ESTRO 2024 - Abstract Book

S2365

Clinical - Urology

ESTRO 2024

807

Digital Poster

MR-guided SBRT in Synchronous Oligometastatic Prostate Cancer: Toxicity and Preliminary Outcomes

Darren MC Poon 1 , Jing Yuan 2 , Oi Lei Wong 2 , Bin Yang 3 , Sin Ting Chiu 4 , George Chiu 4 , Kin Yin Cheung 3 , Siu Ki Yu 3

1 Hong Kong Sanatorium and Hospital, Comprehensive Oncology Centre, Happy Valley, Hong Kong. 2 Hong Kong Sanatorium and Hospital, Research Department, Happy Valley, Hong Kong. 3 Hong Kong Sanatorium and Hospital, Medical Physics Department, Happy Valley, Hong Kong. 4 Hong Kong Sanatorium and Hospital, Department of Radiotherapy, Happy Valley, Hong Kong

Purpose/Objective:

For patients with synchronous oligometastatic prostate cancer (OMPC), prostate primary radiotherapy in addition to androgen-deprivation therapy (ADT) is currently considered the standard of care with improved survival. Simultaneous stereotactic body radiotherapy (SBRT) to both prostate primary and metastatic lesions is hypothesized to further enhance treatment outcome but relevant clinical reports are still scarce. Meanwhile, in the context of SBRT to multiple targets, increasing toxicities may be a concern if the lesions are not synchronously positioned daily. With enhanced soft-tissue resolution and online adaptation, magnetic resonance-guided SBRT (MRgSBRT) may be an ideal treatment modality for patients with OMPC. This study aims to prospectively investigate the feasibility of MRgSBRT to concurrently irradiate both prostate and pelvic metastatic lesions in patients with synchronous OMPC on a 1.5T MR-integrated linear accelerator (MR-LINAC), and to report the toxicity and preliminary clinical outcomes. Patient inclusion criteria were: histologically confirmed PC; age≥18years; no cancer history other than PC; no MRI contraindication; synchronous OMPC (≤5 lymph node LN or/and pelvic bone metastases) detected by PSMA -PET that were diagnosed <6 months after the initial primary PC diagnosis. Exclusion criteria were: patients had MRI contraindications or failed to sign written consent; Metastatic disease was not confirmed by PSMA- PET or ≥5 metastatic lesions or beyond the pelvis; visceral metastases; previous prostate surgery or irradiation; follow- up ≤3 months. MRgSBRT was delivered on a 1.5T MR-LINAC in five fractions (2 fractions/week) of 7.25-8 Gy/fraction to node/bone metastases, and of 6.7-8 Gy/fraction to prostate, additional 8.5Gy/fraction to the dominant intraprostatic lesion (DIL), and 5Gy/fraction to the whole pelvic lymphatics, with daily online adaptation. An isotropic CTV-PTV margin of 5 mm (but 3mm posterior) was applied to prostate. The margins of 3-5mm, 3-5mm, 5-10mm, and 5mm were applied to DIL, nodal metastasis, bone metastasis and lymphatics, respectively. Apart from ADT, the addition of androgen-signaling pathway inhibitors or chemotherapy was allowed. Patients were followed and PSA levels were monitored at 1 month and then every 3 months. Follow-up PSMA-PET scans were arranged when continuous PSA progressions or clinical symptoms were encountered. Toxicities were assessed using the Common Terminology Criteria for Adverse Events (CTCAE) v5.0. Tumor response was assessed using Response Evaluation Criteria in Solid Tumors (RECIST) v1.1. Overall survival (OS), radiographic progression free survival (rPFS) (by PSMA- PET), and biochemical PFS (bPFS, two consecutive increased PSA of ≥50% above the on treatment nadir) were estimated using Kaplan-Meier (KM) method. Material/Methods: