Dynamic whole-body ⁶⁸Ga-DOTATOC PET/CT

Objectives ⁶⁸Ga-DOTATOC PET/CT has established an important role for imaging neuroendocrine tumors (NET) and dynamic PET has been proposed as the preferred acquisition mode (Velikyan 2014). Metastatic NETs can be large and often extend well beyond the axial range of a single PET bed position. We assessed the feasibility of dynamic whole-body (WB) ⁶⁸Ga-DOTATOC PET/CT which involves the acquisition of dynamic PET data over an extended axial range.

Methods Our dynamic WB PET/CT protocol involved WB (eyes to mid-thigh) low-dose CT; ⁶⁸Ga-DOTATOC injection simultaneous with the start of a 6 min, single-bed, dynamic PET over the heart; immediately followed by 8 sequential WB PET scans (8 × 7 bed positions × 1.5 min / bed). 4 patients with metastatic NETs were studied. For each patient the above imaging protocol was repeated within 4 days. The second study was performed following administration of unlabeled octreotide to assess its effect on tracer distribution.

Results Cancer burden was extensive with one tumor measuring 32 cm in the axial direction. Tumor time-activity curves increased throughout the ~90 min imaging period. Tumor SUVs were generally high (SUV 10-60) and very consistent between the repeated studies on the same patient (mean absolute difference 5.7 ± 4.9 % when comparing SUV at last WB pass). Uptake in the spleen was reduced following unlabeled octreotide (mean SUV difference -39.2 ± 25.0 %), with implications for radionuclide therapy. In one patient the extended scan range enabled by the dynamic WB technique revealed unexpected metastases in the thoracic vertebrae. The initial dynamic series over the heart and negligible myocardial uptake in the WB images suggests potential for non-invasive image-derived input function determination, although this capability needs to be carefully validated.

Conclusions A dynamic WB PET/CT protocol provides valuable kinetic information over an extended scan range and is ideally suited for use with ⁶⁸Ga-labelled somatostatin analogs.