[False positive with Tl-201 in the study of Ewing's sarcoma of the astragalus caused by reflex sympathetic dystrophy].

We describe a 14 year old male, diagnosed of Ewing sarcoma in left astragalus. We performed two 201-Thallium scan for evaluating the response to chemotherapy (before and after the treatment). The scan performed after chemotherapy showed patchy uptake in astragalus, calcaneus and some joints of the foot. The following radiology and scintigraphy diagnosed a reflex sympathetic dystrophy in the left foot due to disuse of the limb. The 201-Thallium overestimated viability of the tumour for this reason. We must discard the presence of this kind of associated pathology in order that they could be cause of false positives.