Incidentally Detected Ovarian Teratoma on Fluorodeoxyglucose (FDG) PET/CT Scanning in a Patient With Gastric Cancer

Abstract

This report aims to present the imaging findings of an ovarian teratoma incidentally detected during fluorodeoxyglucose (FDG) positron emission tomography combined with computed tomography (PET/CT) examination performed for the staging of gastric carcinoma, and to discuss these findings in the context of the current literature. A 53-year-old woman with a recently confirmed diagnosis of gastric carcinoma via endoscopic biopsy was referred for PET/CT staging. Imaging revealed pathological FDG uptake along the gastric wall from the cardia to the lesser curvature, consistent with the primary tumor. Additionally, a well-circumscribed, smoothly marginated pelvic mass with no appreciable FDG uptake was identified, demonstrating tooth-like calcifications and fatty fluid components on CT. Given its mixed-density composition, ovarian teratoma was considered the primary differential diagnosis, which was subsequently confirmed on targeted ultrasonography. Mature cystic teratoma typically presents as a metabolically inactive lesion on PET/CT; however, its characteristic CT morphology allows for reliable incidental identification. Awareness of incidental ovarian pathologies encountered during PET/CT examinations performed for unrelated malignancies is of considerable clinical importance.

Introduction

Ovarian teratoma represents the most common germ cell neoplasm of the ovary, accounting for approximately 20% of all ovarian neoplasms, and is predominantly encountered in women of reproductive age [1]. From an embryological standpoint, teratomas arise from pluripotent germ cells and are characterized by the presence of tissue elements derived from all three germ layers - ectoderm, mesoderm, and endoderm [2]. The mature cystic subtype, also referred to as a dermoid cyst, follows a predominantly benign clinical course and is most frequently discovered incidentally, as the majority of patients remain asymptomatic [3]. Histologically, these lesions may contain a diverse array of well-differentiated tissue components, including hair follicles, sebaceous glands, teeth, adipose tissue, and neural elements [4]. Fluorine-18 fluorodeoxyglucose (18F-FDG) positron emission tomography combined with computed tomography (PET/CT) has become an indispensable imaging modality in contemporary oncology, offering the unique advantage of integrating metabolic information with high-resolution anatomical detail. It is widely employed in the staging of malignancies, assessment of treatment response, and detection of disease recurrence [5]. However, the incidental identification of ovarian lesions during PET/CT examinations performed for unrelated indications can present a significant interpretive challenge, particularly when the FDG uptake characteristics of such lesions are equivocal or atypical [6]. The present case illustrates the incidental discovery of a mature cystic teratoma during FDG PET/CT staging of gastric carcinoma. The multimodality imaging features of this lesion are described in detail and contextualized within the relevant literature.

Case presentation

A 53-year-old woman with newly diagnosed poorly differentiated gastric adenocarcinoma confirmed by endoscopic biopsy was referred to the nuclear medicine department for initial staging with FDG PET/CT before initiation of systemic therapy. Patients fasted for at least six hours prior to imaging, and the blood glucose level before FDG administration was within normal limits. Approximately 370 MBq (10 mCi) of 18F-FDG was administered intravenously, and image acquisition was performed 60 minutes after tracer injection. PET/CT imaging extended from the skull base to the mid-thigh. A low-dose non-contrast CT scan was acquired for attenuation correction and anatomical localization, followed by PET acquisition according to the standard institutional protocol. Whole-body PET/CT images demonstrated intense FDG uptake along the gastric wall extending from the cardia to the lesser curvature (maximum standardized uptake value (SUVmax): 12.6), consistent with the known primary gastric malignancy (Figure 1).

Figure 1. Axial PET image (A), CT image (B), fused PET/CT image (C), and coronal fused PET/CT image (D) demonstrating intense FDG uptake along the gastric wall extending from the cardia to the lesser curvature, consistent with the primary gastric malignancy (arrows).

PET: positron emission tomography; CT: computed tomography; FDG: fluorodeoxyglucose

Incidentally, a well-circumscribed left adnexal pelvic mass measuring approximately 7 cm was identified adjacent to the uterus and anterior to the rectosigmoid colon. On CT, the lesion demonstrated a smoothly marginated heterogeneous internal architecture composed of distinct tissue components. The anterosuperior portion contained a region of macroscopic fat attenuation, compatible with mature adipose tissue, without discernible FDG uptake. The posterior component appeared hyperdense without appreciable metabolic activity. The posteroinferior aspect demonstrated a heterogeneous hyperdense area containing tooth-like calcifications and showing only low-level FDG uptake, comparable to background pelvic soft-tissue activity (Figure 2).

Figure 2. Axial PET image (A), CT image (B), fused PET/CT image (C), and coronal fused PET/CT image (D) demonstrating a large well-circumscribed left adnexal pelvic mass with heterogeneous internal composition. The anterior component contains macroscopic fat attenuation without FDG uptake (thick arrow). The posterior component appears hyperdense without appreciable metabolic activity (thin arrow). The posteroinferior aspect demonstrates heterogeneous hyperdense tissue containing tooth-like calcifications (short arrow) with faint low-level FDG uptake (arrowhead), findings characteristic of mature cystic teratoma.

PET: positron emission tomography; CT: computed tomography; FDG: fluorodeoxyglucose

The combination of macroscopic fat attenuation, tooth-like calcifications, and a well-defined capsule strongly suggested mature cystic teratoma. Differential diagnostic considerations included hemorrhagic ovarian cyst, endometrioma, mucinous ovarian neoplasm, and metastatic ovarian involvement; however, the coexistence of fat attenuation and calcified dental elements made these alternatives considerably less likely, prompting further evaluation with targeted ultrasonography. Transabdominal ultrasonography demonstrated a well-circumscribed cystic lesion measuring approximately 7 cm in greatest dimension within the left ovary, showing mixed echogenicity and heterogeneous internal contents. Transvaginal ultrasonography was additionally performed for better lesion characterization and confirmed the presence of echogenic mural components and sebaceous material consistent with a dermoid cyst. Transvaginal ultrasonography was not performed because transabdominal imaging provided sufficient lesion characterization (Figure 3).

Figure 3. Transabdominal ultrasonography of the left ovary demonstrating a smooth-walled cystic lesion with mixed echogenicity and heterogeneous internal contents, raising suspicion for a dermoid cyst.

Power Doppler ultrasonography demonstrated a hyperechoic mural solid component at the posteroinferior aspect of the cyst, corresponding to a Rokitansky nodule (dermoid plug), without internal vascularity on Doppler interrogation. Associated echogenic sebaceous material and multiple echogenic foci producing posterior acoustic shadowing, consistent with calcific or dental elements, were also identified. These sonographic findings were highly characteristic of mature cystic teratoma (Figure 4).

Figure 4. Power Doppler ultrasonography revealing a hyperechoic solid mural component at the posteroinferior aspect of the cystic lesion, accompanied by echogenic sebaceous material characteristic of a dermoid cyst (long arrow). Multiple echogenic foci within the solid component are consistent with heterogeneous calcific or dental elements (short arrows), collectively confirming the diagnosis of mature cystic teratoma.

Taken together, these multimodal imaging findings established a confident diagnosis of mature cystic teratoma, which was subsequently confirmed on surgical pathology as a benign mature cystic teratoma without immature elements or malignant transformation.

Discussion

Mature cystic teratoma is the most frequently encountered ovarian neoplasm in premenopausal women, representing approximately 20% of all ovarian tumors [1]. As documented in the large series reported by Koonings et al., these lesions are often asymptomatic and tend to be discovered incidentally during imaging performed for unrelated clinical indications [2]. Although mature cystic teratomas are generally benign, their management depends on clinical context, including lesion size, patient age, symptoms, and risks such as torsion or rare malignant transformation. In the present case, the patient was undergoing initial staging with FDG PET/CT for a newly diagnosed gastric adenocarcinoma prior to treatment initiation, during which the ovarian lesion was incidentally detected. The characteristic imaging features across multiple modalities often allow for a confident non-invasive diagnosis in appropriate clinical settings, although management decisions may still require individualized clinical and surgical correlation [3]. On CT imaging, mature cystic teratoma produces a highly recognizable pattern defined by the presence of macroscopic fat attenuation, coarse calcifications, dental elements, and a thin, well-defined capsule [1]. These features are considered near-pathognomonic and form the cornerstone of non-invasive diagnosis. Ultrasonography complements CT by demonstrating the cystic architecture, heterogeneous echogenicity, and the characteristic Rokitansky protrusion - a hyperechoic mural nodule representing the dermoid plug - which is highly specific for this entity. From a PET/CT perspective, mature cystic teratomas are generally regarded as metabolically quiescent lesions with absent or only low-level FDG uptake, as was observed in the present case, where only faint peripheral metabolic activity was noted within the lesion [7]. Nevertheless, a small but notable number of mature cystic teratomas may demonstrate unexpectedly elevated FDG uptake, most commonly attributed to metabolically active neuroglial tissue components within the tumor. This finding may mimic malignant ovarian neoplasms or metastatic disease on PET imaging alone, potentially leading to false-positive interpretation in oncologic staging. In such cases, CT morphology plays a crucial role in correct lesion characterization; the presence of macroscopic fat, tooth-like calcifications, and sebaceous components is highly specific for mature teratoma and helps reliably differentiate it from malignant ovarian or metastatic lesions [6,8]. Miyasaka and Kubota described a case of unusually intense FDG uptake in a mature ovarian teratoma and emphasized this as a potential pitfall in PET image interpretation [6]. Similarly, Suh et al. reported markedly increased FDG accumulation in a retroperitoneal mature cystic teratoma in a pediatric patient, again implicating neural tissue as the likely metabolic substrate [8]. The distinction between mature and immature teratoma carries significant clinical and prognostic implications, as immature teratomas behave in a malignant fashion and require aggressive management. Cho et al. conducted a dedicated investigation into the FDG PET/CT characteristics of teratomas and demonstrated that, while immature teratomas generally exhibit higher FDG uptake than their mature counterparts, a degree of metabolic overlap exists, with certain mature teratomas displaying levels of FDG activity comparable to immature lesions [9]. In the present case, the incidental detection of the ovarian lesion during oncologic staging had important clinical implications, as it prompted further targeted imaging evaluation and helped exclude metastatic ovarian involvement from the primary gastric malignancy, thereby preventing potential upstaging and overtreatment. This highlights the pivotal role of PET/CT in oncologic imaging, where metabolic information must always be interpreted in conjunction with morphological CT features. However, PET/CT may also present diagnostic pitfalls, as FDG uptake alone is not specific for malignancy and may be seen in benign entities such as mature teratomas due to metabolically active tissue components. Careful assessment of CT morphology, particularly the presence of macroscopic fat, calcifications, and sebaceous elements, is essential to avoid misclassification and ensure accurate staging and appropriate patient management. This observation underscores the inadequacy of relying solely on FDG uptake for lesion characterization and reinforces the indispensable role of attenuation-correction CT in morphological assessment. In parallel, De Giorgi et al. have highlighted the broader limitations of FDG-PET in the management of germ cell tumors, further supporting the necessity of integrating anatomical and metabolic data [7]. In the context of the present case, the absence of significant FDG uptake within the pelvic mass was reassuring and substantially lowered the suspicion for a metabolically active secondary malignancy or pelvic metastasis from the primary gastric carcinoma, although FDG-PET/CT findings alone cannot definitively exclude malignancy in all clinical settings. Nevertheless, definitive characterization necessitated careful morphological analysis of the CT component, wherein the coexistence of fat attenuation, dental calcifications, and a well-defined capsule collectively pointed unequivocally to a diagnosis of mature cystic teratoma. Subsequent ultrasonography corroborated these findings, demonstrating the classic dermoid cyst appearance with a hyperechoic mural nodule and sebaceous internal contents on Doppler interrogation.

Conclusions

This case underscores the interpretive value of multimodality imaging in the evaluation of incidental pelvic lesions encountered during oncological PET/CT examinations. Mature cystic teratomas are typically metabolically inactive on FDG PET/CT; however, they demonstrate highly characteristic CT features that enable confident diagnosis even in incidental settings. The classic CT triad of macroscopic fat attenuation, tooth-like calcifications, and a well-circumscribed capsule should prompt immediate recognition of this entity, regardless of the primary oncologic indication. Importantly, PET/CT interpretation combining both metabolic and morphological features is critical to avoid false-positive or false-negative findings and to ensure appropriate patient management. Reliance on FDG uptake alone may be misleading, as both benign and malignant ovarian lesions may demonstrate overlapping metabolic patterns. From a practical reporting perspective, incidental ovarian masses detected on oncologic PET/CT should be systematically assessed for the presence of fat and calcific components on CT, as these features are highly specific for mature cystic teratoma and should be explicitly stated in the report to prevent misclassification as metastatic disease. Nuclear medicine physicians and radiologists must remain familiar with the spectrum of FDG uptake patterns and cross-sectional imaging appearances of ovarian lesions to ensure accurate interpretation and optimal patient management in daily oncologic practice.