Case of the Month: Colon

This case first appeared as Performance Improvement Program in Surgical Pathology (PIP) 2015, case 16, and is a diffuse large B cell lymphoma.

The H&E sections show large aggregates of discohesive cells extending from the deep submucosa to the adventitia. The aggregates consist of large to medium sized cells, which form mostly diffuse and vaguely nodular growth patterns. The lymphocytes have vesicular chromatin and indistinct nucleoli. Given the immunophenotype (positive for CD20, CD10, BCL6, BCL2, kappa restricted, ~50% proliferative rate) and molecular testing (positive for t(14;18), but negative for MYC gene rearrangements), this tumor in the colon is diagnosed as diffuse large B cell lymphoma (DLBCL).

Extranodal lymphoma most commonly occurs in the gastrointestinal tract. Within the gastrointestinal tract, the stomach is most often involved by lymphoma, followed by the small intestine and then the colon. Lymphoma only rarely presents in the esophagus, and esophageal lymphomas tend to be follicular lymphoma. In a small subset of cases, multiple parts of the gastrointestinal tract are seeded with lymphoma. In the colon, DLBCL is the most common type of lymphoma. Other less common lymphomas include mantle cell lymphoma (lymphomatous polyposis), extranodal marginal zone lymphoma, follicular lymphoma and Burkitt lymphoma.

DLBCL is a B cell non Hodgkin lymphoma consisting of medium to large size B cells in which the growth pattern must be diffuse, although there can be nodular areas. The size of the tumor cells must be greater than a macrophage nucleus or more than twice the size of a reactive lymphocyte. DLBCL can be a primary tumor, or it can be secondary and transform from a pre-existing low-grade lymphoma, such as follicular lymphoma, marginal zone lymphoma or chronic lymphocytic leukemia/small lymphocytic lymphoma. DLBCL comprises between 30 and 40% of all non Hodgkin lymphoma, and is most commonly seen in older patients with a median age of ~72.

In the large intestine, DLBCL may transform from a follicular lymphoma, but in the stomach, DLBCL typically progresses from marginal zone lymphoma. Treatment of colonic DLBCL typically involves chemotherapy and CHOP (cyclophosphamide, vincristine, doxorubicin, dexamethasone) chemotherapy is the most commonly administered regimen. Anti-CD20 therapy (i.e. rituximab) is also a standard treatment and has improved the prognosis of patients with DLBCL. Within the gastrointestinal tract, surgical resection is also a treatment option, but may be reserved for cases causing perforation, obstruction, or extensive bleeding. Prognosis often depends on the stage at presentation, so sampling of regional lymph nodes is necessary. Regional lymph nodes may be involved by DLBCL or may contain the underlying lower grade lymphoma. DLBCL can be aggressive clinically, but is also curable with chemotherapy treatment.

There are different morphologic variants of DLBCL including centroblastic, immunoblastic, and anaplastic variants. The centroblastic type, as seen in this case, is most common and shows cells with cleaved nuclei that have multiple inconspicuous nucleoli and scant cytoplasm. In the immunoblastic variant, the majority of the neoplastic cells have prominent central nucleoli and more abundant cytoplasm. In the anaplastic variant, the cells typically are very large and pleomorphic with irregular nuclei. These anaplastic cells can morphologically mimic Reed Sternberg cells or carcinoma. The different morphological variants of DLBCL are often diagnosed simply as DLBCL, as there are no significant clinical differences between these variants.

It is the standard of care to utilize flow cytometry and/or immunohistochemistry to phenotype diffuse large B cell lymphoma. However, the large cells of DLBCL sometimes do not survive the processing for flow cytometry. Thus, the malignant populations of diffuse large B cells may be difficult to identify by flow cytometry. DLBCL cells express B-cell antigens (i.e. CD20, CD79a, PAX5). BCL6 and BCL2 are usually positive. Recent gene expression profiling studies have shown that DLBCL can be separated into 'germinal center' and 'activated B-cell' phenotypes, with the latter having a worse prognosis. Some studies have shown that immunohistochemistry for CD10 (a germinal center marker) and MUM1 (a post-germinal center marker) can differentiate between these two phenotypes. However, standard testing for these phenotypes has not reached general acceptance. CD30 may be positive or negative.

Molecular testing for DLBCL shows that the tumor cells are IgH gene rearranged. Other molecular changes include finding a t(14;18) translocation, BCL6 rearrangement and MYC rearrangement. MYC rearrangement is seen in fewer cases, but is associated with a worse prognosis.

Burkitt lymphoma (BL) is within the differential diagnosis since it also consists of medium sized malignant B cells. BL frequently involves the gastrointestinal tract, more common in the stomach and less often in the large intestine. There are usually numerous mitoses and a “starry sky” pattern at scanning magnification. By immunohistochemistry, BL will express B cell markers such as CD20 and PAX5 as well as germinal center markers such as CD10 and BCL6. However, BL usually has a proliferative rate of nearly 100%, whereas it is only ~50% in this case. The proliferative rate for DLBCL usually does not exceed 90%, though rare cases can be near 100% BCL2 can also help distinguish between BL and DLBCL. BCL2 is most often negative or only weakly positive in BL. DLBCL can be BCL2 positive or negative, but is more frequently positive as compared to Burkitt lymphoma. Despite these differences, there are cases that are difficult to distinguish between DLBCL and BL. Unusual cases of DLBCL can morphologically and immunophenotypically resemble BL, and also share MYC translocations. Features that favor DLBCL or BL include BCL2 and BCL6 translocations, unusual MYC translocations, and a complex karyotype. The most recent WHO classification includes the entity 'Lymphoma, unclassifiable, with features intermediate between DLBCL and BL', which may be the best classification for cases that are difficult to categorize. In this case the BCL2 positivity and moderate proliferative rate are both features in favor of DLBCL over BL.

Mantle cell lymphoma (MCL) can also involve the gastrointestinal tract. Lymphomatous polyposis is the term used to describe MCL that is studding the gastrointestinal tract with multiple small nodules. The cells in blastoid mantle cell lymphoma can be larger in size and similar in morphology to DLBCL. Mantle cell lymphoma is typically CD5+ and BCL1+. MCL is effectively excluded in this case since the tumor was reported to be CD5 and BCL1 negative.

Follicular lymphoma is also within the differential diagnosis of DLBCL. In this case, the tumor cells are too large to be interpreted as follicular lymphoma. Follicular lymphoma typically has a nodular architecture at least focally. Follicular dendritic cells (FDC) show tight meshworks in follicular lymphoma; whereas the FDC meshworks are typically lost in DLBCL. FDCs can be highlighted by CD21, CD23 or CD35 immunostains. In this case, the lack of FDCs evidenced on the CD21 immunostain support the diagnosis of DLBCL over follicular lymphoma.

Poorly differentiated carcinoma may also present with diffuse or nodular aggregates of large tumor cells. Poorly differentiated carcinoma is more common than lymphoma in the colon. However, in this case, the tumor cells are negative for keratin markers, cytokeratin AE1/3, and positive for B cell markers, CD20 and PAX5, which favors lymphoma over carcinoma.

1. Immunohistochemistry for BCL2 may be helpful to distinguish between which of the following entities?

   
   

   1. Burkitt lymphoma and diffuse large B cell lymphomaDiffuse large B cell lymphoma and blastic mantle cell lymphoma
   2. Diffuse large B cell lymphoma and follicular lymphoma
   3. Follicular lymphoma and mantle cell lymphoma
   4. Follicular lymphoma and small lymphocytic lymphoma/chronic lymphocytic leukemia
2. Which of the following immunostains would best distinguish between diffuse large B cell lymphoma and Burkitt lymphoma?

   
   

   1. BCL6
   2. CD10
   3. CD20
   4. D45rb
   5. Ki-67
3. Which of the following statements best describes surgical resection used as treatment for colonic lymphoma?

   
   

   1. Surgical resection is always used as the sole treatment to decreased tumor burden.
   2. Surgical resection is never an option for colonic lymphoma.
   3. Surgical resection is only used if the lymphoma is diagnosed as Burkitt lymphoma.
   4. Surgical resection is only used if the patient cannot tolerate chemotherapy.
   5. Surgical resection may be reserved for cases causing perforation, obstruction, or extensive bleeding.

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Benjamin Rosen, DO
Resident in Pathology
National Capital Consortium Pathology Residency Program
Bethesda, MD

Aaron Auerbach, MD MPH
Surgical Pathology Committee
The Joint Pathology Center
Bethesda, MD

1. Burkitt lymphoma and diffuse large B cell lymphoma (a)
2. Ki-67 (e)
3. Surgical resection may be reserved for cases causing perforation, obstruction, orextensive bleeding. (e)