ESTRO 2025 - Abstract Book

S1939

Clinical - Urology

ESTRO 2025

discrete lesion within the prostatic fossa was considered the endpoint of the present study. The proportion of patients with positive findings in the two paired groups was compared with the McNemar test, while agreement with Cohen’s k. The planned sample size was 107 patients. Results: One hundred seven consecutive patients were restaged per protocol from September 2022 to September 2024. The mean PSA values at failure and at restaging were 0.29 ng/ml and 0.53 ng/ml, respectively. Overall, 68 patients (63.5%, 95%CI: 54.1-72.1%) were found to harbor at least one lesion in the prostatic fossa; 24 (35.3%), 4 (5.9%) and 40 (58.8%) were detected only by mpMRI, only by PSMA PET/CT and by both tests, respectively. The percentage of discordant pairs was higher than expected (28/68, 41.2%) and significantly different (p<0.001). The Cohen’s agreement between the two imaging modalities was 0.49, p<0.001. After stratifying by mean PSA at restaging (≤0.53 vs >0.53) and median time between surgery and biochemical failure (<24 vs ≥24 months) mpMRI outperformed PSMA PET/CT in the subgroup with lower PSA and in the subgroup with longer time interval respectively (Figure 1).

Conclusion: mpMRI and PSMA PET/CT show inadequate agreement for the detection of local recurrence at the time of biochemical failure after prostatectomy, with the former detecting a significantly higher number of local lesions than the latter. Multiparametric MRI should be considered the preferred diagnostic modality to local restaging particularly at PSA levels consistent with early salvage RT and 2 or more years after surgery.

Keywords: prostate cancer, SRT, local staging

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Digital Poster Prostate cancer focal boost versus no boost in 20 fraction external beam radiotherapy: a prospective cohort on dosimetry, toxicity and quality of life Arjen van der Veen 1 , Tariq A Lalmahomed 1 , Hans C J de Boer 1 , Gijsbert H Bol 1 , Louk M W Snoeren 1 , Jannette J Bossenga 1 , Wietse S C Eppinga 1 , Corine A van Es 1 , Juus L Noteboom 1 , Sandrine M G van de Pol 1 , Helena M Verkooijen 1,2 , Harm H E van Melick 3 , Jochem R N van der Voort van Zyp 1 1 Radiation Oncology, University Medical Center Utrecht, Utrecht, Netherlands. 2 Division Imaging, Netherlands Cancer Institute, Amsterdam, Netherlands. 3 Urology, St. Antonius Hospital, Utrecht, Netherlands