating the cortex from outside the nodal capsule from multiple directions along the circumference of the node (not via the hilum). 10

| REACTIVE LYMPH NODE APPEARANCE
Reactive inguinofemoral LNs can be sonographically encountered.
They are generally oval shaped but may demonstrate concentric uniform cortical thickening. They usually maintain their nodal core sign (partial or complete) and hilar vascular flow on colour Doppler.
In the post-reactive state, the cortex of LNs can become echogenic and thinner and medulla wider with age. 10

| COMPONENTS OF SONOGRAPHIC WORKSHEET
A sonographic worksheet should consist of a diagram to draw LNs for documentation of the location of inguinofemoral LNs relative to the SFJ. In addition, a suggested table/checklist for reporting inguinofemoral nodal sonographic findings is provided which may be adapted for clinical practice ( Figure 11).

| CONCLUSION
The sonographic assessment of inguinofemoral LNs can be a challenge. A systematic sonographic protocol for the imaging of inguinofemoral LNs is not commonly utilised but is recommended. A consistent format of sonographically reporting LN location, size, shape, echogenicity, and vascular architecture can aid in homogenising the reporting of LN status (whether likely to be metastatic or not). This can also allow specific LNs to be more easily followed up sonographically and/or localised for FNAB. Further studies are required to investigate the reliability and accuracy of the sonographic imaging and reporting of inguinofemoral LNs.