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Microsatellite Instability-High Somatic-Type Malignancy Arising From a Primary Retroperitoneal Germ Cell Tumor: A

Gaku Hayashi1, Tomokazu Kimura1, Satoshi Inoue1

  • 1Department of Urology Nagoya University Graduate School of Medicine Nagoya City Japan.

IJU Case Reports
|August 19, 2026
PubMed
Abstract

Insights

Microsatellite instability-high (MSI-high) status was identified in a rare somatic-type malignancy component of germ cell tumors (GCTs). This finding highlights the importance of MSI testing for potential targeted therapies in GCT patients.

Area of Science:

  • Oncology
  • Genetics
  • Biomarkers

Background:

  • Immune checkpoint inhibitors show limited efficacy in germ cell tumors (GCTs).
  • Predictive biomarkers are needed to guide GCT treatment.
  • Microsatellite instability-high (MSI-high) is a potential biomarker.

Purpose of the Study:

  • To report a rare case of mixed-type GCTs with an MSI-high somatic-type malignancy (STM) component.
  • To emphasize the importance of MSI testing in GCTs.

Main Methods:

  • Case presentation of a 55-year-old man with a retroperitoneal mass.
  • Diagnosis of mixed GCT and treatment with chemotherapy.
  • Identification of STM component and subsequent MSI testing.

Main Results:

  • The somatic-type malignancy component of the mixed GCT was found to be MSI-high.
  • MSI testing of the recurrent tumor confirmed MSI-high status.

Conclusions:

  • Careful tissue selection and timely MSI testing are crucial for identifying actionable cases in GCT patients.
  • MSI-high status in the STM component of GCTs may have therapeutic implications.
Keywords:
DNA mismatch repairPembrolizumabgerm cell tumormicrosatellite instability

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