ESTRO 2025 - Abstract Book

S2865

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

3249

Poster Discussion Spatially Fractionated Radiotherapy: A Planning Feasibility Study for Personalised Lattice Configuration and Enhanced Peak-to-Valley Dose Ratio Veng Jean Heng 1,2 , Amy Parent 2 , Yat Tsang 2 , Makan Farrokhkish 2 , Philip Wong 2,3 , David Kirsch 2,3 , Aruz Mesci 2,3 , Dana Keilty 2,3 , Ezra Hahn 2,3 , David Hodgson 2,3 , Chiaojung Jillian Tsai 2,3 , Peter Chung 2,3 , Laura A Dawson 2,3 , Jelena Lukovic 2,3 , Monica Serban 2,3 1 Department of Radiation Oncology, Stanford University, Palo Alto, USA. 2 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Canada. 3 Department of Radiation Oncology, University of Toronto, Toronto, Canada Purpose/Objective: To optimize and personalize the lattice configuration in Spatially Fractionated Radiotherapy (SFRT) by maximizing the number of high-dose peaks and by adapting their size, spacing and location to tumor shape and organs-at-risk (OARs) proximity. To investigate planning feasibility of increasing the peak-to-valley-dose-ratio (PVDR) along with higher-density lattice in VMAT-based SFRT [1-3]. Material/Methods: This study included the first ten patients treated with SFRT at our institution, following the published 67Gy/20Gy in 5 fractions protocol: 20Gy peripheral “low-dose” covering the anatomical target (GTV, PTV), with integrated 67Gy “high-dose” peaks arranged in a lattice pattern (1.5cm-diameter spheres spaced 6cm apart) forming the PTV PEAKS [4]. GTV volumes ranged from 139-3185cm 3 . A Python script was developed to maximize the number of peaks by optimizing lattice translations/rotations, and by selecting lattice size and spacing based on tumor size. The script incorporates user-defined margins to keep PTV PEAKS away from OARs and defines two configurations: 1.5cm diameter peaks with 6cm spacing and 1cm-diameter peaks with 4cm spacing (Figure-1). Manually placed versus script-generated PTV PEAKS were compared. A treatment planning study was conducted to evaluate feasibility and develop dose prescription and optimization protocols for increasing PVDRs, investigating an additional 67Gy/10Gy prescription (Table-1). Results: Two plans were successfully generated for each patient using 67Gy/20Gy (PVDR=350%) and 67Gy/10Gy (PVDR=700%) prescriptions, totaling 20 plans. The dose prescription protocols and PTV PEAKS coverage strategy for increasing PVDR, even with smaller and higher-density lattice, are summarized in Table-1. 1cm-diameter spheres were appropriate for 5/10 patients. The script allowed for more peaks in 7/10 patients, by an average pairwise increase of 53.5% in number (Table-2). The script used 1.5cm spheres for larger GTVs (average volume 1596cm 3 ); the average GTV volume for 1cm sphere plans was 777cm 3 . GTV D mean (D98%) were ~29Gy (19.5Gy) for the 67Gy/20Gy and 20Gy (9.5Gy) for 67Gy/10Gy prescription (Table-2). Maximum OAR doses remained comparable between the two prescriptions due to the same PTV PEAKS dose, however, dose metrics evaluated on larger volumes were lower with the 67Gy/10Gy prescription. For example, skin D 0.1cm³ differed by 0.5%, while D 10cm³ decreased by 24.2% (5.3Gy) with the 67Gy/10Gy prescription. D 20cm³ to the bladder, bowel, rectum decreased by 47.9% (8.6Gy), 36.4% (7.6Gy) and 42.6% (6.3Gy), respectively (Table-2).