Impact of Ga-PSMA-PET imaging on target volume definition and guidelines in radiation oncology - a patterns of failure analysis in patients with primary diagnosis of prostate cancer.

Background: Ga-PSMA-PET-imaging has proven to be a highly sensitive and specific diagnostic element for patients with prostate cancer (PC). Does the standard clinical target volume (CTV) cover the majority of Ga-PSMA-PET detected lymph nodes (LNs) in a primary setting?

Methods: 25 out of 159 patients with primary PC who underwent Ga-PSMA-PET-imaging were analyzed in the process of this study. These 25 high-risk patients had a total of 126 LNs with positive Ga-PSMA-ligand uptake. A standard CTV according to the 'Radiation Therapy Oncology Group' consensus was delineated and LNs were judged whether they were in- or outside of this target volume. With a Pearson correlation we additionally evaluated whether the Gleason score, the prostate-specific antigen (PSA) value or the risk according to the Roach formula correlate with a higher chance of LNs being outside of the CTV in uncommon LN locations.

Results: 81 (64.3%) of 126 LNs were covered by the CTV with a complete coverage of all positive LNs inside the respective radiation volume in 11 of 25 patients (44%). LNs that were not covered by the CTV included (para-aortic,) common-iliac, pre-sacral, obturatoric, para-rectal, para-vesical and pre-acetabular locations. In a statistical analysis neither the Gleason score, nor the PSA value, nor the calculated risk with the Roach formula correlated with LNs being inside or outside of the CTV in this patient group.

Conclusion: Ga-PSMA-PET-imaging proves to be a valuable asset for patients and physicians for primary diagnosis and treatment planning. In our study, trusting the RTOG consensus for CTV delineation would have led to up to 35.7% of all LNs not to be included in the clinical radiation volume, which might have resulted in insufficient radiation dose coverage.