Fig.12.18).There isamoderateamountofbasophilicoramphophiliccytoplasmthatoftencontainssmall,lipid-filledvacuoles(afeatureappreciatedonsmears).Veryhighratesofproliferationandapoptosisarecharacteristic,thelatteraccountingforthepresenceofnumeroustissuemacrophagescontainingingestednucleardebris.Thesebenignmacrophagesoftenaresurroundedbyaclearspace,creatinga“starry sky” pattern.
Fig. 12.18 Burkittlymphoma—lymphnode.Thetumorcellsandtheirnucleiarefairlyuniform,givingamonotonousappearance.Notethehighlevelofmitoticactivity(arrowheads) andprominentnucleoli.The“starrysky”patternproducedbyinterspersed,lightlystaining,normalmacrophagesisbetterappreciatedatalowermagnification.(Courtesy of Dr. Robert W. McKenna, Department of Pathology, University of Texas Southwestern Medical School, Dallas,Texas.)
Immunophenotype. These tumors express surface IgM, the B cell marker CD20, and the germinal center B cell markers CD10 and BCL6.
Clinical Features. Both the endemic and nonendemic sporadic forms affect mainly children and young adults. Burkitt lymphoma accounts for approximately 30% of childhood NHLs in the United States. The disease usually arises at extranodal sites. Endemic tumors often manifest as maxillary or mandibular masses, whereas abdominal tumors involving the bowel, retroperitoneum, and ovaries are more common in North America. Leukemic presentations sometimes occur and must be distinguished from ALL, which is treated with different drug regimens. Burkitt lymphoma is highly aggressive; however, with very intensive chemotherapy regimens, a majority of patients can be cured.
These B cell proliferations contain neoplastic plasma cells that virtually always secrete a monoclonal immunoglobulin or immunoglobulin fragment, which serve as tumor markers and often have pathologic consequences. Collectively, plasma cell neoplasms and related disorders account for about 15% of the deaths caused by lymphoid neoplasms. The most common and deadly of these neoplasms is multiple myeloma, of which there are about 15,000 new cases per year in the United States.
A monoclonal immunoglobulin identified in the blood is referred to as an M protein, in reference to myeloma. Because complete M proteins have molecular weights of 160,000 or higher, they are restricted to the plasma and extracellular fluid and excluded from the urine in the absence of glomerular damage. However, neoplastic plasma cells often synthesize excess immunoglobulin light chains along with complete immunoglobulins. In unusual cases, tumors may produce only light chains, which are detected and quantified in the blood and the urine by highly sensitive tests.
Terms used to describe the abnormal immunoglobulins associated with plasma cell neoplasms include monoclonal gammopathy, dysproteinemia, and paraproteinemia. These abnormal proteins are associated with several clinicopathologic entities:
Multiple myeloma (plasma cell myeloma), the most important plasma cell neoplasm, usually presents as tumorous masses scattered throughout the skeletal system. Solitary plasmacytoma is an infrequent variant that presents as a single mass in bone or soft tissue. Smoldering myeloma is another uncommon variant defined by a lack of symptoms and a high plasma M component.
Monoclonal gammopathy of undetermined significance (MGUS) is applied to patients without signs or symptoms who have small to moderately large M components in their blood. MGUS is very common in older adults and has a low but constant rate of transformation to a symptomatic monoclonal gammopathy, most often multiple myeloma.