Rhabdomyosarcoma, presenting as disseminated malignancy from an unknown primary site: a retrospective study of ten pediatric cases.

From 1962-1984, ten children were referred to St. Jude Children's Hospital with a metastatic poorly differentiated malignancy; extensive diagnostic workup had failed to disclose the site of the primary tumor. Multiple skeletal metastases as well as bone marrow involvement were common findings. Erythrocytes were detected in the cytoplasm of tumor cells in several cases, and cytochemical stains confirmed that these phagocytic cells did not have features of mononuclear phagocytes. Establishing a pathologic diagnosis in these cases was difficult, and most special studies including cytochemistry and electron microscopy were not helpful in elucidating the diagnosis. A diagnosis of rhabdomyosarcoma was made at presentation in six cases. In the remaining cases, the diagnosis of rhabdomyosarcoma was subsequently made after rebiopsy of new tumor masses during the course of the illness, by ultrastructural examination of a cell line derived from the tumor or at postmortem examination. Based on initial symptoms, clinical features, and postmortem findings, the primary tumor sites were assumed to be in the middle ear, paravertebral area, base of skull, retrobulbar space, chest wall, and retropancreatic area. In four patients the disease was confined to bone marrow, lymph nodes, and meninges so that a primary site could not be assigned. The approach to pediatric patients presenting with disseminated malignancy from an occult primary site should consist of an aggressive pursuit of a specific diagnosis and establishment of a primary site to better direct therapy, particularly for those children whose tumors may be responsive to specific therapy.