Susan Mary Hiniker MD, Presenter: Nothing to Disclose

Andrew Quon MD, Abstract Co-Author: Nothing to Disclose

Maximilian Diehn MD, PhD, Abstract Co-Author: Research Consultant, Varian Medical Systems, Inc Research Grant, Varian Medical Systems, Inc

Billy Wiseman Loo MD, PhD, Abstract Co-Author: Research support, Varian Medical Systems, Inc Research support, RaySearch Laboratories AB

Radiation therapy (RT) is increasingly used in the treatment of thoracic malignancies. Post-RT changes are often difficult to distinguish from local recurrence by FDG-PET or CT. This is a pilot study of 3-deoxy-3-[18F]fluorothymidine (FLT)-PET/CT in patients with equivocal post-RT FDG-PET/CT, to assess the utility of FLT-PET/CT in identifying tumor recurrence.

We prospectively enrolled 10 patients who had a FDG-PET/CT scan that was indeterminate for local disease recurrence after thoracic RT and performed FLT-PET/CT for further characterization. Interpretation criteria for FLT-PET included a semi-quantitative assessment of SUVmax of the lesion and background activity in lung and mediastinum and a qualitative assessment of the focality of the suspicious lesion. The FDG-PET/CT occurring immediately prior to FLT-PET was also evaluated for maximum SUV of the suspicious lesion. Biopsy or clinical follow-up including serial diagnostic contrast CT and FDG-PET/CT imaging served as the reference and outcomes measurement.

10 patients with a median age of 70 years (51-81) underwent FLT-PET/CT. 9 patients received stereotactic ablative RT (SABR), and 1 patient received conventionally fractionated RT. To date, 5 patients have undergone biopsy and 2 patients had serial imaging that resulted in either unequivocal disease progression or resolution of the suspicious lesion. 5/7 were found to have disease recurrence. In 4/5 patients with disease recurrence, the ratio of FLT SUV in the suspicious lesion to FLT SUV in the mediastinal blood pool was > 2.0, while this was true in 0/2 patients without disease recurrence. Focality of the lesion on FLT was a specific marker for disease recurrence, as 3/3 patients with focal lesions were found to have recurrent disease on biopsy or unequivocal progression on subsequent FDG-PET scans.

Adjunctive FLT-PET/CT can complement FDG-PET/CT in distinguishing post-RT changes from disease recurrence in thoracic malignancies. A ratio >2.0 between the SUVmax of the lesion of interest and the mediastinal blood pool may be a useful criteria for diagnosing recurrence in patients treated with thoracic RT.

FLT-PET/CT imaging may differentiate local recurrence from post-treatment changes in patients treated with thoracic RT and with equivocal findings on follow-up FDG-PET/CT imaging.

Hiniker, S, Quon, A, Diehn, M, Loo, B, FLT-PET/CT Differentiates Post-radiation Therapy Changes from Local Recurrence in Thoracic Malignancies.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14019220.html