Engineering topics

Enger, Shirin A.

Publications and source records attributed to Enger, Shirin A..

4 records

DOE OSTI2014

Interstitial rotating shield brachytherapy for prostate cancer

Purpose: To present a novel needle, catheter, and radiation source system for interstitial rotating shield brachytherapy (I-RSBT) of the prostate. I-RSBT is a promising technique for reducing urethra, rectum, and bladder dose relative to conventional interstitial high-dose-rate brachytherapy (HDR-BT). Methods: A wire-mounted 62 GBq{sup 153}Gd source is proposed with an encapsulated diameter of 0.59 mm, active diameter of 0.44 mm, and active length of 10 mm. A concept model I-RSBT needle/catheter pair was constructed using concentric 50 and 75 μm thick nickel-titanium alloy (nitinol) tubes. The needle is 16-gauge (1.651 mm) in outer diameter and the catheter contains a 535 μm thick platinum shield. I-RSBT and conventional HDR-BT treatment plans for a prostate cancer patient were generated based on Monte Carlo dose calculations. In order to minimize urethral dose, urethral dose gradient volumes within 0–5 mm of the urethra surface were allowed to receive doses less than the prescribed dose of 100%. Results: The platinum shield reduced the dose rate on the shielded side of the source at 1 cm off-axis to 6.4% of the dose rate on the unshielded side. For the case considered, for the same minimum dose to the hottest 98% of the clinical target volume (D{sub 98%}), I-RSBT reduced urethral D{sub 0.1cc} below that of conventional HDR-BT by 29%, 33%, 38%, and 44% for urethral dose gradient volumes within 0, 1, 3, and 5 mm of the urethra surface, respectively. Percentages are expressed relative to the prescription dose of 100%. For the case considered, for the same urethral dose gradient volumes, rectum D{sub 1cc} was reduced by 7%, 6%, 6%, and 6%, respectively, and bladder D{sub 1cc} was reduced by 4%, 5%, 5%, and 6%, respectively. Treatment time to deliver 20 Gy with I-RSBT was 154 min with ten 62 GBq {sup 153}Gd sources. Conclusions: For the case considered, the proposed{sup 153}Gd-based I-RSBT system has the potential to lower the urethral dose relative to HDR-BT by 29%–44% if the clinician allows a urethral dose gradient volume of 0–5 mm around the urethra to receive a dose below the prescription. A multisource approach is necessary in order to deliver the proposed {sup 153}Gd-based I-RSBT technique in reasonable treatment times.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

DOE OSTI2012

Exploring {sup 57}Co as a new isotope for brachytherapy applications

Purpose: The characteristics of the radionuclide {sup 57}Co make it interesting for use as a brachytherapy source. {sup 57}Co combines a possible high specific activity with the emission of relatively low-energy photons and a half-life (272 days) suitable for regular source exchanges in an afterloader. {sup 57}Co decays by electron capture to the stable {sup 57}Fe with emission of 136 and 122 keV photons. Methods: A hypothetical {sup 57}Co source based on the Flexisource brachytherapy encapsulation with the active core set as a pure cobalt cylinder (length 3.5 mm and diameter 0.6 mm) covered with a cylindrical stainless-steel capsule (length 5 mm and thickness 0.125 mm) was simulated using Geant4 Monte Carlo (MC) code version 9.4. The radial dose function, g(r), and anisotropy function F(r,{theta}), for the line source approximation were calculated following the TG-43U1 formalism. The results were compared to well-known {sup 192}Ir and {sup 125}I radionuclides, representing the higher and the lower energy end of brachytherapy, respectively. Results: The mean energy of photons in water, after passing through the core and the encapsulation material was 123 keV. This hypothetical {sup 57}Co source has an increasing g(r) due to multiple scatter of low-energy photons, which results in a more uniform dose distribution than {sup 192}Ir. Conclusions: {sup 57}Co has many advantages compared to {sup 192}Ir due to its low-energy gamma emissions without any electron contamination. {sup 57}Co has an increasing g(r) that results in a more uniform dose distribution than {sup 192}Ir due to its multiple scattered photons. The anisotropy of the {sup 57}Co source is comparable to that of {sup 192}Ir. Furthermore, {sup 57}Co has lower shielding requirements than {sup 192}Ir.

07 ISOTOPE AND RADIATION SOURCES↗

DOE OSTI2011

Modeling a Hypothetical {sup 170}Tm Source for Brachytherapy Applications

Purpose: To perform absorbed dose calculations based on Monte Carlo simulations for a hypothetical {sup 170}Tm source and to investigate the influence of encapsulating material on the energy spectrum of the emitted electrons and photons. Methods: GEANT4 Monte Carlo code version 9.2 patch 2 was used to simulate the decay process of {sup 170}Tm and to calculate the absorbed dose distribution using the GEANT4 Penelope physics models. A hypothetical {sup 170}Tm source based on the Flexisource brachytherapy design with the active core set as a pure thulium cylinder (length 3.5 mm and diameter 0.6 mm) and different cylindrical source encapsulations (length 5 mm and thickness 0.125 mm) constructed of titanium, stainless-steel, gold, or platinum were simulated. The radial dose function for the line source approximation was calculated following the TG-43U1 formalism for the stainless-steel encapsulation. Results: For the titanium and stainless-steel encapsulation, 94% of the total bremsstrahlung is produced inside the core, 4.8 and 5.5% in titanium and stainless-steel capsules, respectively, and less than 1% in water. For the gold capsule, 85% is produced inside the core, 14.2% inside the gold capsule, and a negligible amount (<1%) in water. Platinum encapsulation resulted in bremsstrahlung effects similar to those with the gold encapsulation. The range of the beta particles decreases by 1.1 mm with the stainless-steel encapsulation compared to the bare source but the tissue will still receive dose from the beta particles several millimeters from the source capsule. The gold and platinum capsules not only absorb most of the electrons but also attenuate low energy photons. The mean energy of the photons escaping the core and the stainless-steel capsule is 113 keV while for the gold and platinum the mean energy is 160 keV and 165 keV, respectively. Conclusions: A {sup 170}Tm source is primarily a bremsstrahlung source, with the majority of bremsstrahlung photons being generated in the source core and experiencing little attenuation in the source encapsulation. Electrons are efficiently absorbed by the gold and platinum encapsulations. However, for the stainless-steel capsule (or other lower Z encapsulations) electrons will escape. The dose from these electrons is dominant over the photon dose in the first few millimeter but is not taken into account by current standard treatment planning systems. The total energy spectrum of photons emerging from the source depends on the encapsulation composition and results in mean photon energies well above 100 keV. This is higher than the main gamma-ray energy peak at 84 keV. Based on our results, the use of {sup 170}Tm as a brachytherapy source presents notable challenges.

07 ISOTOPE AND RADIATION SOURCES↗

DOE OSTI2011

An algorithm for efficient metal artifact reductions in permanent seed implants

Purpose: In permanent seed implants, 60 to more than 100 small metal capsules are inserted in the prostate, creating artifacts in x-ray computed tomography (CT) imaging. The goal of this work is to develop an automatic method for metal artifact reduction (MAR) from small objects such as brachytherapy seeds for clinical applications. Methods: The approach for MAR is based on the interpolation of missing projections by directly using raw helical CT data (sinogram). First, an initial image is reconstructed from the raw CT data. Then, the metal objects segmented from the reconstructed image are reprojected back into the sinogram space to produce a metal-only sinogram. The Steger method is used to determine precisely the position and edges of the seed traces in the raw CT data. By combining the use of Steger detection and reprojections, the missing projections are detected and replaced by interpolation of non-missing neighboring projections. Results: In both phantom experiments and patient studies, the missing projections have been detected successfully and the artifacts caused by metallic objects have been substantially reduced. The performance of the algorithm has been quantified by comparing the uniformity between the uncorrected and the corrected phantom images. The results of the artifact reduction algorithm are indistinguishable from the true background value. Conclusions: An efficient algorithm for MAR in seed brachytherapy was developed. The test results obtained using raw helical CT data for both phantom and clinical cases have demonstrated that the proposed MAR method is capable of accurately detecting and correcting artifacts caused by a large number of very small metal objects (seeds) in sinogram space. This should enable a more accurate use of advanced brachytherapy dose calculations, such as Monte Carlo simulations.

60 APPLIED LIFE SCIENCES↗