Introduction

In the UK, approximately 2,300 individuals are diagnosed with follicular lymphoma every year. Follicular lymphoma (FL) is the second most common type of slow-growing non-Hodgkin lymphoma (NHL), which takes up 20% of all cases of NHL in the UK.  FL occurs when lymphocytes, a type of white blood cell, become abnormal and grow out of control, causing them to accumulate in lymph nodes or other organs. The clumping of the cells as follicles gives FL its name.¹ 

Histopathological and immunohistochemistry techniques provide crucial information for healthcare providers to diagnose, make a prognosis and plan treatment. The four variants of FL are in situ follicular neoplasia, duodenal-type FL, testicular FL and diffuse variant of FL.² FL is graded according to the percentage of centroblasts into grade 1, grade 2, grade 3, grade 3, grade 3A and grade 3B.  Histopathological tests are an important part of diagnosing FL, allowing the examination of pathological changes in biopsies under the microscope.² 

These tests provide critical information for healthcare providers because some cases of follicular lymphoma can be very indolent, whereas others can be much more aggressive.² Cases involving histological transformation to diffuse large B-cell lymphoma (DLBCL) have a more aggressive clinical course.³ 

To further confirm the diagnosis, immunohistochemistry (IHC) is used.

Histopathological features of follicular lymphoma

Lymph nodes are bean-shaped structures that filter substances in the lymph fluid, which travels through the lymphatic system. They also contain many lymphocytes that can help to fight infection. Follicular lymphoma causes abnormalities in lymph nodes. 

Nodular pattern and cellular composition

Based on its nodular pattern and germinal center-like cellular composition, follicular lymphoma is differentiated from other types of lymphomas.³ For example, the nodular pattern helps distinguish FL from diffuse large B cell lymphoma (DLBCL)  that have a more aggressive course.⁴ 

Follicular lymphoma causes the formation of enlarged nodules that are mostly composed of malignant B cells instead of a balanced population of B and T cells. The malignant B cells occupy the dermis and extend into the deeper layers of subcutaneous fat.⁵  

In healthy individuals, lymphoid follicles are regularly shaped, well-organised and have well-defined borders. With follicular lymphoma, follicles become variable-sized and closely packed, with the development of centrocytes (small cleaved cytes), large cleaved cells, large non-cleaved cells, open chromatin, and multiple nucleoli (centroblasts). 

Other features include an absence of the normal mantle zone and minimal apoptotic cells or tingible body macrophages that are responsible for clearing apoptotic cells.²

Grading of follicular lymphoma

According to the percentage of centroblasts, FL can be graded into grade 1, grade 2, grade 3, grade 3A and grade 3B, where a higher grade indicates a more aggressive course.² The grading of FL helps healthcare provide predict the disease course and make suitable treatment and management plans. The grading criteria are as follows:

• Grade 1: Follicular small cleaved cell lymphoma with 0 to 5 centroblasts per high power field²
• Grade 2: Follicular mixed-cell lymphoma with 6 to 15 centroblasts per high-power field²
• Grade 3A: Follicular large cell lymphoma with more than 15 centroblasts per high-power field and presence of centrocytes²
• Grade 3B: Follicular large cell lymphoma with more than 15 centroblasts per high-power field and solid sheets of centroblasts²

Immunohistochemistry findings

Immunohistochemistry(IHC) findings help confirm the diagnosis for follicular lymphoma by testing for certain markers that indicate the type and presence of tumour cells.³

B-cell markers 

Immunohistochemistry(IHC) findings help confirm the diagnosis of follicular lymphoma by testing for certain markers that indicate the type and presence of tumour cells.³ Immunohistochemistry is important due to the inability of hematoxylin and eosin (H&E) staining to distinguish B cells from T cells.⁶ 

The presence of B-cell markers, including CD20 and CD79a, combined with negative results for T-cell markers such as CD3 and CD5, indicates a positive result for follicular lymphoma.³

CD20 is a pan B-cell marker that is commonly expressed on mature B-cell lymphomas, and CD79a can be used as an alternative. On the other hand, CD3 is a pan T-cell marker found on most mature T/NK-cell lymphomas, and CD5 expression sets follicular lymphoma apart from other low-grade B-cell lymphomas.^{6, 7}

Staining patterns (BCL2 overexpression)

BCL-2 and BCL-6 are important markers associated with germinal center B cells, which inhibits apoptosis of B-cells.⁸ These markers are indicative of the clinical outcome in B-Non-Hodgkin’s lymphoma(NHL). 

The former protein is present in 36.4% of FL cases and is linked to a worse prognosis, whereas the latter is present in 13.6% of cases and is linked to a better prognosis.⁸

Role of Ki-67 proliferation index

Ki-67 is a protein that is found explicitly in cells that are undergoing division, and the Ki-67 proliferation index indicates the percentage of cells that are dividing.⁹  A Ki-67 proliferation index is associated with  a more aggressive disease course for patients with FL.¹⁰

Molecular and genetic features

Some common molecular and genetic abnormalities are associated with the development of follicular lymphoma. The t(14;18)(q32;q21) chromosomal translocation and CREBBP mutations are often found in the early development of follicular lymphoma, whereas MLL2 and TNFSFR14 mutations are found in the later stage of lymphomagenesis.³

The t(14;18)(q32;q21) chromosomal translocation, also known as the IGH-BCL2 translation, is found in almost 100% of cases of follicular lymphoma. 

The translocation causes the oncogene BCL2 to be overexpressed.⁹ The BCL2 gene, in turn, inhibits cell death of B cells, causing them to accumulate and increasing the risk of developing lymphoma. The presence of t(14;18)/IGH-BCL2 can be tested using long-distance polymerase chain reaction or FISH.³

In addition, EZH2, CREBBP and MLL2 genes are histone modifying genes that are often mutated in patients with FL. They encourage the development of lymphomas.³

Transformed follicular lymphoma

Sometimes, follicular lymphoma can transform into more aggressive lymphomas through a series of genetic and environmental changes. These aggressive lymphomas may include diffuse large B-cell lymphoma (DLBCL), high-grade B-cell lymphoma with double-hit, and Epstein-Barr virus-related Hodgkin-like lymphoma.¹⁰

Clinical correlation and impact on treatment

The histopathological findings and molecular findings allow healthcare to provide the most suitable management or treatment methods. FL in later stages, transformed FL (tFL), or FL with higher histopathological grades requires more intense therapies to treat, and vice versa.

F&Q

What is the prognosis of follicular lymphoma?

Most cases of FL are indolent with a good prognosis, where approximately 85% of individuals with FL survive up to 5 years or more after diagnosis. The prognosis of FL is highly variable and depends on many different factors, including the type of FL and the stage of the disease.

What are the treatments for follicular lymphoma?

Treatments for follicular lymphoma may include immunotherapy, chemotherapy, steroids, and radiotherapy. Immunotherapy drugs, including rituximab and obinutuzumab, are frequently used as both a first-line treatment and maintenance treatment.

What are the symptoms of follicular lymphoma?

Some patients with follicular lymphoma may have no symptoms, whereas others may experience them. Symptoms of FL may include the following: 

• Enlarged glands( in the neck, armpits, abdomen, or groin)
• Fatigue
• Breathlessness
• Weight loss
• Night sweats

What is the rate of relapse of follicular lymphoma?

Approximately 20% of patients are found to relapse within 2 years of treatment, and the condition tends to become more resistant to treatment with each subsequent line of treatment.¹³

Summary

Follicular lymphoma is the second most common type of slow-growing non-Hodgkin lymphoma(NHL) in the UK. Lymphocytes become abnormal and clump into “follicles” in lymph nodes or other organs. Histopathology and immunohistochemistry play an important role in the diagnosis of the disease and provide crucial information about the prognosis and the most suitable treatment method for the patients.

Key histopathological features that are indicative of follicular lymphoma include a nodular pattern, germinal center-like cellular composition, enlarged nodules made up of mostly malignant B cells, variable-sized follicles, and so on. Immunohistochemistry(IHC) is used in addition to histopathology to further confirm the diagnosis through the testing of particular markers that indicate the type and presence of tumour cells. Sometimes, follicular lymphoma can transform into more aggressive forms of lymphoma by acquiring a combination of genetic and environmental changes.

A range of genetic markers that are associated with follicular lymphoma have also been identified, such as the t(14;18)(q32;q21) translocation, and mutations in the EZH2, CREBBP and MLL2 genes. Follicular lymphoma can be treated using immunotherapy, radiotherapy, chemotherapy and steroids. A maintenance therapy is often recommended due to its relapse rate of 20%.