A novel QA phantom based on scintillating fiber ribbons with implementation of 2D dose tomography for small‐field radiotherapy

• Published in: Medical physics (Lancaster) 2023-01, Vol.50 (1), p.619-632
• Main Authors: Esteves, Josué, Pivot, Odran, Ribouton, Julien, Jalade, Patrice, Zouaoui, Abdelaali, Desbat, Laurent, Rit, Simon, Blanc, Frédéric, Haefeli, Guido, Hopchev, Plamen, Galvan, Jean‐Marc, Lu, Guo‐Neng, Pittet, Patrick
• Format: Article
• Language: English
• Subjects: 2D dose tomography; COMPUTATIONAL AND EXPERIMENTAL DOSIMETRY; Computer Science; Imaging, Three-Dimensional; Medical Imaging; Phantoms, Imaging; Radiometry; Radiotherapy Dosage; Radiotherapy Planning, Computer-Assisted; small‐field dosimetry; Tomography; Water; water‐equivalent scintillating detectors
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1002/mp.15902

Purpose To develop a novel instrument for real‐time quality assurance (QA) procedures in radiotherapy. The system implements a scintillation‐based phantom and associated signal acquisition and processing modules and aims to monitor two‐dimensional (2D) dose distributions of small fields. Materials and methods For the proposed phantom, we have designed and realized a prototype implementing six high‐resolution tissue‐equivalent scintillating fiber ribbons stacked with in‐plane 30° rotated orientations from each other. Each ribbon output is coupled to a silicon photodiode linear array (with an element pitch of 400 μm) to detect scintillating signal, which represents the projected irradiation profile perpendicular to the ribbon's orientation. For the system providing six acquired projected dose profiles at different orientations, we have developed a two‐step signal processing method to perform 2D dose reconstruction. The first step is to determine irradiation field geometry parameters using a tomographic geometry approach, and the second one is to perform specific penumbra estimation. The QA system prototype has been tested on a Novalis TrueBeam STX with a 6‐MV photon beam for small elliptic fields defined by 5‐ and 10‐mm cone collimators and for 10 × 10‐ and 20 × 10‐mm2 rectangular fields defined by the micro‐multileaf collimator. Gamma index analysis using EBT3 films as reference has been carried out with tight 2%‐dose‐difference (DD)/700‐μm‐distance‐to‐agreement (DTA) as well as 1%‐DD/1‐mm‐DTA criteria for evaluating the system performances. The testing also includes an evaluation of the proposed two‐step field reconstruction method in comparison with two conventional methods: filtered back projection (FBP) and simultaneous iterative reconstruction technique (SIRT). Results The reconstructed 2D dose distributions have gamma index pass rates higher than 95% for all the tested configurations as compared with EBT3 film measurements with both 2%‐DD/700‐μm‐DTA and 1%‐DD/1‐mm criteria. 2D global gamma analysis shows that the two‐step and FBP radiation field reconstruction methods systematically outperform the SIRT approach. Moreover, higher gamma index success rates are obtained with the two‐step method than with FBP in the case of the fields defined with the stereotactic cones. Conclusions The proposed small‐field QA system makes a use of six water‐equivalent scintillating detectors (fiber ribbons) to acquire dose distribution. The developed two‐step signal proc