Estimation of Radiotherapy Dose Fields from a Few Projections: How Many Projections will Ensure Uniqueness?

Pivot, Odran; Clackdoyle, Rolf; Rit, Simon; Desbat, Laurent

doi:10.1109/nss/mic42677.2020.9507859

Cited by 2 publications

(3 citation statements)

References 7 publications

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“…However, these differences remain in all cases submillimeter, which is acceptable. It is also noted that the proposed 2D dose reconstruction method can be extended to more complex shapes defined by the MLC 33 (by using the a priori knowledge of the leaves geometry).…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2022

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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-mm 2 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.

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Section: Discussionmentioning

confidence: 99%

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

et al. 2022

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“…multiple global minima). For 6 projections, non-uniqueness has been demonstrated for as few as N = 32 leaf-pairs (Pivot et al 2020). Increasing the number of measured projections would be required for handling more pairs of leaves.…”

Section: Reconstruction Algorithmmentioning

confidence: 99%

“…the measurements correspond to one unique MLC irradiation field). It has been demonstrated that even for binary objects, uniqueness cannot be ensured but most of the non-unique situations can be avoided by using a detector with enough projection angles (Pivot et al 2020). For six projections, non-uniqueness situations can arise with 32 or more leaf-pairs.…”

Section: Introductionmentioning

confidence: 99%

A tomographic reconstruction algorithm for cross-sectional imaging of IMRT beams from six projections

et al. 2023

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Objective: Patient-specific Quality Assurance (QA) measurements are of key importance in radiotherapy for safe and efficient treatment delivery and allow early detection of clinically relevant errors. Such QA processes remain challenging to implement for complex IMRT radiotherapy fields delivered using a multileaf collimator (MLC) which often feature small open segments and raise QA issues similar to those encountered in small field dosimetry. Recently, detectors based on long scintillating fibers have been proposed to measure a few parallel projections of the irradiation field with good performance for small field dosimetry. The purpose of this work is to develop and validate a novel approach to reconstruct MLC-shaped small irradiation fields from six projections.Approach: The proposed field reconstruction method uses a limited number of geometric parameters to model the irradiation field. These parameters are iteratively estimated with a steepest descent algorithm. The reconstruction method was first validated on simulated data. Real data were measured with a water-equivalent slab phantom equipped with a detector made of 6 scintillating-fiber ribbons placed at 1m from the source. A radiochromic film was used to acquire a reference measurement of a first dose distribution in the slab phantom at the same source-to-detector distance and the treatment planning system (TPS) provided the reference for another dose distribution. In addition, simulated errors introduced on the delivered dose, field location and field shape were used to evaluate the ability of the proposed method to efficiently identify a deviation between the planned and delivered treatments.Main results: For a first small IMRT segment, 3%/3mm, 2%/2mm and 2%/1mm gamma analysis conducted between the reconstructed dose distribution and the dose measured with radiochromic film exhibited pass rates of 100%, 99.9% and 95.7%, respectively. For a second and smaller IMRT segment, the same gamma analysis performed between the reconstructed dose distribution and the reference provided by the TPS showed pass rates of 100%, 99.4% and 92.6% for the 3%/3mm, 2%/2mm and 2%/1mm gamma criteria, respectively. Gamma analysis of the simulated treatment delivery errors showed the ability of the reconstruction algorithm to detect a 3% deviation between the planned and delivered doses, as well as shifts lower than 7mm and 3mm when considering an individual leaf and a whole field shift, respectively.Significance: The proposed method allows accurate tomographic reconstruction of IMRT segments by processing projections measured with six scintillating-fiber ribbons and is suitable for water-equivalent real-time small IMRT segments QA.

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Estimation of Radiotherapy Dose Fields from a Few Projections: How Many Projections will Ensure Uniqueness?

Cited by 2 publications

References 7 publications

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

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

A tomographic reconstruction algorithm for cross-sectional imaging of IMRT beams from six projections

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