METHODSĀ :
A radiological work-up was ordered, which revealed heterogeneous bone lysis in the coccyx on the standard X-ray (figure 1). The CT scan of the pelvis showed an expansive, poorly defined osteolytic lesion of the sacrum with predominantly right lateral extension and invasion of the gluteus maximus and medius (figure 2). The MRI revealed an aggressive sacrococcygeal tumour mass infiltrating and invading the right gluteus maximus (figure 3). The initial clinical and radiological findings indicated a primary or metastatic malignant tumour. A negative extension work-up was conducted, which included a thoraco-abdominal CT scan, cervical ultrasound, mammography, X-ray of the cervical spine (chordoma), bone scan, specialist gynaecological examination, and tuberculosis work-up. The patient underwent a posterior sacral biopsy, which revealed a multivesicular cystic lesion consistent with a hydatid cyst of the sacrum(figure4), subsequently confirmed by anatomopathological examination. The lesion was almost completely excised and the cystic bed was sterilised using hypertonic serum and hydrogen peroxide, with the installation of a drainage system.
CONCLUSION AND RESULTSĀ  :
The immediate postoperative period was uneventful, with resolution of the signs of compression.
At the 10-month follow-up, the patient showed no signs of recurrence, and the cystic bed had progressively ossified.