PT - JOURNAL ARTICLE AU - Irene Burger AU - Ferraro Daniela AU - Jan Ruschoff AU - Michael Messerli AU - Urs Jacob Muehlematter AU - Benedikt Kranzbuehler AU - Lars Husmann AU - Thomas Hermanns AU - Daniel Eberli AU - Niels Rupp TI - <strong>Immunohistochemistry on the primary tumor for PSMA-expression might predict the detection of biochemical recurrence with <sup>68</sup>Ga-PSMA-11-PET. </strong> DP - 2020 May 01 TA - Journal of Nuclear Medicine PG - 1266--1266 VI - 61 IP - supplement 1 4099 - http://jnm.snmjournals.org/content/61/supplement_1/1266.short 4100 - http://jnm.snmjournals.org/content/61/supplement_1/1266.full SO - J Nucl Med2020 May 01; 61 AB - 1266Objectives: Prostate-specific membrane antigen (PSMA) targeted PET has a high detection rate for biochemical recurrence (BCR) of prostate cancer (PCa). Nevertheless, even at high PSA levels (&gt; 3 ng/ml), only 90% of the PSMA-PET scans are positive, probably due to patients with a PSMA-negative PCa. It was the aim to assess whether PSMA-expression of the primary tumor on immunohistochemistry (IHC) can predict the PSMA-PET positivity for BCR. Methods: Retrospective, ethically approved, single center analysis of patients that underwent 68Ga-PSMA-11-PET for BCR after RPE between April 2016 and December 2018, with tumor specimens available for PSMA-IHC. Clinical information (age, PSA-level, previous therapy, Gleason score) and PSMA-IHC of the primary tumor and if available lymph node metastasis (LNM) were collected and correlated with results from PSMA-PET scans (positive/negative), using a multivariate logistic regression analysis. Results: 120 PSMA-PET scans in 74 patients were available for this analysis. The overall detection rate was 62% (74 in 120 scans), with a mean PSA value at scan time of 1 ng/ml (range 0.05-406 ng/ml). From the clinical factors, only PSA-level and status after antiandrogen therapy (ADT) correlated with PSMA-PET positivity. Percentage PSMA-negative tumor area on IHC (PSMA%neg) had a significant inverse correlation to PSMA-PET positivity (p&lt; 0.001), while membranous PSMA-expression did not correlate (p=0.73). The specificity of PSMA%neg above 50% for a negative PSMA-PET was 97% (72/74) and for a PSMA%neg of 80% or more, 100% (74/74). PSMA-expression was more homogeneous in LNM than in the primary tumor specimens, but a strong positive correlation was found between primary tumor and nodes for both PSMA%neg (0.841, p&lt;0.001) and PSMAmemb (0.446, p=0.002). Limitations include retrospective design and the low number of negative PSMA-PET scans. Conclusions: PSMA-negative tumor area was the best predictor of negative PSMA-PET scans, beside PSA-level and ADT. Even at very high PSA levels, PSMA-PET scans were negative in most of the patients with PSMA%neg above 50%.