A seventeen-year-old male presented in rural Montana with a newly appearing left neck mass causing progressive discomfort and airway compression. Magnetic resonance imaging (MRI) showed a left lower neck multi-cystic mass extending into the supraclavicular and subcarinal regions of the mediastinum, consistent with macrocystic lymphatic malformation. Local Otolaryngology attempted aspiration, but insufficient fluid was obtained for relief, prompting transfer out of state for multidisciplinary vascular anomaly center (VAC) care. Past medical, family, and psychosocial history were non-contributory, with no known genetic syndromes or prior malignancy.

VAC evaluation confirmed left cervical cystic masses, the largest measuring 7 × 7 cm. The lesion was non-tender but firm, raising concern for intralesional hemorrhage. Additional history revealed a 40–pound weight loss, emesis, and early satiety over the preceding months. On exam, the abdomen was markedly distended with a palpable mass in the left upper and lower quadrants. Full-body MRI/MRA revealed a large multiloculated cystic abdominal mass suggestive of additional lymphatic malformation sites (Figure 1A). The differential diagnosis included complex lymphatic anomaly, venous-lymphatic malformation, and germ cell tumor, with evolving clinical and procedural findings progressively arguing against a vascular etiology.

Interventional Radiology was consulted for aspiration and possible sclerotherapy. Abdominal fluid was brown-tinged, and cervical fluid was clear, both atypical for lymph and chyle (Figure 1B). Sclerotherapy was initially attempted on the cervical lesion; however, fluid analysis (triglyceride 13 mg/dl; lymphocytes 2%) was not consistent with lymphatic fluid, so further attempts were discontinued.

Multidisciplinary consensus led to surgical resection of the abdominal lesion. A 12-h operation resulted in removal of 95% of a 25 × 25 × 20 cm retroperitoneal mass. The mass had both solid and cystic components and encased critical structures, including the aorta, inferior vena cava, kidneys, ureters, and iliac vessels (Figure 2A). Pathology revealed a mixed germ cell tumor, approximately 98% immature teratoma (Figures 2B, E), with additional mature teratomatous elements including squamous, neural, and gastrointestinal-type epithelium (Figures 2C, D). Foci of embryonal carcinoma were identified (Figure 2F), along with areas consistent with yolk sac tumor (Figure 2G). Tumor markers were elevated: AFP 1,240 ng/ml (ref 0–8), HCG 268 mIU/ml (ref 0–5), confirming the diagnosis. No major diagnostic access barriers were reported.

Following pathologic confirmation and tumor marker elevation, the patient was transitioned to oncology care and initiated on standard Bleomycin, Etoposide, and Cisplatin (BEP) chemotherapy, without dose modification or delay.

The patient was followed clinically and radiographically following initiation of chemotherapy. Follow-up imaging demonstrated regression of residual disease, and tumor markers declined appropriately. Treatment was well tolerated without significant toxicity, dose delays, or interruptions. Adherence was supported through shared decision-making, frequent outpatient follow-up, and active patient engagement in treatment planning.

Schematic diagram showing the diagnostic and treatment timeline from symptom onset to initiation of chemotherapy is showed in Figure 3.

This case highlights the strength of multidisciplinary collaboration, where clinical reasoning, surgical insight, and reevaluation of imaging enabled diagnosis. However, a key limitation was the initial reliance on typical pathways without questioning atypical features, which delayed definitive treatment.

A limited number of reports describe teratomas mimicking vascular anomalies, but few address diagnostic delay related to cognitive bias. Unlike prior cases diagnosed early through surgery, this case illustrates how diagnostic mimicry and specialty silos can delay recognition without active re-evaluation. Diagnostic errors account for approximately 75% of malpractice lawsuits and 38% of total payouts (1), with cognitive biases, particularly confirmation bias, being major contributors. Pediatricians report diagnostic errors affecting patient outcomes at least annually (2–5), and misdiagnosis remains a leading cause of avoidable harm and cost (1, 4, 6, 7).

Lymphatic malformations, typically diagnosed through history, examination, and imaging (4), may demonstrate nonspecific radiologic features and be confused with teratomas (8, 9). Key diagnostic mimickers include complex lymphatic anomalies and congenital cystic neck masses, particularly when cystic morphology is interpreted without integrating systemic symptoms or treatment response. These entities require distinct management strategies, underscoring the importance of accurate differentiation (10–12).

In this case, atypical aspirate characteristics, systemic symptoms, and failure to respond to sclerotherapy prompted diagnostic re-evaluation. Definitive diagnosis of a mixed germ cell tumor was achieved only after surgical intervention, emphasizing the necessity of reassessing assumptions in light of evolving clinical and procedural data.

Anchoring and confirmation bias (Table 1) played key roles. Anchoring refers to over- reliance on initial impressions, while confirmation bias involves selectively interpreting data to support those impressions. Both can coexist and compound diagnostic error (1, 13). Mitigation requires pausing premature closure, actively seeking contradictory evidence, and continuously re-evaluating data. Radiologists and clinicians alike must challenge early assumptions. When a diagnosis fails to fully explain the clinical course, it's essential to ask: What else could this be? What doesn't fit? (1, 13).

Metacognition and cognitive forcing strategies can support more objective decision-making. Avoiding the pseudo-diagnostic effect, relying on irrelevant data to support a diagnosis, is vital. Clinicians should not hesitate to revisit or revise an initial diagnosis if clinical evolution no longer fits. Helpful reflection questions include: What parts of this picture are inconsistent? Have I overlooked alternatives? (10).

Enhancing clinical reasoning means balancing intuitive and analytical thinking (Table 1) while remaining vigilant for bias. Physicians face barriers to consistent analytical reasoning, such as time pressures, sleep deprivation (10), and cognitive overload. Medical education should emphasize debiasing strategies, guided reflection, and structured re- evaluation. Cultivating open discussion, a questioning mindset, and adopting technology such as clinical decision support systems or AI tools can help minimize diagnostic error and improve care (14).