ESTRO 2023 - Abstract Book

S1425

Digital Posters

ESTRO 2023

The aim of this study was to clinically implement a multivariate risk-adjusted Hotelling’s T2 control chart for monitoring the brainstem dose after adjusting for inter-patient variations and variations in patient anatomy. Out-of-control plans were investigated if re-optimization could lower the brainstem dose and improve the plan quality. Materials and Methods For the clinical implementation of the control chart, we acquired 80 head-and-neck VMAT plans previously treated at our institution. The control chart signaled four patients as out-of-control (P34, P43, P67, and P68). P34 and P43 were signaled as out-of-control since they were on the lower end of the brainstem DVH distribution across all the patients, and as a result judged as high-quality plans. For out-of-control patients with high brainstem dose, P67’s and P68’s plans were re-optimized to reduce the dose without compromising PTV coverage and worsening other treatment objectives. To reduce the brainstem dose in P67, three new planning structures were created. Structure 1 accounts for the brainstem volume inside the 42 Gy isodose, structure 2 accounts for the brainstem volume inside the 35 Gy isodose, and structure 3 accounts for the brainstem volume inside the 28.5 Gy isodose. Three new objectives for optimization were added: (structure 1: maximum dose = 41 Gy, structure 2: maximum dose = 33 Gy, and structure 3: uniform dose = 27.5 Gy). To reduce the brainstem dose in P68, five new objectives were added: (brainstem: maximum D2 = 35 Gy and maximum D20 = 25 Gy, PTV 6996: minimum dose = 69.96 Gy, PTV 6000: minimum dose = 60 Gy, and PTV 5700: minimum dose = 57 Gy). The prescription percentage was reduced from 96.5% to 95.9% to maintain the coverage to all three PTVs. Results For P67, the maximum, mean, and minimum brainstem dose was reduced from 57.23 Gy to 47.04 Gy, 18.63 Gy to 16.26 Gy, and 3.57 Gy to 3.52 Gy, respectively. For P68, the maximum, mean, and minimum brainstem dose was reduced from 41.20 Gy to 38.79 Gy, 15.77 Gy to 14.92 Gy, and 3.02 Gy to 2.98 Gy, respectively. Conclusion The clinical implementation of the multivariate risk-adjusted Hotelling’s T2 control chart on our head-and-neck VMAT dataset shows that the control chart can monitor and detect treatment plans with unusual DVH points. Our results show that the out-of-control plans can be re-optimized to improve the plan quality prior to treatment delivery. B. van der Heyden 1 , M. De Saint-Hubert 1 , K. Himschoot 2 , R. Geelen 2 , L. Delombaerde 3 , M. Caprioli 4 , W. Crijns 3 , D. Vandenbroucke 2 , P. Leblans 2 , L. de Freitas Nascimento 1 1 Belgian Nuclear Research Centre (SCK CEN), Research in Dosimetric Application group, Mol, Belgium; 2 Agfa N.V., Corporate Innovation Office, Mortsel, Belgium; 3 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium; 4 KU Leuven, Department of Oncology, Leuven, Belgium Purpose or Objective Thermoplastic fixation masks became standard immobilization technology in the fractionated radiation treatment of brain and head-and-neck cancer patients. This DoseMask project intends to transform masks into passive dosimeters by applying light-insensitive radiophotoluminescence (RPL) coatings before the thermoplastic shaping. A sub-mm thin RPL coating, covering the mask surface, would then store a measure of cumulated dose, which can be readout multiple times non destructively by laser excitation. However, no configurable readout equipment exists today to measure the emitted RPL signal from complexly shaped mask surfaces. This work describes the development of a robotic readout system to measure RPL emission. Materials and Methods RPL sheets (5x5 cm2) were manufactured of Al2O3:C,Mg µ m-powder mixed with a binder and deposited on a polyester substrate. Signal-to-noise measurements in a 2x2 cm2 central region-of-interest were performed on irradiated films (5 Gy) with a series of long- and short-pass optical filter combinations in the robotic optical head (Fig 1). A hand-held structured light Artec Eva 3D scanner was used in HD-mode to digitize an old patient mask as a water-tight mesh model in Artec’s Studio software. The mesh was then loaded in an offline programming package for robots (Robotmaster V7.5) to plan the scanning trajectory of the optical head (incl. RPL detector, 635 nm excitation laser, 650 nm cut-on dichroic mirror, and filter set). Connection pieces were 3D printed in PLA. Scanning configurations and orientations were investigated to cover the fixation mask surface in discrete sub-mm steps. Results The highest signal-to-noise ratio between the RPL-scanned irradiated and non-irradiated films was found with a 700 nm long-pass and 825 nm short-pass filter combination, which were then included in the lens tube of the robotic optical head. A chalk coating had to be deposited on the fixation mask surface to improve digitization accuracy (±0.2 mm) and to avoid scanning artifacts due to white light reflectance. Mesh post-processing was required to align the 518 individual frames in scanning space, to fuse all frames into one single mesh object, to remove polygon outliers and mesh holes. This non automated scanning and post-processing procedure currently requires ±30 minutes for one operator. A realistic simulation model was developed to generate a scanning protocol in sub-mm steps (Fig 2). Computer software was written in C# to communicate in 20 ms periods with the robot (3D+time) and the detector (RPL emission intensity). PO-1711 A robotic dosimetry system by optical scanning of radiophotoluminescence coated immobilization masks