Changes in MR signal intensity and contrast enhancement of therapeutically irradiated soft tissue

Increased MR signal intensity was observed on T₂-weighted, STIR, and Gadolinium-DTPA-enhanced T₁-weighted images of subcutaneous and muscular soft tissue in 9 of 10 children treated with combination chemotheraphy and radiation therapy (RT) for malignancy in the pelvis or an extremity. Total radiation doses ranged from 59.5 to 65 Gy. Eight of the patients with these changes received hyperfractionated RT (seven for Ewing sarcoma and one for perineal rhabdomyosarcoma); one was treated for pelvic hemangiopericytoma with once-daily fractions. Evidence of soft tissue damage became apparent as early as the sixth week of RT and was seen for up to 69 wk post-RT. There was no clear MR evidence of RT-induced soft tissue damage in one patient, who underwent hyperfractionated RT for pelvic rhabdomyosarcoma. Other MR findings in this group included evidence of bladder wall thickening in three of the seven patients given pelvic RT and increased T₁-weighted signal of irradiated marrow in nine patients. All patients had clinical evidence of skin, soft tissue, or epithelial radiation effects. Increased MR signal intensity secondary to RT-induced damage can be differentiated from widespread tumor by geometric borders that conform to the margins of the radiation field.