Objective: This study analyzed the value of dynamic contrast-enhanced and subtraction MR images in detecting residual viable tumor before surgery, with emphasis on timing of enhancement, in patients with high-grade osteosarcoma and Ewing's sarcoma after neoadjuvant chemotherapy.
Subjects and methods: Twenty-one patients with proved osteosarcoma or Ewing's sarcoma were treated with neoadjuvant chemotherapy followed by surgery. After IV administration of gadopentetate dimeglumine, dynamic enhancement patterns on preoperative MR images were compared with corresponding areas on histologic sections of the resected specimens. On dynamic subtraction images obtained with high temporal resolution (1.5-3 sec), the interval between arrival of the bolus of contrast agent in an artery and start of tumoral enhancement was used to distinguish residual viable tumor. Early enhancing foci (interval artery-tumor < 6 sec) and late or nonenhancing areas seen on MR images were correlated with the histopathologic findings in these areas of the resected specimens.
Results: Early and progressively enhancing structures seen on MR images corresponded to feeding arteries, physeal vessels, or residual viable tumor at specific preferential sites on corresponding histologic sections of the resected specimens. Tumor foci as small as 3-5 mm2 could be detected on dynamic MR images. Remnant viable tumor was often located subperiosteally and at the margins of the tumor. Occasionally, active periosteal reaction without presence of viable tumor contributed to early enhancement. Late and gradually enhancing or nonenhancing areas corresponded histopathologically to regions of chemotherapy-induced necrosis, mucomyxoid degeneration, or fibrosis. Alternatively, late or nonenhancing areas were associated with reactive changes such as edema, hemorrhage, or osteomyelitis or with tumor-related extracellular matrices such as abundant osteoid or chondroid. Viable tumor areas with scarce formation of matrix on microscopy, such as small cell osteosarcoma areas or Ewing's sarcoma, showed early enhancement with rapid washout of contrast agent on the dynamic MR images.
Conclusion: Fast dynamic contrast-enhanced sequences are essential for identifying residual tumor before surgery. A short time interval of less than 6 sec between arterial enhancement and tumoral enhancement strongly correlates with presence of viable tumor. Both therapy-related alterations of tissue and tumor-related matrices must be considered when late or lack of enhancement is noted on dynamic MR images.