1
|
Riis HL, Engstrøm KH, Slama L, Dass J, Ebert MA, Rowshanfarzad P. Assessing focal spot alignment in clinical linear accelerators: a comprehensive evaluation with triplet phantoms. Phys Eng Sci Med 2024:10.1007/s13246-024-01450-9. [PMID: 38954381 DOI: 10.1007/s13246-024-01450-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2024] [Accepted: 05/21/2024] [Indexed: 07/04/2024]
Abstract
A fundamental parameter to evaluate the beam delivery precision and stability on a clinical linear accelerator (linac) is the focal spot position (FSP) measured relative to the collimator axis of the radiation head. The aims of this work were to evaluate comprehensive data on FSP acquired on linacs in clinical use and to establish the ability of alternative phantoms to detect effects on patient plan delivery related to FSP. FSP measurements were conducted using a rigid phantom holding two ball-bearings at two different distances from the radiation source. Images of these ball-bearings were acquired using the electronic portal imaging device (EPID) integrated with each linac. Machine QA was assessed using a radiation head-mounted PTW STARCHECK phantom. Patient plan QA was investigated using the SNC ArcCHECK phantom positioned on the treatment couch, irradiated with VMAT plans across a complete 360° gantry rotation and three X-ray energies. This study covered eight Elekta linacs, including those with 6 MV, 18 MV, and 6 MV flattening-filter-free (FFF) beams. The largest range in the FSP was found for 6 MV FFF. The FSP of one linac, retrofitted with 6 MV FFF, displayed substantial differences in FSP compared to 6 MV FFF beams on other linacs, which all had FSP ranges less than 0.50 mm and 0.25 mm in the lateral and longitudinal directions, respectively. The PTW STARCHECK phantom proved effective in characterising the FSP, while the SNC ArcCHECK measurements could not discern FSP-related features. Minor variations in FSP may be attributed to adjustments in linac parameters, component replacements necessary for beam delivery, and the wear and tear of various linac components, including the magnetron and gun filament. Consideration should be given to the ability of any particular phantom to detect a subsequent impact on the accuracy of patient plan delivery.
Collapse
Affiliation(s)
- Hans L Riis
- Department of Oncology, Odense University Hospital, Odense, Denmark.
- Department of Clinical Research, University of Southern Denmark, Odense, Denmark.
- Radiofysisk Laboratorium, Odense University Hospital, Kløvervænget 19, DK-5000 Odense C, Odense, Denmark.
| | - Kenni H Engstrøm
- Department of Oncology, Odense University Hospital, Odense, Denmark
| | - Luke Slama
- Department of Radiation Oncology, Sir Charles Gairdner Hospital, Nedlands, WA, 6009, Australia
| | - Joshua Dass
- Department of Radiation Oncology, Sir Charles Gairdner Hospital, Nedlands, WA, 6009, Australia
- Centre for Advanced Technologies in Cancer Research (CATCR), Perth, WA, 6000, Australia
| | - Martin A Ebert
- Department of Radiation Oncology, Sir Charles Gairdner Hospital, Nedlands, WA, 6009, Australia
- Centre for Advanced Technologies in Cancer Research (CATCR), Perth, WA, 6000, Australia
- School of Physics, Mathematics, and Computing, The University of Western Australia, Crawley, WA, 6009, Australia
| | - Pejman Rowshanfarzad
- Centre for Advanced Technologies in Cancer Research (CATCR), Perth, WA, 6000, Australia
- School of Physics, Mathematics, and Computing, The University of Western Australia, Crawley, WA, 6009, Australia
| |
Collapse
|
2
|
Ono T, Kido T, Nakamura M, Iramina H, Kakino R, Mizowaki T. Automatic measurement of beam-positioning accuracy at off-isocenter positions. J Appl Clin Med Phys 2023; 24:e13844. [PMID: 36420973 PMCID: PMC10018661 DOI: 10.1002/acm2.13844] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/07/2022] [Revised: 10/22/2022] [Accepted: 10/26/2022] [Indexed: 11/25/2022] Open
Abstract
PURPOSE This study performed an automatic measurement of the off-axis beam-positioning accuracy at a single isocenter via the TrueBeam Developer mode and evaluated the beam-positioning accuracy considering the effect of couch rotational errors. METHODS TrueBeam STx and the Winston-Lutz test-dedicated phantom, with a 3 mm diameter steel ball, were used in this study. The phantom was placed on the treatment couch, and the Winston-Lutz test was performed at the isocenter for four gantry angles (0°, 90°, 180°, and 270°) using an electronic portal imaging device. The phantom offset positions were at distances of 0, 25, 50, 75, and 100 mm from the isocenter along the superior-inferior, anterior-posterior, and left-right directions. Seventeen patterns of multileaf collimator-shaped square fields of 10 × 10 mm2 were created at the isocenter and off-axis positions for each gantry angle. The beam-positioning accuracy was evaluated with couch rotation along the yaw-axis (0°, ± 0.5°, and ± 1.0°). RESULTS The mean beam-positioning errors at the isocenter and off-isocenter distances (from the isocenter to ±100 mm) were 0.46-0.60, 0.44-0.91, and 0.42-1.11 mm for the couch angles of 0°, ±0.5°, and ±1°, respectively. The beam-positioning errors increased as the distance from the isocenter and couch rotation increased. CONCLUSION These findings suggest that the beam-positioning accuracy at the isocenter and off-isocenter positions can be evaluated quickly and automatically using the TrueBeam Developer mode. The proposed procedure is expected to contribute to an efficient evaluation of the beam-positioning accuracy at off-isocenter positions.
Collapse
Affiliation(s)
- Tomohiro Ono
- Department of Radiation Oncology and Image‐Applied TherapyKyoto UniversityKyotoJapan
| | - Takahisa Kido
- Department of Information Technology and Medical EngineeringHuman Health SciencesGraduate School of MedicineKyoto UniversityKyotoJapan
| | - Mitsuhiro Nakamura
- Department of Information Technology and Medical EngineeringHuman Health SciencesGraduate School of MedicineKyoto UniversityKyotoJapan
| | - Hiraku Iramina
- Department of Radiation Oncology and Image‐Applied TherapyKyoto UniversityKyotoJapan
| | - Ryo Kakino
- Kansai BNCT Medical Center, OsakaMedical and Pharmaceutical UniversityTakatsukiJapan
| | - Takashi Mizowaki
- Department of Radiation Oncology and Image‐Applied TherapyKyoto UniversityKyotoJapan
| |
Collapse
|
3
|
McCallum-Hee BI, Milan T, White R, Rowshanfarzad P. Dosimetric impact of mechanical movements of the Linac gantry during treatments with small fields. Front Oncol 2022; 12:973431. [DOI: 10.3389/fonc.2022.973431] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2022] [Accepted: 10/20/2022] [Indexed: 11/06/2022] Open
Abstract
ObjectiveCurrent accepted linac Quality Assurance (QA) guidelines used for Volumetric Modulated Arc Therapy (VMAT) suggest a mechanical isocentre tolerance level of 1 mm. However, this tolerance level has not been well-established for the specific case of small field stereotactic VMAT. This study aims to evaluate the clinical impact of mechanical uncertainty on this treatment modality by modelling systematic gantry sag derived isocentre variance in the Treatment Planning System (TPS).ApproachA previously reported dataset of gantry sag values in the literature served as a starting point for this study. Using an in-house developed VMAT arc splitting algorithm, isocentre shifts were applied at a Control Point (CP) level to DICOM-RT treatment plans. Dose distributions for varying isocentre shift magnitudes were calculated for a set of 29 stereotactic VMAT plans using the Eclipse Acuros XB dose algorithm. These plans had a range of Planning Target Volume (PTV) sizes. A quantitative comparison of each plan was conducted by evaluating five Dose Volume Histogram (DVH)-derived plan quality metrics.ResultsAll metrics exhibited a deterioration in plan quality with increasing magnitudes of isocentre shift. At small PTV sizes, these effects were amplified, exhibiting significant changes at 1 mm of average shift when typical targets and tolerances were considered. For plans with PTVs between 0 and 5 cm3, a 1 mm shift reduced PTV coverage by 6.6 ± 2.2% and caused a 12.1 ± 3.8% deterioration in the conformity index. Based on the results of this study, the prevalent tolerance of 1 mm may not be suitable for treatments of small PTVs with small fields.SignificanceIn contrast to commonly accepted values, an absolute mechanical isocentre of 0.5 mm with action level at 0.75 mm is recommended for stereotactic VMAT of PTV sizes below 10 cm3.
Collapse
|
4
|
Pudsey LMM, Biasi G, Ralston A, Rosenfeld A, Poder J. Detection of rotational errors in single-isocenter multiple-target radiosurgery: Is a routine off-axis Winston-Lutz test necessary? J Appl Clin Med Phys 2022; 23:e13665. [PMID: 35713881 PMCID: PMC9512335 DOI: 10.1002/acm2.13665] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2021] [Revised: 03/30/2022] [Accepted: 05/09/2022] [Indexed: 12/02/2022] Open
Abstract
Purpose Recently the use of linear accelerator (linac)‐based stereotactic radiosurgery (SRS) has increased, including single‐isocenter multiple‐target SRS. The workload of medical physicists has grown as a result and so has the necessity of maximizing the efficiency of quality assurance (QA). This study aimed to determine if measurement‐based patient‐specific QA with a high‐spatial‐resolution dosimeter is sensitive to rotational errors, potentially reducing the need for routine off‐axis Winston–Lutz (WL) testing. Methods The impact of rotational errors along gantry, couch, and collimator axes on dose coverage of the gross tumor volume (GTV) and planning target volume (PTV) was determined with a 1‐mm GTV/PTV expansion margin. Two techniques, the off‐axis WL test using the StereoPHAN MultiMet‐WL Cube (Sun Nuclear Corporation, Melbourne, Florida, USA) and patient‐specific QA using the SRS MapCHECK (Sun Nuclear Corporation, Melbourne, Florida, USA), were assessed on their ability to detect introduced errors before target coverage was compromised. These findings were also considered in the context of routine machine QA of rotational axis calibrations. Results Rotational errors significantly impacted PTV dose coverage, especially in the couch angle. GTV dose coverage remained unaffected except for with large couch angle errors (≥1.5°). The off‐axis WL test was shown to be sensitive to rotational errors with results consistently exceeding tolerance levels when or before coverage fell below departmentally accepted limits. Although patient‐specific QA using the SRS MapCHECK was previously validated for SRS, this study showed inconsistency in detection of rotational errors. Conclusions It is recommended that off‐axis WL testing be conducted regularly to supplement routine monthly machine QA, as it is sensitive to errors that patient‐specific QA may not detect. This frequency should be determined by individual departments, with consideration of GTV–PTV margins used, limitations on target off‐axis distances, and routine mechanical QA results for particular linacs.
Collapse
Affiliation(s)
- Lauren M M Pudsey
- Centre for Medical Radiation Physics, School of Physics, University of Wollongong, Wollongong, New South Wales, Australia
| | - Giordano Biasi
- Centre for Medical Radiation Physics, School of Physics, University of Wollongong, Wollongong, New South Wales, Australia.,Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia
| | - Anna Ralston
- St George Hospital Cancer Care Centre, Kogarah, New South Wales, Australia
| | - Anatoly Rosenfeld
- Centre for Medical Radiation Physics, School of Physics, University of Wollongong, Wollongong, New South Wales, Australia
| | - Joel Poder
- Centre for Medical Radiation Physics, School of Physics, University of Wollongong, Wollongong, New South Wales, Australia.,St George Hospital Cancer Care Centre, Kogarah, New South Wales, Australia
| |
Collapse
|
5
|
Wegener S, Schindhelm R, Sauer OA. Implementing corrections of isocentric shifts for the stereotactic irradiation of cerebral targets: Clinical validation. J Appl Clin Med Phys 2022; 23:e13577. [PMID: 35234345 PMCID: PMC9121032 DOI: 10.1002/acm2.13577] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2021] [Revised: 02/01/2022] [Accepted: 02/14/2022] [Indexed: 11/10/2022] Open
Abstract
Purpose: Any Linac will show geometric imprecisions, including non‐ideal alignment of the gantry, collimator and couch axes, and gantry sag or wobble. Their angular dependence can be quantified and resulting changes of the dose distribution predicted (Wack, JACMP 20(5), 2020). We analyzed whether it is feasible to correct geometric shifts during treatment planning. The successful implementation of such a correction procedure was verified by measurements of different stereotactic treatment plans. Methods: Isocentric shifts were quantified for two Elekta Synergy Agility Linacs using the QualiForMed ISO‐CBCT+ module, yielding the shift between kV and MV isocenters, the gantry flex and wobble as well as the positions of couch and collimator rotation axes. Next, the position of each field's isocenter in the Pinnacle treatment planning system was adjusted accordingly using a script. Fifteen stereotactic treatment plans of cerebral metastases (0.34 to 26.53 cm3) comprising 9–11 beams were investigated; 54 gantry and couch combinations in total. Unmodified plans and corrected plans were measured using the Sun Nuclear SRS‐MapCHECK with the Stereophan phantom and evaluated using gamma analysis. Results: Geometric imprecisions, such as shifts of up to 0.8 mm between kV and MV isocenter, a couch rotation axis 0.9 mm off the kV isocente,r and gantry flex with an amplitude of 1.1 mm, were found. For eight, mostly small PTVs D98 values declined more than 5% by simulating these shifts. The average gamma (2%/2 mm, absolute, global, 20% threshold) was reduced from 0.53 to 0.31 (0.32 to 0.30) for Linac 1 (Linac 2) when including the isocentric corrections. Thus, Linac 1 reached the accuracy level of Linac 2 after correction. Conclusion: Correcting for Linac geometric deviations during the planning process is feasible and was dosimetrically validated. The dosimetric impact of the geometric imperfections can vary between Linacs and should be assessed and corrected where necessary.
Collapse
Affiliation(s)
- Sonja Wegener
- Department of Radiation Oncology, University of Wuerzburg, Wuerzburg, Germany
| | - Robert Schindhelm
- Department of Radiation Oncology, University of Wuerzburg, Wuerzburg, Germany
| | - Otto A Sauer
- Department of Radiation Oncology, University of Wuerzburg, Wuerzburg, Germany
| |
Collapse
|
6
|
Michael Gach H, Curcuru AN, Kim T, Yang D. Technical Note: Effects of rotating gantry on magnetic field and eddy currents in 0.35 T MRI-guided radiotherapy (MR-IGRT) system. Med Phys 2021; 48:7228-7235. [PMID: 34520081 DOI: 10.1002/mp.15226] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2021] [Revised: 07/19/2021] [Accepted: 09/04/2021] [Indexed: 01/03/2023] Open
Abstract
PURPOSE The purpose of this study was to identify the cause of severe image artifacts that occurred during gantry rotation in a 0.35 T MRI-Linac by comparing measurements of eddy currents, center frequency, and field inhomogeneities made with the gantry in motion and stationary. METHODS Gradient and B0 eddy currents were calculated from the free induction decays (FIDs) resulting from selective excitation at a temporal resolution of 200 ms/measurement. B0 eddy currents were also calculated from FIDs acquired with nonselective excitation at a temporal resolution of 100 ms/measurement. Center frequencies and B0 inhomogeneities were measured by acquiring FIDs with a repetition time (TR) of 290 ms. Cartesian and radial 2D true fast imaging with steady-state precession (TrueFISP) pulse sequences used in real-time MRI-guided radiation therapy (MR-IGRT) were acquired. To assess artifact severity, the normalized root mean square error (nRMSE) was calculated between a reference MRI (static gantry) and MRIs acquired during gantry rotation for each serial acquisition. Image artifacts were qualitatively graded as nominal, minor, or severe. Measurements were conducted while the gantry was rotated through its entire range for both clockwise and counterclockwise. Measurements during gantry rotation were compared to measurements with a stationary gantry (every 30°). RESULTS Severe image artifacts were observed 22-35% of the time while the gantry was rotating. Short time constant eddy currents were not affected by gantry rotation. The peak to peak center frequency and FWHM rose by factors of 13.2-14.5 and 1.1-1.6, respectively, for the rotating versus stationary gantry. The magnitude of the center frequency offset and field inhomogeneities depended on the direction of the gantry rotation. CONCLUSIONS Image artifacts during gantry rotation were primarily caused by center frequency variations and field inhomogeneities. Therefore, dynamic B0 compensation techniques should be able to reduce artifacts during gantry rotation.
Collapse
Affiliation(s)
- H Michael Gach
- Departments of Radiation Oncology, Radiology, and Biomedical Engineering, Washington University in St. Louis, St. Louis, Missouri, USA
| | - Austen N Curcuru
- Department of Biomedical Engineering, Washington University in St. Louis, St. Louis, Missouri, USA
| | - Taeho Kim
- Department of Radiation Oncology, Washington University in St. Louis, St. Louis, Missouri, USA
| | - Deshan Yang
- Departments of Radiation Oncology and Biomedical Engineering, Washington University in St. Louis, St. Louis, Missouri, USA
| |
Collapse
|