Atypical spindle cell/pleomorphic lipomatous tumor with sarcomatous transformation during recurrence.

Atypical spindle cell/pleomorphic lipomatous tumor (ASPLT) is a recently defined adipocytic neoplasm with broad morphological variability. While generally benign, a small subset of ASPLT can demonstrate locally aggressive behavior with recurrences after incomplete resection. There is only limited description of the radiologic features of ASPLT in the literature. This case report presents a unique longitudinal case of a 75-year-old woman with an ASPLT in the subcutaneous tissue of the left lower leg. Following two incomplete resections, the tumor recurred twice over 38 months, each time with increasing volume and growth rate. The second recurrence demonstrated abrupt histological transition from low-grade ASPLT to high-grade undifferentiated sarcoma, with underlying RB1 deletion, an additional NF1 gene mutation, and a complex genomic profile. The tumors demonstrated low to intermediate signal intensity on T1-weighted imaging and heterogeneous enhancement on T1-weighted fat-saturated images after contrast agent injection. Dynamic contrast-enhanced (DCE)-MRI revealed heterogeneous perfusion patterns correlating to sarcomatous regions. Elevated (semi-)quantitative DCE-MRI parameters, reflecting increased vascularization, perfusion, and high capillary permeability, were observed in histologically high-grade areas compared with low-grade regions. To the best of our knowledge, this is the first in-depth clinicoradiological study documenting transformation of an ASPLT into a high-grade undifferentiated sarcoma following incomplete surgery. This case also emphasizes the diagnostic value of DCE-MRI in identifying biologically aggressive (high-grade) regions within ASPLTs undergoing sarcomatous transformation.