The “collimator monitoring fill factor” of a two‐dimensional detector array, a measure of its ability to detect collimation errors

Stelljes, Tenzin Sonam; Looe, Hui Khee; Harder, Dietrich; Poppe, Björn

doi:10.1002/mp.12106

Cited by 6 publications

(14 citation statements)

References 19 publications

(42 reference statements)

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“…Compared to other arrays a higher conformity of detected drift and actual robot shift could be observed. There appears to be no position dependency when analyzing beam profile shifts using the 1000SRS and the Nonius, confirming previous results [5] now with cylindrical beams. The SRS-Profiler on the other hand shows higher deviations with robot shifts greater than 1mm.…”

Section: Discussionsupporting

confidence: 87%

Usability and accuracy of high-resolution detectors for daily quality assurance for robotic radiosurgery

Loutfi-Krauss

Damme

Stelljes

et al. 2017

Current Directions in Biomedical Engineering

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For daily CyberKnife QA a Winston-Lutz-Test (Automated-Quality-Assurance, AQA) is used to determine sub-millimeter deviations in beam delivery accuracy. This test is performed using gafchromic film, an extensive and user-dependent method requiring the use of disposables. We therefore analyzed the usability and accuracy of high-resolution detector arrays. We analyzed a liquid-filled ionization-chamber array (Octavius 1000SRS, PTW, Germany), which has a central resolution of 2.5mm. To test sufficient sensitivity, beam profiles with robot shifts of 0.1mm along the arrays' axes were measured. The detected deviation between the shifted and central profile were compared to the real robot's position. We then compared the results to the SRS-Profiler (SunNuclear, USA) with 4.0mm resolution and to the Nonius (QUART, Germany), a single-line diode detector with 2.8mm resolution. Finally, AQA variance and usability were analyzed performing a number of AQA tests over time, which required the use of specially designed fixtures for each array, and the results were compared to film. Concerning sensitivity, the 1000SRS detected the beam profile shifts with a maximum difference of 0.11mm (mean deviation = 0.03mm) compared to the actual robot shift. The Nonius and SRSProfiler showed differences of up to 0.15mm and 0.69mm with mean deviation of 0.05mm and 0.18mm, respectively. Analyzing the variation of AQA results over time, the 1000SRS showed a comparable standard deviation to film (0.26mm vs. 0.18mm). The SRS-Profiler and the Nonius showed a standard deviation of 0.16mm and 0.24mm, respectively. The 1000SRS seems to provide equivalent accuracy and sensitivity to the gold standard film when performing daily AQA tests. Compared to other detectors in our study the sensitivity as well as the accuracy of the 1000SRS appears to be superior and more user-friendly. Furthermore, no significant modification of the standard AQA procedure is required when introducing 1000SRS for CyberKnife AQA.

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Section: Discussionsupporting

confidence: 87%

Usability and accuracy of high-resolution detectors for daily quality assurance for robotic radiosurgery

Loutfi-Krauss

Damme

Stelljes

et al. 2017

Current Directions in Biomedical Engineering

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“…In this study, the lateral dose response functions of the two detector arrays are approximated as ideal step functions. As shown in previous studies, in which these lateral dose response functions have been studied for high energetic photon beams, 26,28,45 the functions characterize on one hand the volume-averaging effect due to the extensive dimension of the single detectors and on the other hand the charged particle fluence perturbation due to the presence of the non-water equivalent components of the arrays, including the air cavities of the chambers. In high energetic proton beams, the average energy of the secondary electrons in water is lower than in photon beams and thus their range in the medium and the detector's material is limited.…”

Section: B the Role Of The Detector's Volume Effectmentioning

confidence: 83%

Analysis of the applicability of two‐dimensional detector arrays in terms of sampling rate and detector size to verify scanned intensity‐modulated proton therapy plans

et al. 2020

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The introduction of advanced treatment techniques in proton therapy, such as intensitymodulated proton therapy, leads to an increased need for patient-specific quality assurance, especially an accurate treatment plan verification becomes inevitable. In this study, signal theoretical analysis of dose distributions in scanned proton therapy is performed to investigate the feasibility and limits of two-dimensional (2D) detector arrays for treatment plan verification. Methods: 2D detector arrays are characterized by two main aspects: the distance between the single detectors on the array or the sampling frequency; and the lateral response functions of a single detector. The analysis is based on single spots, reference fields and on measured and calculated dose distributions of typical intensity-modulated proton therapy treatment plans with and without range shifter. Measurements were performed with Gafchromic EBT3 films (Ashland Speciality Ingredients G.P., Bridgewater, NJ, USA), the MatriXX PT detector array (IBA Dosimetry, Schwarzenbruck, Germany) and the OCTAVIUS detector array 1500XDR (PTW-Freiburg, Germany) at an IBA Proteus PLUS proton therapy system (Ion Beam Applications, Louvain-la-Neuve, Belgium). Dose calculations were performed with the treatment planning system RayStation 6 or 8 (RaySearch Laboratories, Sweden). Results: The Fourier analysis of the data of the treatment planning system and film measurements show maximum frequencies of 0.06/mm for the plan with range shifter and 0.083/mm for the plan without range shifter. According to the Nyquist theorem, this corresponds to minimum required sampling distances of 8.3 and 6 mm, respectively. By comparison, the sampling distances of the arrays of 7.6 mm (MatriXX PT) and 7.1 mm (OD1500XDR) are sufficient to reconstruct the dose distributions adequately from measurements if range shifters are used, whereas some fields of the plans without range shifter violated the Nyquist requirement. The lateral dose response functions of the single detectors within the arrays have clearly higher frequencies than the treatment plans and thus the volume effect only slightly influences the measurements. Consequently, the array measurements show high gamma passing rates with at least 96 % and a good agreement between the investigated line profiles. Conclusion: The results indicate that the detector dimensions and sampling distances of the arrays are in most studied cases adequate not to substantially influence the measurement process when they are used for analyzing typical intensity-modulated proton therapy treatment plans. Nevertheless, clinical conditions have been identified, for instance treatment plans without range shifter, under which the Nyquist theorem is violated such that a full representation of the dose distributions with the measurements is not feasible. In these cases, analysis of measurements is limited to pointwise comparisons.

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“…Two‐dimensional detector arrays are used for daily QA measurements and pretreatment plan verifications in clinical routine . Aside from the detector size, the spatial sampling frequency, and the width and shape of the fluence response function of a single 2D‐array detector, the fill factor is another characteristic of a 2D‐array …”

Section: Introductionmentioning

confidence: 99%

“…However, this definition only depends on the FWHM and does not take into account the full shape of the fluence response function. The collimator monitoring fill factor (CM‐FF) has been introduced in a previous work by our working group . It is defined as the probability to detect a collimator position error occurring somewhere on a 2D‐array's surface area.…”

Section: Introductionmentioning

confidence: 99%

“…A collimator error is detectable if its signal change caused by a collimator error of Δ mm, relative to a homogeneously irradiated detector, exceeds a detection threshold d . Based on calculations with simple Monte Carlo detector models, it was shown that for detectors with narrow bell‐shaped response functions, such as diodes and liquid‐filled ionizations chambers, the FWHM fill factor (FWHM‐FF) underestimates the error detection capability of a 2D‐array . For arrays consisting of vented ionization chambers, the differences between both fill factors were less pronounced.…”

Section: Introductionmentioning

confidence: 99%

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Experimental determination of the “collimator monitoring fill factor” and its relation to the error detection capabilities of various 2D‐arrays

et al. 2019

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Purpose: The collimator monitoring fill factor (CM-FF) introduced by Stelljes et al. (2017) and the FWHM fill factor (FWHM-FF) introduced by Gago-Arias et al. (2012) were determined using the measured photon fluence response functions of various 2D-arrays. The error detection capabilities of 2D-arrays were studied by comparing detector signal changes and local gamma index passing rates in different field setups with introduced collimation errors. Methods: The fill factor is defined as the ratio of the sensitive detector area and the cell area of a detector, defined by the detector arrangement on a 2D-array. Gago-Arias et al. calculated the FWHM-FF, using the FWHM² of a detector's fluence response function K M (x) as the sensitive detector area. For the CM-FF a sensitive detector width w(D mm, d%) is calculated. The sensitive detector width is the lateral extent of K M (x), lying inside the detector cell area, along which a collimator error of D mm yields a signal change exceeding a detection threshold of d%. The sensitive area for a single detector is calculated using w(D mm, d%)². The CM-FF is then calculated as the ratio of the sensitive area of a detector within its cell area and the detector cell area. The fluence response functions of the central detector of the OCTAVIUS 729, 1500, and 1000 SRS array (all PTW-Freiburg, Freiburg, Germany) and the Map-CHECK 2 array (Sun Nuclear, Melbourne, US) were measured using a photon slit beam. The FWHM-FF and the CM-FF were calculated and compared for all 2D-arrays under investigation. The error detection capabilities of 2D-arrays in quadratic fields were studied by investigating the signal changes in the detectors adjacent to the collimator edge when changing the collimator position. The change in local gamma index passing rate with respect to the introduced collimator error was investigated for an ionization chamber and a diode array in quadratic and two intensity modulated fields. Results: Values for the CM-FF and FWHM-FF were 1.0 and 0.35, respectively for the area of the liquid-filled 1000 SRS ionization chamber array with a detector to detector distance of 5 mm and 0.32 and 0.04, respectively, for the MapCHECK 2 diode array. For the vented ionization chamber array OCTAVIUS 729 fill factors were calculated as CM-FF = 0.59 and FWHM-FF = 0.53, while the OCTAVIUS 1500 array yielded fill factors of CM-FF = 0.77 and FWHM-FF = 0.72. Signal changes in vented ionization chambers for collimator errors of 1 mm surpassed those of diodes by a factor of 2 in quadratic fields. The gamma index passing rates in quadratic fields reflect those findings. In intensity modulated fields, the decline of the gamma index passing rate is bigger for the ionization chamber array compared to the diode array when introducing collimator errors. Conclusions:The calculated values of the CM-FF correlate with the signal changes in quadratic field setups with introduced collimator position errors of 1 mm, while the FWHM-FF underestimates the error detection capabilities of 2D-arrays. An increased er...

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The “collimator monitoring fill factor” of a two‐dimensional detector array, a measure of its ability to detect collimation errors

Cited by 6 publications

References 19 publications

Usability and accuracy of high-resolution detectors for daily quality assurance for robotic radiosurgery

Usability and accuracy of high-resolution detectors for daily quality assurance for robotic radiosurgery

Analysis of the applicability of two‐dimensional detector arrays in terms of sampling rate and detector size to verify scanned intensity‐modulated proton therapy plans

Experimental determination of the “collimator monitoring fill factor” and its relation to the error detection capabilities of various 2D‐arrays

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