Asian Archives of Pathology 2013; Vol. 9 No.4, 123-136 Special article A practical approach to undifferentiated tumors: A review on helpful markers and common pitfalls Kanapon Pradniwat, MD, Jitsupa Treetipsatit, MD Department of Pathology, Faculty of Medicine, Siriraj Hospital, Mahidol University Corresponding author: Jitsupa Treetipsatit, MD Department of Pathology, Faculty of Medicine, Siriraj Hospital, Mahidol University, 2 Prannok Road, Bangkok Noi, Bangkok 10700, Thailand Phone: 662-419-6520 Fax: 662-411-4260 E-mail address: [email protected] Received: 12 August 2013; Accepted 28 September 2013 ABSTRACT The term “undifferentiated” tumor is commonly used by pathologists when a tumor lacking evidence of lineage differentiation on the basis of routine light microscopic morphology is encountered. Immunohisto- chemistry plays an important role in elucidating the true nature of the undifferentiated tumor that is crucial for clinical management. Since undifferentiated round cell tumors can be mimicked by different types of tumors and often cause diagnostic problems especially in small biopsy specimen, the aim of this review is to provide a practical approach to select helpful immunohistochemical markers, a proposed algorithmic approach and pitfalls that may encountered. Keywords: Undifferentiated tumors, Round cell tumors INTRODUCTION be put in consideration. This article is aimed to Pathologists usually use the term “undif- provide: 1) a concise review on general approach for ferentiated” tumor when a tumor lacks histologic undifferentiated tumors as well as markers that are features which specify its lineage of differentiation. parts of the screening panels used in the recognition However, among these undifferentiated tumors, of the major lineages of the tumor; 2) a proposed a large proportion of cases can be delineated by algorithmic approach using immunohistochemistry; ancillary techniques namely immunohistochemistry. and 3) common pitfalls encountered when dealing To cope with the expense, time-consumption and with undifferentiated tumors with round cell mor- workloads, a systematic approach in using the phology. appropriate immunohistochemical markers should 124 Kanapon Pradniwat, Jitsupa Treetipsatit GENERAL APPROACH FOR characteristics can guide to a diagnosis in some UNDIFFERENTIATED TUMORS cases of undifferentiated tumors, ancillary studies The followings are practical steps of approach especially immunohistochemistry are needed to to the undifferentiated tumor. determine a lineage of differentiation. Immunohis- Step 1: Categorize tumor according to morpho- tochemical studies are particularly helpful in small logic appearances. biopsy specimen by which morphologic assess- Undifferentiated tumors can be categorized ment is limited. A screening panel of immunohisto- according to morphologic appearances into 4 major chemistry for major lineage differentiation often groups: provides the first clue to the nature of an undiffer- 1) Small round cell tumor – In this group, it entiated tumor. comprises heterogeneous groups of tumors that are Step 3: Specify a diagnosis. composed of relatively small, round to oval, closely Once the main lineage of tumor differentia- packed undifferentiated cells with high nuclear- tion is determined, one can proceed to make a much to-cytoplasmic ratio, scant cytoplasm, and round more specific diagnosis. In this step, clinical correla- nuclei1, 2 tion as well as additional ancillary studies is needed. 2) Large round cell/epithelioid cell tumor – The tumors belonged to this group are that of large IMMUNOHISTOCHEMICAL round-oval to polygonal shape with a nesting or MARKERS FOR BROAD LINEAGE sheeting arrangement1. DETERMINATION 3) Spindle cell tumor - The tumors that fall Suggested screening markers for epithelial dif- into this group are composed of spindle-shaped ferentiation neoplastic cells with variable patterns of growth. • Cytokeratins (CKs) 4) Pleomorphic tumor - A cellular component Since the majority of carcinomas, either of these tumors shows high-grade sarcoma-like viable or extensively necrotic, will stain with CKs features with marked cellular pleomorphism and provided that an appropriate CK antibody is applied; marked nuclear atypia. this family of intermediate filaments is crucial in diagnostic immunohistochemistry for the identifi- Step 2: Determine a main lineage of differentia- cation of carcinomatous differentiation and specific tion. carcinoma subtypes3-5. A large number of monoclo- Main lineages of tumor differentiation are nal antibodies that can be used on formalin-fixed listed as follow: paraffin-embedded tissues have been developed 1) Epithelial tumors (carcinomas) and are commercially available; those that play an 2) Germ cell tumors important role in screening for epithelial differen- 3) Melanocytic tumors (malignant mela- tiation are broad-spectrum CK antibodies6. Broad- noma) spectrum CK antibodies that have been used more 4) Hematopoietic and lymphoid tumors often in the published studies and might be helpful 5) Mesenchymal tumors (sarcomas). in diagnostic pathology are briefly discussed as Although morphologic clues such as a follows: specific growth pattern and nuclear or cytoplasmic A practical approach to undifferentiated tumors: A review on helpful markers and 125 common pitfalls Antibody cocktail AE1/AE3 Monoclonal antibody OSCAR This antibody comprises the mouse mono- It is a newly developed broad-spectrum, clonal antibody AE1 that recognizes the acidic (type anti-keratin mouse monoclonal antibody that reacts I) CKs 10, 14, 15, 16, and 19, and AE3 that reacts with CKs 7, 8, 18, and 19, as well as with other with the basic (type II) CKs 1, 2, 3, 4, 5, 6, 7, and CKs not readily identified6. This antibody seems to 87. Although the cocktail contains a wide variety of have an advantage on the formerly mentioned broad- antibodies to important high-molecular weight spectrum CK antibodies that it does not cross-react (HMW) and low-molecular weight (LMW) CKs, with normal brain tissue or gliomas6. it should be noted that this cocktail is not efficient in detecting CK8, which is the major CK in hepa- Antibody cocktail MAK-6 tocellular carcinoma and many other carcinomas5. This cocktail antibody is composed of Also it is inferior to a HMW antibody clone 34ßE12 2 mouse monoclonal antibodies: antibody KA4 (reacts with HMW CKs 1, 5, 10 and 14) in (recognizes CKs 14, 15, 16 and 19) and antibody recognizing tumors with squamous differentiation5. UCD/PR10.11 (reacts with CKs 8 and 18). According Another drawback of this cocktail is that it does to Listrom et al10, MAK-6 reacts with all squamous not react with CK18, which is expressed in many cell carcinomas, and the majority of adenocarcino- carcinomas5, 6. Thus, it is important to keep in mind mas, transitional cell carcinomas, carcinoid tumors, that a negative staining for AE1/AE3 should not and undifferentiated carcinomas. Similar to AE1/ be regarded as sufficient evidence to rule out the AE3 and KL1, MAK-6 may show cross-reactivity possibility of carcinoma. In order to increase the with neural tissue11. sensitivity of the AE1/AE3 cocktail for detecting epithelial differentiation, addition of antibody clones Monoclonal antibody CAM 5.2 that react with CK8 and 18 is suggested3, 6. Cross- CAM 5.2 is a mouse monoclonal antibody reaction with other intermediate filaments, especially that primarily reacts with CK 8. Although it is glial fibrillary acidic protein (GFAP), can result in a technically not a pan-keratin antibody, it can be used false-positive staining result in glial tumors8. as a screening marker for epithelial differentiation because CK 8 is widely expressed among epithelial Monoclonal antibody KL1 tumors and it is highly sensitive for detecting CK 8 This broad-spectrum CK antibody is one of on formalin-fixed, paraffin-embedded tissues6. the most sensitive anti-keratin antibodies currently available for the detection of epithelial differentia- Monoclonal antibody 34ßE12 tion6. KL1 reacts with CKs 1, 2, 5, 6, 7, 8, 11, 14, It is sometimes referred to as keratin 903. 16, 17, and 18; therefore, it has an advantage over This antibody reacts with HMW CKs 1, 5, 10 AE1/AE3 in detecting CK18. Similar to AE1/AE3, and 14. Strong and diffuse staining is observed in cross reaction with normal brain tissue and astrocytic tumors with squamous epithelial differentiation3, 5, 6. tumors may be observed9. Additionally, a wide variety of duct-derived carcino- mas and mesotheliomas are noted to be positive for 34ßE123, 6. When combining with antibody to LMW CKs (e.g. CAM 5.2), 34ßE12 will increase sensiti- vity in detecting epithelial tumors5. 126 Kanapon Pradniwat, Jitsupa Treetipsatit • Supplemental epithelial markers Suggested screening markers for melanocytic These markers, though not specific to epithe- differentiation lial tumors, are useful in confirmation of epithelial • S100 protein or SOX 10 differentiation when other lineages of differentiation S100 protein is regarded as a screening are excluded and CKs are negative or focally posi- marker for melanoma due to its high (>95%) tive. The supplement epithelial markers that might sensitivity for detecting melanoma in primary be useful in corroborating a diagnosis of epithelial and metastatic sites1, 17. The characteristic staining tumor include carcinoembryonic antigen (CEA), pattern is strong and diffuse and occurs in the nucleus epithelial membrane antigen (EMA), epithelial cell and the cell cytoplasm, but sometimes it is reported adhesion molecule (EpCAM) such as