TY - JOUR T1 - Response assessment to treatment with Lu-177 labeled PSMA inhibitor in patients with metastatic castration-resistant prostate cancer: differential response of bone versus lymph node lesions JF - Journal of Nuclear Medicine JO - J Nucl Med SP - 1547 LP - 1547 VL - 57 IS - supplement 2 AU - Harshad Kulkarni AU - AVIRAL SINGH AU - Richard Baum Y1 - 2016/05/01 UR - http://jnm.snmjournals.org/content/57/supplement_2/1547.abstract N2 - 1547Objectives The aim of this study was to assess the response of bone and lymph node lesions on Ga-68 PSMA PET/CT after prostate specific membrane antigen (PSMA) targeted radioligand therapy (PRLT).Methods Ninety-nine patients with progressive metastatic castration-resistant prostate cancer (mCRPC) underwent PRLT using Lu-177 labeled PSMA inhibitor. A total of 61 skeletal and 73 lymph node metastases, also measurable on CT, were included in the analysis. Response after at least 2 cycles of PRLT was determined applying molecular (EORTC) and morphological (RECIST) imaging criteria, using Ga-68 PSMA PET/CT.Results Molecular response (reduction in the SUV of the target lesion on PET/CT by > 25%) after 2 or more PRLT administrations was observed in 32/61 (52.4%) bone lesions and 48/73 (65.7%) lymph node lesions, whereas progression (increase of SUV by > 25%) was noted in 5/61 (8.2%) bone and 10/73 (13.7%) lymph node lesions. On the other hand, CT detected decrease in size in only 4/61 (6.6%) skeletal metastases and 29/73 (39.7%) lymph node metastases. All lesions responding on CT revealed a significant decrease in SUV. There was a progression on CT in 10/73 (13.7%) lymph nodes, all of which showed concordant progression on PET/CT. Interestingly, there was an increase in size on CT of 2 osteoblastic lesions in a patient who had a molecular and biochemical complete remission (Ga-68 PSMA PET negative and PSA undetectable, respectively) after 2 PRLT cycles.Conclusions Lymph node metastases of mCRPC respond better to PRLT than bone metastases. This may be explained by a higher and more uniform absorbed radiation dose by lymph node metastases as compared to bone lesions, probably due to attenuation of radiation in bone lesions due to the normal surrounding bone tissue. In addition, the biological differences in radiation sensitivity must be considered. Ga-68 PSMA PET/CT is superior in response assessment of skeletal metastases compared to CT alone, in which the actual size of the osteoblastic metastases is difficult to measure and change in size difficult to appreciate. The paradoxical increase in size of the osteoblastic lesions in a patient with complete remission was most probably due to osteosclerosis after therapy (flare phenomenon). Even in lymph node metastases, Ga-68 PSMA PET/CT detects response at an earlier stage than CT (i.e. molecular response precedes morphological changes). ER -