Lymph nodes and lymphatic circulation
In addition to the bloodstream, in the human organism there is another important network of vessels, which branches out to reach every part of the body: we are talking about the so-called lymphatic system.
The liquid that flows inside the lymphatic system is called lymph and has a composition very similar to that of plasma (water, electrolytes and proteins), plus a certain percentage of white blood cells, cellular waste products, foreign particles and fats food absorbed in the intestine.
The lymphatic circulation originates at the level of thin capillaries, with a blind bottom, which are distributed in the various tissues to collect the interstitial fluids. The convergent system of lymphatic capillaries gives rise to vessels of a gradually greater caliber, which transport the lymph to the level of the large veins at the base of the neck, where it enters the bloodstream.
Along the complex system of lymphatic vessels there are filters, called lymph nodes or lymph nodes . These filters are located at the confluence points of several lymphatic vessels and filter the lymph by retaining and eliminating foreign particles (such as viruses, bacteria and aged or abnormal cells, including tumors).
To best perform its function, the individual lymph nodes are particularly rich in macrophages (capable of incorporating and digesting pathogens and altered cells) and other white blood cells (lymphocytes).
We can therefore compare the lymph nodes to a sort of customs, capable of intercepting and neutralizing the "anomalous loads" to prevent them from spreading to the rest of the organism.
Lymph nodes are organized in aggregate packets, or in chains arranged along the path of the main veins like pearls strung in a necklace.
Sentinel lymph node
The sentinel node is the first lymph node to receive the lymphatic drainage of the primitive tumor. Depending on the case, the sentinel lymph node may still be free or already be infiltrated with neoplastic cells.
Malignant tumors are so defined because - unlike benign ones - they have cells capable of invading and destroying the surrounding tissues; moreover, the same cells can spread through the blood or the lymphatic capillaries and take root in other organs, giving rise to the so-called metastases (secondary tumors in sites far from the original one).
Some tumors preferentially metastasize through the lymphatic system, and in all these cases the evaluation of the sentinel node takes on great importance.
If the diffusion occurs through the lymphatic circulation, the tumor cells enter the local lymphatic capillaries, which in turn transport them together with the lymph into the sentinel lymph node. At the level of this filter, the local white blood cells try to fight malignant cells to counteract their diffusion in the other sites of the organism; if this defensive line is overcome, the malignant cells spread to the next lymph node and so on, spreading in the body and drastically reducing the possibilities of treatment.
According to the sentinel lymph node theory, if there are no tumor cells inside the sentinel lymph node, the involvement of other nearby lymph nodes can also be excluded.
Clinical meaning
When and why it is important to evaluate the sentinel node
The evaluation of the sentinel node is of great clinical importance in the so-called tumor staging process, which in turn allows us to understand how large and widespread the tumor is. In this way, the medical staff can plan an appropriate treatment and establish a reliable prognosis. For example, in breast cancer the evaluation of the sentinel node allows - in case of negative results - to save unnecessary and harmful operations (axillary dissections) (because they would lead to problems of swelling of the ipsilateral arm); see the dedicated article for further information.
For tumor staging, the so-called TNM system is used, which is based on the evaluation of three elements:
- T: extension of the primitive tumor;
- N: absence or presence of metastases to regional lymph nodes;
- M: absence or presence of distant metastases.
The addition of numbers to these 3 components indicates the extent of the tumor, which is the more serious the greater the number; specifically we have:
- T0 (very small tumor), T1, T2, T3, T4 (very large tumor)
- N0 (no lymph node involved), N1, N2, N3 (many regional lymph nodes involved)
- M0 (no distant metastases), M1 (presence of metastases)
As for the evaluation of sentinel lymph nodes, we have:
- N0 (sn): sentinel node free from metastasis
- N1 (sn): metastasis in the sentinel lymph node
- NX (sn): sentinel node not assessable
The sentinel node must first be identified by the doctor; for this purpose, a marker substance is injected into the area surrounding the tumor, therefore, through imaging techniques, the path of this tracer is observed up to the first lymph node it encounters. Subsequently a biopsy and an anatomo-pathological evaluation of the lymph node is performed to ascertain the presence of malignant cells.
In case of involvement of the sentinel lymph node, the doctor may opt for his surgical removal (lymphadenectomy).
Currently (May 2015) the use of the sentinel node as a diagnostic technique has been widely validated in breast cancer and melanoma . Several studies are underway to understand if it can also be applied to other cancers, such as colon, prostate, testicular, penile and renal cell carcinoma.
Enlarged lymph nodes: when to worry
The enlargement of a lymph node is generally caused by the increase in the number of immune cells within it. In most cases, this phenomenon is due to wholly benign causes (eg a sore throat, other local infectious or inflammatory diseases); only in rare cases can it be the indicator of a neoplastic disease. It argues in favor of the possible presence of a tumor that the swollen lymph nodes:
- remain so for many days;
- they tend to increase in volume with time;
- localize on one side of the body only;
- place themselves above the clavicle;
- are associated with fever;
- are associated with significant weight loss;
- are associated with other signs or symptoms that indicate the possible presence of a tumor.
