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Sheets of centroblasts

Diffuse areas are reported as follows: Follicular = greater than 75% follicular pattern; follicular and diffuse = 25-75% follicular; focally follicular = less than 25% follicular.

Diffuse areas are reported as follows: Follicular = greater than 75% follicular pattern; follicular and diffuse = 25-75% follicular; focally follicular = less than 25% follicular.

Figure 52.6 Immunostains in the case shown in Figure 52.5 demonstrate expression of CD20 (¡eft) and bcl-2 (right). The case also expressed CD10 and bcl-6 (not shown)

variant of low-grade follicular lymphoma is recognized by the WHO classification.

Immunophenotype: Follicular lymphomas are CD5 —, CD10+, CD19 + , and CD20+ B-cells. Light chain restriction can usually be demonstrated; however, a minority of cases may lack detectable surface immunoglobulin.16 Bcl-6 is also expressed in a great majority of follicular lymphomas and is a useful marker when the differential includes other B-cell lymphomas with a nodular pattern such as that may occur in nodular mantle cell lymphoma.17 Bcl-2 is overexpressed in 85% of follicular lymphomas and expression can be useful in distinguishing follicular hyperplasia (negative) from follicular lymphoma (positive) (Figure 52.6). Expression levels depend on cyto-logic grade, with nearly 100% of grade 1 lymphomas expressing bcl-2, while only 75% of grade 3 lymphomas express this oncoprotein.18

Molecular genetics: Follicular lymphoma is typically characterized by t(14;18)(q32;q21). It can be found in approximately 80-90% of follicular lymphomas.19 This brings the BCL2 gene under the influence of IGH and results in overexpression of this antiapoptotic protein. The breakpoints in the BCL2 gene are clustered in the major and minor breakpoint regions of BCL2.20 Thus, PCR assays can be designed to detect this translocation. These can detect t(14;18)(q32;q21) in approximately 60-70% of cases of follicular lymphoma.19 Additional probe sets can be used to detect other less common breakpoints and improve detection of this translocation.21 FISH probes flanking breakpoints have a higher detection rate and are a preferred method for detection of this translocation.22

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