K. Zourari

On the implementation of a recently proposed dosimetric formalism to a robotic radiosurgery system.

PURPOSE The aim of this work is to implement a recently proposed dosimetric formalism for nonstandard fields to the calibration and small field output factor measurement of a robotic stereotactic radiosurgery system. METHODS Reference dosimetry measurements were performed in the nonstandard, 60 mm diameter machine specific reference (msr) field using a Farmer ion chamber, five other cylindrical chambers with cavity lengths ranging from 16.25 down to 2.7 mm, and alanine dosimeters. Output factor measurements were performed for the 5, 7.5, 10, and 15 mm field sizes using microchambers, diode detectors, alanine dosimeters, TLD microcubes, and EBT Gafchromic films. Measurement correction factors as described in the proposed formalism were calculated for the ion chamber and diode detector output factor measurements based on published Monte Carlo data. Corresponding volume averaging correction factors were calculated for the alanine output factor measurements using 3D dose distributions, measured with polymer gel dosimeters. RESULTS Farmer chamber and alanine reference dosimetry results were found in close agreement, yielding a correction factor of k(Q(msr),Q)(f(msr),f(ref)) = 0.999 +/- 0.016 for the chamber readings. These results were also found to be in agreement within experimental uncertainties with corresponding results obtained using the shorter cavity length ionization chambers. The mean measured dose values of the latter, however, were found to be consistently greater than that of the Farmer chamber. This finding, combined with an observed inverse relationship between the mean measured dose and chamber cavity length that follows the trend predicted by theoretical volume averaging calculations in the msr field, implies that the Farmer k(Q(msr),Q)(f(msr),f(ref)) correction is greater than unity. Regarding the output factor results, deviations as large as 33% were observed between the different dosimeters used. These deviations were substantially decreased when appropriate correction factors were applied to the measured microchamber, diode, and alanine values. After correction, all diode and microchamber measured output factors agreed within 1.6% with the corresponding alanine measurements, and within 3.1% with the TLD measurements. The weighted mean output factors were 0.681 +/- 0.001, 0.824 +/- 0.001, 0.875 +/- 0.001, and 0.954 +/- 0.001 for the 5, 7.5, 10, and 15 mm beams, respectively. CONCLUSIONS The comparison of Farmer chamber measurements versus alanine reference dosimetry validates the use of the former for dosimetry in the msr field of this treatment delivery system. The corresponding results of this work obtained using chambers with different cavity lengths, combined with previous literature findings, suggest that a k(Q(msr),Q)(f(msr),f(ref)) Farmer chamber dose response correction factor of 1.01 may improve calibration measurement accuracy when using the proposed dosimetric formalism. The k(Q(msr),Q)(f(msr),f(ref)) correction factor is within 0.5% from unity for ion chambers with cavity lengths less than 10 mm. Substantial improvements in small field output factor measurement accuracy can be obtained when using microchambers and diodes by applying appropriately calculated correction factors to the detector measurements according to the proposed dosimetric formalism, and their routine use is therefore recommended.

Dosimetric accuracy of a deterministic radiation transport based 192Ir brachytherapy treatment planning system. Part I: single sources and bounded homogeneous geometries.

PURPOSE The aim of this work is to validate a deterministic radiation transport based treatment planning system (TPS) for single 192Ir brachytherapy source dosimetry in homogeneous water geometries. METHODS TPS results were obtained using the deterministic radiation transport option of a BRACHYVISION v. 8.8 system for three characteristic source designs (VS2000, GMPlus HDR, and GMPlus PDR) with each source either centered in a 15 cm radius spherical water phantom, or positioned at varying distance away from the phantom center. Corresponding MC simulations were performed using the MCNPX code v.2.5.0 and source geometry models prepared using information provided by the manufacturers. RESULTS Comparison in terms of the AAPM TG-43 dosimetric formalism quantities, as well as dose rate distributions per unit air kerma strength with a spatial resolution of 0.1 cm, yielded close agreement between TPS and MC results for the sources centered in the phantom. Besides some regions close to the source longitudinal axes where discrepancies could be characterized as systematic, overall agreement for all three sources studied is comparable to the statistical (type A) uncertainty of MC simulations (1% at the majority of points in the geometry increasing to 2%-3% at points lying both away from the source center and close to the source longitudinal axis). A corresponding good agreement was also found between TPS and MC results for the sources positioned away from the phantom center. CONCLUSIONS Results of this work attest the capability of the TPS to accurately account for the scatter conditions regardless of the size or shape of a given geometry of dosimetric interest, and the position of a source within it. This is important since, as shown in the literature and summarized also in this work, these factors could introduce a significant dosimetric effect that is currently ignored in clinical treatment planning. It is concluded that the implementation of the deterministic radiation transport option of the BRACHYVISION v. 8.8 system for 192Ir brachytherapy dosimetry in homogeneous water geometries yields results of comparable accuracy to the golden standard of Monte Carlo simulation, in clinically viable calculation times.

On the output factor measurements of the CyberKnife iris collimator small fields: Experimental determination of the kQclin,Qmsrfclin,fmsr correction factors for microchamber and diode detectors

PURPOSE To measure the output factors (OFs) of the small fields formed by the variable aperture collimator system (iris) of a CyberKnife (CK) robotic radiosurgery system, and determine the k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors for a microchamber and four diode detectors. METHODS OF measurements were performed using a PTW PinPoint 31014 microchamber, four diode detectors (PTW-60017, -60012, -60008, and the SunNuclear EDGE detector), TLD-100 microcubes, alanine dosimeters, EBT films, and polymer gels for the 5 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm iris collimators at 650 mm, 800 mm, and 1000 mm source to detector distance (SDD). The alanine OF measurements were corrected for volume averaging effects using the 3D dose distributions registered in polymer gel dosimeters. k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors for the PinPoint microchamber and the diode dosimeters were calculated through comparison against corresponding polymer gel, EBT, alanine, and TLD results. RESULTS Experimental OF results are presented for the array of dosimetric systems used. The PinPoint microchamber was found to underestimate small field OFs, and a k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factor ranging from 1.127 ± 0.022 (for the 5 mm iris collimator) to 1.004 ± 0.010 (for the 15 mm iris collimator) was determined at the reference SDD of 800 mm. The PinPoint k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factor was also found to increase with decreasing SDD; k(Q(clin),Q(msr) ) (f(clin),f(msr) ) values equal to 1.220 ± 0.028 and 1.077 ± 0.016 were obtained for the 5 mm iris collimator at 650 mm and 1000 mm SDD, respectively. On the contrary, diode detectors were found to overestimate small field OFs and a correction factor equal to 0.973 ± 0.006, 0.954 ± 0.006, 0.937 ± 0.007, and 0.964 ± 0.006 was measured for the PTW-60017, -60012, -60008 and the EDGE diode detectors, respectively, for the 5 mm iris collimator at 800 mm SDD. The corresponding correction factors for the 15 mm iris collimator were found equal to 0.997 ± 0.010, 0.994 ± 0.009, 0.988 ± 0.010, and 0.986 ± 0.010, respectively. No correlation of the diode k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors with SDD was observed. CONCLUSIONS This work demonstrates an experimental procedure for the determination of the k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors required to obtain small field OF results of increased accuracy.

A generic high‐dose rate 192Ir brachytherapy source for evaluation of model‐based dose calculations beyond the TG‐43 formalism

PURPOSE In order to facilitate a smooth transition for brachytherapy dose calculations from the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) formalism to model-based dose calculation algorithms (MBDCAs), treatment planning systems (TPSs) using a MBDCA require a set of well-defined test case plans characterized by Monte Carlo (MC) methods. This also permits direct dose comparison to TG-43 reference data. Such test case plans should be made available for use in the software commissioning process performed by clinical end users. To this end, a hypothetical, generic high-dose rate (HDR) (192)Ir source and a virtual water phantom were designed, which can be imported into a TPS. METHODS A hypothetical, generic HDR (192)Ir source was designed based on commercially available sources as well as a virtual, cubic water phantom that can be imported into any TPS in DICOM format. The dose distribution of the generic (192)Ir source when placed at the center of the cubic phantom, and away from the center under altered scatter conditions, was evaluated using two commercial MBDCAs [Oncentra(®) Brachy with advanced collapsed-cone engine (ACE) and BrachyVision ACUROS™ ]. Dose comparisons were performed using state-of-the-art MC codes for radiation transport, including ALGEBRA, BrachyDose, GEANT4, MCNP5, MCNP6, and PENELOPE2008. The methodologies adhered to recommendations in the AAPM TG-229 report on high-energy brachytherapy source dosimetry. TG-43 dosimetry parameters, an along-away dose-rate table, and primary and scatter separated (PSS) data were obtained. The virtual water phantom of (201)(3) voxels (1 mm sides) was used to evaluate the calculated dose distributions. Two test case plans involving a single position of the generic HDR (192)Ir source in this phantom were prepared: (i) source centered in the phantom and (ii) source displaced 7 cm laterally from the center. Datasets were independently produced by different investigators. MC results were then compared against dose calculated using TG-43 and MBDCA methods. RESULTS TG-43 and PSS datasets were generated for the generic source, the PSS data for use with the ace algorithm. The dose-rate constant values obtained from seven MC simulations, performed independently using different codes, were in excellent agreement, yielding an average of 1.1109 ± 0.0004 cGy/(h U) (k = 1, Type A uncertainty). MC calculated dose-rate distributions for the two plans were also found to be in excellent agreement, with differences within type A uncertainties. Differences between commercial MBDCA and MC results were test, position, and calculation parameter dependent. On average, however, these differences were within 1% for ACUROS and 2% for ace at clinically relevant distances. CONCLUSIONS A hypothetical, generic HDR (192)Ir source was designed and implemented in two commercially available TPSs employing different MBDCAs. Reference dose distributions for this source were benchmarked and used for the evaluation of MBDCA calculations employing a virtual, cubic water phantom in the form of a CT DICOM image series. The implementation of a generic source of identical design in all TPSs using MBDCAs is an important step toward supporting univocal commissioning procedures and direct comparisons between TPSs.

Dosimetric accuracy of a deterministic radiation transport based 192Ir brachytherapy treatment planning system: Part III. Comparison to Monte Carlo simulation in voxelized anatomical computational models

PURPOSE To compare TG43-based and Acuros deterministic radiation transport-based calculations of the BrachyVision treatment planning system (TPS) with corresponding Monte Carlo (MC) simulation results in heterogeneous patient geometries, in order to validate Acuros and quantify the accuracy improvement it marks relative to TG43. METHODS Dosimetric comparisons in the form of isodose lines, percentage dose difference maps, and dose volume histogram results were performed for two voxelized mathematical models resembling an esophageal and a breast brachytherapy patient, as well as an actual breast brachytherapy patient model. The mathematical models were converted to digital imaging and communications in medicine (DICOM) image series for input to the TPS. The MCNP5 v.1.40 general-purpose simulation code input files for each model were prepared using information derived from the corresponding DICOM RT exports from the TPS. RESULTS Comparisons of MC and TG43 results in all models showed significant differences, as reported previously in the literature and expected from the inability of the TG43 based algorithm to account for heterogeneities and model specific scatter conditions. A close agreement was observed between MC and Acuros results in all models except for a limited number of points that lay in the penumbra of perfectly shaped structures in the esophageal model, or at distances very close to the catheters in all models. CONCLUSIONS Acuros marks a significant dosimetry improvement relative to TG43. The assessment of the clinical significance of this accuracy improvement requires further work. Mathematical patient equivalent models and models prepared from actual patient CT series are useful complementary tools in the methodology outlined in this series of works for the benchmarking of any advanced dose calculation algorithm beyond TG43.

On the output factor measurements of the CyberKnife iris collimator small fields: Experimental determination of the correction factors for microchamber and diode detectors

PURPOSE To measure the output factors (OFs) of the small fields formed by the variable aperture collimator system (iris) of a CyberKnife (CK) robotic radiosurgery system, and determine the k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors for a microchamber and four diode detectors. METHODS OF measurements were performed using a PTW PinPoint 31014 microchamber, four diode detectors (PTW-60017, -60012, -60008, and the SunNuclear EDGE detector), TLD-100 microcubes, alanine dosimeters, EBT films, and polymer gels for the 5 mm, 7.5 mm, 10 mm, 12.5 mm, and 15 mm iris collimators at 650 mm, 800 mm, and 1000 mm source to detector distance (SDD). The alanine OF measurements were corrected for volume averaging effects using the 3D dose distributions registered in polymer gel dosimeters. k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors for the PinPoint microchamber and the diode dosimeters were calculated through comparison against corresponding polymer gel, EBT, alanine, and TLD results. RESULTS Experimental OF results are presented for the array of dosimetric systems used. The PinPoint microchamber was found to underestimate small field OFs, and a k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factor ranging from 1.127 ± 0.022 (for the 5 mm iris collimator) to 1.004 ± 0.010 (for the 15 mm iris collimator) was determined at the reference SDD of 800 mm. The PinPoint k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factor was also found to increase with decreasing SDD; k(Q(clin),Q(msr) ) (f(clin),f(msr) ) values equal to 1.220 ± 0.028 and 1.077 ± 0.016 were obtained for the 5 mm iris collimator at 650 mm and 1000 mm SDD, respectively. On the contrary, diode detectors were found to overestimate small field OFs and a correction factor equal to 0.973 ± 0.006, 0.954 ± 0.006, 0.937 ± 0.007, and 0.964 ± 0.006 was measured for the PTW-60017, -60012, -60008 and the EDGE diode detectors, respectively, for the 5 mm iris collimator at 800 mm SDD. The corresponding correction factors for the 15 mm iris collimator were found equal to 0.997 ± 0.010, 0.994 ± 0.009, 0.988 ± 0.010, and 0.986 ± 0.010, respectively. No correlation of the diode k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors with SDD was observed. CONCLUSIONS This work demonstrates an experimental procedure for the determination of the k(Q(clin),Q(msr) ) (f(clin),f(msr) ) correction factors required to obtain small field OF results of increased accuracy.

A generic TG-186 shielded applicator for commissioning model-based dose calculation algorithms for high-dose-rate 192Ir brachytherapy

Purpose: A joint working group was created by the American Association of Physicists in Medicine (AAPM), the European Society for Radiotherapy and Oncology (ESTRO), and the Australasian Brachytherapy Group (ABG) with the charge, among others, to develop a set of well‐defined test case plans and perform calculations and comparisons with model‐based dose calculation algorithms (MBDCAs). Its main goal is to facilitate a smooth transition from the AAPM Task Group No. 43 (TG‐43) dose calculation formalism, widely being used in clinical practice for brachytherapy, to the one proposed by Task Group No. 186 (TG‐186) for MBDCAs. To do so, in this work a hypothetical, generic high‐dose rate (HDR) 192Ir shielded applicator has been designed and benchmarked. Methods: A generic HDR 192Ir shielded applicator was designed based on three commercially available gynecological applicators as well as a virtual cubic water phantom that can be imported into any DICOM‐RT compatible treatment planning system (TPS). The absorbed dose distribution around the applicator with the TG‐186 192Ir source located at one dwell position at its center was computed using two commercial TPSs incorporating MBDCAs (Oncentra® Brachy with Advanced Collapsed‐cone Engine, ACE™, and BrachyVision ACUROS™) and state‐of‐the‐art Monte Carlo (MC) codes, including ALGEBRA, BrachyDose, egs_brachy, Geant4, MCNP6, and Penelope2008. TPS‐based volumetric dose distributions for the previously reported “source centered in water” and “source displaced” test cases, and the new “source centered in applicator” test case, were analyzed here using the MCNP6 dose distribution as a reference. Volumetric dose comparisons of TPS results against results for the other MC codes were also performed. Distributions of local and global dose difference ratios are reported. Results: The local dose differences among MC codes are comparable to the statistical uncertainties of the reference datasets for the “source centered in water” and “source displaced” test cases and for the clinically relevant part of the unshielded volume in the “source centered in applicator” case. Larger local differences appear in the shielded volume or at large distances. Considering clinically relevant regions, global dose differences are smaller than the local ones. The most disadvantageous case for the MBDCAs is the one including the shielded applicator. In this case, ACUROS agrees with MC within [−4.2%, +4.2%] for the majority of voxels (95%) while presenting dose differences within [−0.12%, +0.12%] of the dose at a clinically relevant reference point. For ACE, 95% of the total volume presents differences with respect to MC in the range [−1.7%, +0.4%] of the dose at the reference point. Conclusions: The combination of the generic source and generic shielded applicator, together with the previously developed test cases and reference datasets (available in the Brachytherapy Source Registry), lay a solid foundation in supporting uniform commissioning procedures and direct comparisons among treatment planning systems for HDR 192Ir brachytherapy.

On the experimental validation of model-based dose calculation algorithms for 192Ir HDR brachytherapy treatment planning

There is an acknowledged need for the design and implementation of physical phantoms appropriate for the experimental validation of model-based dose calculation algorithms (MBDCA) introduced recently in 192Ir brachytherapy treatment planning systems (TPS), and this work investigates whether it can be met. A PMMA phantom was prepared to accommodate material inhomogeneities (air and Teflon), four plastic brachytherapy catheters, as well as 84 LiF TLD dosimeters (MTS-100M 1  ×  1  ×  1 mm3 microcubes), two radiochromic films (Gafchromic EBT3) and a plastic 3D dosimeter (PRESAGE). An irradiation plan consisting of 53 source dwell positions was prepared on phantom CT images using a commercially available TPS and taking into account the calibration dose range of each detector. Irradiation was performed using an 192Ir high dose rate (HDR) source. Dose to medium in medium, [Formula: see text], was calculated using the MBDCA option of the same TPS as well as Monte Carlo (MC) simulation with the MCNP code and a benchmarked methodology. Measured and calculated dose distributions were spatially registered and compared. The total standard (k  =  1) spatial uncertainties for TLD, film and PRESAGE were: 0.71, 1.58 and 2.55 mm. Corresponding percentage total dosimetric uncertainties were: 5.4-6.4, 2.5-6.4 and 4.85, owing mainly to the absorbed dose sensitivity correction and the relative energy dependence correction (position dependent) for TLD, the film sensitivity calibration (dose dependent) and the dependencies of PRESAGE sensitivity. Results imply a LiF over-response due to a relative intrinsic energy dependence between 192Ir and megavoltage calibration energies, and a dose rate dependence of PRESAGE sensitivity at low dose rates (<1 Gy min-1). Calculations were experimentally validated within uncertainties except for MBDCA results for points in the phantom periphery and dose levels  <20%. Experimental MBDCA validation is laborious, yet feasible. Further work is required for the full characterization of dosimeter response for 192Ir and the reduction of experimental uncertainties.

SU-F-BRA-11: An Experimental Commissioning Test of Brachytherapy MBDCA Dosimetry, Based On a Commercial Radiochromic Gel/optical CT System

Purpose: To implement a 3D dose verification procedure of Model-Based Dose Calculation Algorithms (MBDCAs) for 1 9 2Ir HDR brachytherapy, based on a novel Ferrous Xylenol-orange gel (FXG) and optical CT read-out. Methods: The TruView gel was employed for absolute dosimetry in conjunction with cone-beam optical CT read-out with the VISTA scanner (both from Modus Medical Inc, London, ON, Canada). A multi-catheter skin flap was attached to a cylindrical PETE jar (d=9.6cm, h=16cm) filled with FXG, which served as both the dosimeter and the water equivalent phantom of bounded dimensions. X- ray CT image series of the jar with flap attached was imported to Oncentra Brachy v.4.5. A treatment plan consisting of 8 catheters and 56 dwell positions was generated, and Oncentra-ACE MBDCA as well as TG43 dose results were exported for further evaluation. The irradiation was carried out with a microSelecton v2 source. The FXG dose-response, measured via an electron irradiation of a second dosimeter from the same batch, was linear (R2>0.999) at least up to 12Gy. A MCNP6 input file was prepared from the DICOM-RT plan data using BrachyGuide to facilitate Monte Carlo (MC) simulation dosimetry in the actual experimental geometry. Agreement between experimental (reference) and calculated dose distributions was evaluated using the 3D gamma index (GI) method with criteria (5%-2mm applied locally) determined from uncertainty analysis. Results: The TG-43 GI failed, as expected, in the majority of voxels away from the flap (pass rate 59% for D>0.8Gy, corresponding to 10% of prescribed dose). ACE performed significantly better (corresponding pass rate 92%). The GI evaluation for the MC data (corresponding pass rate 97%) failed mainly at low dose points of increased uncertainty. Conclusion: FXG gel/optical CT is an efficient method for level-2 commissioning of brachytherapy MBDCAs. Target dosimetry is not affected from uncertainty introduced by TG43 assumptions in 192Ir skin brachytherapy. Research co-financed by the ESF and Greek funds through the Operational Program Education and Lifelong Learning Investing in Knowledge Society of the NSRF. Research Funding Program: Aristeia. Modus Medical Devices Inc. provided a TruView dosimeter batch and Nucletron, and Elekta company, provided access to Oncentra Brachy v4.5, for research purposes.

A comparative assessment of inhomogeneity and finite patient dimension effects in 60 Co and 192 Ir high-dose-rate brachytherapy

Purpose To perform a comparative study of heterogeneities and finite patient dimension effects in 60Co and 192Ir high-dose-rate (HDR) brachytherapy. Material and methods Clinically equivalent plans were prepared for 19 cases (8 breast, 5 esophagus, 6 gynecologic) using the Ir2.A85-2 and the Co0.A86 HDR sources, with a TG-43 based treatment planning system (TPS). Phase space files were obtained for the two source designs using MCNP6, and validated through comparison to a single source dosimetry results in the literature. Dose to water, taking into account the patient specific anatomy and materials (Dw,m), was calculated for all plans using MCNP6, with input files prepared using the BrachyGuide software tool to analyze information from DICOM RT plan exports. Results A general TG-43 dose overestimation was observed, except for the lungs, with a greater magnitude for 192Ir. The distribution of percentage differences between TG-43 and Monte Carlo (MC) in dose volume histogram (DVH) indices for the planning target volume (PTV) presented small median values (about 2%) for both 60Co and 192Ir, with a greater dispersion for 192Ir. Regarding the organs at risk (OARs), median percentage differences for breast V50% were 3% (5%) for 60Co (192Ir). Differences in median skin D2cc were found comparable, with a larger dispersion for 192Ir, and the same applied to the lung D10cc and the aorta D2cc. TG-43 overestimates D2cc for the rectum and the sigmoid, with median differences from MC within 2% and a greater dispersion for 192Ir. For the bladder, the median of the difference is greater for 60Co (~2%) than for 192Ir (~0.75%), demonstrating however a greater dispersion again for 192Ir. Conclusions The magnitude of differences observed between TG-43 based and MC dosimetry and their smaller dispersion relative to 192Ir, suggest that 60Co HDR sources are more amenable to the TG-43 assumptions in clinical treatment planning dosimetry.