ESTRO meets Asia 2024 - Abstract Book

S264

Interdisciplinary – SBRT

ESTRO meets Asia 2024

Purpose/Objective:

Stereotactic ablative body radiotherapy (SABR) delivers high doses of radiation in few fractions, aiming for improved local control and survival in oligometastatic cancer. Oligometastatic disease (OMD) is defined by NHS England SABR commissioning criteria 1 as having three or fewer extracranial metastases. SABR is an established treatment option for liver oligometastases. However, the optimal dose for liver OMD SABR remains unclear.

Material/Methods:

This retrospective study analysed patients who underwent liver SABR between 2018 and 2023 at a single UK oncology centre. All patients were treated with prescription doses of 30-60Gy in 3-8 fractions. Patients with primary hepatocellular carcinoma were excluded. Clinical baseline characteristics, treatment details, and dosimetry data were collected. All volumes were peer-reviewed with consultant clinical oncologists and clinical radiologists to ensure robust target volume quality assurance. The mean PTV biologically effective dose (BED) was calculated using an alpha-beta ratio of 10. Physician-recorded acute (occurring ≤3 months post SABR) and chronic (occurring >3 months post SABR) toxicities were documented according to CTCAEv5.0.

The primary endpoint was local progression-free survival (LPFS), with overall survival (OS) as a secondary endpoint. Kaplan-Meier survival estimates were stratified by BED 10 .

Results:

A total of 71 patients with 94 liver metastases were included in the study, with a median follow-up of 13 months. The most common histological diagnoses were colorectal adenocarcinoma (n=54), non-small cell lung cancer (n=5), and breast adenocarcinoma (n=3). Twenty-nine patients (41%) had more than one metastatic site of disease. Notably, nearly two-thirds (n=47) of patients required prescription dose compromise, defined as a reduced prescription dose to all or part of the PTV to <50Gy/5# equivalent, to meet OAR constraints. This was primarily due to the proximity of critical structures such as the heart and gastrointestinal tract, and to meet mean liver dose constraints.

The mean gross tumour volume (GTV) was 33.4cm 3 , with a mean GTV dose (BED planned PTV dose (BED 10 ) of 55.2Gy (median 115.2Gy; IQR 91.5-146.4Gy).

10 ) of 58.5Gy (128.5Gy) and a mean

SABR was delivered as planned in 94% of patients. Two patients terminated SABR prematurely due to disease progression, while an additional two patients discontinued treatment early because of concerns regarding organ at-risk (OAR) doses detected on daily pre-treatment cone beam CT imaging.

No grade 3 or higher acute toxicities were reported, and no chronic toxicities were reported. Grade 1-2 toxicities were recorded in 51% of patients, predominantly fatigue and gastrointestinal symptoms.

In our cohort, median LPFS was not reached. Median OS was 25 months. Higher mean PTV-BED 10 (>115.2Gy versus ≤115.2Gy) was associated with increased LPFS (HR 3.74, 95% CI 1.20-11.66, p=0.03) and OS [median OS 39 versus 21 months (HR 2.40, 95% CI 1.15-5.04, p=0.03)]