OSTEOSARCOMA AND EWINGS-SARCOMA AFTER NEOADJUVANT CHEMOTHERAPY - VALUE OF DYNAMIC MR-IMAGING IN DETECTING VIABLE TUMOR BEFORE SURGERY

OBJECTIVE. This study analyzed the value of dynamic contrast-enhanced and subtraction MR images in detecting residual viable tumor before su rgery, with emphasis on timing of enhancement, in patients with high-g rade 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, dynami c enhancement patterns on preoperative MR images were compared with co rresponding 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 spec imens. RESULTS. Early and progressively enhancing structures seen on M R images corresponded to feeding arteries, physeal vessels, or residua l viable tumor at specific preferential sites on corresponding histolo gic sections of the resected specimens. Tumor foci as small as 3-5 mm( 2) could be detected on dynamic MR images. Remnant viable tumor was of ten located subperiosteally and at the margins of the tumor, Occasiona lly, active periosteal reaction without presence of viable tumor contr ibuted to early enhancement. Late and gradually enhancing or nonenhanc ing 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 extracel lular matrices such as abundant osteoid or chondroid, Viable tumor are as with scarce formation of matrix on microscopy, such as small cell o steosarcoma areas or Ewing's sarcoma, showed early enhancement with ra pid washout of contrast agent on the dynamic MR images. CONCLUSION. Fa st dynamic contrast-enhanced sequences are essential for identifying r esidual tumor before surgery, A short time interval of less than 6 sec between arterial enhancement and tumoral enhancement strongly correla tes with presence of viable tumor. Both therapy-related alterations of tissue and tumor-related matrices must be considered when late or lac k of enhancement is noted on dynamic MR images.