Sunday, January 01, 2006

Bone marrow aspirates and trephine biopsies

Everyone has seen a marrow bone. The inside of a bone is colored red or yellow, depending on how much fat there is. On the butcher's slab it looks like a jellied honeycomb, but in life its temperature is 98.6 and it is a liquid surrounding a network of bony spicules. It is the site of blood production, and under the microscope you can see all the precursor cells of the blood. There are also the cells of the supporting structures, but the commonest cell is a fat cell. The long bones in the legs and arms mainly have fat cells in their bone marrow. A small proportion of the marrow cells are stem cells; they can mature into any type of blood cell, but some have the possibility of maturing into cells of other tissue. Just how complete this plasticity is, is still a matter of conjecture.

To examine the bone marrow there are two separate procedures, though they are often combined in a single operation. Marrow aspiration involves sucking out the liquid marrow through a small hole in the bone; marrow trephine means taking a core of bone through a hollow needle. It is usual to take a specimen from the iliac bone. This is the large flat bone that forms part of the pelvis. Either the front or back of that bone is a suitable site. I learned to take marrow aspirates from the sternum or breast bone, which in my view is very much easier, but being in front of the patient's nose, is perhaps more frightening.

The marrow aspirate needle follows a design by Salah or Klima (the difference is how the guard that stops you going in too far) is fastened. It is a hollow needle with a solid insert that is removed after it has been inserted into the bone. A syringe is fastened to the needle and suction is applied. This is a painful procedure. I am told that there is a special sort of pain when the suction is applied. However, it is a very rapid procedure that can be over in about 30 seconds after the needle is inserted. Anesthetic is essential. The skin and periosteum (the membrane that surrounds the bone) have nerve endings that must be dulled. Before the procedure begins lignocaine (lidocaine) is infiltrated at the site. Now comes the mistake that is most common. you have to wait until the local anesthetic works. Many operators are too hasty.

The marrow trephine needle is based on a design by Jamshidi. I learned to do trephines with a Sacker-Nordin needle. This was a horrific implement, about a half in diameter; it comprised a saw and a spike. We seldom did trephines in those days. The Jamshidi is another hollow needle with a solid trochar, but about 3 times the size of an aspirate needle. The trick is to have a tapered end so that after a core of marrow is cut out, it is retained within the needle while the needle is withdrawn. A trephine is generally taken from the back of the iliac bone at the top. It is more painful and takes longer than an aspirate. Good analgesia is essential. Some people prefer to be put out with a shot of midazolam; others prefer a pre-med with pethidine (meperidine). Again it is very important to wait long enough for the local anesthetic to work. Some people have tough bones and it takes considerable force to get the needle through the outer table of the bone. It is not an easy technique and beginners should always render their patients unconscious before they have perfected the method. Patients should always opt for an experienced operative if they intend to stay awake for the operation. Ideally it should be possible to obtain a core of bone at least an inch long.

The purposes of the two procedures are different. The trephine obtains a sample for histology. The core of marrow can be sliced and stained and examined down the microscope, but first it has to be decalcified, otherwise it would damage the knife. The decalcification process alters the structure of the marrow and it may mean that certain immunostains don't work well. The histology allows an accurate assessment of the anatomy of the bone marrow and a good assessment of how much of the marrow is replaced by tumor. The aspirate obtains a suspension of cells, which may be spread on a slide rather like a blood smear or used for other examinations like flow cytometry or cytogenetics or bacterial culture. You get a much better picture of the nature of the cells and their immunoreactivity, but not their relationship to each other. It is also subject to sampling error, so the assessment of percentage of a particular cell is not so accurate.

What is the need for a bone marrow in CLL? In most cases a bone marrow is not needed to make the diagnosis, which is made perfectly well from the blood smear. In cases of SLL, diagnosed by lymph node histology, a bone marrow trephine may be done to see if the bone marrow is involved (it almost always is). A bone marrow trephine used to be used to determine the prognosis, but this is now out of date. Four histological patterns were described: interstitial, nodular, nodular and interstitial, and diffuse. Only the diffuse pattern carried a poor prognosis. However, this has now been superseded by the modern prognostic factors.In clinical trials it is important to document the state of play before and after the treatment. Bone marrow trephines are recommended before and after treatment. It is impossible to attain a complete remission according to the NCI criteria if a bone marrow is not done. Trials that have declared CRs without a bone marrow trephine consistently get a 'better' success rate than those that insist on one. Outside clinical trials there is some debate as to whether a trephine should be done. If it is the intention of treatment to get the best possible remission then a trephine should be done so as to know when to stop treatment. However, it may be that in the future estimation of MRD will replace the trephine.

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