Rhabdomyosarcoma and pleomorphic sarcoma in the same location: Recurrence or new entity?

Soft tissue sarcomas (STS) represent a small group of adult solid malignancies, with risk factors such as environmental factors, genetic predisposition, and prior radiotherapy. In STS patients with a novel swelling, differential diagnoses include recurrence, second primary cancer, metastasis from unknown primary cancer, and radiation-associated STS, the latter usually occurring approximately 10 years after radiotherapy. We present the case of a 64-year-old male patient with pleomorphic rhabdomyosarcoma, who underwent resection and radiotherapy. The patient presented again 5 years later with painful swelling in the area of the prior sarcoma, raising suspicion of recurrence. Resection was performed and a diagnosis of pleomorphic sarcoma (not otherwise specified [NOS]) was made. The patient was treated with radiotherapy and remained sarcoma-free for the following 7 years. A molecular analysis of both neoplasms, using RNA next-generation sequencing, did not detect any specific fusions. Due to the lack of rhabdomyoblastic differentiation in the second sarcoma and the low likelihood of a second primary in the same previously irradiated location, the diagnosis of a radiation-associated STS was suggested. This short report illustrates the difficult diagnostic work-up of a presumably radiation-associated STS, as these neoplasms lack characteristic morphological and immunohistochemical features. In our case, the suggested diagnosis may have pointed against another course of radiotherapy in an already irradiation-harmed region. Therefore, a relatively low latency period between surgery, radiotherapy, and diagnosis of another STS should not automatically point towards recurrence and may prompt further in-depth investigation.