3D isocentricity analysis for clinical linear accelerators

Velten, Christian; Wang, Yifang; Adamovics, J

doi:10.1002/mp.14039

Cited by 6 publications

(19 citation statements)

References 27 publications

(53 reference statements)

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“…With the 3D coordinates of the paired spots known, a set of lines representing the central axes of beam rays in 3D space can be calculated. The center of the smallest sphere intersecting with all these lines represents the location of the radiation isocenter 24,25 . To find this smallest sphere, the Ritter algorithm was employed.…”

Section: Methodsmentioning

confidence: 99%

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A two‐layer cylinder phantom developed for film‐based isocenter verification of radiotherapy machine

Song

Yan

et al. 2021

Medical Physics

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Purpose A two‐layer cylinder (TLC) phantom was developed for simplifying film‐based isocenter verification of linear accelerators in radiotherapy. Methods and materials The phantom mainly consists of two parts: (1) two nested solid cylinders between which a radiochromic film can be inserted and irradiated; (2) a tungsten ball supported by a thin rod and located at the phantom center for alignment with the mechanical isocenter. In practice, the phantom was first positioned by the room laser to align the tungsten ball to the mechanical isocenter of the linear accelerator. Then, a radiochromic film was precisely inserted into the gap between the two cylinders of the phantom and irradiated by beams with preset gantry and couch angles. Later the irradiated film was scanned and processed by an in‐house developed analysis software. Finally, the offset of the radiation isocenter from the mechanical isocenter was determined by the built‐in three‐dimensional (3D) reconstruction algorithms. The accuracy of this method was evaluated by positioning the phantom with a known couch shift, then checking the residual error after couch shift correction. The reliability of this method was evaluated by comparing the calculated offset with the corresponding result determined by the traditional star‐shot method. Results For the accuracy test, the residual errors were −0.14 ± 0.03 mm, 0.05 ± 0.06 mm, and 0.05 ± 0.06 mm in the lateral, longitudinal, and vertical axes, respectively. For the reliability test, the differences between the calculated offset and the result determined by the star‐shot method were −0.10 mm, 0.12 mm, and 0.12 mm in the lateral, longitudinal, and vertical axes, respectively. Conclusion The proposed method is able to reconstruct beams in 3D with one film, which is more time‐saving and accurate. Additionally, with this design, the phantom positioning, film loading, beam delivery, and data analyzing are simpler. This phantom and analysis software provides an efficient and effective way to perform film‐based isocenter verification of linear accelerators in radiotherapy.

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Section: Methodsmentioning

confidence: 99%

“…The center of the smallest sphere intersecting with all these lines represents the location of the radiation isocenter. 24,25 To find this smallest sphere, the Ritter algorithm was employed. The radius of the smallest sphere r s can be calculated and used to measure the rotational stability of linear accelerators.…”

Section: D Reconstructionmentioning

confidence: 99%

A two‐layer cylinder phantom developed for film‐based isocenter verification of radiotherapy machine

Song

Yan

et al. 2021

Medical Physics

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“…This is an unusually high threshold, but with 9 to 11 fields per plan, this choice avoids detecting small geometric shifts at the beam edges far from the target and restricts the analysis to the more relevant region where dose contributions from different beams overlap. All planned dose distributions were calculated and exported using (1 mm) 3 voxel size. While Low and Dempsey 15 suggest using a spatial criterion as three times the calculation grid size or more for accurate calculation of gamma values in steep dose gradient regions, here, for the calculation grid size of 1 mm a stricter criterion of 2%/2 mm was chosen, as in Wack et al 4 (see this reference for a further discussion of the choice).…”

Section: Ta B L Ementioning

confidence: 99%

“…Despite all efforts to improve patient positioning and delivery accuracy, mechanical imperfections of these machines remain. [3][4][5][6][7][8] With the increasing consideration of irradiation of multiple metastases as a treatment option, 9 and the increasing use of singleisocenter multi-target irradiations, 10 geometric problems come even more into the focus.…”

Section: Introductionmentioning

confidence: 99%

Implementing corrections of isocentric shifts for the stereotactic irradiation of cerebral targets: Clinical validation

Wegener

Schindhelm

Sauer

2022

J Applied Clin Med Phys

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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 cm 3 ) 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.

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“…5,6 The radiochromic response observed in PRESAGE dosimeters following their exposure to ionizing radiation is the result of the radiolytic oxidation of the leuco dye to dye via the generation of free radicals from the dissociation of the covalent bond in the free radical initiator. Current dose distribution studies involving PRESAGE have utilized them as either cuvettes [7][8][9][10] or as cylindrical dosimeters [11][12][13][14] formulated using leucomalachite green (LMG) as the leuco dye with varying concentrations of chloroform or bromoform as the radical initiator. Current studies in the investigation of PRESAGE dosimetry have been principally in the measurement of 3D dose distributions using optical CT (OCT) scanning.…”

Section: Introductionmentioning

confidence: 99%

Temperature dependence and temporal stability of stacked radiochromic sheets for three‐dimensional dose verification

et al. 2020

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Purpose Recently a novel radiochromic sheet dosimeter, termed as PRESAGE sheets, consisting of leuco crystal violet dye and radical initiator had been developed and characterized. This study examines the dosimeter’s temporal stability and storage temperature dependence postirradiation, and its applicability for dose verification in three dimensions (3D) as a stack dosimeter. Methods PRESAGE sheets were irradiated using 6 MV photons at a dose range of 0–20 Gy with the change in optical density measured using a flatbed scanner. Following their irradiation, PRESAGE sheets were stored in different temperature environments (–18 °C, 4 °C, and 22 °C) and scanned at different time points, ranging from 1 to 168 h postirradiation, to track changes in measured signal and linearity of dose response. Multiple PRESAGE sheets were bound together to create a 12 × 13 × 8.7 cm3 film stack, with EBT3 film inserted between the sheets in the central region of the stack, that was treated using a clinical VMAT plan. Based on the results from the time and storage temperature study, two‐dimensional (2D) relative dose distribution measurements in PRESAGE were acquired promptly following irradiation at selected planes in the coronal, sagittal, and axial orientation of the film stack and compared to the treatment planning system calculations in their respective axes. Dose distribution measurements on the coronal axis of the stack dosimeter were also independently verified using EBT3 film. Results The dose response was observed to be linear (R2> 0.995) with sheets stored in colder temperatures retaining their signal and dose response sensitivity for extended periods postirradiation. Sheets stored in 22 °C environment should be measured within an hour postirradiation. Sheets stored in a 4 °C and −18 °C environment can be scanned up to 20‐ and 72 h postirradiation, respectively, while preserving the integrity of their dose response sensitivity and linearity of dose response within a mean absolute percent error of 2.0%. For instance, at 20 h postirradiation the dose response sensitivity for sheets stored in a −18 °C, 4 °C, and 22 °C temperature environment was measured to be 97%, 91%, and 77% of their original values measured within an hour postirradiation, respectively. The 2D gamma pass rate for central slices exceed 95% for PRESAGE film stack compared with treatment planning system on selected planes in the axial, coronal, and sagittal orientation and EBT3 film in the coronal orientation using a 2D gamma index of 2%/2mm. The gamma pass rate in comparing the calculated dose distribution with the measured dose distribution from PRESAGE‐LCV was observed to decrease in sheets scanned at later elapsed times postirradiation. In one example, the gamma pass rate for 2%/2mm criteria in the coronal plane was observed to decrease from 97.7% pass rate when scanned within an hour postirradiation to 92.1% pass rate when scanned at 20 h postirradiation under room temperature conditions. Conclusions This is the first study to demonstrate that the temporal s...

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3D isocentricity analysis for clinical linear accelerators

Cited by 6 publications

References 27 publications

A two‐layer cylinder phantom developed for film‐based isocenter verification of radiotherapy machine

A two‐layer cylinder phantom developed for film‐based isocenter verification of radiotherapy machine

Implementing corrections of isocentric shifts for the stereotactic irradiation of cerebral targets: Clinical validation

Temperature dependence and temporal stability of stacked radiochromic sheets for three‐dimensional dose verification

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