—  SHORT COURSE #39  —

Neoplastic Disorders Of The Spleen

Section 5 - Lymphoid Neoplasms of the Spleen

Attilio Orazi
Dennis P. O'Malley


Malignant lymphomas may involve the spleen in any one of three clinical settings. 1. The rarest, true primary splenic lymphoma, where the tumor is confined to the spleen or splenic hilar lymph nodes, without evidence of involvement of other sites. 2. The most common setting, in which the spleen is involved as part of generalized lymphomatous spread. 3. The lymphomatous process is characterized by prominent or predominant splenomegaly and often distinctive clinicopathologic features.

True Primary Splenic Lymphomas

Lymphomas which are defined primary on the basis of the above definition are rare, accounting for less than 1% of all lymphomas. A number of histologic subtypes and clinical settings (e.g. HIV infection) have been described in the literature [1]. The gross findings and the histologic characteristics are similar to those observed in spleens secondarily involved by malignant lymphoma with one exception: a high incidence of cases of large cell lymphomas [1, 2, 3, 4, 5]. Not unexpectedly, most cases show a B–cell phenotype; a small number of primary T-cell lymphomas is also reported [6]. Primary Hodgkin lymphoma of the spleen is also extremely rare [8, 9, 10]. The course of primary splenic lymphoma is hard to predict because of the rarity of cases and the disparate histologic types included in published case reports or small series.

Secondary Splenic Involvement by Lymphoma

Hodgkin Lymphoma (HL)
The spleen is the most common non-nodal organ involved by HL [11]. Involvement of the liver and bone marrow rarely is found in the absence of splenic involvement [11]. All histologic subtypes of HL may involve the spleen, although nodular sclerosis and mixed cellularity are the most common [11]. Involvement by lymphocyte predominant HL is rare [12]. The rare lymphocyte-depleted HL, a poorly characterized subtype difficult to distinguish from non-Hodgkin lymphomas with anaplastic or pleomorphic large cell morphology, characteristically presents with subdiaphragmatic disease and splenic involvement [13].

Hodgkin lymphoma produces either small, miliary nodules, or more frequently, solitary or multiple tumor masses in the spleen [14]. Splenic involvement is generally detectable grossly but may be subtle, only a few millimeters in size [15]. For this reason, the gross examination of the spleen must be meticulous. The early lesions of HL in the spleen are found microscopically in the PALS or in the marginal zones [15]. As the disease progresses, the nodules expand to efface the lymphoid follicles and also may produce involvement of the red pulp.

Sarcoid type granulomas may be found in the spleens of patients who have Hodgkin lymphoma. It has been suggested that granulomas occur more frequently in spleens uninvolved by HL than in those involved by disease [16]. Grossly, they may be so large as to mimic involvement by Hodgkin lymphoma. Microscopically, the granulomas are composed of clusters of epithelioid histiocytes that occur in the white pulp in close association with the arterial circulation. Their significance is unknown [16, 17].

The criteria for the diagnosis of HL in the spleen are the same as those as for other non-nodal sites, i.e. in a patient with a previous nodal diagnosis, the documentation of HL can be made if abnormally nucleolated large cells (mononuclear variant of Reed-Sternberg cells) are seen in a typical, mixed inflammatory background. A classic Reed-Sternberg cell is not necessary for the diagnosis in this setting. If the patient does not have a previous nodal diagnosis, however, diagnostic Reed-Sternberg cells must be found. Immunohistochemical studies can be helpful in confirming the diagnosis of HL by showing, in atypical large cells, an immunophenotype appropriate for a given HL subtype (see IMMUNOHISTOCHEMICAL EVALUATION OF SPLEEN).

The subclassification of HL in the spleen is sometimes difficult and is unnecessary in a case with a previous nodal diagnosis [18]. However, the unique morphologic and immunophenotypic characteristics of nodular lymphocyte predominance HL frequently allow its distinction from the other subtypes. In difficult cases, large panels of markers may have to be used to reach a correct diagnosis.

Non-Hodgkin Lymphoma
Clinical assessment of the likelihood of splenic involvement by malignant lymphomas may be difficult. The weights of involved spleens vary widely; one study found that approximately one third of non-palpable spleens were involved by lymphoma at staging laparotomy [19].

Non-Hodgkin lymphomas of different types involve the spleen with variable frequency. Splenic involvement is particularly frequent in low-grade B-cell lymphomas in which involvement at diagnosis is seen in > 50% of cases [1]. As was mentioned earlier, evaluation of splenic hilar lymph nodes is very important. Histologic findings of lymphoma that are ambiguous or incompletely diagnostic in splenic sections may be more distinctive in splenic hilar lymph nodes. Liver involvement by lymphoma is rare in the absence of splenic disease.

B-Cell Lymphomas
The morphology of the spleen involved by malignant lymphomas of B-cell lineage has been well characterized [14, 15]. The B-cell lymphomas involve the spleen in one of two main patterns, either with uniform nodular expansion of all the white pulp, as seen in cases of small B-cell lymphomas (small lymphocytic lymphoma, lymphoplasmacytic lymphoma, splenic marginal zone lymphoma, mantle cell lymphoma, and follicular lymphoma) or with the formation of single or multiple tumor masses as seen in most cases of large B-cell lymphomas [14, 20]. Small B-cell lymphomas have a predictable higher incidence of splenic involvement at presentation than diffuse large B-cell lymphomas. Occasionally, the spleen will be the site of large cell transformation of a small B-cell lymphoma.

Most B-cell lymphomas predominantly involve and replace the white pulp in a nodular pattern. Cytologically, there are differences in cell composition, but these can be subtle and occasionally obscured by residual benign cells. The features of each individual entity will be discussed below, but it should be remembered that the differential diagnosis can be difficult even for expert hematopathologists. Reference to the clinical and previous hematopathological characterization of the case including peripheral blood and marrow findings cannot be overemphasized. In this area, immunohistology and other ancillary diagnostic techniques are particularly valuable [Table 5-1 and 5-2].

Small Lymphocytic Lymphoma/ Chronic Lymphocytic Leukemia (SLL/CLL)
In the spleen, the morphology of small lymphocytic lymphoma is indistinguishable from that of splenic involvement with chronic lymphocytic leukemia [15]. SLL/CLL often produces grossly visible nodules in a miliary pattern or shows, in advanced stages, a more diffuse appearance [15]. Typically, no germinal centers or rare atrophic ones are present, infiltration of large vessels or trabeculae may be prominent. Although a variable number of prolymphocytes and paraimmunoblasts is usually present intermingled with the predominant small lymphocytes, well formed aggregates of these cells (proliferation centers or pseudofollicles) are only exceptionally seen in the spleen. An increased number of prolymphocytes and paraimmunoblasts may raise the question of high-grade transformation of preexisting CLL/SLL. Studies have addressed this issue and concluded that except in peripheral blood, increased prolymphocytes or paraimmunoblasts are not necessarily associated with accelerated disease [21]. The spleen may, however, be the site of large cell transformation (Richter's syndrome), in CLL/SLL. Rarely, a marginal zone-like differentiation can be observed. (See SPLENIC MARGINAL ZONE LYMPHOMA).

Early splenic involvement by SLL may be difficult to detect because the white pulp nodules resemble those of the immunologically unstimulated spleen [15]. The presence of scattered prolymphocytes and paraimmunoblasts and focal infiltration of the red pulp may aid in the recognition of these cases. In some cases, however, the diagnosis rests on examination of splenic hilar lymph nodes and flow cytometric or immunohistochemical confirmation.

Lymphoplasmacytic Lymphoma / Waldenstrom Macroglobulinemia (LPL/WM)
Hepatosplenomegaly often is a feature of lymphoplasmacytic lymphoma (LPL). The majority of patients with LPL/WM have a significant (>3gm/dl) monoclonal IgM paraprotein with hyperviscosity, anemia, and often bleeding [15]. The histologic features observed in spleen sections of patients with LPL/WM is similar to those described for SLL/CLL and splenic marginal zone lymphoma with both white and red pulp involvement. The most common cellular pattern seen in LPL in the spleen is that of a polymorphic infiltrate of lymphocytes, plasmacytoid lymphocytes, and plasma cells that may have numerous Dutcher and Russell bodies. A marginal zone-like differentiation can be observed. LPL/WM is a rare disorder; it should be considered a diagnosis of exclusion after marginal zone lymphoma or plasmacytoid variants of CLL/SLL have been excluded. Transformation to diffuse large cell lymphoma has been described to occur in the spleen in LPL cases [22].

Mantle Cell Lymphoma (MCL)
Mantle cell lymphoma frequently involves the spleen [23, 24, 25, 26]. Typically, the white pulp is uniformly expanded, with neoplastic cells proliferating in widened mantle zones around benign, atrophic-appearing germinal centers. This pattern may superficially mimic reactive secondary splenic follicles. The latter are tripartite, however, with central follicles, mantle, and marginal zones. The presence of spillage of lymphoma cells into the red pulp may aid in the diagnosis of malignancy.

Eventually, the neoplastic cells invade and replace the germinal centers. Such cases may closely resemble SLL/CLL or SMZL. Rarely, a marginal zone-like differentiation can also be observed and distinction with SMZL may be an issue. Cyclin D1 is the most useful diagnostic stain (to confirm a diagnosis of MCL).

Splenic involvement with blastoid variants of mantle cell lymphoma may occur as a "de novo" process or represent a morphologic transformation of a previously diagnosed mantle cell lymphoma of small cell morphology. In the blastoid variant, the cells may look similar to lymphoblasts or display a pleomorphic morphology. The documentation of cyclin D1 positivity and lack of TdT expression is particularly useful in such cases. Truly leukemic forms of blastoid mantle cell lymphoma have also been described (See later section on LYMPHOID LEUKEMIAS).

Follicular Lymphoma (FL)
Splenic involvement is common in follicular lymphomas. Grossly, the spleen shows a miliary pattern of involvement due to the expansion of abnormal white pulp follicles, although, occasionally, the neoplastic nodules may coalesce to form larger tumor masses [14, 15]. In follicular lymphoma, grade I (small cleaved cell type), high-power examination reveals a monotonous population of small-cleaved lymphocytes with coarse chromatin and inconspicuous nucleoli. The diagnosis of follicular lymphoma, grade 2 (mixed small and large cell type) may be more difficult, because the admixture of cells may superficially resemble reactive germinal centers. Different findings seen in these two conditions are summarized in Table 4-3. Additionally, in FL, the red pulp does not display the plasmacytosis usually seen in reactive lymphoid hyperplasia and may contain satellite nodules composed of lymphoma cells. In difficult cases subtle red pulp involvement can be effectively demonstrated by immunostaining with CD20 or other B-cell antibodies. Immunologic and/or molecular studies to demonstrate monoclonality and BCL-2 gene rearrangement are also useful in difficult cases both for confirmation of malignancy and for lymphoma subtyping. As with other B cell lymphomas, the spleen may represent the initial site of high-grade transformation of follicular lymphoma to diffuse large B-cell lymphoma.

Marginal Zone Lymphoma (MZL)
Secondary splenic involvement by a previously established nodal (NMZL) or extranodal marginal zone B-cell lymphoma is rare [15, 25]. When it occurs, it is usually seen with advanced disease [27]. The involvement is predominantly in the white pulp, with a variable extension of the lymphoma cells into the red pulp. The white pulp often shows residual germinal centers and follicular mantles surrounded by pale cells with small nuclei, abundant cytoplasm and distinct cell margins expanding the marginal zone [1, 15, 20, 28]. In cases of MZL with monocytoid morphology, the lymphoma cells may closely resemble the cells of hairy cell leukemia (HCL). The two diseases, however, show different patterns of splenic infiltration, i.e. white versus red pulp. In addition, the tartrate resistant acid phosphatase reaction in MZL is negative. CD103 expression is restricted to HCL. A word of caution: the paraffin reactive antibody DBA.44 may be positive in MZL as well as in HCL.

Diffuse Large B Cell Lymphoma (DLBCL)
Diffuse large B-cell lymphoma in the spleen characteristically produces solitary or multiple tumor masses, which are usually well demarcated and may show areas of necrosis. Prominent splenomegaly may be a presenting feature [1, 15, 29]. These tumors do not show a predilection for involvement of the white pulp and may involve the red pulp as well. A predominant red pulp involvement with an associated reactive histiocytosis may be observed in some cases [29, 30]. In one such case, the lymphoid infiltration was composed predominantly of small reactive T lymphocytes with occasional large atypical cells positive for B-cell antigens. This case seems to correspond to the morphologic variant of diffuse large B-cell lymphoma known as T-cell-rich B-cell lymphoma [30]. Cases of splenic DLBCL with a predominant red pulp involvement must be distinguished from cases of histiocytic sarcoma and hepatosplenic T-cell lymphoma [15]. Immunohistologic stains confirm the B-cell nature of the lymphoma cells in these cases.

Burkitt Lymphoma (BL)
Involvement of the spleen, lymph nodes, or liver is uncommon in Burkitt lymphoma (BL). Grogan and associates [31] reported splenic involvement in two patients, one of whom was leukemic; and Banks and colleagues [32] found splenic involvement in 10 of 17 cases of sporadic BLs. Most cases have both red and white pulp involvement, although occasionally more selective involvement of the white pulp, either in the Malpighian corpuscles or in the marginal zones, is seen. Immunohistochemistry for TdT can be useful in separating BL (TdT negative) from lymphoblastic malignancies. Mib-1 (Ki67) staining is also valuable in the distinction with "small cell variants" of DLBCL formerly known as Burkitt-like (presently called high grade B-cell lymphoma, unclassifiable). In difficult cases, molecular demonstration of c-myc rearrangement may be crucial to confirm the diagnosis, particularly in cases of atypical BL.

T-Cell Lymphomas
Mature T-cell malignancies are relatively uncommon and there have been only few descriptive studies of splenic involvement in these lymphomas.

Mycosis Fungoides / Sezary Syndrome (MF/SS)
Splenic involvement is seen in cases of advanced stage mycosis fungoides / Sezary syndrome. Splenic involvement in MF usually affects the white and red pulp alike [33]. The marginal zones and the periarteriolar lymphoid sheaths (PALS) are infiltrated by large atypical cells, associated with both diffuse and patchy nodular involvement in the red pulp [34, 35]. Not all cells have cerebriform nuclear contours, and a variable proportion of the tumor cells may appear blastic.

Peripheral T-cell Lymphomas, Unspecified (PTCL-U)
The pathological features seen in cases of secondary splenic involvement by peripheral T-cell lymphomas unspecified are not well described in the literature. The pattern of splenic involvement is different from that seen in B cell lymphomas. Expansion of the PALS producing discrete masses, occasionally mimicking the pattern of involvement seen in MF, can be seen in these cases [15, 35]. Burke [35] reported several cases in which the lymphomatous involvement was mostly found at the periphery of the white pulp; in one case the red pulp was the predominant site of involvement.

Lymphoepithelioid (Lennert's) cell variant, a cytologic subtype of PTCL unspecified characterized by a high content of epithelioid histiocytes may produce a characteristic, but not specific splenic morphology. The epithelioid histiocytes may show a ring-like arrangement at the periphery of the involved white pulp (periphery of follicle and PALS); although they occasionally form clusters, similar to ones seen in involved lymph nodes [15]. Although originally thought to be characteristic for this type of lymphoma, the ring-like arrangement of epithelioid cells may be seen in other forms of lymphoma of both B and T-cell type.

Angioimmunoblastic T-cell Lymphoma (AILT)
In addition to generalized lymphadenopathy, fever, weight loss, and a pruritic rash, hepatosplenomegaly often is a presenting feature in patients who have AILT [15]. Splenectomy occasionally is performed in cases associated with severe hemolytic anemia. Early splenic involvement by AILT is characterized by reactive follicles with expanded perifollicular areas containing a polymorphic cellular infiltrate in which occasional aggregates of medium to large size atypical lymphoid cells with clear cytoplasm may be identified. The abnormal cells may also extend into the surrounding red pulp [15, 36]. The increased reticulin content in the involved areas may produce perifollicular fibrosis. Other cases, with a more extensive pattern of involvement, show large lymphoid nodules involving both white and red pulp. The arborizing vascular network typically seen in lymph nodes is not seen in the spleen.

Anaplastic Large Cell Lymphoma (ALCL)
Anaplastic large cell lymphoma is a heterogeneous entity, which includes cases that are both ALK-positive and those that are ALK-negative. Both types can have splenic involvement [37, 38, 39]; rare cases may present with splenic rupture. The pattern of splenic involvement (focal or diffuse) is variable. Hodgkin lymphoma and other pleomorphic variants of B- and T-cell lymphoma should be considered in the differential diagnosis of ALCL, as well as rare non-hematopoietic neoplasms with anaplastic morphology. In such cases, immunophenotyping for CD30, Alk protein, pan-B and pan-T antigens, and CD45 will usually suffice. In the ALK-negative subgroup, the differential diagnosis with other subtypes of peripheral T cell lymphoma is particularly difficult and probably unwarranted in a spleen setting.

Malignant Lymphomas Presenting with Prominentsplenomegaly
Non-Hodgkin lymphoma presenting with prominent splenomegaly appears to be a clinical syndrome but not a specific histologic entity. Almost any histologic type of lymphoma can present initially with isolated splenomegaly. There are, however, several types of lymphoma that present with splenomegaly and distinct clinicopathological characteristics such as splenic marginal zone lymphoma, and hepatosplenic T-cell lymphoma, that require separate discussion [Table 2].

Splenic Marginal Zone Lymphoma (SMZL)
Splenic marginal zone lymphoma (SMZL) usually presents with splenomegaly, anemia, and weight loss; most cases involve marrow and liver at the time of diagnosis [40]. SMZL is indolent and splenectomy may be followed by prolonged survival. Rare cases with minimal or no splenomegaly and absent bone marrow or liver involvement have also been reported ("indolent variant") that have been incidentally identified in splenectomy specimens [41, 42].

The cut surface of the spleen in SMZL shows a characteristic miliary expansion of the white pulp. Histologically, nodular involvement of the white pulp centered on preexisting follicles is observed. The mantle zones are usually lost. Germinal centers are also rarely identifiable, being often surrounded and occupied by small lymphocytes. This central zone merges at the periphery with a zone of medium-sized cells with more disperse chromatin, less regular nuclei and relatively abundant pale cytoplasm which resemble marginal zone cells. Toward the periphery of the neoplastic nodule, a variable numbers of large atypical lymphoid cells are usually seen. When residual germinal centers are identified, the neoplastic cells are arranged into broad concentric bands around the germinal center (reactive or hyalinized), a pattern which may superficially resemble reactive marginal zone hyperplasia, particularly in those cases with minimal or no splenomegaly. However, in marginal zone hyperplasia, mantle zones are preserved and lymphoid infiltration of the germinal center is not observed. In addition, red pulp involvement, which is a common feature in SMZL, is not seen in reactive hyperplasia.

Immunohistochemically the lymphoma cells express IgM and often IgD and various pan B-cell markers. Molecular analysis shows lack of BCL-2 or BCL-1 rearrangement.

A variant of SMZL, which is characterized by predominant red pulp involvement, needs to be distinguished from hairy cell leukemia [43]. The distinction is helped by the presence, in this variant, of otherwise typical SMZL-associated phenotypic (e.g. TRAP negativity) and bone marrow findings (i.e. multinodular involvement versus interstitial involvement in hairy cell leukemia).

Transformation of SMZL to large B-cell lymphoma occurs in a proportion of cases [44]. This may be associated with an increased number of epithelioid histiocytes and reactive T cells, with some of the transformed cases resembling T-cell/histiocyte rich large B-cell lymphoma. p53 overexpression is detectable in a high proportion of the transformed cases [44].

The differential diagnosis of SMZL includes all other small B-cell lymphomas, since a marginal zone growth pattern can also be seen in non-MZL subtypes [15, 25, 45]. The main differential diagnostic points are summarized in Table 5-3. The main morphologic feature of discriminatory value is the different cytological composition of the white pulp lymphomatous nodules. SMZL only shows a characteristic dimorphic cytology, different from the monomorphic cytology of MCL, and the distinctive mixture of centroblasts and small-cleaved cells, which is seen in FL. A monotonous small lymphocytic cytology in conjunction with the presence of scattered prolymphocytes and paraimmunoblasts should point towards SLL/CLL. Do not count on the presence of proliferation centers which are only rarely encountered in spleens with SLL/CLL. However, when present, they may facilitate the diagnostic interpretation.

Although SMZL and LPL have overlapping histologic features, cases diagnosed as LPL are usually associated with the clinical syndrome known as Waldenstrom's macroglobulinemia, and, usually, show a more prominent degree of plasmacytic differentiation and higher levels (>3gm/dl) of monoclonal IgM serum paraprotein than cases of SMZL.

A careful integration of morphology and immunohistochemistry is recommended for the differential diagnosis of small B cell lymphoma in spleen [Table 5-3].

Hepatosplenic T-cell Lymphoma
Hepatosplenic T-cell lymphoma (HTCL) is a distinct clinical entity within the spectrum of peripheral T-cell lymphomas. It is characterized by cytopenias, sinusoidal tropism and, in most cases, T-cell gamma/delta receptor phenotype of the malignant cells [46, 47]. It typically occurs in young adults, more frequently males, and is associated with a poor prognosis. This tumor largely overlaps with a rare type of T-cell lymphoma that also involves the spleen termed erythrophagocytic T-gamma lymphoma [48].

Macroscopically, the spleen is markedly enlarged with a homogeneous cut surface. Histologically, the neoplastic cells infiltrate the red pulp cords in a predominantly intrasinusoidal fashion. The lymphoid cells are usually medium-sized and have oval or folded nuclei with chromatin less condensed than that of small lymphocytes, and a moderate amount of pale cytoplasm. Occasional cases have larger, more blastic appearing cells.

The neoplastic cells stain with various pan-T cell markers, but are often "double" negative with CD4 and CD8. Most cases express the gamma/delta T-cell receptor, although some cases also express the alpha/beta receptor [49]. Evidence has suggested a strong association between HTCL and the presence of isochromosome 7q10 [50].

The differential diagnosis includes both B-cell and T-cell neoplasms involving the red pulp. A careful review of the morphology (e.g. lack of blood lakes) and immunohistochemical findings will allow separation from B-cell neoplasms (e.g. HCL). Immunohistochemistry can also be helpful in the differential diagnosis with T-PLL, by demonstrating in the latter, the frequent co-expression of CD4 and CD8. Immunohistochemistry can also be used to separate HTCL from rare cases of nonleukemic NK/T-cell lympholiferative disorders as well as from other types of peripheral T-cell lymphoma with predominantly involvement of the splenic red pulp. Cases of splenic DLBCL with a predominant red pulp involvement must also be distinguished as well as cases of histiocytic sarcoma [15]. Immunohistologic stains confirm the B-cell (e.g. CD20) or histiocytic nature (e.g. CD68) of the malignant cells respectively, in these cases.

Plasma Cell Neoplasms
Plasma cell neoplasms comprise a heterogeneous group of neoplastic conditions associated with monoclonal immunoglobulin production. These conditions can affect both the red and white pulp of the spleen. A common finding is of the presence of plasma cells or plasmacytoid lymphocytes with PAS-positive cytoplasmic globules (Mott cells) and intranuclear and cytoplasmic immunoglobulin inclusions (Dutcher and Russell bodies, respectively).

Plasma Cell Myeloma (PCM)
Splenomegaly is rarely clinically significant in patients who have plasma cell myeloma but is commonly observed in cases of POEMS syndrome [15], in which an osteosclerotic variant of myeloma occurs in association with polyneuropathy, organomegaly, endocrinopathy, and hyperpigmentation of the skin.

The myeloma cells may either infiltrate the red pulp diffusely or, rarely, produce grossly visible nodules. The spleen may be extensively involved, particularly in the presence of plasma cell leukemia, and splenic rupture may be seen in those cases [51]. The pattern of involvement of the spleen in plasma cell leukemia resembles that seen in other leukemic processes (i.e. red pulp/ intravascular). Splenomegaly in MM may also result from amyloid deposition. In rare cases, light chain deposition only and not amyloid has been described in myeloma patients. Rarely, excess light chain is taken up by histiocytes, causing so-called crystal-storing histiocytosis [52]. This can be accompanied by a foreign body type of granulomatous reaction.

Primary Amyloidosis (PAL)
Amyloid deposition in spleen may occur in cases of systemic amyloidosis. Although both primary and secondary amyloidosis may involve the spleen, splenic involvement is more common in the latter. Early in the disease, amyloid is deposited in the walls of small blood vessels. A nodular deposition of amyloid in the white pulp produces the miliary, or "sago," pattern, whereas the "lardaceous" spleen results from larger, more expansive sheets of amyloid in the red pulp. Functional hyposplenism may occur with extensive amyloid deposition. In a large percentage of PAL cases, a small number of monoclonal plasma cells with monoclonal light chain restriction can be demonstrated. Splenic rupture has also been reported in association with splenic amyloidosis [53].

Heavy Chain Diseases (HCD)
Splenic involvement may be a feature of both gamma (GHCD; Franklin 's disease) and mu heavy chain diseases (MuHCD). Splenic involvement has not been reported in alpha heavy chain disease. Gamma heavy chain disease is a variant of lymphoplasmacytic lymphoma and share many morphologic similarities with the latter entity [54]. A polymorphous lymphoplasmacytoid infiltrates, which also includes eosinophils and immunoblasts is characteristically seen in GHCD. MuHCD is a B-cell lymphoproliferative disorder which resembles chronic lymphocytic leukemia [15]. Its marrow and splenic morphology, however, differs from that seen in CLL, and is characterized by a mixture of small lymphocytes and plasma cells with vacuolated cytoplasms. Immunophenotypically, both forms of HCD share more similarities with LPL than CLL (e.g. CD5 is negative).

Precursor B and T-Cell Leukemic Neoplasms

Acute Lymphoblastic Leukemia (ALL)/lymphoblastic Lymphoma (LBL)
Although enlargement of the spleen often occurs during the course of acute lymphoblastic leukemia, it rarely approaches clinical significance. Since splenectomy is only exceptionally performed, the features of ALL/LBL in spleen are not well described. In early stages, the disease is localized adjacent to the follicles, especially in the regions of the PALS [15, 20]. In more advanced stages of splenic infiltration, extensive red pulp involvement results in a more diffuse pattern with obliteration of splenic architecture.

ALL/LBL can be easily confused with both Burkitt lymphoma, and mantle cell lymphoma, blastoid variant, particularly in suboptimal histologic preparations. Immunohistochemistry with TdT, CD10, CD3, CD5, CD20, and cyclin D1 may be valuable in distinguishing between these entities.

B-Cell Leukemic Neoplasms

Chronic Lymphocytic Leukemia (CLL)
The morphologic features of CLL in spleen are histologically identical to those seen in small lymphocytic lymphoma, as previously described (see small B cell lymphoma section).

B-cell Prolymphocytic Leukemia (B-PLL)
B-cell prolymphocytic leukemia (B-PLL) is a B cell malignancy characterized by a marked lymphocytosis (often greater than 100 x 109/L) in which > 55% of the circulating lymphocytes are prolymphocytes [15, 55]. Splenomegaly is prominent, with hypersplenism, anemia and thrombocytopenias, and the absence of significant lymphadenopathy as associated features [55].

The pattern of infiltration in the spleen is similar to that seen in SLL/CLL in that, in addition to splenic white pulp, the red pulp also shows significant involvement [15]. Histologic distinction from cases of leukemic blastoid mantle cell lymphoma (see next section) as well as other types of small B cell lymphomas with an increased number of large cells may be difficult. Cases of B-PLL with t(11;14) are at present considered as indistinguishable from blastoid MCL (See NEXT SECTION).

Mantle Cell Lymphoma in Leukemic Phase with Prominent Splenomegaly
Leukemic manifestations of mantle cell lymphoma are seen in a minority of cases, usually associated with extensive tumor. However, studies have documented the existence of unusual leukemic cases of mantle cell lymphoma, characterized by prominent splenomegaly in the lack of significant peripheral lymphadenopathy [56]. Not uncommonly, a blastoid morphology and aggressive clinical behavior is seen in these cases. There is still confusion in the literature on how to distinguish leukemic blastoid mantle cell lymphoma from cases of PLL associated with 14q32 rearrangements. It is likely that some of the cases previously reported as B-PLL with t(11;14) may be examples of leukemic MCL rather than B-PLL.

Hairy Cell Leukemia (HCL)
Patients with hairy cell leukemia present with prominent splenomegaly associated with pancytopenia, insignificant lymphadenopathy, and characteristic hairy cells in the peripheral blood [15, 57]. The gross appearance of the spleen is homogeneous and dark red, with variably sized hemorrhagic areas (blood lakes lined by hairy cells [58]. The white pulp is inconspicuous. The hairy cells, in splenic touch smears, appear as medium sized atypical lymphoid cells with homogeneous chromatin, inconspicuous nucleoli, moderate amounts of clear to pink cytoplasm, and typical surface projections. A pericellular clear halo is often seen in histology sections. Tumor cells infiltrate both the splenic cords and the red pulp sinuses, and subendothelial invasion of the trabecular veins may be prominent. The infiltrate in HCL may resemble large granular lymphocytic leukemia (LGL) or hepatosplenic T-cell lymphoma. In HCL, characteristic tartrate-resistant acid phosphatase (TRAP) reactivity and the positivity (by flow) with CD103, CD11c, and CD25 allow for an easy separation from LGL or T cell neoplasms. Mastocytosis in the spleen may also occasionally resemble HCL; however, the characteristic trabecular distribution and the associated fibrosis of mastocytosis are not seen in HCL. In difficult cases CD117 and/or tryptase (both negative in HCL) can be very helpful. TRAP staining, however, may be positive in a proportion of cases of mastocytosis.

Hairy Cell Variant (HCL-V)
The disorder is characterized by a high leukemic count without associated cytopenias [59, 60]. Hairy cell variant displays histologic findings similar to HCL; however, its cytologic features are different. The tumor cells of HCL-v have a higher nuclear-cytoplasmic ratio than typical hairy cells, a less condensed nuclear chromatin, and a prominent nucleolus. The cells often lack TRAP, CD25, and/or CD103. The distinction from B-PLL or SMZL with predominant red pulp infiltration may be difficult in spleen and correlation with the peripheral blood findings is often essential. Pleomorphic/large cell and blastic HCL-v subtypes have also been reported.

T-Cell Leukemic Neoplasms

T-cell Prolymphocytic Leukemia (T-PLL)
T-cell prolymphocytic leukemia (T-PLL) is a rare leukemia which presents with high lymphocyte count, generalized lymphadenopathy and hepatosplenomegaly [61]. The circulating lymphocytes may be small and resemble CLL. In many cases, however, they show nuclear irregularity and visible nucleoli. As opposed to LGL cases, the cells of T-PLL do not have cytoplasmic granules. Heavy Infiltration of the splenic red pulp with variable degrees of white pulp replacement is commonly seen in T-PLL. The diagnosis of T-PLL is difficult to make by spleen (or bone marrow) histology alone and examination of peripheral blood is strongly recommended. The presence of "double positivity" for CD4 and CD8 may be helpful in distinguishing T-PLL from other mature T-cell proliferations involving the spleen. TdT negativity helps in differentiating T-PLL from T-ALL.

Large Granular Lymphocytic Leukemia (LGL)
Large granular lymphocytic leukemia defines a spectrum of proliferations of large granular lymphocytes of T cell (85%) or, less frequently, NK cell phenotype. Some patients with T-cell LGL have only a moderate lymphocytosis with neutropenia and minimal splenomegaly. Others have progressive disease with marked blood and bone marrow lymphocytosis and prominent splenomegaly. Peripheral blood values of LGLs greater than 5 x 109/L are a strong presumptive evidence of a non-reactive cause. Values of LGLs greater than 2 x 109/L are still consistent with this diagnosis while lower values (less than 2 x 109/L) require confirmation of clonality by molecular techniques or karyotype. T-LGL cases needs to be distinguished from reactive large granular lymphocytosis, a condition which is most frequently observed in patients with aplastic anemia, rheumatoid arthritis or other autoimmune disorders

Splenic involvement in both types of LGL (T-cell and NK-cell) is similar [62]. It is confined to the red pulp, in which LGL cells infiltrate both cords and sinuses. The histopathologic features of LGL may mimic hairy-cell leukemia and T-PLL. However, the blood lakes characteristic of HCL are not present in LGL leukemia. In addition, immunophenotypic findings are distinctively different in the three disorders.

TABLE 5-1 Features of B cell lymphoproliferative disorders in the spleen
White Pulp Red Pulp Critical IHC
SLL/CLL

PLL 		Homogeneous nodules of small, round lymphocytes.
Variable proportions of prolymphocytes 		Frequent infiltration with intravascular involvement CD5+*, Cyclin D1-
MCL

Conventional 		Follicles with expanded mantle zones Frequent infiltration Cyclin D1+
MCL

Blastoid/Pleomorphic 		Blastoid cells in white pulp Frequent infiltration with prominent intravascular involvement Cyclin D1+
FL Expansion of white pulp by neoplastic follicles Rare, subtle Bcl-2+ follicles
NMZL Nodules with monocytoid lymphocytes Variable infiltration IgD -
SMZL
LPL 		Homogeneous nodules with small, round lymphocytes with occasional larger cells found at periphery Frequent infiltration often with small nodules IgD+

Distinctive pattern of Ki67 staining found in SMZL (see ref. 50bis)
HCL Involvement very rare Diffuse with formation of pseudosinuses and blood lakesTRAP+, DBA.44+

* CD5 is less frequently positive in PLL than in CLL/SLL.

TABLE 5-2 Bone Marrow and Peripheral Blood Findings In Lymphomas with prominent splenic involvement
Blood Bone Marrow
SMZL Atypical lymphocytes with condensed chromatin and moderate amounts of cytoplasm. Cytoplasmic projections, if present, are often polar and coarser than those in HCL Nodular/interstitial aggregates of atypical lymphocytes with abundant. Pale cytoplasm. Rarely, follicles with well-formed marginal zones.
HCL Round nuclei with partially condensed chromatin and abundant cytoplasm. Numerous hairy projections. Monocytopenia common. Interstitial/diffuse infiltrates of small lymphocytes with abundant cytoplasm ('fried-egg' appearance). Spindle cell morphology can also be seen.
HSTCL Occasional leukemic forms may show small lymphocytes, LGLs or blastoid-appearing forms. Hypercellular marrow with often subtle, intrasinusoidal infiltrate of T cells.

TABLE 5-3 Differential Diagnosis of Splenic Marginal Zone Lymphoma and Small B-cell Lymphomas with MZ-like Pattern in Spleen
Pattern of splenic involvement Cytology Useful Immuno-Phenotypic Markers Cytogenetics
SMZL Expansion of marginal zones - often with sparing of follicles. Frequent red pulp and intravascular involvement. Small lymphocytes; frequent monocytoid morphology; occasional larger cells. Mitoses rare IgD+, CD5-, CD23-, CD10-, CD43- del(7q), del(3), others
CLL/SLL Diffuse replacement of white pulp with neoplastic cells. Proliferation centers occasionally seen. Frequent red pulp and intravascular involvement. Small round lymphocytes with condensed chromatin. Prolymphocytes present. Mitoses rare. CD5+, CD23+, CD43+, CD10- +12, del(13q14)
FL Neoplastic follicles replacing normal white pulp. Mixture of small irregular, cleaved lymphocytes, and larger centroblasts. Mitoses dependent on grade. CD10+, CD5-, CD23-, CD43- t(14;18)
MCL (conventional type) Diffuse replacement of white pulp with neoplastic cells occasionally surrounding atrophic germinal centers. Frequent red pulp and intravascular involvement. Small irregular lymphocytes with dense chromatin. Mitoses common. Cyclin D1 +, CD5+, CD43+, CD23-, CD10- t(11;14)
LPL Expansion of marginal zone and other white pulp areas. Frequent red pulp involvement. Mixture of small round lymphocytes, plasma cells and plasmacytoid lymphocytes. Mitoses rare. Dutcher bodies seen occasionally. CD43+/-, CD5-, CD23-, CD10-, IgD- t(9;14)

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