diagnostics Article WT1 and Cyclin D1 Immunohistochemistry: A Useful Adjunct for Diagnosis of Pediatric Small Round Blue Cell Tumors on Small Biopsies Lucia Salvatorelli 1, *, Rosalba Parenti 2 , Giuseppe Broggi 1 , Giada Maria Vecchio 1 , Giuseppe Angelico 1 , Lidia Puzzo 1 , Andrea Di Cataldo 3 , Vincenzo Di Benedetto 4 , Rita Alaggio 5 and Gaetano Magro 1   Citation: Salvatorelli, L.; Parenti, R.; Broggi, G.; Vecchio, G.M.; Angelico, G.; Puzzo, L.; Di Cataldo, A.; Di Benedetto, V.; Alaggio, R.; Magro, G. WT1 and Cyclin D1 Immunohistochemistry: A Useful Adjunct for Diagnosis of Pediatric Small Round Blue Cell Tumors on Small Biopsies. Diagnostics 2021, 11, 2254. https://doi.org/10.3390/ diagnostics11122254 Academic Editor: Paola Gazzaniga Received: 15 November 2021 Accepted: 29 November 2021 Published: 2 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Medical and Surgical Sciences and Advanced Technologies, “G. F. Ingrassia”, Anatomic Pathology, University of Catania, 95123 Catania, Italy; giuseppe.broggi@gmail.com (G.B.); giadamariavecchio@gmail.com (G.M.V.); giuangel86@hotmail.it (G.A.); lipuzzo@unict.it (L.P.); g.magro@unict.it (G.M.) 2 Department of Biomedical and Biotechnological Sciences, Physiology Section, University of Catania, 95123 Catania, Italy; parenti@unict.it 3 Pediatric Hematology and Oncology Unit, Department of Pediatrics, University of Catania, 95123 Catania, Italy; adicata@unict.it 4 Pediatric Surgery Unit, Department of Medical and Surgical Sciences and Advanced Technologies, “G. F. Ingrassia”, University of Catania, 95123 Catania, Italy; vdb@chirpedunict.it 5 Pathology Unit, Bambino Gesù Children’s Hospital, IRCCS, 00165 Rome, Italy; rita.alaggio@opbg.net * Correspondence: lucia.salvatorelli@unict.it Abstract: Pediatric small round blue cell tumors (SRBCTs) are a heterogeneous group of neoplasms with overlapping morphological appearance. Accordingly, their diagnosis is one of the most difficult in the field of surgical pathology. The most common tumors include rhabdomyosarcoma, Ewing’s sarcoma, neuroblastoma, lymphoblastic lymphoma and Wilms’ tumor (the blastemal component). Over time their diagnosis has become more difficult due to the increasing use of small biopsies. However, the advent of immunohistochemistry has improved the quality of diagnosis in most cases by the application of an adequate panel of immunomarkers. Recently, WT1 and Cyclin D1 have been shown to be useful in the differential diagnosis of SRBCTs on surgically-resected specimens, showing a diffuse cytoplasmic positivity of the former in all RMSs and a diffuse nuclear staining of the latter in both EWS and NB. The aim of the present study was to investigate the expression of WT1 and Cyclin D1 on small biopsies from a series of 105 pediatric SRBCTs to evaluate their diagnostic utility. Both immunomarkers were differentially expressed, with a diffuse and strong cytoplasmic staining for WT1 limited to all cases of RMS, and a diffuse nuclear staining for cyclin D1 restricted to all cases of EWS and NB. Notably, the expression of WT1 and cyclin D1 was also retained in those cases in which the conventional tumor markers (myogenin, desmin and MyoD1 for RMS; CD99 for EWS; NB84 for NB) were focally expressed or more rarely absent. The present study shows that WT1 and Cyclin D1 are helpful immunomarkers exploitable in the differential diagnosis of pediatric SRBCTs on small biopsies, suggesting their applicability in routine practice. Keywords: immunohistochemistry; WT1; cyclin D1; small round blue cell tumors; small biopsy 1. Introduction Pediatric small round blue cell tumors (SRBCTs) are a heterogeneous group of neo- plasms which include rhabdomyosarcoma (RMS), Ewing’s sarcoma (EWS), neuroblastoma (NB), lymphoblastic lymphoma (LL) and Wilms’ tumor. These neoplasms may show overlapping morphological and sometimes immunohistochemical features [1–6]. In daily practice, these tumors are often diagnosed on small biopsies, making diagnosis more diffi- cult, especially if the pathologist is not familiar with these lesions or if tumors show unusual morphology and/or immunohistochemical profile and/or clinical presentation [2,4,7–9]. Diagnostics 2021, 11, 2254. https://doi.org/10.3390/diagnostics11122254 https://www.mdpi.com/journal/diagnostics