Value of post-therapy 177 Lu-PSMA images for accurate interpretation of therapy response with 68 Ga-PSMA PET/CT.

A 54-year-old man with progressive prostate cancer underwent a 68 Ga-PSMA PET/CT, which showed lymph node and bone metastases. After 2-cycles of 177 Lu-PSMA therapy, the repeated 68 Ga-PSMA PET/CT showed decreased radiotracer uptake in lymph node and bones metastases, but there were new lesions which may be compatible with progression or tumour sink-effect. A review of 177 Lu-PSMA-therapy images revealed that new lesions in the second PET/CT were the metastatic lesions that progressed after the first PET/CT, and subsequently showed a good response. The patient received additional cycles of 177 Lu-PSMA therapy, and the disease regressed further, with a PSA of 0.06ng/ml. Response evaluation of new therapeutic diagnostics (theranostic) agents needs a review of not only diagnostic PET/CT images, but also post-therapy images and laboratory results.