Clinical feasibility of early scanning after administration of ⁶⁸Ga-DOTATOC

Objectives: PET/CT with ⁶⁸Ga-DOTATOC has been widely accepted as somatostatin receptor imaging. The scanning is usually performed at 1-h post-injection; however, due to the expected higher tumor-to-background ratio by rapid clearance of blood activity, the waiting time for scanning after administration of DOTATOC might be shortened without affecting diagnostic performance. The purpose of this study was to investigate the feasibility of early scanning at 30-min post-injection.

Methods: Thirty-eight patients (M:F=18:20, age range 31 - 78, mean 59.2 y.o.) who underwent DOTATOC-PET/CT between April 2016 and December 2016 were analyzed. After administration of approximately 140 MBq of ⁶⁸Ga -DOTATOC, data acquisition was performed twice at 30-min post-injection and at 60-min post-injection. For both image datasets, we compared the number of known or suspected pathological lesions, quantitative values of those lesions, and quantitative values of physiological uptake in pituitary gland, liver, spleen, adrenal glands, and pancreatic uncus. As quantitative values in pathological lesions, SUVmax, SUVpeak, metabolic tumor volume with a threshold of 40% of SUVmax (MTV), and total lesion uptake (TLU) were calculated. When a patient had more than 10 lesions in one organ, 10 lesions were assessed.

Results: There were 125 known or suspected pathological lesions (17 primary tumors, 22 liver metastases, 39 liver metastases, 5 lung metastases, 37 bone metastases, and 5 other lesions) in each dataset, resulting in no difference of the number of findings between the two datasets. The SUVmax, SUVpeak, MTV, and TLU were highly reproducible between the two datasets, with Spearman's rho of 0.983, 0.986, 0.918, and 0.981, respectively. The average of percent difference (%DIFFave) defines as difference of the values divided by value at 1-h post-injection was 11.1% in SUVmax, 6.6% in SUVpeak, 13.1% in MTV, and 20.8% in TLU. As for the physiological uptake, the same reproducibility was observed between the two datasets in pituitary gland (Spearman's rho=0.954, %DIFFave=11.9%), liver (0.989, 4.1%), spleen (0.970, 6.4%), adrenal glands (0.879, 13.6%), and pancreatic uncus (0.946, 13.7%).

Conclusion: Our preliminary data indicate that comparable images were obtained at 30-min post-injection, compared to those at 1-h post-injection, resulting in no difference of detection rate of lesions. We need to keep in mind that there should be some differences of quantitative values, although they are minimum between 30-min post-injection and 1-h post-injection in DOTATOC-PET/CT. Research Support: A Grant-in-Aid for Scientific Reseach from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (16K10346).

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