Case Report Uncommon Presentation of Granulomatosis with Polyangiitis Mimicking Metastatic Lung Cancer Edyta Maria Urbanska 1, * , Johanna Elversang 2 , Bonnie Colville-Ebeling 2 , Johan Olof Löfgren 3 , Karl Emil Nelveg-Kristensen 4 and Wladimir M. Szpirt 4, *   Citation: Urbanska, E.M.; Elversang, J.; Colville-Ebeling, B.; Löfgren, J.O.; Nelveg-Kristensen, K.E.; Szpirt, W.M. Uncommon Presentation of Granulomatosis with Polyangiitis Mimicking Metastatic Lung Cancer. Clin. Pract. 2021, 11, 293–302. https://doi.org/10.3390/ clinpract11020042 Received: 15 April 2021 Accepted: 11 May 2021 Published: 14 May 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 Oncology, Rigshospitalet, University of Copenhagen, 2100 Copenhagen, Denmark 2 Department of Pathology, Rigshospitalet, University of Copenhagen, 2100 Copenhagen, Denmark; johanna.elversang.01@regionh.dk (J.E.); bonnie.colville-ebeling.01@regionh.dk (B.C.-E.) 3 Department of Clinical Physiology, Nuclear Medicine & PET, Rigshospitalet, University of Copenhagen, 2100 Copenhagen, Denmark; johan.olof.loefgren@regionh.dk 4 Department of Nephrology, Rigshospitalet, University of Copenhagen, 2100 Copenhagen, Denmark; karl.emil.nelveg-kristensen.02@regionh.dk * Correspondence: Edyta.Maria.Urbanska@regionh.dk (E.M.U.); wladimir.mietek.szpirt.01@regionh.dk (W.M.S.); Tel.: +45-3545-0604 (E.M.U.); +45-3545-1767 (W.M.S.) Abstract: Diagnosis of anomalous intrathoracic lesions may be challenging and require a multidis- ciplinary approach. We present a case of granulomatosis with polyangiitis (GPA) clinically and radiologically mimicking metastatic lung cancer with a bilateral pulmonary mass, mediastinal and cervical lymph node involvement, and pleural effusion. Surgical biopsy of the thoracic lesion revealed necrotic granulomatous inflammation, and the final diagnosis was subsequently confirmed by kidney biopsy and biochemical parameters. This case illustrates how comprehensive diagnosis secures timely and relevant treatment. Systemic vasculitis may be one of the key differential diagnoses in patients with multiorgan involvement, especially with pattern-mimicking lung cancer. Keywords: intrathoracic lesions; granulomatosis with polyangiitis; GPA; vasculitis; lung cancer differential diagnosis 1. Introduction Differential diagnosis of abnormal thoracic lesions constitutes an important part of routine clinical work-up and can include a wide spectrum of diseases. As soon as standard diagnostic procedures fail to recognize a patient’s diagnosis, a collaborative approach is required to secure timely and relevant treatment. Granulomatous diseases can easily mimic a malignant phenotype as in the current case of granulomatosis with polyangiitis (GPA, Wegener’s granulomatosis). GPA, micro- scopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA) are defined as small-vessel vasculitides associated with antineutrophil cytoplasmic antibodies (ANCAs) [1]. ANCA-associated vasculitis (AAV) is characterized by inflammation of small vessel walls with preferential involvement of the upper and lower airways as well as of the kidneys. However, it can essentially affect any part of the body [2–4]. AAV carries a significant risk of mortality and morbidity notwithstanding adequate treatment, and the time from initial symptoms to actual diagnosis is positively correlated with the disease outcome [5,6]. GPA and MPA are rare diseases with a collective incidence of approximately 20 cases per million per year [7]; they are characterized by similar renal histopatholog- ical lesions [8] and share a putative ANCA-associated pathogenesis [9]; however, their associated ANCA serology (anti-proteinase 3 and anti-myeloperoxidase) differs in genetic predisposition [10], treatment response, risk of relapse [11], and prognosis [12,13]. Due to the granulomatous phenotype including pseudotumors and lung granulomas as well as the frequently reported longstanding prodromal constitutional symptoms, the diagno- sis of GPA often represents a diagnostic challenge which may delay adequate treatment Clin. Pract. 2021, 11, 293–302. https://doi.org/10.3390/clinpract11020042 https://www.mdpi.com/journal/clinpract