ESTRO 2023 - Abstract Book

S1193

Digital Posters

ESTRO 2023

Observed results confirmed efficacy and safety of post-prostatectomy hypofractionated radiotherapy in 16 daily fractions. Adjuvant and early salvage radiotherapy indication was an independent predictor of favorable long-term biochemical control.

PO-1473 PSMA PET imaging of non-acinar histological variants of prostate cancer

C. Hassanzadeh 1 , G. Ravizzini 2 , B. Chapin 3 , A. Aparicio 4 , T. Bathala 5 , D.S. Surasi 2 , C. Tang 1

1 The University of Texas MD Anderson, Radiation Oncology, Houston, USA; 2 The University of Texas MD Anderson, Nuclear Medicine, Houston, USA; 3 The University of Texas MD Anderson, Urology, Houston, USA; 4 The University of Texas MD Anderson, Medical Oncology, Houston, USA; 5 The University of Texas MD Anderson, Diagnostic Imaging, Houston, USA Purpose or Objective Prostate-specific membrane antigen-targeted positron emission tomography (PSMA-PET) has become widely adopted in the settings of biochemically recurrent and metastatic prostate cancer. However, roughly 20% of patients with biochemical recurrence have PSMA-PET negative disease. Limited data exists to identify characteristics that predispose to PSMA occult prostate cancer. Non-acinar histological variants such as ductal adenocarcinoma (DAC) and neuroendocrine prostate cancer (NEPC) are associated with poorer outcomes and may remain undetectable using PSMA diagnostic imaging. Conflicting case reports have challenged the ability of PSMA-PET to accurately identify lesions in patients with histological variants. We analyzed the effect of non-acinar histology on PSMA-PET radiotracer, [18F]DCFPyL radiographic findings. Materials and Methods Patients who underwent [18F]DCFPyL PSMA-PET/CT imaging at a single institution from 8/2021 to 8/2022 were reviewed. Among these, 21 patients with non-acinar histology were included for analysis. Patient and clinical characteristics were collected and compared with PSMA-PET imaging findings. Chi square analysis was used to evaluate the association of clinical features with PSMA-PET results. Results Among 21 patients, 14 patients (67%) had DAC while 7 patients (33%) had NEPC. Median age of diagnosis was 63 years (range, 49-79). Reason for undergoing PSMA-PET included surveillance after biochemical recurrence (12 patients), assessing response with metastatic disease (8 patients), and initial staging in localized high risk prostate cancer (1 patient). Median injected dose was 9.3 mCi (range, 5.5-9.9) while median uptake time was 60 minutes (range, 55- 80). Median PSA at time of PSMA-PET was 1.8 ng/mL (range, 0-221.3), with 7 patients (33%) on active ADT at time of PSMA-PET. PSMA-PET was positive in 16 patients (76%) and negative in 5 patients. Among the 5 patients with PSMA-PET occult disease, 5 had DAC and 1 had NEPC with a median PSA of 0.5 (range, 0-1.8). PSA at the time of PSMA-PET was significantly associated with PSMA PET positivity (p=0.01), while initial Gleason grade group, initial treatment, injected dose, uptake time, and receipt of ADT at time to PET were not associated with PET positivity. Ultimately, PSMA-PET altered clinical management in 14 patients (67%). Conclusion PSMA-PET identified lesions in patients with non-acinar prostate cancer and remains a useful clinical tool in these challenging cases. Lower PSA levels appeared to impact detection by PSMA-PET and should be considered in the context of PSMA-PET interpretation. L. Cavallini 1 , C. Malizia 2 , G. Macchia 3 , M. Ntreta 4 , A. Arcelli 4 , F. Mammini 1 , E. Galietta 1 , S. Paolinelli 1 , V. Laghi 1 , D. Vallerossa 1 , E. Natoli 1 , S. Cilla 5 , S. Bisello 1 , A. Cortesi 6 , S. Cammelli 1 , G.P. Frezza 7 , A.G. Morganti 1 , F. Deodato 8 , M. Buwenge 1 1 Radiation Oncology, Department of Experimental, Diagnostic and Specialty Medicine-DIMES, Alma Mater Studiorum University of Bologna, IRCCS Azienda Ospedaliero-Universitaria di Bologna, Bologna, Italy; 2 Nuclear Medicine, IRCCS Azienda Ospedaliero-Universitaria di Bologna, Bologna, Italy; 3 Radiation Oncology Unit, Gemelli Molise Hospital – Università Cattolica del Sacro Cuore, Campobasso, Italy; 4 Radiation Oncology, IRCCS Azienda Ospedaliero-Universitaria di Bologna, Bologna, Italy; 5 Medical Physics Unit, Gemelli Molise Hospital – Università Cattolica del Sacro Cuore, Campobasso, Italy; 6 Radiotherapy Unit, IRCCS Istituto Romagnolo per lo Studio dei Tumori (IRST) "Dino Amadori", Meldola, Italy; 7 Radiotherapy Department, Ospedale Bellaria, Bologna, Italy; 8 Radiation Oncology Unit, Istituto di Radiologia, Gemelli Molise Hospital – Università Cattolica del Sacro Cuore, Università Cattolica del Sacro Cuore , Campobasso, Roma, Italy Purpose or Objective The aim of this study was to analyze the prognostic impact on toxicity, in patients with prostate cancer treated with adjuvant radiotherapy, of several patients and treatment characteristics. Materials and Methods Both acute and late toxicity were assessed in this observational study (ICAROS-1). The recorded and evaluated patients related characteristics were age and Charlson’s comorbidity index. Analyzed treatment characteristics were: delivery of prophylactic lymph nodes irradiation, previous TURP, use of adjuvant ADT and its type (LH-RH analogues or high-dose Bicalutamide) and duration, RT fractionation and technique (including type of image-guidance systems), and Equivalent Dose in 2 Gy/fraction (EQD2) to the prostate bed and pelvic lymph nodes. Late toxicity-free survival curves were calculated by the Kaplan-Meier product-limit method and compared with the log-rank test. Variables with p value less than 0.05 or with a trend (p < 0.1) at univariate analysis were entered into a multivariate Cox’s regression model. Acute toxicity was assessed by RTOG scale while late toxicity was evaluated with the RTOG/EORTC scale. PO-1474 Adjuvant radiotherapy of prostate cancer: a comprehensive analysis of toxicity predictors