An Adolescent Sacroiliac Spindle Cell Tumor With PDGFB Fusion

This case report describes a diagnostically and therapeutically challenging instance of spindle cell neoplasm located in the sacroiliac region of a 16-year-old male. The patient presented with numbness in the right hip and radiating pain in the lower extremities. Diagnostic imaging and laboratory tests localized the disease to a sacroiliac tumor; however, accurate identification of the tumor type remained difficult. Following tumor excision and curettage, RNA sequencing revealed the presence of the COL1A1::PDGFB fusion, which is considered characteristic of dermatofibrosarcoma protuberans (DFSP). Although DFSP is typically a superficial, low-grade tumor with low metastatic potential, this case exhibited aggressive clinical features. Consequently, it was recommended to classify it as a "sacroiliac spindle cell tumor with COL1A1::PDGFB fusion," which may represent a variant within the broader spectrum of DFSP. Following recurrence and metastasis, the patient was treated with imatinib mesylate and radiotherapy, resulting in a sustained major partial response. This report presents a rare case of aggressive sacroiliac spindle cell sarcoma with COL1A1::PDGFB fusion, expands the clinicopathologic spectrum of DFSP and its variants, and underscores the importance of molecular studies for definitive diagnosis, as well as the potential for targeted therapies in managing complex sarcoma cases.