IgG4 Related Disease: case report and 6-year follow up of an elusive diagnosis mimicking malignancySara Melo Oliveira1, Inês Trigo1, Ana Sofia Oliveira1, Isabel Gomes1, Rita Neto Lopes11Family Health Unit of Barrinha, Esmoriz, ACeS Baixo-Vouga, Aveiro, PortugalCorrespondence: Sara Melo Oliveira, Family Health Unit of Barrinha, ACeS Baixo-Vouga, Aveiro, Portugal. Email:[email protected] Clinical Message: IgG4-related disease is a rare and emerging pathology, characterized by the appearance of pseudotumors. Due to the ability to mimic other pathologies, it is essential to consider it as a differential diagnosis in multisystemic processes. The diagnosis is challenging, requiring a multidisciplinary approach, to minimize the associated morbidity and mortality.Abstract: IgG4-related disease (IgG4-RD) is a rare, emerging, systemic and chronic pathology,characterized by the appearance of pseudotumors resulting from tissue infiltration by IgG4-positive plasmablasts that promote eosinophilic inflammation of the tissue with subsequent fibrosis. We present the case of a male, 45-year-old patient, who consulted his Family Health Unit with complaints of postprandial discomfort in the left hypochondrium with a feeling of fullness, weight loss, chronic fatigue and hyperhidrosis that had lasted for a month. During physical examination, he was pale, and had pain at palpation of the left hypochondrium. Laboratory data showed increased inflammation markers, abdominal ultrasound and CT demonstrated numerous enlarged lymph nodes in the upper quadrants, raising concern for a malignant lymphoproliferative process. Laparoscopic excisional biopsy revealed features of IgG4-related lymphadenopathy and excluded malignant lymphoproliferative disease. The increase in serum IgG4 supported the final diagnosis of IgG4-RD.Keywords: IgG4-related disease; Pseudotumor; Immune-mediated; Systemic; Chronic.FUNDING INFORMATION: This work did not receive any type of financial support from any entity in the public or private domain.CONFLICT OF INTEREST STATEMENT: The authors have no conflicts of interest to declare.DATA AVAILABILITY STATEMENT: The data that support the findings of this study are available from the corresponding author upon reasonable request.ETHICS STATEMENT: All procedures performed in this case were in accordance with the Helsinki Declaration (as revised in 2013). Written informed consent was obtained from the patient for the publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.CONSENT: Written informed consent was obtained from the patient to publish this report in accordance with the journal’s patient consent policy.PATIENT PERSPECTIVE: The patient was anxious and worried about his health condition, mainly due to the possibility of a neoplastic disease. However, he felt calmer as soon as he was informed about the diagnosis and experienced symptomatic improvement after starting treatment. During the follow-up, he remained cooperative and assiduous in regular consultations.ORCID: Sara Melo Oliveira:https://orcid.org/0000-0003-0649-2694