About 1 in 20 uterine leiomyosarcomas were mismatch repair deficient in a large sequencing database

Design

Retrospective single-institution clinicopathological and molecular series

Population

39 MMR-deficient sarcomas; 1,531 sarcomas in the sequencing database

Primary outcome

Prevalence of MMR deficiency by sarcoma type; histology and outcome

Effect

1.2% overall; uterine leiomyosarcoma 5.6%; undifferentiated pleomorphic 2.3%; 8 index cancers in Lynch syndrome

A US group studied 39 sarcomas with mismatch repair deficiency, identified by sequencing and confirmed by immunohistochemistry. In their sequencing database of 1,531 sarcomas, 1.2% were deficient overall. Rates were higher in uterine leiomyosarcoma (7 of 124, 5.6%), undifferentiated or unclassified pleomorphic sarcoma (6 of 259, 2.3%) and pleomorphic rhabdomyosarcoma (1 of 5). Staining 20 further cases of each confirmed deficiency in all three groups. Eight sarcomas were index neoplasms among the 15 patients with documented Lynch syndrome.

Two patterns emerged among the unclassified tumours. A lobulated, heavily inflamed pattern had a good outlook, with all six followed patients free of disease and two complete responses to checkpoint inhibitors. An epithelioid to rhabdoid pattern did poorly.

The authors propose routine MMR immunohistochemistry for pleomorphic rhabdomyosarcoma, uterine leiomyosarcoma, unclassified or undifferentiated pleomorphic sarcoma, and any sarcoma in a patient with Lynch syndrome or a family history of it. This is a retrospective series from one institution and the proposal is not yet a guideline, but the test is cheap, widely available and has consequences for both treatment and family screening. It was published in June 2026.

• Consider MMR immunohistochemistry on uterine leiomyosarcoma, pleomorphic rhabdomyosarcoma and undifferentiated or unclassified pleomorphic sarcoma.
• Consider MMR staining for sarcomas in patients with a personal or family Lynch history, as the authors propose.
• Report MMR loss so oncology can consider checkpoint inhibitor eligibility.
• Recommend genetics referral where the loss pattern suggests a germline cause.
• Note a lobulated, inflammation-rich pattern in unclassified sarcoma as a prompt to stain.

Why it matters

A cheap stain can uncover Lynch syndrome and open a route to immunotherapy in sarcomas.

Don't overread it

This is a single-institution retrospective series; routine sarcoma MMR testing is the authors' proposal, not a guideline.

The statistics, in plain English

Rates come from small subgroups, for example 7 of 124 uterine leiomyosarcomas, so the true proportion could be somewhat higher or lower. The pleomorphic rhabdomyosarcoma figure rests on only 5 sequenced cases plus 20 stained. Outcome observations in a few patients are descriptive, not comparative.

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