ESTRO 2023 - Abstract Book

S1237

Digital Posters

ESTRO 2023

Purpose or Objective The role of metastasis-directed radiotherapy (MDT) in hormone-sensitive prostate cancer is being explored actively. We report on an approach of utilising Ga68-PSMA PET-CT response to ADT for selecting patients for adding metastasis-directed RT (MDT) in node-positive and oligometastatic prostate cancer. Materials and Methods In this IRB-approved study, patients diagnosed with prostate adenocarcinoma with metastases to pelvic nodes or oligometastases, as seen on baseline staging Ga68-PSMA PETCT, were identified. All patients received long-term ADT and image-guided conformal RT to the prostate. Eligible patients had a second Ga68-PSMA PETCT scan after 4-6 months of ADT prior to RT. MDT was added only to the metabolically active residual metastases on the second scan. Change in maximum standardised uptake value (SUVmax) was estimated for prostate, nodes, and bone metastases from the baseline to pre-RT Ga68-PSMA PETCT scans. Response to ADT was categorised as complete/partial/stable/progression (CR/PR/SD/PD) using EAU-EANM consensus criteria. Results Total 148 patients (N1M0 = 65, M1a = 44, M1b = 39) with paired Ga68-PSMA PETCT scans were eligible and analysed, with median 6 months ADT (IQR 5-6) between the scans. 90% of patients received ADT alone without systemic intensification. Median relative decline in SUVmax was 47% (IQR 5-72) for prostate, 100% (51-100) for nodes, and 100% (8-100) for bone metastases (Table 1). The proportion of CR/PR/SD/PD in prostate, nodes, and bones is shown in figure 1. Using this PSMA PETCT response-adapted approach, 23/148 (15.5%) patients needed MDT to metabolically active residual lesions. With a median follow-up of 38 months, 26/148 (17.6%) patients developed castration-resistant prostate cancer (CRPC).

Conclusion Ga68-PSMA PETCT-based response assessment in HSPC showed nodal and bone metastases responding better to ADT than primary prostate. Only a small proportion of patients with metabolically active residual metastases required MDT. This approach of selective risk-adapted MDT needs to be explored and validated prospectively.

PO-1522 The validity of treatment plan using contours propagated by DIR on MR-Linac in prostate cancer.

M. Saito 1 , K. Abe 1 , A. Kodate 1 , A. Kanazawa 1 , M. Kurokawa 1 , R. Harada 1 , M. Watanabe 1 , M. Tsuneta 1 , T. Uno 1

1 Chiba University, Diagnostic Radiology and Radiation Oncology, Chiba, Japan

Purpose or Objective In stereotactic body radiation therapy (SBRT) of prostate cancer, the management of the inter- and intra- fractionation motions is needed. MR-guided radiotherapy can administrate daily adaptive radiotherapy and motion monitoring during treatment, while can be both time and resource intensive such as manual delineation. Contours can be propagated from planning MRI to session MRI via a deformable image registration (DIR). This study evaluates the validity of treatment planning using contours propagated by DIR against contours manually delineated in daily session on the Elekta Unity MR Linac. Materials and Methods Fifty fractions in total were analysed from 10 patients of prostate cancer who were treated with SBRT (37.5 Gy in 5 fractions). All patients underwent placement of hydrogel spacer. Clinical target volume (CTV) encompassed prostate and the proximal 1cm of the seminal vesicle and planning target volume (PTV) was expanded CTV isotropically by 5 mm. Organ at risk was bladder and rectum (sigmoid or intestine was not involved). Contours (bladder, rectum, prostate, seminal vesicle, CTV, and PTV) of DIR and manual were evaluated by DICE coefficient. Dose constrains of manual contours were evaluated by treatment plan using contours propagated by DIR on session MRI.