Mucinous urachal adenocarcinoma: A potential nonfluorodeoxyglucose-avid pitfall on ¹⁸fluorine-fluorodeoxyglucose positron emission tomography/computed tomography

Abstract

Mucinous adenocarcinoma of the urachal remnant is a nonurothelial malignancy that may be asymptomatic until locally advanced or metastatic. We describe a 37-year-old woman with invasive ductal breast carcinoma who underwent ¹⁸Fluorine-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET) computed tomography (CT) demonstrating a non-FDG avid pelvic mass, initially suspected to represent a pedunculated uterine fibroid. Magnetic resonance imaging revealed a mixed solid-cystic mass separate from the uterus, suspicious for urachal neoplasm, confirmed as mucinous adenocarcinoma on histopathology. Urachal tumors may not be FDG-avid and represent a potential pitfall on FDG PET/CT.

Keywords

¹⁸Fluorine-fluorodeoxyglucose - mucinous neoplasm - positron emission tomography/computed tomography - urachal adenocarcinoma

Introduction

Urachal cancer is a rare clinical entity usually identified as a midline, supravesical mass, with mixed solid-cystic components that may be difficult to confidently diagnose from other benign urachal pathology or more commonly encountered urothelial bladder neoplasms. Adenocarcinoma accounts for up to 80% of urachal tumors with over two-thirds producing mucin. On computed tomography (CT), urachal adenocarcinoma can demonstrate low-attenuation due to mucinous or necrotic components. Intrinsic calcification can be seen in the majority of cases with solid tumor demonstrating variable enhancement on postintravenous contrast imaging. On magnetic resonance imaging (MRI), solid components of urachal adenocarcinoma exhibit intermediate T2 signal with the presence of hyperintense T2 signal suggesting necrotic constituent, high mucin content, or cystic change.

Case Report

A 37-year-old woman with a history of the right invasive ductal breast carcinoma surgically managed with right mastectomy and axillary lymph node dissection presented for follow-up. Laboratory hematological and serum biochemical analyses were unremarkable. 18Fluorine-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET) CT was performed for the purpose of restaging. No hypermetabolic disease was demonstrated, however, a calcified, non-FDG-avid pelvic lesion was revealed, possibly representing a subserosal pedunculated partially calcified uterine leiomyoma [Figure 1]. Following multidisciplinary discussion, further cross-sectional imaging was considered.

Figure 1 Sagittal positron emission tomography (a) and computed tomography (b) images, coronal positron emission tomography (c) and computed tomography (d) images, as well as axial positron emission tomography (e) and computed tomography (f) images, demonstrate a well-defined soft-tissue density (arrows) in the midline pelvis, superior to the bladder, exhibiting coarse central and peripheral calcification, and without abnormal fluorodeoxyglucose avidity

Subsequently performed pelvic MRI demonstrated a well-defined, multilobulated, predominantly T2 hyperintense mass arising from the anterosuperior bladder dome, extending into the space of Retzius suspicious for a primary mucinous neoplasm [Figure 2]. Percutaneous CT-guided biopsy of the mass was performed. Histopathology showed the presence of invasive adenocarcinoma with mucinous features invading into the lamina propria with no evidence of urothelial component consistent with urachal mucinous adenocarcinoma. Immunohistochemistry was positive for CK7 and CK20 and negative for GATA3.

Figure 2 Sagittal (a) and axial (b) T2 weighted magnetic resonance imaging of the pelvis demonstrating a lobulated supravesical lesion (arrows) with heterogeneous hyperintense signal abnormality representing mucin

The partial bladder dome cystectomy and excision of the urachus and umbilicus were performed. The final Sheldon stage was T3N0.

Discussion

The urachus is a fibrous cord extending from the anterosuperior bladder dome to umbilicus that obliterates during normal fetal development. Incomplete obliteration of this channel can result in persistent urachal pathology. Urachal adenocarcinoma, a rare nonurothelial tumor accounting for <0.5% of bladder cancers, can develop in this vestigial remnant and occurs almost twice as common in men with a median age of onset between 45 and 56 years.[1],[2] Urachal adenocarcinoma can present with nonspecific lower urinary tract symptoms with almost 30% of patients present with locally advanced or distant metastatic disease at the time of the initial investigation.[2],[3]

The current series have demonstrated a 10-year survival rate of up to 49% with surgical treatment. Salvage chemotherapeutic agents are rarely effective with a <10% success rate for patients with systemic metastases, which occur most commonly in the liver and lung.[1],[2],[3]

Urachal remnant pathology demonstrates a characteristic location in the midline, superior to the bladder dome, extending into Retzius space toward the umbilicus. On CT, urachal adenocarcinoma usually exhibits central or peripheral calcifications, seen in up to 70% of cases. Urachal cancer may contain both solid and cystic components, demonstrating low attenuation and heterogeneous enhancement on postcontrast imaging. Potentially mimicking other benign and malignant pelvic pathology.[2],[3],[4],[5]

Notably, the majority (69%) of urachal adenocarcinomas are mucin-producing.[2],[5] Although FDG PET/CT may be helpful in diagnosing urachal adenocarcinoma,[6],[7],[8] it is important to highlight that up to 59% of mucinous malignancies demonstrate low FDG avidity because of tumor hypocellularity compared to other nonmucinous tumor types, which may limit the diagnostic efficacy of FDG PET/CT.[9],[10]

This case highlights the challenge of differentiating mucinous urachal neoplasms from other benign low-FDG avid pelvic pathology on FDG PET/CT.

Physicians and PET readers should be aware of the limitations of FDG PET/CT in detecting mucinous tumors and consider further evaluation of non-FDG avid calcified pelvic pathology with additional cross-sectional imaging, such as MRI, to address the possibility of mucinous malignancy.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Conflict of Interest

There are no conflicts of interest.

Financial support and sponsorship

This research was in part funded through the NIH/NCI Cancer Center Support Grant P30 CA008748.

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Publication History

Received: 26 July 2019

Accepted: 25 October 2019

Article published online:
19 April 2022