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Rhabdomyosarcoma (RMS) is an aggressive malignancy and the most common
soft tissue sarcoma in childhood. It can arise from any part of the
body, but the commonest sites are the head-neck region (35-40%),
genito-urinary tract (20%), and limbs and trunk (20%). Two main
histological subtypes are recognized: embryonal RMS (75-80% of cases),
and the more aggressive alveolar RMS (20-30%), characterized by the
PAX3/7-FOXO1 translocation, which often presents as metastatic at onset.
At diagnosis, approximately 20% of children have distant metastases,
the most frequent sites being the lungs, bone marrow, bones, and distant
lymph nodes. The survival rate for these patients remains
unsatisfactory, and less than 30% of patients are cured (1,2).
In a few cases, patients present with disseminated disease, but no
evidence of a primary tumor mass, sometimes mimicking other cancers such
as acute lymphoblastic leukemia. These cases of RMS with unknown primary
tumor site have rarely been investigated, and only a few series and case
reports are available in the literature (9-24). A common North
American-European analysis on metastatic RMS patients included 12
patients with no evident primary tumor site, whose 3-year event-free
survival (EFS) was reportedly 8% (2).
To contribute further information on this rare condition, we report a
series of patients with metastatic RMS with unknown primary tumor site
registered in the MTS 2008 protocol coordinated by the European
pediatric Soft tissue sarcoma Study Group (EpSSG).