Contributed by Davsheen Bedi, MD and Nidhi Aggarwal, MD DIAGNOSIS
Anaplastic Lymphoma Kinase positive Anaplastic Large Cell Lymphoma (ALK+ ALCL)
DISCUSSION
ALK positive Anaplastic Large Cell Lymphoma is a T-cell lymphoma that consists of large, pleomorphic, CD30+ T-cells that have abundant cytoplasm and horse-shoe shaped nuclei. These cells express the ALK protein and show chromosomal translocation involving the ALK gene.
ALK+ ALCL accounts for 3% of non-Hodgkin lymphoma in adults and about 20-30% lymphomas in children (1). It is seen more commonly in males and usually in the first three decades of life (1). It can involve both lymph nodes and extra-nodal sites, and clinically patients with ALK+ ALCL present with advanced stage disease usually.
Morphologically, these lymphomas are characterized by the hallmark cells i.e. large cells with horse-shoe shaped or kidney shaped nuclei, with prominent nucleoli and abundant amount of vacuolated to eosinophilic cytoplasm. There are various morphological patterns of ALK+ ALCL, namely the common pattern (60%), lympho-histiocytic pattern (10%), small-cell pattern (5-10%), Hodgkin-like pattern (3%) and composite pattern (15%) where more than one patterns can be seen in a single lymph node (1).
Immunohistochemically, these tumor cells are positive for CD30, EMA and ALK. The pattern of ALK staining can vary depending on the type of translocation the tumor harbors and will be mentioned subsequently. Most ALK+ ALCLs express one or more T cell markers. While CD3 is negative in >75% of the cases, CD2, CD5 and CD4 are positive in about 70% cases (1). Most cases are also positive for cytotoxic antigens like TIA1, Granzyme B and/or perforin.
Some cases of the ALK+ ALCLs may not be positive for any T-cell markers but show T-cell rearrangements at the genetic or molecular level. Such cases are called Null cell ALK+ ALCLs (2).
The most frequently occurring genetic alteration in ALK+ ALCLs is the translocation t(2;5)(p23;q35), between the ALK gene on chromosome 2 and the NPM1 gene on chromosome 5. The NPM-ALK chimeric protein is constitutively expressed from the NPM promoter, leading to the overexpression of the ALK catalytic domain and thus tumorigenesis. Many other varying rearrangements involving the ALK gene have recently been shown to be associated with ALCL, including ALO17-ALK, TRK-fused gene (TFG)-ALK, moesin (MSN)-ALK, Tropomyosin 3 (TPM3)-ALK, Tropomyosin 4 (TPM4)-ALK, ATIC-ALK, myosin 9(MYH9)-ALK and CLTC-ALK (3). While the NPM-ALK mutation causes a diffuse cytoplasmic and nuclear staining pattern for ALK, the TPM3-ALK results in a cytoplasmic ALK+ staining with membranous/ peripheral intensification (1) and the CTCL-ALK rearrangement results in a granular cytoplasmic staining (4). Most other translocations cause a cytoplasmic ALK staining. Therefore, strong nuclear and cytoplasmic ALK staining is to some extent indicative of the ALK-NPM1 translocation which can be confirmed on FISH studies using break apart probe or by genetic studies.
Amongst the important differential diagnoses of ALK+ ALCLs are the ALK+ Large B cell lymphomas, ALK+ systemic histiocytosis, inflammatory myofibroblastic tumor (IMT) and metastatic carcinoma. ALK+ Large B cell lymphomas are positive for B cell markers like CD20, PAX5, are usually CD30 negative and are more commonly associated with CTCL-ALK translocation that results in the granular cytoplasmic staining and not the nuclear ALK staining seen in ALK+ T-cell ALCL with NPM1-ALK translocation. ALK+ histiocytosis is also CD30 negative and shows expression for histiocytes markers like CD68. ALK fusions seen in IMTs are varied like TPM3-ALK, TPM4-ALK, CLTC-ALK , ATIC-ALK , CARS-ALK, RANBP2-ALK, SEC31L1-ALK (5). Inflammatory myofibroblastic tumors can be differentiated by a more commonly seen cytoplasmic ALK+ staining, and rarely nuclear membrane ALK+ staining in cases with RANBP2-ALK fusion. Metastatic carcinomas are usually positive for a number of keratins and also positive for the site specific primary tumor markers, whereas ALCLs are usually positive for EMA (1,6). Keratin positivity in lymphomas can be a common diagnostic pitfall and diagnosis is made based of various clinical, radiological and morphological findings and a battery of immunohistochemical stains (6). ALK gene has been found to play a role in several carcinomas. Esophageal squamous cell carcinoma harbors TPM4-ALK fusion, whereas lung adenocarcinomas are known to harbor ALK translocation with more than 19 different fusion partners, including EML4, KIF5B, KLC1, and TPR (7-9). Other ALK positive carcinomas include breast cancer (ELM4-ALK fusion in 2.4% cases), colon cancer (ELM4-ALK fusion in 2.5% cases), thyroid carcinoma (point mutations in exon 23 of the ALK gene, seen in 11% cases), renal cell carcinoma (VCL-ALK fusion, seen in pediatric RCCs) and neuroblastomas (ALK gene point mutations) (3,7-9). Many of these carcinomas can be ruled out based on the clinical history, thorough radiological and clinical examination to rule out other primary sites and on further confirmation with the site specific immunohistochemical stains as was also done in the present case as detailed earlier.
Our present case is an ALK+ ALCL with the characteristic NPM1-ALK fusion. It shows the characteristic strong nuclear and cytoplasmic staining for ALK immunostaining and the FISH studies confirmed the presence of ALK rearrangement based on break apart probe. Though, the partner gene cannot be determined with the break apart probe on FISH analysis, we could infer the partner gene to be NPM1 based on the strong diffuse nuclear and cytoplasmic ALK staining, which is not seen with any other ALK fusions.
There has been recently great interest in ALK related targeted immunotherapy. ALK fusion proteins and the constitutive ALK tyrosine kinase activity represent a therapeutic target in many malignancies with ALK rearrangement. Since ALK is not widely expressed in adult tissue, few toxic effects might be expected from treatment aimed at blocking ALK function (8). ALK small-molecule inhibitor compounds have demonstrated extremely robust anti-tumor efficacy in various trials. Crizotinib is one such small molecule inhibitor that is an FDA approved drug for ALK+ lung cancers. Brigatinib is used in patients who are intolerant to Crizotinib. Many other trials are undergoing for various ALK+ tumors (9,10).
The patient was started on chemotherapy including Cyclophosphamide, Hydroxydaunorubicin, Prednisone and Brentuximab, and after 5 cycles the size of the tumor reduced from 10.1 x 6.5cm to 4.6 x 3.4 cm. The patient is doing well except for the common side effects of nausea, vomiting which also resolved after the first 2 cycles.
REFERENCES
Contributed by Davsheen Bedi, MD and Nidhi Aggarwal, MD
  |
