Background and Clinical Significance: Rhabdomyosarcoma (RMS) is a rare and aggressive malignant soft-tissue sarcoma (STS) arising from skeletal connective tissues and is most commonly seen in the pediatric population. The pleomorphic subtype is mostly seen in adults in the sixth and seventh decades of life, representing 1% of all histological types of RMS and having a very poor prognosis. Case Presentation: This report presents the case of a 63-year-old male with a medical history of papillary thyroid cancer, who presented with an ulcer-hemorrhagic malignant tumor, namely, a poorly differentiated desmin-positive pleomorphic rhabdomyosarcoma (PRMS), with impressive dimensions located on the posterior thoracic wall. This tumor was surgically removed via a wide resection, followed by palliative chemotherapy and radiotherapy. However, the patient relapsed locally, with pulmonary, bone, and lymph node metastases. The peculiarity of this case is represented by the rapid growth, aggressive nature, and high metastatic potential of the adult RMS, as well as its poor response to treatment. Conclusions: The presented case underscores the need for early diagnosis, multidisciplinary management, and exploration of molecular profiling for therapeutic planning.

Rhabdomyosarcoma (RMS) is a rare and aggressive malignant soft-tissue sarcoma (STS) arising from skeletal connective tissues, most commonly seen in the pediatric population.The pleomorphic subtype is mostly seen in adults in the sixth and seventh decades of life, representing 1% of all histological types of RMS, having a very poor prognosis. This report presents the case of a 63-year-old male with medical history of papillary thyroid cancer, who presented an ulcer-hemorrhagic malignant tumor, a poorly differentiated desmin-positive pleomorphic rhabdomyosarcoma (PRMS) with impressive dimensions, located on the posterior thoracic wall, which was surgically removed while performing a wide resection, followed by palliative chemotherapy and radiotherapy, relapsed locally, with pulmonary, bone, and lymph node metastases. The peculiarity of this case is represented by the faster-growing feature, aggressive nature and high metastate potential as well as the poor treatment response of the adult RMS. It underscore the need for early diagnosis, multidisciplinary management and exploration of molecular profiling in therapeutic planning.