Dosimetric Characteristic Research Articles

The Impact of Deep Inspiration Breath Hold (DIBH) Implementation on the Hybrid Technique in Left-Sided Whole Breast Irradiation: A Dosimetric Characteristic Study of 3D-CRT Hybrid VMAT in DIBH and Free Breathing Conditions, and VMAT in Free Breathing Conditions.

To investigate the impact of DIBH for heart sparing effect on left sided breast postoperative whole breast irradiation by comparing the dosimetric characteristics of 3D-CRT hybrid VMAT and pure VMAT treatment planning under DIBH condition. The primary CT data sets from previously treated left sided early breast cancer were used for pure volumetric arc therapy (VMAT) technique re-planning for the dosimetric characteristics comparison. A treatment plan of 3D-CRT hybrid VMAT technique was re-planned on the free breath (FB) condition for the investigation of the dosimetric characteristics comparison on DIBH condition. The prescribed dose for all the treatment plans was 42.5Gy in 16 fractions. All plans were optimized to cover 100% of the PTV by 95% of prescribed dose. The dosimetric differences among the 3 treatment plans for the 20 patients were analyzed using Wilcoxon signed-rank test, with p value<0.05 considered statistically significant. 3D-CRT hybrid VMAT using DIBH technique yielded the best results on the conformity index (CI) and homogeneity index (HI). By comparing this 3D-CRT hybrid VMAT technique using FB and DIBH technique, the mean heart dose (MHD) was reduced from 5.38Gy to 1.65Gy, respectively (p =0.001) and the left anterior descending coronary artery (LAD)0.03cc dose was reduced from 27.87Gy to 9.41Gy, respectively (p =0.001). 3D-CRT hybrid VMAT using DIBH technique significantly reduced the V5, V20 and D mean of the ipsilateral lung and D mean of the contralateral lung. The D5 of right breast was significantly reduced by 3D-CRT hybrid VMAT compared with VMAT using DIBH technique. The incorporation of DIBH into 3D-CRT hybrid VMAT technique provides the best benefits for the heart and the OAR with respect to the radiation dose-sparing effect without compromising the target conformity and homogeneity in the treatment planning.

ДОЗИМЕТРИЧЕСКАЯ ХАРАКТЕРИСТИКА КОГОРТЫ ПЕРСОНАЛА СИБИРСКОГО ХИМИЧЕСКОГО КОМБИНАТА В ПЕРИОД 1950–2010 гг.

Purpose: To give a dosimetric characterization of the personnel of the Siberian Chemical Plant (SCP) subjected to prolonged technogenic occupational exposure to ionizing radiation (IR) in the period 1950–2010, as well as the cohort of the personnel of the SCP involved in working with uranium compounds. Material and methods: SCP is one of the largest and oldest complexes of nuclear industry enterprises in the world, having experience in continuous dosimetric monitoring of personnel for more than 60 years. The database of the regional medical-dosimetric register of employees of the SCP (RMDR) contains information about all employees of the SCP for the entire history of the enterprise, including personal data of medical, dosimetric and professional nature of 65.350 employees of the SCP, of which more than 32.000 people were exposed to chronic technogenic occupational radiation in the range of low doses. The archive of the medical documentation of the Seversk Biophysical Research Center contains 55.569 medical histories of the employees of the SCP, 29.800 outpatient records and 11.953 autopsy protocols. Results: Employees of the radiochemical, chemical-metallurgical, sublimate and separation industries of the SCP were subjected to combined (external and internal) irradiation, while the personnel of the reactor production were exposed exclusively to external irradiation. Auxiliary production workers were mainly exposed to non-radiation factors. Individual dosimetric control (IDC) of external exposure was carried out for all workers who were in the zone of exposure to external radiation sources. The average accumulated dose of external radiation for employees of the SCP was 28.3 mSv. IDC of internal irradiation was carried out for all workers employed at the production site, where the excess of the regulatory level of the concentration of radionuclides in the air of the working area was detected. The average value of the activity of radionuclides in the urine of SCP workers does not exceed 0.74 Bq. The life status has been clarified for 80.8 % of the employees of the SCP. About 3.500 employees of the agricultural complex have an IDC for internal irradiation from uranium based on the results of a biophysical examination. The life status is established for 75 % of employees of the uranium cohort. Conclusion: For the first time, a dosimetric characteristic of a cohort of 65.350 workers of the SCP (21 % women) who started work in the period 1950–2010 was given. The cohort of the staff of the SCP is suitable for conducting epidemiological studies in order to establish the risks associated with the impact on the human body of technogenic occupational exposure to IR.

Dosimetric characterization for optimum concentration in rare earth doped hafnium oxide nanophosphors for medical radiotherapy applications

Rare earth (RE) doped HfO2 NPs was prepared using the co-precipitation method. This study focuses on the dosimetric characteristic of rare earth (Gd3+,Eu3+,Ho3+) activated HfO2 nanoparticles as potential thermoluminescence (TL) dosimeter for radiation therapy dosimetry. The selected dopant concentrations of 2.5 mol% of Gd, 7.5 mol% of Eu and 2.5 mol% of Ho were irradiated with a clinical gamma photon beam. From the study of dosimetric characterization, it is observed that there is an enhancement of dose response by incorporating rare earth materials. The results obtained in this work show that the Eu3+(7.5 mol%) satisfies the dosimetric characteristic (expect fading) other than an optimized dopant concentration of Gd and Ho. Eu3+ showed that the dosimetric features of reproducibility are high, the linear behavior for photon dose is a good and high degree of fading. The fading factors for the proposed dosimeters at room temperature, which explicit the high loss of TL signal is due to the un-stabilization of trapped charged effects. This detailed study can be useful for the further development of highly efficient ionizing radiation dosimetry.

Fading of pMOS dosimeters over a long period of time

The fading of radiation-sensitive p-channel metal-oxide-semiconductor field-effect transistors (known as RADFETs or pMOS dosimeters) over a long time period of 10 years after irradiation has been investigated. Fading is, in addition to sensitivity, another characteristic of pMOS radiation dosimeters. It is considered as the recovery of threshold voltage of irradiated pMOS dosimeters during ambient annealing without gate polarization. Usually, there are the fading data for few months after irradiation only. Although fading is a very important dosimetric characteristic, here it is given for the first time in the literature in such a long period of time. Two types of pMOS dosimeters with oxide thicknesses of 400 and 1000 nm are used. They are irradiated without and with a 5 V polarization on the gate. For the first time, the authors came to a very significant result that the key role in fading has the voltage applied during irradiation, but not the thickness of the oxide, and that the pMOS dosimeters irradiated with gate voltage of 5 V have greater fading than pMOS dosimeters irradiated without a gate voltage. Fitting of threshold voltage shift and fading, performed using the radiation-induced Eγ and Es traps, shows very good agreement with the experimental values.

Dosimetric Characteristics of 6 MV Medical Linac at BAEC

Dosimetric characteristic is one of the essential parameters of a medical linear accelerator (LINAC), which must be obtained before clinical use. The dosimetric characteristics for 6 MV photon beam were measured and compared with the corresponding published data. The study was done using a Varian linear accelerator (Model Clinac-iX) at the Institute of Nuclear Medical Physics (INMP), AERE, Savar, Dhaka, under the Bangladesh Atomic Energy Commission (BAEC). The data is taken for 10 field sizes (2 × 2, 3 × 3, 5 × 5, 7 × 7, 10 × 10, 15 × 15, 20 × 20, 25 × 25, 30 × 30 and 40 × 40 cm2) at same conditions. The measured Percent Depth Dose (PDD) curves were obtained for 6 MV photon beams with the field as mentioned above and compared with the calculated PDD curves. The measured depth dose (Dmax) for reference field size (FS) 10 × 10 cm2 is 15.99 mm, and the PDD at 10 cm depth (D10) is 66.87% for 6 MV photon energies that are found to be compatible with the published report BJR supplement 25. The measured PDD curves for photon energies show a good agreement with the standard PDD curves. The photon beam dosimetry data found in the current study are compatible and all the tolerances are within the clinically acceptable tolerance limit.

A novel procedure for determining the optimal MLC configuration parameters in treatment planning systems based on measurements with a Farmer chamber

Modelling of the multi-leaf collimator (MLC) in treatment planning systems (TPS) is crucial for the dose calculation accuracy of intensity-modulated radiation therapy plans. However, no standardised methodology for their configuration exists to date. In this study we present a method that separates the effect of each dosimetric characteristic of the MLC, offering comprehensive equations for the determination of the configuration parameters used in the TPS model. The main advantage of the method is that it only requires prior knowledge of the nominal leaf width and is based on doses measured with a Farmer chamber, which is a very well established and robust methodology. Another significant advantage is the required time, since measuring the tests takes only about 30 minutes per energy. Firstly, we provide a theoretical general formalism in terms of the primary fluence constructed from the transmission map obtained from an MLC model for synchronous and asynchronous sweeping beams. Secondly, we apply the formalism to the RayStation TPS as a proof of concept and we derive analytical expressions that allow the determination of the configuration parameters (leaf tip width, tongue-and-groove width, x-position offset and MLC transmission) and describe how they intertwine. Finally, we apply the method to Varian’s Millennium120 and HD120 MLCs in a TrueBeam linear accelerator for different energies and determine the optimal configuration parameters. The proposed procedure is much faster and streamlined than the typical trial-and-error methods and increases the accuracy of dose calculation in clinical plans. Additionally, the procedure can be useful for standardising the MLC configuration process and it exposes the limitations of the implemented MLC model, providing guidance for further improvement of these models in TPSs.

Lattice radiation therapy - its concept and impact in the immunomodulation cancer treatment era.

Voluminous tumors represent a challenge in radiation oncology, particularly when surgical resection is not possible. Lattice radiotherapy (LTR) is a technique that may provide equivalent or superior clinical response in the management of large tumors while limiting toxicity to adjacent normal tissues. LRT can precisely deliver inhomogeneous high doses of radiation to different areas within the gross tumor volumes (GTV). The dosimetric characteristic of LTR is defined by the ratio of the valley dose (lower doses - cold spots) and the peak doses, also called vertex (higher doses - hot spots), or the valley-to-peak dose ratio. The valley-to-peak ratio thereby quantifies the degree of spatial fractionation. LRT delivers high doses of radiation without exceeding the tolerance of adjacent critical structures. Radiobiological experiments support the role of radiation-induced bystander effects, vascular alterations, and immunologic interactions in areas subject to low dose radiation. The technological advancements continue to expand in Radiation Oncology, bringing new safety opportunities of treatment for bulky lesions.

Dosimetric Evaluation of TF-3DCRT, FIF-Forward IMRT and FF Inversely Optimized IMRT for Breast Conserving Treatment

Objective: To compare the dosimetric characteristic of tangential field 3D conformal (TF-3DCRT, TF), field-in-field intensity-modulated (FIF-IMRT, FIF) and fixed-field inversely optimized intensity-modulated Radiotherapy (FFIO-IMRT, FFIO) for breast conserving treatment, and explore the benefit and efficiency for the three techniques. Materials and Methods: TF-3DCRT, FIF-IMRT and FFIO-IMRT treatment plans were analyzed for 16 breast patients (8 right-sided and 8 left-sided) after breast-conserving surgery. The target and organs at risk (OARs) were contoured by the same physician in the CT images. The prescription dose was 50Gy/25f. TF-3DCRT and FIF-IMRT were designed using Varian Eclipse Ver10.0 planning system, and FFIO-IMRT in the planning system of Pinnacle Ver9.6. Treatment plans were compared according to dose volume histogram (DVH) analysis in terms of PTV homogeneity and conformity indices (HI and CI) as well as OARs dose and volume parameters, and the efficiency was also evaluated. Results: In all cases, the treatment plans showed statistically significant difference between TF-3DCRT, FIF and FFIO-IMRT. The MUs were 244.9±8.3MU vs 285.9±20.3MU vs 534.0±56.2MU (p<0.001), the CIs of dose distribution and the target were 0.40±0.12 vs 0.48±0.12 vs 0.57±0.12 (p<0.01), and the HIs were 0.20±0.02 vs 0.13±0.02 vs 0.17±0.02 (p<0.01). Compared with TF and FFIO, FIF-IMRT showed smaller in the dose of D2 and volume of V107 and V110 in the target. FFIO-IMRT generally increased the D mean, V10 and V20 of ipsilateral lung, the D1 of contralateral breast and the mean dose of contralateral lung, heart, esophagus, and spinal cord relative to TF and FIF techniques. Conclusion: In breast-conserving RT, FIF-IMRT improved the overall quality of dose distribution and delivery efficiency, and the patients are most likely to benefit from FIF-IMRT.

Investigation of dosimetric characteristic of NIPAM polymer gel using x-ray CT

Introduction: Polymer gel dosimeters contain chemical materials sensitive to the radiation which are polymerized by the radiation as a function of absorbed dose. So information of spatial dose distribution can be extracted by imaging from irradiated gel. Among imaging techniques, computed tomography (CT) poses as an attractive method because of practical advantages such as accessibility, easy and speed of image acquisition and dose-response is stable and relatively insensitive to environmental factors. The most major limitation that prevents this method from clinical application is high noise in images that obtained from this system. In this study, imaging parameters in purpose of noise reduction of CT image and then concentration impact of each component of NIPAM gel on dose-response have been investigated. Finally, imaging appropriate parameter for read-out gel and NIPAM gel composition with maximum dose sensitivity for X-ray CT imaging of gel are introduced which would be an attempt to facilitate the application of this method in clinical applications. Materials and Methods: NIPAM gel was made according to the method described by Chain et al.; after the preparation, they were poured inside the vials and retained in refrigerator at 4° C for 24 hours. After this stage, gel-contained vials were subject to irradiation with a cobalt- 60 machine in dose ranging 0-50 Gy. After this, the irradiated gels were imaged with the CT scan. Ultimately, resultant DICOM images were processed in MATLAB and CT mean value of the vials as well as corresponding standard deviation were taken. Results: In this study, the imaging protocol for the Siemens SOMATOS PLUSE 4 CT scanner was investigated to reduce image noise. Parameters of 140 kV, 206 mAs, 5 mm slice thickness and 10AH kernel-based reconstruction are suitable options to evaluate polymer gel using aforementioned device. A number of 25 images for averaging could reduce the noise to an acceptable level. NIPAM polymer gel with 5% gelatin, 10 mM THPC, 17% NIPAM monomer and 3% BIS indicates the biggest changes in the CT value at 0-50 Gy ranging dose for CT imaging method. Conclusion: high-percentage monomer NIPAM polymer gel shows high sensitivity to CT imaging method and has good reproducibility and linear response range; as well as since appropriately choosing imaging protocols and thus achieved image processing reduce the noise to an acceptable level, standing these factors along with applicable advantages of CT method such as accessibility in radiotherapy centers, ease and speed of the image provision, as well as being low-cost exhibits that NIPAM polymer gel dosimeter with CT imaging method has a good potential to become a three-dimensional dosimeter in radiation therapy clinical applications.

Monte Carlo Simulation of Electron Beams produced by LIAC Intraoperative Radiation Therapy Accelerator.

One of the main problems of dedicated IORT accelerators is to determine dosimetric characteristics of the electron beams. Monte Carlo simulation of IORT accelerator head and produced beam will be useful to improve the accuracy of beam dosimetry. Liac accelerator head was modeled using the BEAMnrcMonte Carlo simulation system. Phase-space files were generated at the bottom of the applicators. These phase-space files were used as an input source in DOSXYZnrc and BEAMDP codes for dose calculation and analysis of the characteristic of the electron beams in all applicators and energies. The results of Monte Carlo calculations are in very close agreement with the measurements. There is a decrease in the peak of the initial spectrum when electrons come from the end of accelerator wave guide to the end of applicator. By decreasing the applicator diameter, the mean energy of electron beam decreased. Using applicators and increasing their size, X-ray contamination will increase. The percentage of X-ray contamination increases by applicator diameter. This is related to the increase of the mean energy of electron beams. Application of PMMA collimator leads to, although well below accepted level, the production of bremsstrahlung. The results of this study showed that special design of LIAC head accompanying by PMMA collimator system cause to produce an electron beam with an individual dosimetric characteristic making it a useful tool for intraoperative radiotherapy purposes.

DOSIMETRIC CHARACTERISTICS OF THE NOVEL LiF:Mg,Cu,Ag PHOSPHOR.

In this study, the preparation procedure and dosimetric characteristic of LiF activated by Mg, Cu and Ag were investigated for the first time. It was observed that with the thermal annealing at 240°C for 10 min the sensitivity of the novel LiF:Mg,Cu,Ag powder was more than that of the TLD-100 powder. Also, the thermoluminescence intensity decreased with the increase of annealing temperature from 240 to 300°C. The glow curve structure of this sample that is similar to TLD-100H shows a mean peak at ~243°C along with four overlapping peaks. Also, the absorbed dose response increases linearly at low dose ranges. Other thermoluminescence characteristics of this phosphor such as low residual signal, low fading rate and good stability at repeatability and reusability make it useful for the dosimetric applications.

Dosimetric characteristic of physical wedge versus enhanced dynamic wedge based on Monte Carlo simulations.

Physical wedges (PWs) are widely used in radiotherapy to obtain tilted isodose curves, but they alter beam quality. Dynamic wedges (DWs) using moving collimator overcome this problem, but measuring their beam data is not simple. The main aim of this study is to obtain all dosimetric parameters of DWs produced by Varian 2100CD with Monte Carlo simulation and compare them to those from PWs. To simulate 6 MV photon beams equipped with PW and DW, BEAMnrc code was used. All dosimetric data were obtained with EDR2 films and two-dimensional diode array detector. Good agreement between simulated and measured dosimetric data for PW and DW fields was obtained. Our results showed that percentage depth dose and beam profiles at nonwedged direction for DWs are the same as open fields and can be used to each other. From Monte Carlo simulations, it can be concluded that DWs in spite of PW do not have effect on beam quality and are good options for treatment planning system which cannot consider hardening effect produced by PWs. Furthermore, BEAMnrc is a powerful code to acquire all date required by DWs.

Dosimetric characteristics and day-to-day performance of an amorphous-silicon type electronic portal imaging device

This study investigated the dosimetric characteristics of an amorphous-silicon (aSi) electronic portal imaging device (EPID) and the utility and consistency of its daily output. This investigation utilised the portal dose prediction function of aSi-1000 EPID panel to test the panel for linearity, field size, dose rate and source to detector distance (SDD). To check daily output consistency, to acquire fluence by aSi EPID and reading calibration unit (CU) value every working day. Percentage variance of daily output also was measured using alternative two different types of dosimeter, EBT3 film and a multiple-channels output check device. Total of 165 daily output readings of EPID showed an average variance of 0.36% ± 0.53 from July 2014 to February 2015. The other two results between EBT3 film with BeamChecker Plus were −0.17% ± 2.36 and 0.35% ± 0.14, respectively. From correlation analysis, results of EPID is significantly correlated with BeamChecker Plus's (0.625, p << 0.001). Furthermore, the dosimetric characteristic of EPID indicated that CU values were a linear function of monitor units (MUs). When field size or dose rate varied, the panel's response was within 2%. The inverse square relation was between variable SDD and responding signal of panel. As compared with other dosimeters, the daily output monitor of aSi EPID was found to be reasonably consistent with commercial daily output device and replace with complex film-based procedure.

SU‐C‐BRB‐01: Development of Dynamic Gimbaled X‐Ray Head Swing Irradiation Technique

Purpose:The Vero4DRT has a unique gimbaled x‐ray head with rotating around orthogonal two axes. The purpose of this study was to develop a new irradiation technique using the dynamic gimbaled x‐ray head swing function.Methods:The Vero4DRT has maximum field size of 150Χ150 mm2. The expanded irradiation field (expanded‐field) for the longitudinal direction which is vertical to the MLC sliding direction, was created by the MLC motion and the gimbaled x‐ray head rotation. The gimbaled x‐ray head was rotated ± 35 mm, and the expanded‐field size was set as 150Χ220 mm2. To irradiate uniform dose distribution, the diamond‐shaped radiation field was created and continuously moved for the longitudinal direction. It was achieved by combination of opening and closing of the MLC and gimbal swing rotation. To evaluate dosimetric characteristic of the expanded‐field, films inserted in water‐equivalent phantoms at 100 mm depth were irradiated and the field size, penumbra, flatness and symmetry were analyzed.In addition, the expanded‐field irradiation technique was applied to virtual wedge irradiation. Wedged beam was acquired with the delta–shaped radiation field. 150Χ 220 mm2 fields with 15, 30, 45, and 60 degree wedge were examined. The wedge angles were measured with irradiated film and compared with assumed wedge angles.Results:The field size, penumbra, flatness and symmetry of the expanded‐field were 150.0 mm, 8.1–8.4 mm, 2.8% and −0.8% for the lateral direction and 220.1 mm, 6.3–6.4 mm, 3.2% and −0.4% for the longitudinal direction at 100 mm depth. The measured wedge angles were 15.1, 30.2, 45.2 and 60.2 degrees. The differences between assumed and measured angles were within 0.2 degrees.Conclusion:A new technique of the gimbal swing irradiation was developed. To extend applied targets, especially for whole breast irradiation, the expanded‐field and virtual wedge irradiations would be effective.

TH‐C‐12A‐03: Development of Expanded Field Irradiation Technique with Gimbaled X‐Ray Head

Purpose:The Vero4DRT has a maximum field size of 150×150 mm2. The purposes of this study were to develop an expanded field irradiation technique using a unique gimbaled x‐ray head of Vero4DRT and to evaluate its dosimetric characteristic.Methods:The expanded field irradiation consisted of four separate fields with 2.39 degree gimbal rotation around orthogonal two axes. The central beam axis for each field shifted 40 mm from the isocenter for longitudinal and lateral directions, and thus, the field size was expanded up to 230×230 mm2. Adjacent region were created at the isocenter (center‐adjacent expanded‐field) and 20 mm from isocenter (offadjacent expanded‐field). To create flat dose distribution in the combined piecewise‐fields, the overlapping and gaps regions on the isocenter plane were adjusted with the gimbal rotating and the MLC. To evaluate dosimetric characteristic of the expanded‐field, films inserted in water‐equivalent phantoms at 50, 100 and 150 mm depth were irradiated and the field size, penumbra, flatness and symmetry were analyzed.In addition, the expandedfield irradiation technique was applied to IMRT. A head and neck IMRT field, which was planned for the conventional linac (Varian Clinac iX), was reproduced with the expanded‐field of the Vero4DRT. The simulated dose distribution for the expanded IMRT field was compared to the measured dose distribution.Results:The field size, penumbra, flatness and symmetry of center‐ and off‐ adjacent expanded‐fields were 230.2–232.1 mm, 7.8–10.7 mm, 2.3–6.5% and –0.5–0.4% at 100 mm depth. The 82.1% area of the expanded IMRT dose distribution was within 5% difference between measurement and simulation, which was analyzed upper 50% dose area, and the 3%/3 mm gamma pass rate was 98.4%.Conclusions:The expandedfield technique was developed using the gimbaled x‐ray head. To extend applied targets, such as whole breast irradiations or head and neck IMRT, the expanded‐field technique would be effective.

SU‐E‐T‐671: Is There Really a Favorable Image Data Set Between Free‐Breathing, Average Or Maximum Intensity Projections for Peripheral Lung SBRT?

Purpose: This work investigates the dosimetric impact of 4D‐CT treatment planning for lung SBRT. We compare treatment planning and dosimetric characteristic between plans optimized on free‐breathing (FB) scans then, computed on maximum and average intensity projections (MIP and AIP). Which data set is more favorable for planning and dose calculations between fully‐optimized plans on MIP or AIP versus FB? Methods: A small number of patients with peripheral lung lesions treated with SBRT in our center with an Oncor‐160MLC system, were selected for this study. All patients received a FB helical scan to the entire chest and a 4D‐CT scan to a smaller volume that includes the lesion. Internal target volume (ITV) was first delineated on the MIP and a 5mm margin to it, defined the PTV. Multi‐field (7–11) inversely optimized planning was done first on the FB set, and then it was transferred to registered MIP and AIP for re‐calculation (with heterogeneities), after having propagated and segmented onto MIP and AIP sets all organs‐at‐risk (OARs), respectively. Additonal plans were created with re‐optimization of the beam fluences on AIP and MIP sets. All plans were compared based on dosimetric parameters, distributions and dose‐volume histograms (DVHs). Results: Dosimetric characteristics of FB with re‐calculated AIP and MIP plans were similar overall with those of MIP being slightly higher of the three. Interestingly, the re‐optimized plans give lower target and involved lung doses most likely due to different average lung CT‐numbers between FB, MIP and AIP sets. Conclusion: For peripheral lung lesions, planning for SBRT with intensity modulation is challenges as different imaging data sets have similar dosimetric characteristics. Re‐optimization of the plans on MIP and AIP give a better indication as to which data set is appropriate to be utilized. A more robust protocol is needed for lung 4D‐CT treatment planning.

SU-E-T-348: Dosimetric Characteristic of Zeiss IntraBeam Flat and Surface Applicators

Purpose: To verify the dosimetric characteristics of Flat and Surface applicators (two of each, 2cm and 4cm in diameters) for Zeiss Intrabeam systemMethods: Dose rate vs. distance from surface of the applicators were measured with the 1D water tank and PTW ion chamber provided by IntraBeam system. Gafchromic films and solid water were used to determine the 2D dose distribution and beam divergence of the applicators. Results: The highest dose rate is from 2cm Surface applicator (3.4Gy/min at 1.6mm from applicator surface, it drops to 0.52Gy/min at 10mm). The 2cm Flat applicator dose rate is 1.47Gy/min at 1.6mm; it drops to 0.36Gy/min at 10mm. The lowest dose rate is from the 4cm Flat applicator (0.49Gy/min at 1.6mm; it drops to 0.18Gy/min at 10mm). The 4cm Surface applicator dose rate is 1.36Gy/min at 1.6mm; it drops to 0.29Gy/min at 10mm. All the data were verified and confirmed with the manufacture factory measurements.Two dimensional dose profiles (parallel to applicator surface) at different depths were plotted from scanned Gafchromic films. Dose divergence of approximately 85 degree for 4cm applicators; and 65 degree for 2cm applicators; can also be seen from exposed films perpendicular to the applicator surface. Conclusion: Dosimetric characteristic were studied for both Flat and Surface applicators from Intrabeam system. Although all the applicators in this study are still pending FDA approval for clinical use, the data collected thus far can serve as a framework for both radiation oncologists and medical physicists to build the future clinical trials and treatment programs.

SU‐E‐T‐619: Dosimetric Comparison Between Static IMRT and VMAT for Cervical Cancer

Purpose: To evaluate the plan quality of static IMRT and VMAT for cervical cancer from comparing the dosimetric characteristic of IMRT and VMAT plans. Methods: For 16 patients with cervical cancer,7‐field step &amp; shoot IMRT plan and 360° dual‐arc VMAT plan were optimized using Pinnacle 9.0 TPS(prescribed 95% PTV dose 50Gy).Plans were evaluated according to CI(conformity index),HI (homogeneity index),volume of the OARs and dose of the normal tissues from DVH. Results: The CI and HI of IMRT and VMAT plans were 0.82,0.86 and 1.09,1.09,respectively. Both of IMRT and VMAT could achieve the same homogeneity,while the VMAT had a better performance in conformity than IMRT. The V20,V30and V40 of rectum for VMAT were 1.4%, 6.9% and 4.5% less than those for IMRT, and the mean dose was lower by 1.3Gy. In VMAT, the V20, V30 and V40 of bladder were 4.3%, 6.5% and 3.5% less than those for IMRT plans, and mean dose was reduced by 1.6Gy. The V5 and V50 of rectum and bladder for IMRT and VMAT were comparable with less than 1% in difference. The V10 and V20 of intestine for VMAT were 11% and 4.5% less than those for IMRT, while the V10 of colon for VMAT was 6.6% higher than that for IMRT plans. For sigmoid the V30 and V40 of VMAT were 3.2% and 5.3% less than of IMRT, but the V50 of VMAT was slightly higher than that of IMRT by 1.3%. The V50 of pelvic for VMAT was 5.2% higher than that for IMRT, but the mean dose was reduced by 0.4Gy. For normal tissues, the V10 of VMAT was 2% higher than that of IMRT, while the V20 of VMAT was 5.4% less than that of IMRT. Conclusion: The dosimetric comparison indicated that VMAT can deliver prescribed dose more conformity onto the patient and spare rectum and bladder better than IMRT. Considering advantages of fewer MUs and faster treatment, the VMAT is a better choice for cervical cancer. National Natural Foundation of China

Kinetic parameters of lithium tetraborate based TL materials

Lithium tetraborate (LTB) based thermoluminescent materials are attractive from the clinical and personnel dosimetry point of view because of their near tissue equivalence. Lithium tetraborate phosphors doped with different dopants like Copper (Cu), Silver (Ag), Phosphorus (P), Indium (In) and Manganese (Mn) or their combinations were synthesized by solid state route. Thermoluminescence (TL) glow curves and their emission spectra were recorded. The dosimetric characteristic of thermoluminescence materials mainly depend on the kinetic parameters like the trap depth, frequency factor and the order of kinetics. In this paper the kinetic parameters of lithium tetraborate based phosphors with different dopants were calculated using different standard methods namely variable heating rate method, initial rise method, peak shape method and isothermal decay method. A comparison was made for the trap depth obtained by the four methods for all the phosphors studied. It was found that the trap depth obtained by Isothermal decay method alone varies considerably for all the phosphors studied. The TL glow curves of LTB:Mn was generated from the estimated parameters by integrating the general order kinetic equations numerically using Adaptive Simpson Quadrature algorithm.

SU‐E‐T‐393: A Novel 4D Dosimetric Characterization of a Common 103‐Pd Brachytherapy Source with Mobile Internal Components

Purpose: A new method, based on the Finite Element (FE) modeling, has been introduced for dosimetric assessment of the brachytherapy sources with mobile internal component. Methods: Dosimetric characteristic of a commercially available brachytherapy source, Best Double‐Wall Model 2335 103Pd seed, were calculated by Monte Carlo (MC) simulation using the MCNP4c2 and MCNP5 codes according to the updated AAPM Task Group 43 (TG43U1) protocol, following finite element (FE) modeling to predict displacements of the seedˈs internal radioactive components in different orientations of the seed encountered in clinical practice (4D dosimetry). The complete seed geometry and materials together with a published 103Pd spectrum were used to perform finite‐element modeling using ABAQUS v.6.8.1 software and dosimetric characterization of the source using MCNP4c2 and MCNP5 codes. Results: Using FE modeling, change of seed orientation resulted in displacements of the seedˈs internal components. For the case of the Model 2335 103Pd seed, this has a negligible effect on the dose rate constant and radial dose function (&lt;1%) but the consequences on the 2D anisotropy function was greater (up to 16.2% at small angles). Conclusions: There are no clinically significant variations in the dose rate constant and radial dose function due to the uncertainty of the internal position of the structures for the Model 2335 103Pd seed. However, the uncertainty in the 2D anisotropy function at smaller angles is larger than what has been reported, due to the motion of the internal components. Using this 4D dosimetry method is suggested to determine the dosimetric characteristics of each new brachytherapy seed with mobile internal components.Keywords: Palladium‐103, Brachytherapy, Monte Carlo simulation, Finite Element modeling, TG43U1