THYMIC HYPERPLASIA AS A CASE OF FALSE-POSITIVE PET CT IN A FEMALE WITH LYNCH SYNDROME

Abstract

SESSION TITLE: Disorders of the Mediastinum 1 SESSION TYPE: Med Student/Res Case Rep Postr PRESENTED ON: 10/09/2018 01:15 pm - 02:15 pm INTRODUCTION: Lynch syndrome is an autosomal dominant genetic condition of defective DNA mismatch repair predisposing for malignancies. Here we present a case of Lynch syndrome with endometrial cancer and a hypermetabolic superior mediastinal lymph node on positron emission tomography-computed tomography (PET- CT). CASE PRESENTATION: An Asymptomatic 59-year old female tested positive for Lynch syndrome (PMS2 mutation) during family screening prompted by the death of her son by colon cancer after being diagnosed with Lynch syndrome. She underwent a prophylactic hysterectomy and was found to have well-differentiated adenocarcinoma of the endometrium, pT1b NxM0. Screening mammogram, colonoscopy, and upper endoscopy were negative. CT chest/abdomen/pelvis showed several lung nodules, some of which were calcified suggesting granulomas. PET-CT showed an isolated nodule (1.6 x 1.1 cm) in the upper left paratracheal area with standardized uptake values (SUV) of 11.1. She underwent ultrasound-guided biopsy of the nodule. Histopathology demonstrated scant lymphoid tissue. Flow cytometry was negative for lymphoma. Due to her Lynch syndrome and high activity on PET-CT, she underwent a second ultrasound-guided biopsy the result was suggestive of benign thymic tissue. There was no evidence of metastatic carcinoma. Repeat PET CT showed persistent hypermetabolic activity. She was referred to a thoracic surgeon for mediastinoscopy. She chose conservative observation strategy and is being followed closely. DISCUSSION: Superior mediastinal masses range from benign thymic hyperplasia to malignant etiologies such as lymphoma. Patients with Lynch syndrome are at increased risk for colorectal, endometrial, ovarian, gastric, brain, and skin cancers. However, they are not associated with chest or mediastinal malignancies.The thymus is a lymphoid organ that is active in the early years of life and atrophies in adults. Thymic hyperplasia is the proliferation of cells and increase in activity and size with normal histology seen on biopsy and is seen in young adults. Rebound thymic hyperplasia has been described in young patients treated for malignancy. (1) Increased FDG uptake was previously reported, however, the SUV ranged from 1.1 to 2.2 (2). Sakasi et al (3) reported the mean SUV to be 7.2 ± 2.9 in patients with thymic cancer. Thymic hyperplasia with SUV of 11.1 has not been reported before.Our patient had two negative core biopsies for malignancy despite having a very high SUV on PET-CT, thereby confirming a false positive scan. Our patient also did not have any obvious reasons for hyperplasia and was never treated with chemotherapy, adding to the uniqueness of the case. CONCLUSIONS: Thymic malignancies are typically associated with highly elevated SUVs on PET-CT however, on occasion, hyperplasia can cause mild elevations of an SUV. Our patient had a strongly positive PET CT but benign thymic tissue on biopsy. Reference #1: Sarji A. Physiological uptake in FDG PET simulating disease. Biomed Imaging Interv J. 2006 Oct-Dec; 2(4): e59. Reference #2: Wittram C, Fischman A, Mark E, et al. Thymic Enlargement and FDG Uptake in Three Patients: CT and FDG Positron Emission Tomography Correlated with Pathology. American Journal of Roentgenology. 2003; 180: 519-522. Reference #3: Sasaki M, Kuwabara Y, Ichiya Y, et al. Differential diagnosis of thymic tumors using a combination of 11C-methionine PET and FDG PET. J Nucl Med 1999; 40:1595-1601. DISCLOSURES: No relevant relationships by Amit Diwakar, source=Web Response No relevant relationships by Ahmad JABRI, source=Web Response no disclosure on file for Jason Lane; No relevant relationships by Colin McLaughlin, source=Web Response