Predictive Value of Ga68-PSMA PETCT-Based Response to Neoadjuvant Androgen Deprivation Therapy in Node Positive Prostate Cancer Treated with Radical Radiotherapy.

To evaluate the utility of Ga68-PSMA PETCT based response to neoadjuvant androgen deprivation therapy (ADT) as a predictive biomarker in patients with node positive prostate cancer treated with definitive radiotherapy Patients with newly diagnosed non-metastatic adenocarcinoma prostate with regional nodal involvement on index Ga68-PSMA PETCT scan were identified from prospectively maintained institutional database. All patients were started on neoadjuvant ADT and treated with definitive image-guided conformal radiotherapy with long term ADT. Eligible patients also had a response assessment Ga68-PSMA PETCT scan prior to radiotherapy. Patients who had a single PSMA PETCT scan, or who received prior or concurrent abiraterone ordocetaxel were excluded. Imaging parameters including maximum standardized uptake values (SUV max), PSMA tumor volume (PSMA-TV), and total lesion PSMA (TL-PSMA) were calculated separately for prostate and pelvic nodes in the paired Ga68-PSMA PETCT scans done pre-ADT and pre-radiotherapy. Absolute and relative response for these parameters was calculated. Biochemical failure (BCF) was defined by the Phoenix criteria and its association with absolute and relative response in Ga68-PSMA PETCT parameters was tested using the logistic regression analysis. Total 93 patients were eligible for analysis. Median duration of neoadjuvant ADT was 6 months. Response observed in the paired Ga68-PSMA PETCT scans is detailed in Table 1. Complete metabolic response was observed in 62% patients for pelvic nodes and 7% for prostate. Partial metabolic response was observed in 23% for pelvic nodes and 55% for prostate, while stable disease was seen in 11% and 18% for pelvic nodes and prostate respectively. Progressive disease was observed in 4% for pelvic nodes and 14% for prostate. At a median follow up of 37 months, 15 patients (16%) experienced BCF. Median absolute and relative decline in SUV max for pelvic nodes was 8.29 (IQR:3.5-20.3) and 100% (IQR:61.3-100) respectively. Both the absolute and the relative decline in SUV max for pelvic nodes showed statistically significant association with BCF post treatment using logistic regression (p=0.05 each). No significant association was observed for other volumetric parameters. A steeper decline in absolute and relative nodal SUV max on Ga68-PSMA PETCT with neoadjuvant ADT is associated with lower probability of BCF post radiotherapy in node positive prostate cancer. [ABSTRACT FROM AUTHOR]