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Extended commissioning and validation of the comprehensive motion management (CMM) system on an Elekta Unity MR-Linac

Wed, 01 Jan 2025 00:00:00 GMT

Title: Extended commissioning and validation of the comprehensive motion management (CMM) system on an Elekta Unity MR-Linac Abstract: Background The Elekta Unity MR-Linac (MRL), operational at Sir Anthony Mamo Oncology Centre (SAMOC) since July 2024, enables daily adaptive radiotherapy (ART) by adjusting treatment plans to account for tumour and anatomical changes. The Comprehensive Motion Management (CMM) system allows real-time motion tracking and gated beam delivery using different strategies, prompting the need for extended commissioning and validation to characterise performance under varied clinical conditions. Objectives To design and deliver extended commissioning and validation tests on the Elekta CMM system, to quantify its positional and dosimetric accuracy across three selected gating strategies (respiratory, exhale navigated, and non-respiratory), and identify any potential limitations. Research Methodology The MRI4D MODUS QUASARTM phantom was commissioned to verify its positional accuracy, forming the basis for subsequent tests. A number of motion traces were used to challenge the system for the three gating strategies. Selected traces were used to quantify the Anatomic Position Monitoring (APM) positional accuracy, measure beam gating latency for both beam-on/off transitions, and assess the dosimetric impact of these parameters during different strategies. The dosimetric impact of different gating thresholds, volumetric overlapping criterion (VOICE) percentages, and complex waveforms was characterised using EBT4 film dosimetry. Results APM analysis confirmed reliable target localisation within typically <±1 mm across all strategies. In the respiratory strategy, regular waveforms met the manufacturer’s latency specification, while irregular waveforms showed degraded performance. Film measurements for respiratory strategy indicated that regular breathing traces maintained target coverage (−0.40±0.22mm change in prescription isodose), while irregular breathing may compromise dose conformity and reduce clinical target volume (CTV) coverage (−1.22±0.23mm change). A trade-off was evident between gating window and efficiency: tighter gating thresholds (2mm) and VOICE 100% improved coverage (0.25±0.20 and 0.08±0.21mm) compared with 5mm and VOICE 95% (–1.13±0.29 and –1.53±0.21mm), but at the cost of delivery efficiency (3.3-5.5× longer). All gating scenarios retained gamma pass rates >97% (3%/2 mm), but a slight reduction was observed for challenging deliveries. Conclusions and Recommendations The CMM system demonstrated robust performance under regular conditions but limitations where characterised under irregular motion including CTV coverage reduction and may need to be accounted for clinically but further investigation is required. Future work should validate CMM in lung-equivalent 4D phantoms and further assess the exhale-navigated strategy. Description: M.Sc. Med.Phy.(Melit.)

TLD-based organ dosimetry in abdomino-pelvic CT

Wed, 01 Jan 2025 00:00:00 GMT

Title: TLD-based organ dosimetry in abdomino-pelvic CT Abstract: Background Ideally, patient organ dosimetry should follow a direct in-vivo approach. However, owing to a lack of feasibility of such an approach, Monte-Carlo (MC) simulations are the gold standard for estimating organ absorbed doses. Such simulation software must be verified via measurements using physical dosimeters calibrated at the beam energy. However, calibration methods of such dosimeters often use high-energy sources without inclusion of lower-energy scattered photons, which reduces calibration accuracy. A direct calibration using Computed Tomography (CT) spectra is desired. Objectives This research investigated how organ doses within the abdomino-pelvic region are affected by CT beam energy. A calibration method was proposed to include the impact of scatter. Research Methodology Thermoluminescent Dosimeter (TLD) dose measurements were made in 9 separate organs within an adult male anthropomorphic phan tom on an abdomino-pelvis protocol. Readings were taken at two different beam energies (120kVp, 140kVp). For each energy, the crystals were read using 2 different calibration profiles, one developed on a planar radiography system (without scatter) and using a 3D-printed insert and CT Dose Index (CTDI) phantom directly on the CT scanner (with scatter). Results An increase of 11.3% mGy normalized mean was observed in organ doses between the two energy levels. There was a significant proportional bias between results from the two calibration methods. Conclusions and Recommendations To our knowledge, this is the first study to consider calibrating TLD crystals directly at a CT scanner. Description: M.Sc. Med.Phy.(Melit.)

Validation of a motion management and gating protocol on the Elekta Unity MR LINAC for margin reduction in prostate cancer

Wed, 01 Jan 2025 00:00:00 GMT

Title: Validation of a motion management and gating protocol on the Elekta Unity MR LINAC for margin reduction in prostate cancer Abstract: Background: The recent installation of Elekta Unity MR-LINAC (MRL) at the Sir Anthony Mamo Oncology Centre (SAMOC) integrates magnetic resonance imaging (MRI) with radiotherapy delivery. The system offers superior soft tissue visualisation and motion management capabilities, particularly relevant for prostate cancer, where intrafractional motion compromises dose coverage and increased exposure to organs at risk (OAR). Objectives: This study aimed to evaluate the feasibility, performance, and limitations of a reduced margin protocol for prostate radiotherapy on the Elekta Unity MRL. Therefore, the validation of treatment margin reduction was essential to ensure safe margin reduction and effective treatment delivery in a typical clinical setting for prostate treatments. Research Methodology: The analysis of 360 treatment fractions was performed to characterise prostate motion using Unity log files. Then, a treatment planning study with a phantom was performed to simulate prostate displacements and evaluate dose–volume histogram (DVH) metrics using Monaco® TPS. Lastly, radiochromic film dosimetry validated treatment delivery under static and motion scenarios, with gamma analysis used for dose comparison. Results: The log file analysis showed minimal motion in the LR and AP directions, but higher variability in the SI axis, especially during the final 30 seconds of treatment. DVH analysis revealed that most treatment plans met dosimetric criteria, though extremely inferior displacements reduced target coverage. Film dosimetry and gamma analysis showed that the film exposed during motion with 2 baselineshifts (BLS) and the film exposed at 97% VOICE performed the best and poorest, respectively. Conclusions and Recommendations: This study shows that reducing prostate PTV margins to 2 mm is possible. Moreover, it was concluded that BLS can restore margin coverage. Clinically, combining margin optimisation with adaptive work-flows and quality assurance QA practices enhances safety and precision. Future research should validate results in larger cohorts, consider seminal vesicle motion, and apply such strategies across other anatomically mobile sites. Description: M.Sc. Med.Phy.(Melit.)

Environmental dose evaluation following the installation of a SPECT/CT scanner in a major public hospital using Monte Carlo simulation

Wed, 01 Jan 2025 00:00:00 GMT

Title: Environmental dose evaluation following the installation of a SPECT/CT scanner in a major public hospital using Monte Carlo simulation Abstract: Background: Accurate shielding design in nuclear medicine requires reliable dose estimates to maintain occupational and public exposures as low as reasonably achievable. Shielding calculations are conventionally performed analytically. A Monte Carlo (MC) approach offers more detailed, site-specific insight but has not yet been systematically applied locally. Objectives: Acquire proficiency in FLUKA; develop a detailed model of a site-specific SPECT/CT facility; simulate a typical 99mTc imaging protocol; benchmark the model against experimental data and national dose regulations; extend the framework to 177Lu. Research Methodology: A hybrid approach combining simulation, experimental measurements, and analytical calculations was adopted. The facility and scanner were modelled based on technical drawings and material data. Ambient dose equivalents were physically measured at multiple locations using thermoluminescent dosimeters exposed to a 99mTc-injected phantom for model validation. Quantities scored in FLUKA included ambient dose equivalents, fluence distributions, regional energy deposition, and fluence-energy spectra. Dose equivalents were normalised to clinical workloads to derive annual dose estimates. Results: A high-fidelity MC model of the SPECT/CT facility was successfully developed, simulated and validated. Weighted mean ratios and two-tailed paired t-tests (n = 11) showed values of 1.17 ± 0.02 (analytical vs experiment, significant, p < 0.03) and 1.20 ± 0.01 (simulation vs experiment, not significant, p = 0.10). The ratio findings indicated that the MC method performed at least as well as the analytical approach, while the paired t-test suggested superior performance. Workload-adjusted annual dose equivalents verified compliance of installed shielding with national dose limits and regional energy deposition revealed strong phantom self-absorption (62% for 99mTc, 87% for 177Lu). Conclusions and Recommendations: This study introduced and validated an MC facility model for nuclear medicine shielding assessment. Future planning should combine analytical, experimental, and MC methods for accurate, yet time-efficient assessments. Further work should address validation against alternative MC codes, CT source modelling, and dynamic patient and equipment motion. Description: M.Sc. Med.Phy.(Melit.)