ESTRO 2022 - Abstract Book

S1170

Abstract book

ESTRO 2022

1 Baskent University Faculty of Medicine, Adana Dr Turgut Noyan Research and Treatment Center, Department of Radiation Oncology, Adana, Turkey; 2 Iskenderun Gelisim Hospital, Division of Radiaiton Oncology, Hatay, Turkey; 3 Baskent University Faculty of Medicine, Adana Dr Turgut Noyan Research and Treatment Center, Department of Nuclear Medicine, Adana, Turkey Purpose or Objective The role of the gallium-68 ( 68 Ga)-labeled prostate-specific membrane antigen (PSMA) ligand in treatment response evaluation after definitive radiotherapy (RT) has not been evaluated in prostate cancer (PC) patients. The aim of this study is to evaluate standardized uptake value (SUV) changes after definitive RT and to assess the factors affecting SUV changes in primary tumors (SUV p ) and metastatic lymph nodes (SUV ln ) and PSA response rates. Materials and Methods The clinical data of 108 PC patients receiving neoadjuvant ADT and definitive intensity-modulated RT between March 2015 and May 2020 were retrospectively analyzed. All patients underwent 68 Ga-PSMA-PET/CT before the start of ADT for initial staging and RT planning and after the completion of RT. Patients receiving ADT or chemotherapy before 68 Ga-PSMA-PET/CT, patients undergoing radical prostatectomy, and patients with clinical and radiological evidence of distant metastasis were excluded from the study. Differences in SUV p , SUV ln , and PSA before and after RT were evaluated. Receiver operating characteristic curves were generated for primary tumor pre- and posttreatment SUV response to determine the cutoff for predicting recurrence. Results The median serum PSA level at a median of 3.4 months (range: 2.7–4.9 months) after ADT initiation was 0.3 ng/mL (range: 0–53.2 ng/mL), and the median PSA response rate was 98.8% (range: 1.4–100%). A PSA response at a median of 4.1 months (range: 2.8–4.8 months) after RT was observed in all patients, with a median decrease of 99.9% (range: 51.1–100%) and a median PSA level of 0 ng/mL (range: 0–14.2 ng/mL) (Fig 1). Posttreatment 68 Ga-PSMA-PET/CT imaging was performed at a median of 9.7 months after ADT initiation. Decreases of a median of 100% in SUV p and a median of 100% in SUV ln were seen in 101 and 46 patients, respectively. The response rates in serum PSA, SUV p , and SUV ln did not differ significantly according to the Gleason score, PSA groups, and clinical T and N stages. A decrease in SUV ln was observed in 46 patients, with a median of 100% (range: 45.2–100%). One patient showed no change in SUV ln , and one showed a 53% increase. Bone metastasis was observed on the posttreatment 68 Ga-PSMA-PET/CT of two patients. According to the PERCIST, 55 (52.4%) patients had CR, 36 (34.3%) had PR, 9 (8.6%) had SD, and 5 (4.7%) had PD. Disease progression was observed in 20% of the patients. The progression rate was significantly higher in patients with a SUV p response rate of <61.7% than in those with a ≥ 61.7% response rate (46.6% vs. 9.2%; p<0.001). Patients with higher SUV response rates also had significantly better PSA responses after definitive RT and ADT than patients with lower SUV responses (99.7 ± 0.8% vs. 95.6 ± 10.5%; p = 0.001). Conclusion The findings suggest that 68 Ga-PSMA-PET/CT could be used as a quantitative imaging modality for therapy monitoring after neoadjuvant ADT and RT in PC patients. The findings should be validated by prospective studies. Purpose or Objective Stereotactic body radiotherapy (SBRT) precisely delivers a high radiation dose towards an extracranial target in 1 to 5 fractions. The purpose of this study is to describe the treatment characteristics and acute toxicity in a cohort composed of prostate cancer patients treated with CBCT guided SBRT without fiducial markers. Materials and Methods We performed a retrospective analysis of patients treated with SBRT between 2020 – 2021 at Carlos Van Buren Hospital in Valparaíso with a flattening filter free volumetric modulated arc therapy. CTV was defined as: 1) Prostate (low risk patients) 2) Prostate + 1 cm Seminal Vesicles (S.V) (intermediate risk) 3) Prostate + 2 cm S.V. PTV = CTV +0.5 cm (0.3 cm posteriorly). Prescription consisted of 36.25 Gy in 5 fractions (alternate days) to Prostate PTV and 27.25 Gy in 5 fractions to S.V PTV. On-line correction was performed before each session. Patients were treated in a Elekta, Versa HD. Acute toxicity was registered according to the RTOG definition (<90 days since treatment end). The study was approved by the ethics committee. Results 82 patients were included. Median PSA was 14.83 (IQR 7.19-23). 32 (39.02 %) patients had high risk disease. In 62 (77.5%) patients radiotherapy planning was done based on MRI / CT fusion. Median treatment delivery time (Beam On Time) was 139.7 s. 71.3% of patients suffered some form of acute toxicity; 2 patients G3 acute gastrointestinal toxicity and 2 patients G3 acute genitourinary toxicity, no G4-G5 toxicity was observed. Only 1 patient had to discontinue treatment due to acute urinary obstruction caused by disease progression. PO-1381 SBRT in prostate cancer: annalysis of CBCT guided radiotherapy and on-line correction J. Olivares 1 , J. Solis Campos 2,1 , G. Lazcano Alvarez 1 , G. Veillon Contreras 2,1 , B. Tudela Staub 2,1 , I. Perrot Rosenberg 1 1 Universidad de Valparaiso, Oncology, Valparaiso, Chile; 2 Carlos Van Buren Hospital, Oncology, Valparaiso, Chile