Benign and malignant soft-tissue tumors: posttreatment MR imaging.

Soft-tissue sarcoma requires aggressive treatment, often with a combination of radiation therapy, chemotherapy, and surgical resection. Even after multimodality treatment, local recurrence is common, and regular follow-up imaging at short intervals is required. Interpretation of posttreatment magnetic resonance (MR) images may be complicated by changes in the surgical bed or treatment field. The challenge of distinguishing posttreatment change from recurrent tumor may be minimized by using an organized, systematic approach to imaging, with emphasis on the patient's clinical and surgical history and a review of pretreatment images. Common changes that result from radiation therapy include soft-tissue trabeculation, increased fatty marrow, and focal marrow abnormalities. Rarely, radiation-induced malignancies may develop within the treatment field. Chemotherapy also influences posttreatment imaging appearance. Occasionally, it causes a substantial increase in tumor size that is a result of chemotherapy-induced hemorrhage. Although myocutaneous flaps used in reconstructive surgery may mimic a mass, they demonstrate time-dependent changes in size, signal intensity, and enhancement on MR images. Recurrent tumor is characterized by the presence of a discrete nodule or mass with signal characteristics that typically mirror those of the original tumor. MR imaging sequences such as unenhanced T1-weighted fat-suppressed and gradient-echo sequences may help differentiate posttreatment hemorrhage from local tumor recurrence. A consistent imaging approach combined with a detailed knowledge of the patient's history, familiarity with pretreatment images, and an understanding of the various posttreatment changes enables optimal monitoring of the treatment bed and maximizes accuracy in the detection of recurrence. (c) RSNA, 2009.