Since lymphatic tumour spread is common in pelvic malignancy, it is important to identify the anatomical location of all the pelvic nodal groups and to be aware of the usual pathway of lymphatic drainage for each organ involved.
There are perivisceral nodes in the meso-rectal fat, parametrium, and paravesical fat. While meso-rectal lymph node metastases are often identified in rectal cancer, the other perivisceral nodes are only rarely seen.
Nodal metastases are most frequently identified in the pelvic sidewall chains, which are the external iliac, internal iliac and common iliac nodal groups, each named after the artery it accompanies.
• External iliac lymph nodes are composed of medial, anterior and lateral chains. Intermediate nodes lie just posterior to the external iliac vein and medial to the obturator internus. These are known as the ' surgical obturator' group, since they are adjacent to the obturator vessels. Some authorities consider this group to be part of the internal iliac nodes. The medial chain lies between the external iliac artery and vein. The lateral chain is drain to the middle and lateral common iliac chains. lateral to the external iliac artery. The external iliac lymph nodes
• Internal iliac lymph nodes accompany the internal iliac vessels and drain to the common iliac chain. This group also includes two outlying nodal sites, the sacral nodes and the 'anatomical obturator' nodes. The former accompany the median and lateral sacral vessels and may drain directly into the lumbar lymphatics. The latter lie in the obturator foramen but are identified in less than 10% of the population.
• Common iliac lymph nodes are composed of medial, middle and lateral nodes. The medial chain lies between both common iliac arteries and includes nodes anterior to the sacral promontory. The lateral chain lies lateral to the common iliac artery. The middle (posterior) chain lies posterior to the common iliac vessels, between the psoas muscle and the spine. Enlargement of this chain produces the filled-in fat sign at the pelvic brim. The common iliac lymph nodes drain to the left and right lateral aortic chains, which are part of the lumbar (upper retroperitoneal) nodal chain.
The other local nodal group of relevance in pelvic cancer is the inguinal chain composed of superficial and deep subgroups. The superficial inguinal nodes lie immediately below the inguinal ligament and drain to the external iliac chain. The deep inguinal nodes lie medial to the femoral vein and also drain to the external iliac chain.
The usual pathways of lymphatic drainage for each of the pelvic organs are listed in Table 13.1. However certain tumour types may skip nodal stations, giving rise to non-contiguous nodal spread. For example, in cervical carcinoma the pelvic nodes may appear normal on imaging but retroperitoneal and supra-clavicular lymph node enlargement may be identified. Also, surgery can radically alter lymphatic drainage, which is relevant in the assessment of nodal metastases in recurrent disease.
Pelvic metastatic lymph node involvement alters tumour staging, treatment options and prognosis. For example, the 5-year survival of women with FIGO stage IB node-positive cervical cancer has been reported as 45-55%, compared to 85-95% for stage IB node-negative tumours. In bladder cancer, 5-year survival for T1N1 tumours is 15% but is 85% for T1N0 tumours.
Accuracy of imaging in the detection of lymph node metastases
Ultrasound, CT and MRI rely on morphological features.mainly lymph node size, to suggest the presence of lymph node metastases. Ultrasound is of little value in lymph node staging of pelvic malignancy as it has a relatively poor sensitivity for the detection of retroperitoneal and pelvic nodes. Assessing the relative accuracy of CT and MRI is difficult despite the numerous published studies because they vary in the criteria adopted for malignant infiltration and in the scan technique employed. Overall, the range of accuracies quoted for detection of lymph node metastases with MRI and CT are 85-93%
Table 13.1. Normal sites of lymphatic drainage for the pelvic organs Organ Lymph drainage
Ovary • Para-aortic via lymphatic chain accompanying the ovarian vessels, on the left to the renal vein and on the right to the IVC at L1
• External and common iliac, obturator via the broad ligament
• Superficial inguinal via round ligament
Uterus • Fundus—Para-aortic via lymph chain accompanying the ovarian vessels
• Body—External and internal iliac ^ common iliac
• Region of Fallopian tube—Superficial inguinal via round ligament
Cervix • Parametrial, obturator and pre-sacral (via the utero-sacral ligament) initially
• External iliac, internal iliac ^ common iliac
Vagina • Upper A—Internal and external iliac accompanying uterine artery
• Middle A—Internal iliac accompanying vaginal artery
• Lower A—Superficial inguinal Prostate • Obturator most frequently
• Internal and external iliac ^ common iliac
Bladder • Anterior/lateral paravesical and pre-sacral initially, followed by
• Obturator and external iliac ^ common iliac Rectal • Pararectal/meso-rectal initially
• Upper — Pararectal at origin of inferior mesenteric artery via the lymph chain accompanying the superior rectal artery
• Lower —Internal iliac via lymph chain accompanying the middle rectal artery Anus • Above dentate line—Perirectal, internal iliac
• Below dentate line—Superficial inguinal and 65-80%, respectively. The sensitivities of MRI and CT range from 50-73% and 44-86%, respectively with specificities of 83-98% and 78-97%, respectively. CT and MRI are therefore equivalent in the detection of lymph node metastases.
There has been some work on the use of an ultra-small super-paramagnetic iron oxide (SPIO) contrast agent in lymph node assessment. This is taken up by the reticulo-endothelial system and acts to shorten T1, T2 and T2* resulting in decreased signal intensity on T2-or T2*-weighted images. In metastatic lymph nodes, the normal reticuloendothelial system macrophages are replaced by tumour cells, which do not take up SPIO and show no signal drop post SPIO injection. Using this agent there have been some encouraging results in differentiating benign from malignant lymph nodes.
18Fluorodeoxyglucose positron emission tomography (FDG PET) is a functional imaging modality relying on increased glucose metabolism in cancer cells. In the detection of lymph node metastases, FDG PET sensitivities range from 24 to 91 % and specificities from 77 to 100%. While many studies demonstrate increased staging accuracy of FDG PET compared to cross-sectional imaging, its availability is limited and CT and MRI remain the primary imaging modalities in lymph node assessment.
The body coil is used to obtain images of the upper retroperitoneal nodal stations and a pelvic phased array coil should be used to assess the pelvic lymph node stations if possible. Intravenous hyoscine butylbromide (Buscopan) may be given to reduce artifacts caused by bowel peristalsis, and improve image quality. T1 -weighted sequences enable identification of lymph nodes against the high signal intensity of the pelvic fat. T2-weighted sequences allow the signal intensity characteristics of the nodes to be compared with those of the primary tumour. A fat suppressed sequence may make pelvic lymph nodes more conspicuous. Intravenous MRI contrast agents and specific lymphatic contrast agents such as SPIOs are not routinely indicated.
Normal pelvic lymph nodes may appear homogenous or have a central fatty hilum. They are best detected on T1-weighted images, where they appear of homogenous low/intermediate signal contrasting well with the surrounding high signal fat, or have a high signal hilum consistent with intra-nodal fat, surrounded by an intermediate signal rim giving a characteristic target appearance. On T2-weighted images lymph nodes may be less conspicuous due to reduced contrast between their intermediate/high signal and the high signal of the surrounding fat. On a fat suppressed short tau inversion recovery (STIR) sequence lymph nodes tend to be well seen as high signal structures. The STIR sequence is particularly helpful for differentiating between hilar fat and central nodal necrosis.
When differentiating benign from metastatic nodes on CT and MRI, lymph node size is the only imaging criterion widely accepted to be useful. However, there are other helpful imaging features including shape, site, clustering and asymmetry, contour and signal intensity, which should also be considered when deciding if a node is involved by metastatic disease.
When assessing lymph node size it is important to measure the maximum short axis diameter (MSAD), since that remains relatively constant irrespective of nodal orientation in the plane of the scan. Moreover diseased nodes are known to expand by becoming rounder before they become longer. Currently there is no universally agreed normal pelvic lymph node size in the imaging literature and the range varies between 6.0 mm and 15.0 mm with the most frequently used upper limit of normal short axis diameter being 10.0 mm. This is because studies have been performed on a mix of patient groups, such as normal controls or patients with early stage cancers who were eligible for surgical correlation. In these groups, normal lymph nodes often have a small short axis diameter (5.0 mm or less). However, in later stage larger tumours, which are often necrotic and infected,the regional lymph nodes are more likely to undergo reactive hyperplasia and there may be a need for a higher size threshold.
Nodal site influences normal size limits. United Kingdom guidelines quote normal short axis diameter lymph node size as 9.0 mm in the common iliac chain, 7.0 mm in the internal iliac chain, 10.0 mm in the external iliac chain, 8.0 mm in the obturator chain and 15.0 mm in the inguinal region.
Normal lymph nodes are kidney bean shaped or oval. Round nodes should be regarded with suspicion. Site
The normal lymph drainage pathway of the pelvic organs should be considered when assessing a lymph node for metastatic involvement. If a node is detected in a recognised drainage site, although borderline on size criteria, it should be considered with caution. For example, the presence of a prominent obturator node in a patient with bladder, prostate or cervical cancer should be considered suspicious for disease involvement.
Clustering, asymmetry and contour
Asymmetry of normal lymph nodes can be seen in up to 10% of patients and therefore this feature cannot be solely relied upon. However, clustering of numerous small nodes is a suspicious finding. In the absence of local acute inflammation, a node with irregular margins suggests extra-capsular spread of tumour. In such cases over 75% of nodes are already enlarged and will have been considered abnormal on size criteria.
If the nodal tissue is of similar signal to the primary tumour on T2-weighted images then one should be suspicious of metastatic involvement. Likewise, central nodal necrosis is a good predictor of metastases in patients with squamous cell carcinoma or teratoma. This appears as central high signal on T2-weighted images and low signal on T1-weighted images but is best appreciated on contrast-enhanced T1 -weighted images, when the nonenhancing necrotic centre of the node contrasts well against the enhancing periphery.
• Lymph node hyperplasia can be confused with metastatic lymph node enlargement. In many cases, no clear differentiation can be made but if the node is enlarged and does not have a signal intensity similar to the primary tumour, then reactive nodal hyperplasia should be considered.
• Normal anatomical structures may be mistaken for lymph nodes on MR imaging. These include the ovaries, bowel, tortuous vessels, ureters and prominent iliopsoas bursae. A helpful landmark in identification of the ovary is the round ligament. This extends from the groin into the pelvis, passing medial to the external iliac vessels, and links to the ipsilateral adnexa. The ovarian follicles may be clearly visible on T2-weighted scans in premenopausal women. Bowel, tortuous blood vessels, vessels with slow flow and the ureters may be differentiated from lymph nodes by scrolling through the images to confirm they are tubular structures. In addition, fast flowing blood produces a flow void particularly in arteries. A ureter may have high signal urine within the lumen on T2-weighted images. An enlarged iliopsoas bursa is low signal on T1-weighted images and high signal on T2-weighted images, smoothly demarcated and characteristically positioned posterolateral to the iliofemoral vessels.
• Post-surgical complications such as haematomas or lymphocoeles may mimic enlarged lymph nodes. Haematomas may demonstrate the concentric ring sign on T1-weighted images. Lymphocoeles form in less than 5% of patients postoperatively. They are collections of lymph fluid, which typically lie adjacent to the pelvic sidewall and are invested by parietal peritoneum. They return low signal on T1-weighted images and high signal on T2-weighted images.
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