C Shi

A boundary-representation method for designing whole-body radiation dosimetry models: pregnant females at the ends of three gestational periods--RPI-P3, -P6 and -P9.

Fetuses are extremely radiosensitive and the protection of pregnant females against ionizing radiation is of particular interest in many health and medical physics applications. Existing models of pregnant females relied on simplified anatomical shapes or partial-body images of low resolutions. This paper reviews two general types of solid geometry modeling: constructive solid geometry (CSG) and boundary representation (BREP). It presents in detail a project to adopt the BREP modeling approach to systematically design whole-body radiation dosimetry models: a pregnant female and her fetus at the ends of three gestational periods of 3, 6 and 9 months. Based on previously published CT images of a 7-month pregnant female, the VIP-Man model and mesh organ models, this new set of pregnant female models was constructed using 3D surface modeling technologies instead of voxels. The organ masses were adjusted to agree with the reference data provided by the International Commission on Radiological Protection (ICRP) and previously published papers within 0.5%. The models were then voxelized for the purpose of performing dose calculations in identically implemented EGS4 and MCNPX Monte Carlo codes. The agreements of the fetal doses obtained from these two codes for this set of models were found to be within 2% for the majority of the external photon irradiation geometries of AP, PA, LAT, ROT and ISO at various energies. It is concluded that the so-called RPI-P3, RPI-P6 and RPI-P9 models have been reliably defined for Monte Carlo calculations. The paper also discusses the needs for future research and the possibility for the BREP method to become a major tool in the anatomical modeling for radiation dosimetry.

Use of the VIP-Man model to calculate energy imparted and effective dose for x-ray examinations.

Abstract— A male human tomographic model was used to calculate values of energy imparted (&egr;) and effective dose (E) for monoenergetic photons (30–150 keV) in radiographic examinations. Energy deposition in the organs and tissues of the human phantom were obtained using Monte Carlo simulations. Values of E/&egr; were obtained for three common projections [anterior-posterior (AP), posterior-anterior (PA), and lateral (LAT)] of the head, cervical spine, chest, and abdomen, respectively. For head radiographs, all three projections yielded similar E/&egr; values. At 30 keV, the value of E/&egr; was ∼1.6 mSv J−1, which is increased to ∼7 mSv J−1 for 150 keV photons. The AP cervical spine was the only projection investigated where the value of E/&egr; decreased with increasing photon energy. Above 70 keV, cervical spine E/&egr; values showed little energy dependence and ranged between ∼8.5 mSv J−1 for PA projections and ∼17 mSv J−1 for AP projections. The values of E/&egr; for AP chest examinations showed very little variation with photon energy, and had values of ∼23 mSv J−1. Values of E/&egr; for PA and LAT chest projections were substantially lower than the AP projections and increased with increasing photon energy. For abdominal radiographs, differences between the PA and LAT projections were very small. All abdomen projections showed an increase in the E/&egr; ratio with increasing photon energy, and reached a maximum value of ∼13.5 mSv J−1 for AP projections, and ∼9.5 mSv J−1 for PA/lateral projections. These monoenergetic E/&egr; values can generate values of E/&egr; for any x-ray spectrum, and can be used to convert values of energy imparted into effective dose for patients undergoing common head and body radiological examinations.

SAF values for internal photon emitters calculated for the RPI-P pregnant-female models using Monte Carlo methods

Estimates of radiation absorbed doses from radionuclides internally deposited in a pregnant woman and her fetus are very important due to elevated fetal radiosensitivity. This paper reports a set of specific absorbed fractions (SAFs) for use with the dosimetry schema developed by the Society of Nuclear Medicines Medical Internal Radiation Dose (MIRD) Committee. The calculations were based on three newly constructed pregnant female anatomic models, called RPI-P3, RPI-P6, and RPI-P9, that represent adult females at 3-, 6-, and 9-month gestational periods, respectively. Advanced Boundary REPresentation (BREP) surface-geometry modeling methods were used to create anatomically realistic geometries and organ volumes that were carefully adjusted to agree with the latest ICRP reference values. A Monte Carlo user code, EGS4-VLSI, was used to simulate internal photon emitters ranging from 10 keV to 4 MeV. SAF values were calculated and compared with previous data derived from stylized models of simplified geometries and with a model of a 7.5-month pregnant female developed previously from partial-body CT images. The results show considerable differences between these models for low energy photons, but generally good agreement at higher energies. These differences are caused mainly by different organ shapes and positions. Other factors, such as the organ mass, the source-to-target-organ centroid distance, and the Monte Carlo code used in each study, played lesser roles in the observed differences in these. Since the SAF values reported in this study are based on models that are anatomically more realistic than previous models, these data are recommended for future applications as standard reference values in internal dosimetry involving pregnant females.

Specific absorbed fractions for internal photon emitters calculated for a tomographic model of a pregnant woman.

Specific absorbed fractions are essential for calculation of radiation dose from internal emitters. Existing specific absorbed fractions for pregnant women were calculated using the stylized models; in this work, a partial-body tomographic model for a pregnant woman was constructed from a rare set of CT images. Based on this tomographic model, the Monte Carlo code, EGS4-VLSI, was used to derive specific absorbed fractions. Monoenergetic, isotropic photon emitters from 15 keV to 4 MeV were distributed in different source organs, and doses were calculated to many target regions in the body. Even though the results showed general agreement with previous studies for higher energies, significant differences were also found, especially for lower energies. The main reasons for the differences are due to the variation of mass, geometry, and organ distances, and they demonstrate the influence of more realistic body models on dose calculations.

SU‐GG‐T‐38: Dosimetry and Inverse Treatment Planning for 3D Intensity Modulated Brachytherapy

Purpose: To propose a dosimetry algorithm for three‐dimensional (3D) intensity modulated brachytherapy (IMBT) and to develop an inverse treatment planning method applying the dosimetry algorithm. Method and Materials: A Matlab based prototype 3D IMBT treatment planning system was developed. The system consisted of three main components: (1) a comprehensive source commissioning method for intensity modulated sources based on Monte Carlo (EGSnrc) simulations; (2) a “modified TG43” (mTG43) dose calculation algorithm for IMBT dosimetry; (3) an inverse IMBT treatment planning method based on Dose Volume Histogram (DVH) or DoseSurface Histogram (DSH) constraints and simulated annealingoptimization algorithm. The system was applied for planning of an intracavitary accelerated partial breast irradiation (APBI) case treated with Xoft Axxent electronic brachytherapy. Plan quality, planning and delivery time of the IMBT plan were compared with the original plan used for the patients treatment. Results: For the patient studied, IMBT plan showed better plan quality compared with the original plan. With similar coverage to the target, high dose region V200 was decrease by 16.1%. Maximum doses to skin and ribs were reduced by 56 cGy and 104 cGy in one fraction respectively. Mean dose to ipsilateral and contralateral breasts and lungs were also slightly reduced by IMBT. Conclusion: Application of three‐dimensional intensity modulation in brachytherapytreatment planning is both feasible and promising. 3D IMBT improves the quality of APBI brachytherapy treatment plan, increasing dose uniformity in target and reducing the dose to critical structures.

SU‐FF‐T‐386: Validation of the Delta4 Dosimetry Phantom Against Ionometric Measurements

Purpose: To validate a 3D dose calculation methodology used by the 3D Delta4™ phantom. Method and materials: Measurements were performed using a TomoTherapy™ HiArt™ and a Varian 2300C/D™. A pinpoint PTW N31006 chamber with a sensitive volume of 0.016cc was use for point dose measurements. The Delta4™ phantom (ScandiDos AB, Uppsala, Sweden) was modified to accept the ion chamber. Eight (n=8) quality assurance plans were created in both planning systems. Plans covered a range of doses from 20% to 160% (20% steps) of the prescribed dose using Pinnacle3 TPS. Tomotherapy plans covered doses from 20% to 100% (20% steps) and a DQA plan with 200% of the prescribed dose. Treatment plan, DQA plan, DQA dose and structures were exported via DICOM RT to the Delta4™ software. At each delivery, a point dose measurement was recorded. Point dose measurements and calculated doses by the Delta4™ were compared against the calculated dose of the treatment planning systems. Furthermore, point dose measurements were compared against the Delta4™ calculated dose. Results: The preliminary results showed that chamber measurements agree with Pinnacle3 within 0.3% to 2%. The Delta4™ system have an excellent response at higher doses while at lower doses the percent differences are compared to the ones obtained by the pinpoint chamber. Tomotherapy delivery results showed good agreement between the pinpoint chamber and Delta4™ with percent differences ranging from 0.03% to 5% with better response at high doses Conclusion: Delta4™ showed good agreement with the pinpoint chamber in the dose calculation, which shows that the system 3D dose calculation methodology is able to predict the same or better doses compared to the pinpoint chamber measurements.

Specific absorbed fractions for internal electron emitters derived for a set of anatomically realistic reference pregnant female models.

The specific absorbed fraction (Phi), defined by the Medical Internal Radiation Dose Committee, is generally applied to evaluate the average absorbed dose in a target organ as a result of radioactive materials deposited in a source organ. This paper reports a new set of Phi values for internal electron emitters ranging from 10 keV to 4 MeV from various internal organs of the mother to the fetus based on three newly developed pregnant female tomographic models, called RPI-P3, RPI-P6 and RPI-P9. The results show a linear log relationship between Phi values and electron energy. The linear log coefficients have been derived and reported. The relationship between Phi values and mean distances between source organs and the fetus were also determined to allow for individual dosimetry. Since the RPI-P models have finer details of human anatomy and more realistic organ volumes and geometries, which follow the latest ICRP reference values, the newly derived Phi values could be used as reference values in determination of the dose to the fetus from internal electron emitters.

SU-FF-T-267: Total Skin Electron Therapy Skin Dose Validation Using Optically Stimulated Luminescent Dosimeters

Purpose: Total Skin Electron Therapy skin dose validation using optically stimulated luminescent dosimeters Methods and Materials: The institutions standard of care for total skin electron therapy (TSE) follows the Stanford Technique on a Varian 2300 EX linac that treats the patient standing in six different positions. The patient stands 140 cm from the beams isocenter. A 9MeV HDTSe electron beam delivers 200 cGy to the skins surface up to a depth of 0.8 cm with a rapid drop‐off of dose. A 1.2 cm acrylic beam spoiler 30 cm was placed in front of the patient and a sheet of 1mm of aluminum is at the gantry head. Phantom measurements were made to determine suitability of optically stimulated luminescence dosimeters (OSLDs) for in vivodosimetry of TSE patients. A 30×30×30 cm3 solid water phantom was placed at 100 cm SSD. TLDs, OSLDs and calibrated plane parallel ion chamber were placed at the center of the phantom under 2.1 cm of build up. A dose of 100 cGy was delivered for calibration. For validating the response of the OSLDs, both TLDs and OSLDs were located at different locations around the torso of a Rando phantom. Following the technique described above, the Rando phantom was irradiated to deliver a surface dose of 200 cGy. In addition, in vivo measurements were made for a TSE patient at seven different body points. Results: OSLD phantom measurements were 3% agreement with the ROOS chamber and 4.5% with TLDs during calibration.Skin dose validation of the Rando phantom between TLDs and OSLDs differed by as much as 5.6%. In vivodosimetry using OSLDs for the patient agreed within 7% for TLDs.Conclusions: Results for both the phantom and patient measurements shows that OSLDs are suitable for in vivodosimetry in Total SkinElectron beam therapy.

SU‐FF‐T‐05: A Monte Carlo Based Dose Calculation and Evaluation Toolkit for Electronic Brachytherapy: Feasibility of IMBT

Purpose: To develop a Monte Carlo based dose calculation and evaluation toolkit for electronic brachytherapy sources, capable of simulating Intensity Modulated Brachytherapy (IMBT). Material and methods: This toolkit used Monte Carlo code EGS4 to calculate the dose distribution for a treatment in a realistic virtual human phantom converted from the patients CTimages and contours. An in‐house Matlab program was developed to analyze the dose distribution and generate DVHs and isodoses. Results: The system was benchmarked by comparing the calculated radial dose functions (in water) with experimental data published. Difference was within 3%. A typical intracavitary accelerated partial breast irradiation (APBI) treatment plan using Xoft Axxent electronic brachytherapy source was simulated using this toolkit. DVHs and isodoses revealed that the dose to the ribs were high in this plan due to proximity of balloon to the chest wall and the high absorption coefficient of bone to low energy X‐rays. A simple IMBT plan using partial block could conform the isodose distribution to the target, reduce the dose to ribs and chest wall without compromising the dose homogeneity to the target or increasing the dose to other critical structures. Conclusion: A Monte Carlo based dose calculation and evaluation system was developed for electronic brachytherapy sources. Benchmarked through published data, this system is capable of producing reliable and detailed dose distributions for both isotropic and intensity modulated sources. The feasibility of intensity modulation in improving the plan quality was proved.

SU‐FF‐T‐260: In Vivo Dose Measurements for Total Body Irradiation Using Optically Stimulated Luminescent Dosimeters

Purpose: In vivo dose measurements for Total Body Irradiation using optically stimulated luminescent dosimeters Method and Materials: The institutions standard of care for total body irradiation uses a 6 MV Varian 600C linac with the gantry angle at 90 degrees and field size of 40×40 cm2. The patients midline is at 350 cm. A 1.2 cm acrylic spoiler is place 40 cm from the surface of the patient and an acrylic tray holding lead compensators is at the head of the gantry. Phantom measurements were made to determine suitability of optically stimulated luminescence dosimeters (OSLDs) for in vivodosimetry of patients undergoing total body irradiation (TBI). A 30×30×30 cm3 solid water phantom was placed at 350 cm SAD. A calibrated plane parallel ROOS ion chamber was placed at the center of the phantom under 1 cm of build up. OSLDs and TLDs were also place on the phantom adjacent to the ROOS chamber at 1 cm depth. A treatment plan was created to deliver 100 cGy to the midplane of the phantom for 1 field. All three dosimeters were irradiated. A similar setup was created using an anthropomorphic phantom at 350 SAD,TLDs and OSLDs at three different locations. In addition, in vivo measurements were made for three patients undergoing total body irradiation at nine different body points. Results: OSLD phantom measurements were in agreement of the ROOS chamber (2.6%) and TLDs (3.8%) when using the solid water phantom. Comparative measurements between TLDs and OSLDs differed by as much as 4.5% for the anthropomorphic phantom irradiation. In vivodosimetry using OSLDs for the three patients agreed within (7.6%) for TLDs.Conclusions: Results for both the phantom and patient measurements confirm that OSLDs are both suitable and recommended for required in vivodosimetry in Total Body Irradiation.