Dosimetric Performance Research Articles

Voxel-based automatic multi-criteria optimization for intensity modulated radiation therapy

BackgroundAutomatic multi-criteria optimization is necessary for intensity modulated radiation therapy (IMRT) because of low planning efficiency and large plan quality uncertainty in current clinical practice. Most studies focused on imitating dosimetrists’ planning procedures to automate this process and ignored the fact that organ-based objective functions typically used in commercial treatment planning systems (such as dose-volume function) usually lead to sub-optimal plans. To guarantee the optimum results and to automate this process, we incorporate an improved automation strategy and a voxel-based optimization algorithm to generate a novel automatic multi-criteria optimization framework. We then evaluate it in clinical cases.MethodsThis novel automatic multi-criteria optimization framework incorporates a ranked priority-list based automatic constraints adjustment strategy and an in-house developed voxel-based optimization algorithm. Constraints are sequentially adjusted following a pre-defined priority list. Afterward, a voxel-based fluence map optimization (FMO) with an orientation to the newly updated constraints is launched to find a Pareto optimal solution. Loops of constraints adjustment are repeated until each of them could not be relaxed or tightened. The feasibility of the framework is evaluated with 10 automatic generated gynecology (GYN) cancer IMRT cases by comparing the dosimetric performance with the original.ResultsPlan quality improvement is observed for our automatic multi-criteria optimization method. Comparable DVHs are found for the planning target volume (PTV), but with better organs-at-risk (OAR) dose sparing. Among 13 evaluated dosimetric endpoints, 5 of them show significant improvements in automatically generated plans compared with the original plans. Investigation of improvement tendency during optimization exhibits gradual change as the optimization stage proceeds. An initial voxel-based optimization stage and in-low-priority dosimetric criteria tighten can significantly contribute to the optimization procedure.ConclusionsWe have successfully developed an automatic multi-criteria optimization framework that can dramatically reduce the current trial-and-error patterned planning workload while affording an efficient method to assure high plan quality consistency. This optimization framework is expected to greatly facilitate precise radiation therapy because of its advantages of planning efficiency and plan quality improvement.

13 Response to high energy photons of the redesigned PTW T31022 PinPoint 3D ion chamber

Introduction The aim of this study was to investigate the dosimetric performances of the new PTW T31022 PinPoint 3D which replaces the PTW T31016 PinPoint 3D. Methods Experimental data were collected using a Varian TrueBeam linear accelerator (Varian Medical Systems, Palo Alto) at nominal energies of 6 and 10 MV with (WFF) and without a flattening filter (FFF) (6WFF, 10WFF and 6FFF, 10FFF, respectively). Measurements of absolute and relative dosimetry were undertaken. Ion collection efficiency associated with Jaffe plots for different dose per pulse, polarity effect, beam profiles, depth dose curves (PDD) and output factors associated to small field output correction factors were investigated. Results The T31022 Jaffe plots were found to be more linear than those of the T31016. The use of the two-voltage technique is sufficient for clinical practice and is consistent with the accuracy of the Jaffe plots technique. Less polarity effect was found for T31022 compared to T31016. The polarity correction factor of the T31022 was dependent on the field size, although it was at a lesser extent than for the T31016. The largest deviation of polarity factors was obtained for the largest field size and for WFF beams. From 3x3 cm2 to 40x40 cm2 field sizes, excellent agreement was found between T31022 output ratios corrected from polarity and T31010 Semiflex results, exhibiting maximum deviations of 0.2% for all energies. Without polarity correction maximum deviations are also relatively low reaching 0.7%, −0.4%, 0.7% and −0.4% for 6WFF, 6FFF, 10WFF and 10FFF respectively. The use of the chamber in small fields ( ⩽ 1x1 cm2) leads to an underestimation of output factors but presents lower field output correction factors than its predecessor. PDD curves present good agreement but discrepancies around 2% were observed for the largest field size and have been identified as a polarity dependence. This effect was moreover emphasized by keeping the electrometer at a distance less than 5 m from the water phantom. Conclusions The T31022 PinPoint 3D ion chamber is an improvement over its predecessor, the T31016, in terms of polarity, ion recombination and in small field dosimetry. The T31022 polarity correction factor has to be considered when measuring output factors and PDD for large field size.

Dose verification of volumetric-modulated arc therapy using one-dimensional and two-dimensional dosimeters

AbstractPurposeTo verify dose delivery and quality assurance of volumetric-modulated arc therapy (VMAT) for head and neck (H&N) cancer.MethodThe Imaging and Radiation Oncology Core Houston (IROC-H) H&N phantom with thermoluminescent dosimeters (TLDs) and films, were imaged with computed tomography scan and the reconstructed image was transferred to pinnacle treatment planning system (TPS). On TPS, the planning target volume (PTV), secondary target volume (STV) and organ at risk (OAR) were delineated manually and a treatment plan was made. The dose constraints were determined for the concerned organs according to IROC-H prescription. The treatment plan was optimised using adoptive convolution algorithm to improve dose homogeneity and conformity. The dose calculation was performed using C.C Convolution algorithm and a Varian True Beam linear accelerator was used to deliver the treatment plan to the H&N phantom. The delivered radiation dose to the phantom was measured through TLDs and GafChromic external beam radiotherapy 2 (EBT2) films. The dosimetric performance of the VMAT delivery was studied by analysing percent dose difference, isodose line profile and gamma analysis of the TPS-computed dose and linac-delivered doses.ResultThe percent dose difference of 3.8% was observed between the planned and measured doses of TLDs and a 1.5-mm distance to agreement (DTA) was observed by comparing isodose line profiles. Passed the gamma criteria of 3%/3 mm was with good percentages.ConclusionThe dosimetric performance of VMAT delivery for a challenging H&N radiotherapy can be verified using TLDs and films embedded in an anthropomorphic H&N phantom.

Setup and Characterization of a 137CS Dosimetry Calibration Source in a Space-constrained Environment.

Proper characterization of Cs sources used for dosimeter calibration and performance testing is crucial for accurate and precise knowledge of air kerma rate and delivered dose. A 269 GBq Cs source was relocated to a new facility, which had a footprint of approximately 2.8 m × 3.6 m and a 3.4 m high ceiling. A small room size, such as in the new facility, may significantly increase backscatter from the walls or room return. Due to the limited source strength, a relatively close irradiation position of 1.00 m from the source was selected to decrease required exposure times. Proximity to the small but cylindrical source has the potential to alter the 1/r relationship (inverse square law) of air kerma rate with distance associated with point sources. Practical tutorials concerning dosimetry irradiation facilities are largely absent from the archival literature. For those reasons, standard characterization experiments were repeated multiple times, great care was taken with positioning, several experiments were added to the standard ones, an ion chamber and dosimeters were used rather than film for greater accuracy and precision, and practical details of the process were recorded. The impacts of room size and finite source dimensions were quantified and mitigated by the efforts reported here. Recommendations for simple but thorough facility characterization, beam calibration, and quality control resulted.

Dosimetric Performance of a New Ring-Gantry Linear Accelerator for IMRT and VMAT Prostate Plans

We aimed to evaluate dosimetric performance and delivery efficiency of a new ring-gantry linac characterized by a staggered dual-layer multi-leaf collimator (MLC). Performance of treatment plans generated using a cloud-based planning system is reported here for the first time. Using IMRT & VMAT prostate plans, we compared performance of the ring gantry linac with that of a C-arm based linac. We hypothesized that both IMRT and VMAT treatments on the new system would yield similar target coverage and dose conformity, albeit with differences in treatment time and organ-at-risk (OAR) sparing. Ten prostate cancer patients previously planned using a treatment planning system (TPS) and treated with VMAT on a conventional C-arm linac (2300EX) at a dose rate of 300 MU/min were retrospectively re-planned for the new linac using a newer version of the TPS. Each plan was re-planned for IMRT (7 and 9 fields) and VMAT (2 and 3 arcs). Prescription dose for the target was standardized to 66.6 Gy. Plans were normalized to deliver 100% prescription dose to 95% of the target volume. Due to limitations in the current version of the ring gantry system, plans were generated using a single layer MLC (leaf width of 1 cm). Metrics of plan quality such as dose conformity, OAR sparing, and monitor units (MU) for new IMRT & VMAT plans were compared with those of the clinically delivered plans. Wilcoxon signed-rank test was used to compare differences in plan quality. Compared to the clinically delivered plans, rectum/bladder sparing and dose conformity were comparable for the new 9-field IMRT & VMAT (2- and 3-arc) plans, but were inferior for the 7-field IMRT plan (p <0.05). Both new IMRT plans achieved significantly better femoral head sparing than the delivered plans and the new VMAT plans (p <0.05). A significantly higher MU was required for all new plans (P < 0.05), to compensate for the wider leaf width. However, delivery time for the new VMAT plans is about 3 times shorter on the new system due to faster gantry and MLC motion. The 9-field IMRT and 2- and 3-arc VMAT prostate plans generated for the new ring-gantry linac offer dose coverage and OAR sparing that is comparable to the VMAT plans delivered on the conventional linac. When comparing ring-gantry IMRT and VMAT plans, the 9-field IMRT plans achieved better OAR sparing while maintaining similar dose coverage.Abstract TU_28_3176; Table 1Dosimetric comparison between clinically treated plans vs. plans for the ring-gantry linacClinically DeliveredPlans for new ring-gantry linac7-field IMRT9-field IMRT2-arc VMAT3-arc VMATCI1.02 ± 0.021.10 ± 0.051.06 ± 0.031.05 ± 0.031.04 ± 0.02HI1.05 ± 0.011.06 ± 0.011.05 ± 0.011.06 ± 0.011.05 ± 0.01MU666 ± 179931 ± 216948 ± 2321123 ± 4651151 ± 271Rectum V70 (%)33.3 ± 11.435.8 ± 14.931.5 ± 11.631.7 ± 10.133.1 ± 13.4Bladder V70 (%)32.2 ± 21.433.9 ± 20.828.8 ± 20.030.6 ± 18.431.1 ± 20.3Left Femoral Head V30 (%)19.1 ± 15.27.6 ± 10.49.7 ± 9.419.0 ± 14.317.9 ± 15.0Right Femoral Head V30 (%)18.3 ± 17.98.83 ± 15.54.5 ± 7.921.7 ± 11.916.0 ± 11.4 Open table in a new tab

Treatment plan complexity does not predict IROC Houston anthropomorphic head and neck phantom performance

Previous works indicate that intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) plans that are highly complex may produce more errors in dose calculation and treatment delivery. Multiple complexity metrics have been proposed and associated with IMRT QA results, but their relationships with plan performance using in situ dose measurements have not been thoroughly investigated. This study aimed to evaluate the relationships between IMRT treatment plan complexity and anthropomorphic phantom performance in order to assess the extent to which plan complexity is related to dosimetric performance in the IROC phantom credentialing program. Sixteen complexity metrics, including the modulation complexity score (MCS), several modulation indices, and total monitor units (MU) delivered, were evaluated for 343 head and neck phantom irradiations, comprising both IMRT (step-and-shoot and sliding window techniques) and VMAT. Spearman’s correlations were used to explore the relationship between complexity and plan performance, as measured by the dosimetric differences between the treatment planning system (TPS) and thermoluminescent dosimeter (TLD) measurement, as well as film gamma analysis. Relationships were likewise determined for several combinations of subpopulations, based on the linear accelerator model, TPS used, and delivery modality. Evaluation of the complexity metrics presented here yielded no significant relationships (p > 0.01, Bonferroni-corrected) and all correlations were weak (less than ±0.30). These results indicate that complexity metrics have limited predictive utility in assessing plan performance in multi-institutional comparisons of IMRT plans. Other factors affecting plan accuracy, such as dosimetric modeling or multileaf collimator (MLC) performance, should be investigated to determine a more probable cause for dose delivery errors.

Dosimetric comparison between three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT) in the treatment of different stages of nasopharyngeal carcinoma

AbstractBackgroundThis work aims to compare the dosimetric performance of three-dimensional conformal radiotherapy (3D-CRT), a relatively available technique in developing countries, to intensity-modulated radiotherapy (IMRT) in the treatment of different stages of nasopharyngeal carcinoma (NPC).Materials and MethodsAccording to the diagnostic stages, 40 NPC patients were divided into two equal groups. Three planning techniques such as 3D-CRT, seven-field IMRT (7F-IMRT) and nine-field IMRT (9F-IMRT) were compared. Dose prescriptions of 70 and 66 Gy were delivered in 35 fractions to gross planning target volume (PTV1) and bilateral retropharyngeal carcinoma (PTV2), respectively.ResultsStage I dose data for almost all of the three investigated planning techniques obey the international recommendations. The dose delivered to PTV1 and PTV2 for 3D-CRT and 7F-IMRT are statistically similar, whereas 9F-IMRT is significantly better than 3D-CRT. For organs at risk (OARs), the delivered dose is significantly better for 9F-IMRT compared with the other two techniques, whereas 7F-IMRT is significantly better than 3D-CRT.Conclusions3D-CRT is an acceptable alternative treatment technique for stage I NPC patients in developing countries suffering from the lack of advanced radiotherapy treatment techniques. 3D-CRT and 7F-IMRT have comparable performance in PTVs, while 9F-IMRT is superior in PTVs and OARs.

OA030] Improved dose sensitivity of N-(isobutoxymethyl) acrylamide polymer gel dosimeters for radiation therapy

Purpose A new and very low dose sensitivity of polymer gel dosimeter containing N-(isobutoxymethyl) acrylamide was introduced (Salah et al., 2017). A significant increase in dose response as well as dose sensitivity of N-(isobutoxymethyl) acrylamide polymer gel dosimeter has been reported in this study. Methods The N-(isobutoxymethyl) acrylamide (NIBMA) gels were fabricated under a fume hood in normal atmospheric conditions. Gels were irradiated by X-ray beams of a medical linear accelerator. The irradiated gel dosimeters were evaluated using nuclear magnetic imaging (NMR) techniques in terms of relaxation time ( T 2 ) of hydrogen protons within the water molecule. Results The dose response as well as dose sensitivity were significantly increased by increasing NIBMA concentration. Slight effects of dose rate, beam energy and post-irradiation stability were noticed on the performance of NIBMA polymer gel dosimeters. Conclusions High dose sensitivity with low toxicity polymer gel dosimeters containing N-(isobutoxymethyl) acrylamide (NIBMA) were developed for radiotherapy treatment planning system.

Performance of an inorganic optical fiber dosimeter for external beam radiation therapy

Modern radiation therapy treatment techniques and delivery methods introduce new challenges to existing dosimetry systems. As a result, much research has been conducted on the development of a radiation dosimetry system based on scintillating optical fibre. They offer many advantages over the currently employed dosimeters for real time dosimetry. Optical Fibre dosimeters (OFD) have small dimensions, are immune to electromagnetic fields and can be used to remotely monitor radiation in real time. For this study a novel OFD based on an inorganic scintillating material for external beam radiotherapy was developed. The dosimeter was constructed using a PMMA (polymethyl methacrylate) plastic optical fibre. The core of the PMMA was micro machined to make a 700 μm diameter and 7 mm deep cavity. The scintillating material, terbium doped gadolinium oxysulphide (Gd2O2S:Tb) was filled in the cavity and sealed with an epoxy. The scintillation material fluoresces on exposure to ionising radiation and the resultant emitted fluorescent light is detected using a multi-pixel photon counting module. Measurements to date using the novel dosimeter demonstrate very good repeatability and reproducibility with only a maximum of 0.17% and 0.68% difference from the average value, respectively. The results also show excellent linearity of the output signal with radiation doses and the dosimeter response was independent of dose rate. However, the percentage depth dose measurements showed an over response of the OFD compared to an ionization chamber. These differences are being investigated by means of Monte Carlo simulations.

A Monte Carlo study of the impact of phosphor optical properties on EPID imaging performance

We have developed a Monte Carlo computational model of a clinically employed electronic portal imaging device (EPID), and demonstrated the impact of phosphor optical properties on the imaging performance. The EPID model was built with Geant4 application for tomographic emission. Both radiative and optical transport were included in the model. Modulation transfer function (MTF), normalized noise-power spectrum times the incident x-ray fluence (qNNPS), and detective quantum efficiency (DQE) were calculated for simulated and measured data, and their agreement was quantified by the normalized root-mean-square error (NRMSE). MTF was computed using a 100 µm wide slit tilted by 1.5° and qNNPS was estimated using the Fujita–Lubberts–Swank method. DQE was calculated from MTF and qNNPS data. The NRMSE value was 0.0467 for MTF, 0.0217 for qNNPS, and 0.0885 for DQE, showing good agreement between measurement and simulation. Five major optical properties, phosphor grain size, phosphor thickness, phosphor refractive index, binder refractive index, and packing ratio were tested for their influence on the qNNPS, MTF, and DQE(0) of the model. Generally, the effect on the qNNPS is greater than MTF, and no impact on DQE(0), except from phosphor thickness, was observed. Multiple applications, such as imager design optimization and investigations of the dosimetric performance, are expected to benefit from the validated model.

Characterization of the exradin W1 plastic scintillation detector for small field applications in proton therapy

Accurate dosimetry in small field proton therapy is challenging, particularly for applications such as ocular therapy, and suitable detectors for this purpose are sought. The Exradin W1 plastic scintillating fibre detector is known to out-perform most other detectors for determining relative dose factors for small megavoltage photon beams used in radiotherapy but its potential in small proton beams has been relatively unexplored in the literature. The 1 mm diameter cylindrical geometry and near water equivalence of the W1 makes it an attractive alternative to other detectors. This study examines the dosimetric performance of the W1 in a 74 MeV proton therapy beam with particular focus on detector response characteristics relevant to relative dose measurement in small fields suitable for ocular therapy. Quenching of the scintillation signal is characterized and demonstrated not to impede relative dose measurements at a fixed depth. The background cable-only (Čerenkov and radio-fluorescence) signal is 4 orders of magnitude less than the scintillation signal, greatly simplifying relative dose measurements. Comparison with other detectors and Monte Carlo simulations indicate that the W1 is useful for measuring relative dose factors for field sizes down to 5 mm diameter and shallow spread out Bragg peaks down to 6 mm in depth.

Dosimetric performance of the new PTW 31022 PinPoint 3D ionization chamber in high energy photon beams

This paper is an investigation of the dosimetric performances of the new PTW T31022 PinPoint 3D which supersedes T31016. Measurements have been performed in small and large fields conditions, with (WFF) and without flattening filter (FFF) for two energies. The study includes characterization of ion collection efficiency (Jaffé plots), polarity effect and measurements of beam profiles, depth dose curves and output factors associated to small field correction factors. The T31022 Jaffé plots were found to be more linear than those of the T31016 and the use of the two-voltage technique was validated. The use of the chamber in small fields (≤1 × 1 cm2) leads to an underestimation of output factors but presents lower field output correction factors than its predecessor. The chamber was found to present a field size dependence of the polarity correction factor (<1%). This polarity effect is particular notable when measuring a depth dose curve of a 40 × 40 cm2 field and 6WFF photon beams.

Monte Carlo simulations for dose enhancement in cancer treatment using bismuth oxide nanoparticles implanted in brain soft tissue.

The objective of this work is to study the dosimetric performances of bismuth oxide nanoparticles implanted in tumors in cancer radiotherapy. GEANT4 based Monte Carlo numerical simulations were performed to assess dose enhancement distributions in and around a 1 × 1 × 1 cm3 tumor implanted with different concentrations of bismuth oxide and irradiated with low energies 125I, 131Cs, and 103Pd radioactive sources. Dose contributions were considered from photoelectrons, Auger electrons, and characteristic X-rays. Our results show the dose enhancement increased with increasing both bismuth oxide concentration in the target and photon energy. A dose enhancement factor up to 18.55 was obtained for a concentration of 70mg/g of bismuth oxide in the tumor when irradiated with 131Cs source. This study showed that bismuth oxide nanoparticles are innovative agents that could be potentially applicable to in vivo cancer radiotherapy due to the fact that they induce a highly localized energy deposition within the tumor.

The impact of the oxygen scavenger on the dose-rate dependence and dose sensitivity of MAGIC type polymer gels

Recent developments in radiation therapy aimed at more precise dose delivery along with higher dose gradients (dose painting) and more efficient dose delivery with higher dose rates e.g. flattening filter free (FFF) irradiation.Magnetic-resonance-imaging based polymer gel dosimetry offers 3D information for precise dose delivery techniques. Many of the proposed polymer gels have been reported to exhibit a dose response, measured as relaxation rate ΔR2(D), which is dose rate dependent. A lack of or a reduced dose-rate sensitivity is very important for dosimetric accuracy, especially with regard to the increasing clinical use of FFF irradiation protocols with LINACs at high dose rates. Some commonly used polymer gels are based on Methacrylic-Acid-Gel-Initiated-by-Copper (MAGIC). Here, we report on the dose sensitivity (ΔR2/ΔD) of MAGIC-type gels with different oxygen scavenger concentration for their specific dependence on the applied dose rate in order to improve the dosimetric performance, especially for high dose rates. A preclinical x-ray machine (‘Yxlon’, E = 200 kV) was used for irradiation to cover a range of dose rates from low min = 0.6 Gy min−1 to high max = 18 Gy min−1. The dose response was evaluated using R2-imaging of the gel on a human high-field (7T) MR-scanner. The results indicate that all of the investigated dose rates had an impact on the dose response in polymer gel dosimeters, being strongest in the high dose region and less effective for low dose levels. The absolute dose rate dependence of the dose response in MAGIC-type gel is significantly reduced using higher concentrations of oxygen scavenger at the expense of reduced dose sensitivity. For quantitative dose evaluations the relative dose rate dependence of a polymer gel, normalized to its sensitivity is important. Based on this normalized sensitivity the dose rate sensitivity was reduced distinctly using an increased oxygen scavenger concentration with reference to standard MAGIC-type gel formulation at high dose rate levels. The proposed gel composition with high oxygen scavenger concentration exhibits a larger linear active dose response and might be used especially in FFF-radiation applications and preclinical dosimetry at high dose rates. We propose in general to use high dose rates for calibration and evaluation as the change in relative dose sensitivity is reduced at higher dose rates in all of the investigated gel types.

Assessment of multi-criteria optimization (MCO) for volumetric modulated arc therapy (VMAT) in hippocampal avoidance whole brain radiation therapy (HA-WBRT).

This study compared the dosimetric performance of (a) volumetric modulated arc therapy (VMAT) with standard optimization (STD) and (b) multi‐criteria optimization (MCO) to (c) intensity modulated radiation therapy (IMRT) with MCO for hippocampal avoidance whole brain radiation therapy (HA‐WBRT) in RayStation treatment planning system (TPS). Ten HA‐WBRT patients previously treated with MCO‐IMRT or MCO‐VMAT on an Elekta Infinity accelerator with Agility multileaf collimators (5‐mm leaves) were re‐planned for the other two modalities. All patients received 30 Gy in 15 fractions to the planning target volume (PTV), namely, PTV30 expanded with a 2‐mm margin from the whole brain excluding hippocampus with margin. The patients all had metastatic lesions (up to 12) of variable sizes and proximity to the hippocampus, treated with an additional 7.5 Gy from a simultaneous integrated boost (SIB) to PTV37.5. The IMRT plans used eight to eleven non‐coplanar fields, whereas the VMAT plans used two coplanar full arcs and a vertex half arc. The averaged target coverage, dose to organs‐at‐risk (OARs) and monitor unit provided by the three modalities were compared, and a Wilcoxon signed‐rank test was performed. MCO‐VMAT provided statistically significant reduction of D100 of hippocampus compared to STD‐VMAT, and Dmax of cochleas compared to MCO‐IMRT. With statistical significance, MCO‐VMAT improved V30 of PTV30 by 14.2% and 4.8%, respectively, compared to MCO‐IMRT and STD‐VMAT. It also raised D95 of PTV37.5 by 0.4 Gy compared to both MCO‐IMRT and STD‐VMAT. Improved plan quality parameters such as a decrease in overall plan Dmax and total monitor units (MU) were also observed for MCO‐VMAT. MCO‐VMAT is found to be the optimal modality for HA‐WBRT in terms of PTV coverage, OAR sparing and delivery efficiency, compared to MCO‐IMRT or STD‐VMAT.

A fast dual wavelength laser beam fluid-less optical CT scanner for radiotherapy 3D gel dosimetry I: design and development

Three dimensional dosimetry by optical CT readout of radiosensitive gels or solids has previously been indicated as a solution for measurement of radiotherapy 3D dose distributions. The clinical uptake of these dosimetry methods has been limited, partly due to impracticalities of the optical readout such as the expertise and labour required for refractive index fluid matching. In this work a fast laser beam optical CT scanner is described, featuring fluid-less and dual wavelength operation. A second laser with a different wavelength is used to provide an alternative reference scan to the commonly used pre-irradiation scan. Transmission data for both wavelengths is effectively acquired simultaneously, giving a single scan process. Together with the elimination of refractive index fluid matching issues, scanning practicality is substantially improved. Image quality and quantitative accuracy were assessed for both dual and single wavelength methods. The dual wavelength scan technique gave improvements in uniformity of reconstructed optical attenuation coefficients in the sample 3D volume. This was due to a reduction of artefacts caused by scan to scan changes. Optical attenuation measurement accuracy was similar for both dual and single wavelength modes of operation. These results established the basis for further work on dosimetric performance.

Characterization of the first RF coil dedicated to 1.5 T MR guided radiotherapy

The purpose of this study is to investigate the attenuation characteristics of a novel radiofrequency (RF) coil, which is the first coil that is solely dedicated to MR guided radiotherapy with a 1.5 T MR-linac. Additionally, we investigated the impact of the treatment beam on the MRI performance of this RF coil.First, the attenuation characteristics of the RF coil were characterized. Second, we investigated the impact of the treatment beam on the MRI performance of the RF coil. We additionally demonstrated the ability of the anterior coil to attenuate returning electrons and thereby reducing the dose to the skin at the distal side of the treatment beam.Intensity modulated radiation therapy simulation of a clinically viable treatment plan for spinal bone metastasis shows a decrease of the dose to the planned tumor volume of 1.8% as a result of the MR coil around the patient. Ionization chamber and film measurements show that the anterior and posterior coil attenuate the beam homogeneously by 0.4% and 2.2%, respectively. The impact of the radiation resulted in a slight drop of the time-course signal-to-noise ratio and was dependent on imaging parameters. However, we could not observe any image artifacts resulting from this irradiation in any situation.In conclusion, the investigated MR-coil can be utilized for treatments with the 1.5 T-linac system. However, there is still room for improvement when considering both the dosimetric and imaging performance of the coil.

Verification of high-dose-rate brachytherapy treatment planning dose distribution using liquid-filled ionization chamber array.

PurposeThis study aims to investigate the dosimetric performance of a liquid-filled ionization chamber array in high-dose-rate (HDR) brachytherapy dosimetry. A comparative study was carried out with air-filled ionization chamber array and EBT3 Gafchromic films to demonstrate its suitability in brachytherapy.Material and methodsThe PTW OCTAVIUS detector 1000 SRS (IA 2.5-5 mm) is a liquid-filled ionization chamber array of area 11 x 11 cm2 and chamber spacing of 2.5-5 mm, whereas the PTW OCTAVIUS detector 729 (IA 10 mm) is an air vented ionization chamber array of area 27 x 27 cm2 and chamber spacing of 10 mm. EBT3 films were exposed to doses up to a maximum of 6 Gy and evaluated using multi-channel analysis. The detectors were evaluated using test plans to mimic a HDR intracavitary gynecological treatment. The plan was calculated and delivered with the applicator plane placed 20 mm from the detector plane. The acquired measurements were compared to the treatment plan. In addition to point dose measurement, profile/isodose, gamma analysis, and uncertainty analysis were performed. Detector sensitivity was evaluated by introducing simulated errors to the test plans.ResultsThe mean point dose differences between measured and calculated plans were 0.2% ± 1.6%, 1.8% ± 1.0%, and 1.5% ± 0.81% for film, IA 10 mm, and IA 2.5-5 mm, respectively. The average percentage of passed gamma (global/local) values using 3%/3 mm criteria was above 99.8% for all three detectors on the original plan. For IA 2.5-5 mm, local gamma criteria of 2%/1 mm with a passing rate of at least 95% was found to be sensitive when simulated positional errors of 1 mm was introduced.ConclusionThe dosimetric properties of IA 2.5-5 mm showed the applicability of liquid-filled ionization chamber array as a potential QA device for HDR brachytherapy treatment planning systems.

Experimental verification of a 3D in vivo dose monitoring system based on EPID.

PurposeTo evaluate the Edose system, a novel three-dimensional (3D) in vivo dose monitoring system based on electronic portal imaging device (EPID), prior to clinical application, we analyzed the preliminary clinical data using Edose system in patients receiving intensity-modulated radiation therapy (IMRT).Materials and methodsAfter the physical modeling, the measured results from the Edose system were examined in homogeneous and inhomogeneous phantoms, respectively. To verify the accuracy of the Edose system, we compared its results with testing results from ionization chamber, measurement matrix (Delta4) and dosimetric films. The dosimetric performance of the Edose system was evaluated in 12 randomly selected patients with IMRT and VMAT, and the measured results were compared with the treatment plans.ResultsCompared with the measured results, the dose difference at the center of target volume was (0.12±0.91)% and (0.03±0.85)%, the γ pass rate was (94.18±1.69)% and (95.24±1.62)% (3mm/3%)for homogeneous and inhomogeneous phantoms, respectively. For IMRT patients, the dose difference at the center of target volume was (0.75±1.53)%, and the γ pass rates were (89.11±3.24)% (3mm/3%) and (96.40±1.47)% (3mm/5%), respectively. Compared with the results of DVH, the maximum differences of PTVs and mostly organs at risk were all within 3%. For VMAT patients, the γ pass rates were (93.04 ± 2.62)% (3mm/3%) and (97.92 ± 1.38)% (3mm/5%), respectively.ConclusionsIn vivo dose monitoring may further improve the safety and quality assurance for radiation therapy. But rigorous clinical testing is required before putting the existing commercial systems into clinical application. In addition, more clinical experiences and better workflows for using the Edose system are needed.

Comparison of different treatment planning optimization methods for vaginal HDR brachytherapy with multichannel applicators: A reduction of the high doses to the vaginal mucosa is possible

PurposeA direct planning approach with multi-channel vaginal cylinders (MVCs) used for HDR brachytherapy of vaginal cancers is particularly challenging. Purpose of this study was to compare the dosimetric performances of different forward and inverse methods used for the optimization of MVC-based vaginal treatments for endometrial cancer, with a particular attention to the definition of strategies useful to limit the high doses to the vaginal mucosa. MethodsTwelve postoperative vaginal HDR brachytherapy treatments performed with MVCs were considered. Plans were retrospectively optimized with three different methods: Dose Point Optimization followed by Graphical Optimization (DPO + GrO), Inverse Planning Simulated Annealing with two different class solutions as starting conditions (surflPSA and homogIPSA) and Hybrid Inverse Planning Optimization (HIPO). Several dosimetric parameters related to target coverage, hot spot extensions and sparing of organs at risk were analyzed to evaluate the quality of the achieved treatment plans. Dose homogeneity index (DHI), conformal index (COIN) and a further parameter quantifying the proportion of the central catheter loading with respect to the overall loading (i.e., the central catheter loading index: CCLI) were also quantified. ResultsThe achieved PTV coverage parameters were highly correlated with each other but uncorrelated with the hot spot quantifiers. HomogIPSA and HIPO achieved higher DHIs and CCLIs and lower volumes of high doses than DPO + GrO and surflPSA. ConclusionsWithin the investigated optimization methods, HIPO and homoglPSA showed the highest dose homogeneity to the target. In particular, homogIPSA resulted also the most effective in reducing hot spots to the vaginal mucosa.