Round Cell Tumors: Classification and Immunohistochemistry

Published online: 2021-07-04

Review Article

Abstract Shweta Sharma, Round cell tumors as the name suggest are comprised round cells with increased nuclear‑cytoplasmic Kamala R1, ratio. This group of tumor includes entities such as peripheral neuroectodermal tumor, Divya Nair2, rhabdomyosarcoma, synovial sarcoma, non‑Hodgkin’s lymphoma, neuroblastoma, hepatoblastoma, 3 Wilms’ tumor, and desmoplastic small round cell tumor. These round cells tumors are characterized Raju Ragavendra T , 4 by typical histological pattern, immunohistochemical, and electron microscopic features that Swapnil Mhatre , can help in differential diagnosis. The present article describes the classification and explains the Robin Sabharwal5, histopathology and immunohistochemistry of some important round cell tumors. Basanta Kumar 6 Keywords: Ewing sarcoma, lymphoma, round cell tumor Choudhury , Vivek Rana7 Department of Conservative Introduction 4. Embryonal rhabdomyosarcoma Dentistry and Endodontics, (ERMS) Eklavya Dental College and The term round cell tumor describes a 4 5. Small cell carcinoma Hospital, Kotputli, Department group of highly aggressive malignant of Pedodontics, RR Dental 6. Lymphoma tumors composed of relatively small College, Udaipur, Rajasthan, 7. Leukemic infiltrate. 1 and monotonous undifferentiated cells Department of Oral Medicine B. Septate or lobulated round cell pattern and Radiology, ESIC PGIMSR with increased nuclear‑cytoplasmic 1. Small round cells are divided by Medical College and Hospital, ratio.[1] Malignant small round cell tumors Bengaluru, Karnataka, 2PDM fibrous/fibrovascular septate (MSRCT) is a term used for tumors Dental College and Research 2. Ewing’s sarcoma 5 composed of malignant round cells that Institute, Bahadurgarh, BRS 3. Alveolar rhabdomyosarcoma Dental College and Hospital, are slightly larger or double the size of red 3 (ARMS). Sultanpur, Haryana, Oral blood cells in air‑dried smears.[2] Pathology Division Oral Basic C. Alveolar/pseudoalveolar round cell Clinical Sciences, College This group of neoplasms is characterized pattern of Dentistry, Qassim Private

by small, round, relatively undifferentiated This pattern includes focal, poor Colleges, Buraidah 51411, KSA, 6 cells. Differential diagnosis of small round cohesion of the round cell population Department of Oral Medicine and Radiology, Institute of cell tumors is particularly difficult due to resulting in pseudo alveolar appearance Dental Sciences and Sum their undifferentiated or primitive character. 1. ARMS Hospital, Bhubaneswar, Odisha, Tumors that show good differentiation are 2. PNET. 7Private Practioner, Lajpat generally easy to diagnose, but identification D. Round cell pattern with rosettes Nagar, New Delhi, India of the diagnostic, morphological features A rosette’ is like a flower, with the cells is difficult when a tumor is poorly being arranged radially around a central differentiated, therefore, no definitive area diagnosis may be possible.[3] Fine‑needle 1. Flexner’s (also called aspiration cytology (FNAC) plays an Flexner ‑ Winterstein, true important role in the diagnosis of these rosettes) ‑ contain clearly delineated tumors.[2,4,5] empty central lumen Address for correspondence: e.g., neuroblastoma, PNET Dr. Vivek Rana, Classification 2. Homer Wright rosette‑center has Private Practioner, On the basis of round cell pattern no lumen, but abundant fibrillary Lajpat Nagar, New Delhi, India. material E‑mail: [email protected] A. Diffuse round cell pattern e.g., neuroblastoma. 1. Ewing’s sarcoma E. Round cell pattern with Access this article online 2. Primitive neuroectodermal tumor hemangiopericytomatous vascular (PNET) pattern Website: www.ijmpo.org 3. Merkel cell carcinoma e.g., poorly differentiated synovial DOI: 10.4103/ijmpo.ijmpo_84_16 sarcoma, Mesenchymal chondrosarcoma. Quick Response Code: F. Round cell pattern with other components This is an open access article distributed under the terms of the Creative Commons Attribution‑NonCommercial‑ShareAlike 3.0 License, which allows others to remix, tweak, and build upon the How to cite this article: Sharma S, Kamala R, Nair D, work non‑commercially, as long as the author is credited and the Ragavendra TR, Mhatre S, Sabharwal R, et al. Round cell new creations are licensed under the identical terms. tumors: Classification and immunohistochemistry. Indian For reprints contact: [email protected] J Med Paediatr Oncol 2017;38:349-53.

1. Pseudo glands ‑ poorly differentiated synovial sarcoma cells have a high nucleocytoplasmic ratio. Two population 2. Cartilage ‑ mesenchymal chondrosarcoma. of cells have been described large chief cells and smaller dark cells.[2] According to size of round cell The cytoplasm of these cells is pale blue and 1. Small round cell ‑ Squamous cell carcinoma, contains variable numbers of punched‑out vacuoles PNET, Ewing’s sarcoma, melanoma, which correspond to glycogen deposits, can be well rhabdomyosarcoma (RMS), Langerhans cell disease, demonstrated by periodic acid–Schiff (PAS) staining. lymphoma, adenocarcinoma, neuroendocrine carcinoma, However, the presence of large amounts of intracellular Merkel cell carcinoma, olfactory neuroblastoma glycogen is not a specific finding as while up to 35% 2. Large round cell ‑ Squamous cell carcinoma, of all Ewing’s sarcoma cases do not contain detectable adenocarcinoma, melanoma, RMS, lymphoid tumors, glycogen, many other childhood tumors do contain paraganglioma. detectable glycogen.[2] On the basis of origin The tumor shows variable numbers of pseudorosettes; I. Neurogenic origin: Ewing’s sarcoma/PNET, fibrillary matrix and Homer Wright rosettes are seen at neuroblastoma, retinoblastoma, medulloblastoma, Merkel times, and mitotic figures are rarely detected. cell tumor, paragangliomas, small cell tumor of lung Radiographic features II. Mesenchymal origin 1. Myogenic differentiation Onionskin or sunburst appearance. a. ERMS Special stains b. ARMS. 2. Osteoid differentiation PAS with diastase (glycogen present in 75% of cases), a. Small cell osteosarcoma. immunohistochemistry. 3. Chondroid differentiation a. Mesenchymal chondrosarcoma. Table 1: Histological characteristics of Ewing Sarcoma 4. Adipose tissue like differentiation and PNET a. Myxoid/round cell liposarcoma. Histological types Characterization

Hematolymphoid origin General Sheets and large nests of uniform, small, polygonal cells with scanty cytoplasm and a. Lymphoma/“reticulum cell sarcoma.” indistinct cell borders are present Malignant soft tissue tumors of uncertain type Dispersed chromatin with hyperchromasia and variable mitotic figures a. Desmoplastic small round cell tumor (DSRCT) Rosettes are absent b. Poorly differentiated synovial sarcoma. Typical Round cells with varying proportions of large clear cells and smaller hyperchromatic Ewing’s Sarcoma and Primitive Neuroectodermal cells are present Tumor Cytoplasm is ill‑defined, scanty, pale Ewing’s sarcoma is a sarcoma of bone classically described staining and vacuolated as the result of under small round cell tumors. There is considerable intracellular deposits of glycogen clinical and histologic overlap between this tumor and the Hemorrhage with vascular lakes or sinuses PNET. Ewing’s sarcoma arises within the bone, but can are seen also occur within the soft tissue (extraosseous Ewing’s Filigree pattern (association of distinct sarcoma) and PNET arises within soft tissues.[6] This vascular structures with degenerated or neoplasm mainly affects the pelvis and the femur region necrotic ghost cells) larger tumor cells and predominates in the second decade of life [Table 1]. Metaplastic bone or cartilage Atypical Cells have increased nuclear size or cellular PNET is a small round cell malignancy of primitive, atypism neuroectodermal tissue or pluripotential, migratory Moderate amount of glycogen neural crest cells that arises from the soft tissue or bone, Lobular architecture, increased extracellular commonly affecting older children and adults.[7] The term, matrix, or alveolar pattern with no evidence “PNET” includes MSRCTs of the thoracopulmonary region of myoblastic differentiation (Askin’s tumor), extraskeletal Ewing’s sarcoma, peripheral [2,8] Increased mitoses (>2/HPF) and cellular neuroblastoma, and peripheral neuroepithelioma. pleomorphism FNAC reveals the presence of tumor cells that are arranged Spindle cells, usually at the tumor margin, in relatively small, tight clusters with the presence of but not diffuse round or irregular nuclei lacking nucleoli. These small blue HPF – High‑power field

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Neuroblastoma Table 2: Histological characteristics of Neuroblastoma It is the third most common malignant extracranial solid and variants tumor of childhood. Neuroblastoma and its related variants Features Classic Ewing’s Atypical Ewing’s PNET are derived from primitive neural crest cells that migrate sarcoma sarcoma from the mantle layer of the developing spinal cord and Cell shape Uniform, round Irregular Irregular populate the primordial of the sympathetic ganglia and Chromatin Fine Coarse Coarse adrenal medulla [Table 2].[9] Nucleoli Pinpoint More prominent Prominent Glycogen Abundant Moderate Scanty Rhabdomyosarcoma Rosettes Absent Absent Present PNET – Primitive neuroectodermal tumor RMS is the most commonly found soft tissue sarcoma in children wherein the cancer cells are thought to arise from skeletal muscle progenitors. These tumors are currently Table 3: Immunohistochemical Data on Desmoplastic classified into ERMS, ARMS, and pleomorphic RMS Round Cell Tumor (PRMS) subtypes.[9] Histological types Characterization General Neuroblasts with varying stages of Subtypes differentiation I. ERMS: Mostly affects the children younger than Homer‑wright rosettes 10 years of age. Features include: Neuroblastoma Undifferentiated type ‑ no ganglionic a. Varying degree of cellularity with alternating differentiation densely packed hypercellular areas and loosely Poorly differentiated type ‑ <5% textured myxoid areas differentiating cells b. A mixture of poorly oriented small undifferentiated Differentiating type ‑ >5% differentiating hyperchromatic round‑ or spindle‑shaped cells and cells varying number of differentiated cells with eosinophilic Ganglioneuroblastoma, Has primitive neuroblasts along with cytoplasm characteristic of rhabdomyoblasts[9,10] nodular maturing ganglion cells c. A matrix containing little collagen and varying Contains gross nodules of amount of myxoid material. neuroblastoma abutting large expanses

of ganglioneuroma. Also known as II. ARMS: Composed of large aggregates of poorly composite neuroblastoma differentiated round or oval tumor cells that show central Ganglioneuroblastoma, Consist of nests of neuroblasts situated loss of cellular cohesion and formation of irregular intermixed in the ganglioneuromatous stroma alveolar spaces. The individual cellular aggregates are Ganglioneuromna Mature and fully differentiated tumor separated and surrounded by dense hyalinized fibrous characterized by a mixture of Schwann septa cells and ganglion cells Cells at the periphery of alveolar spaces adhere in a Special stains/immunoperoxidase Characterization single layer to the fibrous septa while the cells at the Immunoperoxidase NSE, neurofilament protein, center of the alveolar spaces are loosely arranged or S100, desmin [11] free floating. NSE – Neuron‑specific enolase III. PRMS: Composed of spindle‑shaped cells arranged in a fascicular pattern with eosinophilic stringy cytoplasm. oval cells embedded in the hypervascular desmoplastic Special stains stroma. Large tumor cell nests have central necrosis [Table 3].[15] 1. PAS with or without diastase: For intracellular glycogen 2. Colloidal iron and alcian blue: For extracellular The arrangement of the cells mucinous material 1. Large nests with central necrosis 3. Masson’s trichrome, phosphotungstic acid hematoxylin. 2. Tubular like structures 3. Trabeculae separated by fibrovascular septa reminiscent Desmoplastic Small Round Cell Tumor[12] of a “Zellballen” pattern. DSRCT is a rare neoplasm that was first described by Other features include Gerald and Rosai in 1989.[12,13] It is a high‑grade tumor 1. Tumor cells have cleared out cytoplasm or a signet ring that mostly affects abdominal cavity and visceral organs. appearance It differs from other childhood tumors due to its clinical 2. Rhabdoid like foci in which tumor cells have features, morphology, and its immunohistochemistry paranuclear intracytoplasmic hyaline inclusions staining pattern.[2,14] Tumors are composed of sharply composed of aggregates of intermediate filaments demarcated nests of varying size with small round or 3. Wright like rosettes.

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Immunohistochemical Data on Desmoplastic Table 4: Histological variants of Willm’s tumor [15] Round Cell Tumors Histological types Characterization General‑favorable Mixture of cell types differentiating into Marker Number of positive histology blastema, epithelium, and stroma cases (%) Desmin (dot like pattern) 39/39 (100) This triphasic pattern is the most common but Cytokeratin (CAM 5.2 and AE1/AE3) 37/39 (95) mono‑ and bi‑phasic tumors are also identified EMA 24/25 (96) Blastemal Resembles condensed mesenchyme of the embryonic kidney Vimentin 22/27 (81) CD57 (Leu7) 10/15 (67) Small closely packed and mitotically active NSE 18/25 (72) cells with minimal differentiation Diffuse blastemal Large sheets of blastema Synaptophysin[15] 3/19 (16) EMA – Epithelial membrane antigen; NSE – Neuron‑specific enolase May extend beyond kidney and diffusely infiltrative Willm’s Tumor Serpentine Frequent pattern with undulating cords of blastemal blastemal cells in a loose, myxoid stroma[16] Willm’s tumor (WT) or nephroblastoma is the cancer of kidneys Nodular blastemal Blastemal islands are rounded that typically occurs in children rarely in adults [Table 4]. Basaloid blastemal Serpentine or nodular patterns are outlined in Markers for Wilm’s tumor a distinctive epithelial layer Epithelial Recapitulates various stages of normal Cytokeratin, Desmin, WT‑1 protein expression, NB84, nephrogenesis resembling collecting ducts or CCN‑3 protein.[17] nephrons and glomeruli Heterologous elements of mucin, squamous, Immunohistochemistry of Round Cell Tumors and ciliated epithelium may occur Markers:[9] Stromal Myxoid and spindle cells resembling 1. CD 99 embryonic mesenchyme are present • CD 99 is a transmembrane glycoprotein of 30–32 Skeletal muscle most common element KDa Various elements including cartilage, adipose • It plays a role in cellular adhesion and regulation of tissue, bone, mature ganglion cells, and neural

cellular proliferation tissue • Normal tissue that commonly displays strong expression of CD99 include: myocardium, skeletal muscle and neurons, α‑β 1. Cortical thymocytes isotype found in chondrocytes, glia and skin 2. Sertoli cells adnexae, β‑β isotype found in Langerhans and 3. Endothelium Schwann cells. 4. Pancreatic islets 4. Desmin 5. Ependyma • It is the intermediate filament protein associated 6. Epithelium (urothelium, squamous epithelium, with both smooth and skeletal muscle columnar epithelium) differentiation • It is specific for: • Rarely expressed by myofibroblasts and their 1. Ewing’s sarcoma ‑ 90% corresponding tumors 2. Lymphoblastic lymphoma ‑ 90% • In skeletal muscles desmin is localized to Z‑zone 3. Synovial sarcoma ‑ >75% between myofibrils when it serves as binding 4. Mesenchymal chondrosarcoma ‑ 50% material for contractile apparatus. In smooth muscles, 5. Osteosarcoma and desmoplastic round cell it is associated with cytoplasmic dense bodies tumor ‑ Rare • Desmin can also be expressed by nonmuscle cells 6. Neuroblastoma ‑ Never reported (‑). including fibroblastic reticulum cells of lymph 2. NB 84 nodes, submesothelial fibroblast, and endometrial • It is sensitive marker for neuroblastoma (75%), stromal cells Ewing’s sarcoma (16%–25%) • Expressed in PNET, desmoplastic round cell tumors, • Also positive for RMS, WT, osteosarcoma, neuroblastoma, mesothelial cells and tumors, WT. desmoplastic round cell tumor. 5. Cytokeratins 3. S‑100 • Used for distinguishing epithelial from nonepithelial • It is a marker for benign and malignant nerve sheath tumors (lymphomas, sarcomas, melanomas) tumors • Expressed in: Carcinoma, epithelial sarcoma, • Composed of two subunits α and β that combine leiomyosarcoma, mesothelioma to form 3 isotypes: α‑α isotype found in the • Also expressed by round cell tumors such as

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Table 5: Screening for undifferentiated round cell tumors Antibody to Small cell Melanoma Lymphoma PNET Rhabdomyosarcoma Poorly Desmoplastic Neuroblastoma carcinoma differentiated round cell synovial sarcoma tumor Pan cytokeratin + ‑ ‑ Variable ‑ + + ‑ S‑100 protein ‑ + ‑ ‑ Variable Variable ‑ Rare CD45 ‑ ‑ + ‑ ‑ ‑ ‑ ‑ Desmin ‑ ‑ ‑ Rare + Variable + ‑ CD99 ‑ ‑ Variable + Variable + Rare ‑

Myogenin/MyoD1 ‑ ‑ ‑ + + + ‑ ‑ NB‑84 ‑ ‑ ‑ Rare Rare ‑ Rare + PAX5 + + + +: Reactive, -: Non reactive, PNET: Primitive neuroectodermal tumor

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