Reviewing and understanding your bone marrow report can be confusing and difficult.
This is because there is variability in the testing done for the evaluation and there is variability in how laboratories and pathologists report the information. Finally, the reports typically use medical words that patients are unfamiliar with. It is common for patients to call me after having reviewed their bone marrow report and ask for my interpretation in wording they are able to understand.
The following provides the very basics of bone marrow evaluation for patients with CLL and SLL. In untreated patients with CLL, the bone marrow virtually always has leukemia cells present, including at initial diagnosis.
The bone marrow is an organ that resides inside bones, and is where the cells that circulate in the blood are formed. The cells that make blood cells are referred to as stem cells. There are many different types of cells in the marrow, including fat cells. On the inside, bones are like a sponge, with hollow spaces where these cells reside. Blood vessels circulate blood with nutrients and oxygen in and out of the marrow. Normally, when stem cells mature they go through changes in their size, shape, and surface proteins, analogous to an individual maturing from an infant, toddler, child, teenager, etc., and ultimately they die of old-age. It is only the “adult” forms of cells that normally circulate in blood.
The boney structure that makes up the marrow space does not change, but the proportions and numbers of the different types of cells in the marrow do change, especially in patients with leukemia. The leukemia cells in CLL reside in the marrow space and this is where they multiply, which can result in “crowding out” of the normal stem cells and cause other blood cell counts, like red cells and platelets, to drop, since they are unable to grow. CLL cells also reside in lymph nodes, liver, and spleen, which is why these organs enlarge with active and progressive CLL. Unlike these organs, which can enlarge, the bone marrow cannot.
Bone marrow evaluation usually consists of collecting 2 different samples, the marrow aspirate and the bone marrow biopsy.
The marrow aspirate
The marrow aspirate is obtained by placing a needle in the bone space and removing the “liquid” portion of the marrow. This sample is usually taken first, and the liquid aspirate is smeared on microscope slides and stained so that the cells can be seen under a microscope. The aspirate is where the cell “differential” is obtained. The differential provides the proportion of each type of cell in the marrow and typically the technician reviews 500 total cells to obtain the percentage of each. Normally, there are many different types of cells counted in the differential, including lymphocytes, many different forms of immature neutrophils and red blood cells and megakaryocytes, the cells that produce platelets; and there are expected normal ranges in which these cells should be present.
There are many different types of cells that can be described in the differential that reflect the different maturation stages of the different types of normal cells that grow in the marrow. Cells such as metamyelocytes, promyelocytes, normoblasts, and pronormoblasts are reported for example. In the marrow of a normal individual, the proportion and types of cells present is very typical, and different diseases, including leukemia are reflected in too many of a particular type of cell, too few of a particular cell or cells, or abnormalities in the shape, size or structure of the cells present. The details of this are far beyond this review.
CLL or SLL cells are counted as lymphocytes because they look like normal lymphocytes, which should be in the marrow. In CLL, there is typically an abnormally high percentage of lymphocytes (>30%), and the cells must be studied by flow cytometry, a method to evaluate the different proteins on the cell surface, to determine if the cells are actually CLL cells. In a patient with CLL who needs to begin treatment, it is common to have more than 90% lymphocytes on the differential. In order to be considered in complete remission after treatment, there must be less than 30% lymphocytes on the marrow differential, even though the differential of a normal individual should not have more than 17% lymphocytes.
Flow cytometry is done on the marrow aspirate, and is the method used to characterize the proteins on the surface of the cells, as well as to evaluate the size and shape of cells. Cells normally have certain proteins on their surface and this is one way we identify the type of cells present, especially for lymphocytes. Most of these proteins are designated with “CD” followed by a number, like “CD19”, which is a protein found on B cells. There are hundreds of different proteins that can be identified. CLL cells have the following proteins on their surface, which is how we can tell that they are present: CD5, CD19, and CD23. CLL cells will also all have either kappa or lambda light chain (another protein but without a “CD” designation) on their surface, which is also a marker used in making the diagnosis. If CD38 is present on the CLL cells, this has been associated with more rapidly growing CLL. Flow cytometry is the method we use to determine what types of lymphocytes are present in the marrow aspirate and if there are any CLL cells present. Bone marrow evaluation is not required to make a diagnosis of CLL. This can be done by performing flow cytometry on blood, since the CLL cells are virtually always present in blood. Flow cytometry is very sensitive and can detect down to 1 CLL cell in 10,000 normal cells. Very low level of detectable CLL is referred to as minimal residual disease (MRD), which is assessed upon completing treatment to determine the quality and depth of remission.
Fluorescence in situ hybridization (FISH) is a test done on the cells of the aspirate, or on blood to identify specific chromosome abnormalities that can be found in CLL, like deletion 17p, deletion 11q, trisomy 12 and deletion 13q. More aggressive CLL has been associated with presence of deletion 17p and 11q. The bone marrow report may have information about FISH included. These chromosome abnormalities may go away when a patient goes into complete remission.
Immunoglobulin heavy chain variable gene (IGHV) mutation analysis as well as sequencing to detect mutations on other genes like TP53 can be done on the marrow aspirate or on blood. These results will also appear in the bone marrow report.
The bone marrow biopsy
The bone marrow biopsy is actually done by removing a small piece of the bone with a large needle. This piece of bone includes the hard outer surface, as well as the inner marrow (the “sponge” with cells and fat in it). The biopsy is first decalcified in the laboratory so that it can be thinly sliced, stained and visualized under the microscope. The biopsy allows us to see the architecture of the bone, the outer dense bone, and the inner “spongy” marrow, as well as all the cells in the spongy space of the marrow.
The cells in the spongy space are normally surrounded by fat, and when the bone marrow report indicates the “cellularity” this refers to the amount of the space that has cells in it; the remainder is fat. In normal individuals, as we age, we lose cells from the marrow and the space is replaced with fat. The “rule of thumb” to give the expected normal cellularity (“normocellular”) for an individual is 100-age. For example, a normal individual who is 70 years old should have a cellularity of roughly 30% (70% fat). When CLL is present in the marrow, the cellularity is typically increased, so the example of a 70-year old with CLL may have a marrow cellularity up to 80 or 90% (“hypercellular”), with the majority of the cells being CLL cells. A hypocellular marrow (too few cells) can be seen in patients with CLL who have been over-treated with chemotherapy. In order for patients to be considered in remission after treatment, not only must they have <30% lymphocytes, their marrow must also be normocellular for their age.
In CLL and SLL, there are typically too many lymphocytes present in the biopsy. The bone marrow report will also provide information about where the lymphocytes are distributed (pattern) in the space, which correlates with how aggressive the disease might be.
- A “nodular” pattern indicates that the CLL cells are seen in clusters, with normal cells and fat between them. This pattern is associated with slow-growing CLL.
- An “interstitial” pattern indicates that the CLL cells are mixed in with the normal cells and fat and not clustered. There are too many of them but they are evenly integrated. This pattern is associated with an intermediate growth rate.
- A “diffuse” pattern is where most of the cells seen under the microscope are CLL cells, that replace the normal cells and fat. The cellularity in these cases is typically 95-100%, and this pattern is associated with aggressively growing CLL.
The bone marrow report reflects what is happening with the CLL in the bone marrow. If it is early in the disease, there may not be a lot of CLL, and there may be adequate numbers and proportion of normal cells, giving normal blood counts. Prior to treatment when the disease is active, the marrow is typically full of CLL, with few normal cells and low blood counts (except the lymphocyte count). Treatment is intended to clear the bone marrow of leukemia, allowing the normal cells to return, grow, and produce normal numbers of blood cells. Bone marrow evaluation can also provide information about other diagnoses or problems that can result in low blood counts.
William G. Wierda, MD, PhD is Professor of Medicine in the Department of Leukemia, Division of Cancer Medicine at The University of Texas MD Anderson Cancer Center in Houston, TX. He is a member of the Medical Advisory Board for the CLL Society.
Originally published in The CLL Tribune Q3 2016.