ESTRO 2023 - Abstract Book

S1572

Digital Posters

ESTRO 2023

We evaluated the treatment response and clinical predictive factors for overall survival (OS) and relapse in patients with liver metastases who underwent stereotactic body radiotherapy (SBRT) in our institution. Materials and Methods Clinical information was retrospectively collected for 25 patients (35 lesions) with liver metastases, who were treated with SBRT in our center between September 2017 and January 2022. After abdominal dampening with thermoplastic mask and 4DCT-simulation with contrast or expiration CT with and without contrast and MRI fusion, patients received SBRT doses of 30-60 Gy in 3-5 fractions in Synergy®, Infinity® LINAC and Cyberknife®. Results Median age was 69 years (33-90). The primary tumors were CRC in 52.4%. All patients had controlled or controllable primary tumor and 14 patients (56%) had received previous chemotherapy treatment. All patients completed the planned radiotherapy treatment, and the median follow-up duration was 12 months (range, 2-40 months). The local PFS at 6, 12 and 18 months was 96%, 92% and 92%. After the follow-up period there were 44% distant relapses (11 patients). Median OS from initial diagnosis was 43 months (range 4-151 months). The univariate model indicated that the following were significant independent predictors of OS: total dose (p<0.035) and dose for fraction (p<0.041). The univariate model indicated that the following were significant independent predictors of relapse: level of AST pretreatment (p<0.05), and more than 2 lesions (p= 0.021). Conclusion SBRT is an effective treatment for patients with liver metastases. Notably, total dose and dose for fraction are independent prognostic factors for survival, and pretreatment AST and more than 2 lesions are independent prognostic factors for relapse. This data establish hypothesis for new studies.

PO-1838 Characterization of a ultra-high dose rate proton beam for preclinical experiments.

M. Schwarz 1 , J. Saini 1 , F. Vander Stappen 2 , L. Palmer 3 , J. Zeng 1

1 Fred Hutchinson Cancer Center, Radiation Oncology, Seattle, USA; 2 IBA, Proton Therapy, Louvain La Neuve, Belgium; 3 IBA, Proton therapy, Seattle, USA Purpose or Objective As a first step towards implementing ultra-high dose rate (UHDR) proton therapy treatments, we characterized a proton beam for pre-clinical UHDR experiments on a pencil beam scanning beamline used in clinical practice. Materials and Methods After modifications to the beamline were applied to our gantry proton room (Proteus Plus, IBA Proton therapy, Belgium) to maximize the beam current at the isocenter, we characterized the UHDR beam in terms of: a) Spot and shape in three configurations; pristine beam, 75 mm water-equivalent-thickness (WET) range shifter (RS), and custom-built 135 mm WET RS to be mounted 310 mm upstream of aperture in the snout housing.

b) Beam transport efficiency and beam output;

c) Dose homogeneity, beam penumbra, and dose rate for the fields to be used in pre-clinical irradiations.

All measurements were performed at isocenter, either in air or at 1 cm depth, and using the highest energy, corresponding to a nominal range of 32.9 cm in water. Results The range measurements acquired with a multi-layer ionization chamber are consistent within 1 mm with the nominal range. In UHDR mode, the spot sigma at the isocenter ranges from 4.5 mm for the pristine beam to 9.2 mm for the 135 mm RS. The same energy is associated with a 2.9 mm sigma in clinical mode. The spot is constant for beam intensities ranging from 300 nA to 700 nA at the cyclotron. The results are summarized in figure 1.