Role of FDG PET/CT in rare bone tumor: Chordoma

Introduction: Chordoma is an extremely rare bone tumor with an incidence rate of 0.3-0.8 per million population. It is the most common primary bone tumor involving the spine. Most common site of involvement is sacrum-coccyx followed by skull base and mobile spine. Although it’s a slow growing tumor, poor prognosis associated with it may be attributed to local invasion and risk of local recurrence (68%). Metastasis is a rare occurrence, noted in only 3-48% cases, with most common site being lungs. Few solitary case reports have documented the potential role of F-18 FDG PET/CT in Chordoma. We aimed at exploring the use of FDG PET/CT in staging, response assessment and surveillance of chordoma.

Methods: This was an ambispective study, which included patients with chordoma who underwent FDG PET/CT in our institute since January 2019. Prospectively recruited patients of chordoma in our oncology unit, were further screened and referred for PET/CT under three indications: staging, response assessment and surveillance post treatment completion. F18 FDG PET/CTs were acquired 60-80minutes after intravenous injection of 8-10mCi of F-18 FDG. PET/CT scan was acquired from vertex to mid-thigh. All scans were reported by two nuclear physicians independently. Scan findings were corroborated with clinical and radiological findings wherever necessary.

Results: Five patients and 11 FDG PET/CT scans of chordoma were included in the present study. Mean age of 38.8 (±7.8) years and strong male (4:1) preponderance was noted. All included cases were of sacro-coccygeal chordoma. One of the cases was referred for baseline staging and two each for response monitoring and post treatment surveillance. Local residual or recurrent disease was present in all 11 studies. Metastatic disease was found in all but one (4/5) cases. Most commonly seen sites were lung (2/4) and bone (2/4), followed by occasional lymph nodes, peritoneal deposits and meningeal metastases. Six scans were available for response assessment, two of which revealed disease progression and the remaining four showed stable disease. FDG PET/CT was responsible for change in line of management in one case. Both the post surveillance scans revealed local recurrence and one showed meningeal deposits as well.

Conclusions: FDG PET/CT may find appropriate use in the staging, response and surveillance of rare tumor chordoma. Despite of lack of much supporting literature and limited sample size, this study indicates that FDG PET/CT can efficiently detect metastatic disease, occult residual/ recurrent local disease and guide management in chordoma.