ESTRO 2025 - Abstract Book

S2136

Clinical - Urology

ESTRO 2025

4443

Digital Poster Evaluation of the Radiological Response in Intraprostatic Nodules of Prostate Cancer Following Radiotherapy Bennur Zeynan Devran 1 , İlknur Alsan Cetin 2 , Tunc Ones 3 , Canan Cimsit 4 1 Radiation Oncology, Diyarbakır Gazi Yasargil Training and Research Hospital, Diyarbakır, Turkey. 2 Radiation Oncology, Marmara University Faculty of Medicine, Istanbul, Turkey. 3 Nuclear Medicine, Marmara University Faculty of Medicine, Istanbul, Turkey. 4 Radiology, Marmara University Faculty of Medicine, Istanbul, Turkey Purpose/Objective: To evaluate the radiologic response in intraprostatic nodules of prostate cancer patients who underwent radiotherapy using post-treatment multiparametric MRI (mp-MR) and 68 GaPSMA-PET/CT imaging. Material/Methods: This study included 38 unfavorable intermediate and high-risk prostate cancer patients who underwent intraprostatic dose escalation radiotherapy between November 2020 and April 2024. At 3 months post radiotherapy, after the resolution of acute radiation effects, control mp-MR and 68 GaPSMA-PET/CT scans were performed to assess radiologic treatment response. Complete response on 68 GaPSMA-PET/CT was defined as the absence of PSMA or physiological uptake. For mp-MR, the absence of residual lesions causing diffusion restriction in the lesion area was classified as a complete response. If no complete response was observed at 3 months, imaging was repeated 6 months post-treatment. The Mann-Whitney U and Chi-Square tests were used to compare patients' complete and partial responses to assess whether the difference between the groups was statistically significant. Results: The mean age of the patients was 70 years (57 – 81). The median pre-treatment PSA level was 14.5 ng/mL (3.9 – 174), while the median post-radiotherapy PSA level was 0.1 ng/mL (0.01 – 31.9). Pre-treatment 68 GaPSMA-PET/CT showed a mean SUVmax of 9.6 (2.6 – 32.2), which decreased to a mean SUVmax of 2.5 (0 – 19.7) at 3 months post-radiotherapy. The mean DIL volume identified by mp-MR (GTV-MR) was 4.33 cc (1.0 – 14.6 cc), while determined by PSMA-PET (GTV PET) was 5.4 cc (1.1 – 15.0 cc). Combined imaging-defined DIL volume (GTV-DIL) mean 7.49 cc (2.3 – 22.6). At 3 months post-radiotherapy, complete response on 68 GaPSMA-PET/CT and mp-MR imaging were observed in 22 patients (57.9%) and 25 patients (65.8%). The mean tumor volume (GTV-DIL) of patients with complete response on 68 GaPSMA- PET/CT for 3 months was 4,67 cc, while it was 8,18 cc in patients’ incomplete response. Patients with complete response had significantly smaller pre-treatment intraprostatic tumor volumes (p=0.004). Post radiotherapy PSA levels were significantly lower in patients who achieved a complete response on 68 GaPSMA-PET/CT (median: 0.03 ng/mL) compared to those with an incomplete response (median: 0.3 ng/mL) (p <0.001). The presence of seminal vesicle invasion was significantly associated with lower radiologic complete response rates (p =0.02). Conclusion: We found that smaller pre-treatment tumor volumes and lower post-treatment PSA levels are associated with higher complete response rates. Conversely, the seminal vesicle invasion was significantly linked to poorer radiologic outcomes. Additionally, 68 Ga PSMA-PET/CT and mp-MRI are effective tools in evaluating the response to radiological treatment in intraprostatic nodules after radiotherapy.

Keywords: prostat cancer, DIL, radiological response