A dosimetric uncertainty analysis for photon‐emitting brachytherapy sources: Report of AAPM Task Group No. 138 and GEC‐ESTRO

The purpose of this study is to evaluate patient setup accuracy and quantify individual and cumulative positioning uncertainties associated with different hardware and software components of the stereotactic radiotherapy (SRS/SRT) with the frameless 6D ExacTrac system. A statistical model is used to evaluate positioning uncertainties of the different components of SRS/SRT treatment with the Brainlab 6D ExacTrac system using the positioning shifts of 35 patients having cranial lesions. All these patients are immobilized with rigid head‐and‐neck masks, simulated with Brainlab localizer and planned with iPlan treatment planning system. Stereoscopic X‐ray images (XC) are acquired and registered to corresponding digitally reconstructed radiographs using bony‐anatomy matching to calculate 6D translational and rotational shifts. When the shifts are within tolerance (0.7 mm and 1°), treatment is initiated. Otherwise corrections are applied and additional X‐rays (XV) are acquired to verify that patient position is within tolerance. The uncertainties from the mask, localizer, IR ‐frame, X‐ray imaging, MV, and kV isocentricity are quantified individually. Mask uncertainty (translational: lateral, longitudinal, vertical; rotational: pitch, roll, yaw) is the largest and varies with patients in the range false(−2.07−3.71mm,−5.82−5.62mm,−5.84−3.61mm;−2.10−2.40∘,−2.23−2.60∘,and−2.7−3.00∘false) obtained from mean of XC shifts for each patient. Setup uncertainty in IR positioning (0.88, 2.12, 1.40 mm, and 0.64°, 0.83°, 0.96°) is extracted from standard deviation of XC. Systematic uncertainties of the frame (0.18, 0.25, −1.27mm, −0.32∘, 0.18°, and 0.47°) and localizer (−0.03, −0.01, 0.03 mm, and −0.03∘, 0.00°, −0.01∘) are extracted from means of all XV setups and mean of all XC distributions, respectively. Uncertainties in isocentricity of the MV radiotherapy machine are (0.27, 0.24, 0.34 mm) and kV imager (0.15, −0.4, 0.21 mm). A statistical model is developed to evaluate the individual and cumulative systematic and random positioning uncertainties induced by the different hardware and software components of the 6D ExacTrac system. The uncertainties from the mask, localizer, IR frame, X‐ray imaging, couch, MV linac, and kV imager isocentricity are quantified using statistical modeling.PACS number(s): 87.56.B‐, 87.59.B‐

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Quantitative evaluation of patient setup uncertainty of stereotactic radiotherapy with the frameless 6D ExacTrac system using statistical modeling

Keeling

Hossain

Jin

et al. 2016

“…As reported in literature, 14 , 15 the HDR VCBT has some significant uncertainties. The uncertainties which existed in this study include source strength (2%); treatment planning (3%); medium dosimetry corrections (1%); volume contouring (3%); and dose delivery, including registration of applicator geometry to anatomy (5%), and interfraction/intrafraction changes between imaging and dose delivery (5%).…”

Section: Discussionmentioning

confidence: 94%

Dosimetric impact of cylinder size in high‐dose rate vaginal cuff brachytherapy (VCBT) for primary endometrial cancer

Zhang

Gopalakrishnan

Lee

et al. 2016

The purpose of this study was to evaluate the dosimetric impact of cylinder size in high‐dose‐rate (HDR) vaginal cuff brachytherapy (VCBT). Sample plans of HDR VCBT in a list of cylinders ranging from 2.5 to 4 cm in diameter at 0.5 cm increment were created and analyzed. The doses were prescribed either at the 0.5 cm depth with 5.5 Gy for 4 fractions or at the cylinder surface with 8.8 Gy for 4 fractions, in various treatment lengths. A 0.5 cm shell volume called PTV_Eval was contoured for each plan and served as the target volume for dosimetric evaluation. The cumulative and differential dose volume histograms (c‐DVH and d‐DVH), mean doses (D‐mean) and the doses covering 90% (D90), 10% (D10), and 5% (D5) of PTV_Eval were calculated. In the 0.5 cm depth regimen, the DVH curves were found to have shifted toward the lower dose zone when a larger cylinder was used, but in the surface regimen the DVH curves shifted toward the higher dose zone as the cylinder size increased. The D‐means of the both regimens were between 6.9 and 7.8 Gy and dependent on the cylinder size but independent of the treatment length. A 0.5 cm variation of diameter could result in a 4% change of D‐mean. Average D90s were 5.7 (ranging from 5.6 to 5.8 Gy) and 6.1 Gy (from 5.7 to 6.4 Gy), respectively, for the 0.5 cm and surface regimens. Average D10 and D5 were 9.2 and 11 Gy, respectively, for the 0.5 cm depth regimen, and 8.9 and 9.7 Gy, respectively, for the surface regimen. D‐mean, D90, D10, and D5 for other prescription doses could be calculated from the lookup tables of this study. Results indicated that the cylinder size has moderate dosimetric impact, and that both regimens are comparable in dosimetric quality.PACS number(s): 87.61.‐c, 87.53.Jw, 87.19.xj

“…The dose rate constants varied with different coating thicknesses, coating mass densities, photon interaction cross‐section libraries, and photon emission spectrum types 16, 26. Aryal P et al30 found that varying 125 I coating thickness, coating mass density, photon cross‐section library, and photon emission spectrum for the model IA125I A 125 I seed changed the dose rate constant by up to 0.9%, about 1%, about 3%, and 3%, respectively, in comparison to the proposed standard value of 0.922

c G y \cdot h^{- 1} \cdot U^{- 1}

.…”

Section: Discussionmentioning

confidence: 99%

“…The uncertainties associated with our TLD measurements were estimated by following the guidelines of the AAPM TG‐138 and TG‐43U1 reports 16, 26. The uncertainty associated with TLD‐seed relative positioning should take into account the 0.05 mm machining tolerance of the PMMA phantom reported by the manufacturer.…”

Section: Methodsmentioning

confidence: 99%

Dosimetric characterization of GMS BT‐125‐1 ¹²⁵I radioactive seed with Monte Carlo simulations and experimental measurement

Zhao

Yang

Ren

et al. 2017

PurposeTo investigate the dosimetric characteristics of the new GMS BT‐125‐1 125I radioactive seed, including dose rate constant, radial dose functions, and anisotropy functions.MethodsDosimetric parameters of GMS BT‐125‐1 125I seed including dose rate constant, radial dose functions, and anisotropy functions were calculated using the Monte Carlo code of MCNP5, and measured with thermoluminescent dosimeters (TLDs). The results were compared with those of PharmaSeed BT‐125‐1, PharmaSeed BT‐125‐2 125I, and model 6711 125I seeds.ResultsThe dose rate constant of GMS BT‐125‐1 125I seed was 0.959 cGy·h−1·U−1, with the difference of 0.94%, 0.83%, and 0.73% compared with the PharmaSeed BT‐125‐1 125I seed, PharmaSeed BT‐125‐2 125I seed, and Model 6711 125I seed, respectively. For radial dose function, the differences between the Monte Carlo and the experimental g(r) results were mostly within 10%. Monte Carlo results of g(r) for GMS BT‐125‐1 125I seed were found in agreement (within 3.3%) with corresponding results for the PharmaSeed BT‐125‐2 125I seed. The largest differences were 8.1% and 6.2% compared with PharmaSeed BT‐125‐1 125I seed and model 6711 125I seed, respectively. For anisotropy function, the difference between GMS BT‐125‐1 125I seed and PharmaSeed BT‐125‐2 125I seed was typically <10%.ConclusionsThe measured dose rate constant, radial dose functions, and two‐dimensional anisotropy functions for the GMS BT‐125‐1 125I seed showed good agreement with the Monte Carlo results. The dose rate constant of the GMS BT‐125‐1 125I seed is similar to that of the PharmaSeed BT‐125‐1 125I seed, the PharmaSeed BT‐125‐2 125I seed, and the model 6711 125I seed. For radial dose functions and two‐dimensional anisotropy functions, the GMS BT‐125‐1 125I seed is similar to the PharmaSeed BT‐125‐2 125I seed but different from the PharmaSeed BT‐125‐1 125I seed and the model 6711 125I seed.