Soft Tissue Equivalents Research Articles

High mobility single crystal diamond detectors for dosimetry: Application to radiotherapy

Diamond exhibits properties of interest for applications in the medical field. It is a very attractive material for detector fabrication due to its intrinsic properties and particularly its soft-tissue equivalence ( Z = 6 compared to Z = 7.42 for human tissue), mechanical robustness and radiation hardness. Detectors fabricated from natural diamonds are used in several hospitals as dosimetric tools for the dose measurement received by the patient during radiotherapy and for beam calibration. Natural diamond based devices are expensive and long delivery times are common. The use of synthetic single crystal diamond is a promising issue for point dosimeter. Here we report on the growth of synthetic diamond using the CVD technique to fabricate free standing single crystals. Samples were characterized from their optical and electronic properties (Raman, TOF) and mounted as solid ionisation chambers with blocking contacts, for the evaluation of their dosimetric properties. Clinical tests were conducted in a medical facility at the Institute Gustave Roussy (IGR) in France specialised in the medical treatment of tumours. The results obtained demonstrate that our single crystal diamond detectors comply with the required specifications for radiotherapy applications.

Synthesis and Characterization of a Soft-tissue Substitute for Neutron Dosimetry

To develop a physical phantom for neutron dosimetry, a solid soft-tissue substitute was synthesized. The synthesized tissue substitute, NAN-JAERI, is improved in both hydrogen and oxygen elemental composition in comparison with existing tissue substitutes. To examine the radiation characteristics of the new soft-tissue substitute, absorbed dose distributions in NAN-JAERI were measured using a 252Cf neutron source. The measured absorbed dose distributions of neutrons and photons agree with those calculated by a Monte Carlo simulation code MCNP. The agreement between the experiment and the simulation verifies this method of evaluating the soft-tissue equivalence of NAN-JAERI for 252Cf neutrons. Similar simulations for some mono-energetic neutron sources showed that the newly developed tissue substitute has soft-tissue equivalent characteristics in the neutron energy range from 1 MeV up to 14 MeV, in terms of the absorbed dose distributions in a slab phantom.

Synthesis and Characterization of a Soft-tissue Substitute for Neutron Dosimetry

To develop a physical phantom for neutron dosimetry, a solid soft-tissue substitute was synthesized. The synthesized tissue substitute, NAN-JAERI, is improved in both hydrogen and oxygen elemental composition in comparison with existing tissue substitutes. To examine the radiation characteristics of the new soft-tissue substitute, absorbed dose distributions in NAN-JAERI were measured using a 252 Cf neutron source. The measured absorbed dose distributions of neutrons and photons agree with those calculated by a Monte Carlo simulation code MCNP. The agreement between the experiment and the simulation verifies this method of evaluating the soft-tissue equivalence of NAN-JAERI for 252 Cf neutrons. Similar simulations for some mono-energetic neutron sources showed that the newly developed tissue substitute has soft-tissue equivalent characteristics in the neutron energy range from 1 MeV up to 14 MeV, in terms of the absorbed dose distributions in a slab phantom. Neutron dosimetry has become more important for radi- ation protection than it has been in the past, since neutron sources, obtained from nuclear reactors or accelerators, have been widely used in solid state physics, material science, nuclear physics and neutron therapy. Experimental tech- niques of neutron diffraction or neutron scattering are so useful that they are applicable to other research fields men- tioned above. To perform experiments safely using the neu- tron sources, neutron dosimetry of high precision is required. Under the present circumstances, however, neutron dosimetry in radiation protection depends highly on numerical methods. Dose conversion coefficients relating neutron fluence to or- gan doses or to the effective dose have been used for dose assessments of workers and the public. The dose conversion coefficients have been calculated using mathematical anthro- pomorphic human models and radiation transport simulation techniques. Although it is almost impossible to construct re- alistic human models, experimental verification of the dose assessment technique is needed. To perform accurate neutron dose assessments based on ex- periments, elements such as appropriate neutron sources, de- tectors and phantoms are indispensable. Among these three elements, the appropriate phantom is one of the most impor- tant elements for dose assessments and for the calibration of detectors as an alternative to the human body. For photon dosimetry, various phantoms have been developed. The tis- sue substitutes of phantoms for photons are made of materi- als whose electron densities are close to those of the body tis- sues, because the photon deposits its energy through interac- tions with atomic electrons. In the case of neutrons, it is ideal that phantoms are made of materials with elemental compo- sitions that are identical to those of the body tissues, because the neutron interacts with the atomic nuclei. In ICRU Report

Radiation dose from DXA scanning to reproductive tissues of females.

The objective of this study was to use an anatomically arrayed whole-body phantom to measure radiation exposure to the ovaries and uterus during standard dual-energy Xray absorptiometry (DXA) scanning. DXA instrument manufacturers' published entrance skin exposure is about 3 mR (0.77 microC/kg), which is equivalent to the radiation exposure received during a transcontinental plane trip. Nonetheless, since DXA scanning is used more frequently with very young females, the need for pregnancy testing has become an issue that requires attention and formulation of research guidelines. We attached thermoluminescent dosimeters (TLDs) to anatomically arrayed balloon models for ovaries and the uterus, and placed these in the appropriate sites within a small human skeleton along with appropriate amounts of aqueous and fat soft tissue equivalents. Whole-body scanning with a Hologic QDR-2000W was performed 10 times with the pencil beam mode and, using separate TLD detectors, 10 times with the fan beam mode. Overall, the average exposures at skin entrance were 0.89 mR (0.23 microC/kg) with doses for the ovaries of 0.52 mrad (5.2 microgy) and 0.59 mrad (5.9 microgy) for the uterus. These doses are equivalent to 2 d of ambient background radiation in central Pennsylvania or 1 h of flying at 39,000 ft. Although different DXA models by Hologic and DXA instruments by other manufacturers will have different radiation outputs, we believe that these low radiation levels do not require pregnancy testing or questioning of whether the scan subject might be pregnant.

Measured lung dose correction factors for 50 MV photons

Some clinically relevant measurements of lung tissue/water equivalent interfaces have been performed for a 50 MV therapeutic x-ray beam. The purpose was to investigate the severity of dose perturbation effects in lung tissue and adjacent tissues using an energy well above the common clinical practice in thoracic irradiations. The phantoms were constructed of solid water, PMMA and white polystyrene as soft tissue (water) equivalents, and cork was used as the lung tissue equivalent. Measurements were performed using radiographic film and a cylindrical ionization chamber. The results show that the degradation of the 20/80% beam penumbra in the lung region is severe, up to 2.5 times the penumbra in water for a 10 cm thick lung with a density of . The lack of electronic equilibrium in the low-density region can cause underdosage at the lung/tumour interface of up to 30% of maximum target dose, and the build-up depth to 95% of target dose in unit density tissue behind the lung may be as large as 22 mm. It is also shown that these figures strongly depend on patient anatomy and beam size and why a careful calculation of the individual dose distribution is needed for optimal choice of photon beam energy in thoracic treatments.

Influence of patient's weight on dual-photon absorptiometry and dual-energy X-ray absorptiometry measurements of bone mineral density

Lumbar spine bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry (DXA) (Hologic QDR 1000) and by 153Gd dual-photon absorptiometry (DPA) (Novo Lab 22a) in 120 postmenopausal women. Though a high correlation existed between the two techniques, the ratio between DXA and DPA values was not constant. Using DXA we observed a higher dependence of BMD on weight than in the DPA measurements. To investigate the different behaviour of DXA and DPA machines with weight, we analysed the effects of increasing thickness of soft tissue equivalents on the BMD of the Hologic spine phantom and on the BMD equivalent of an aluminium standard tube. Increasing tissue-equivalent thickness caused the phantom BMD measured by DPA to decrease significantly but had not effect on the DXA measurements. The different behaviour of DPA and DXA equipment with regard to the phantoms could account for the differences observed in the relations between BMD and weight in the patients. Using multiple regression we studied the influence of weight and body mass index on the relation between BMD measured by the two techniques. The introduction of either of these variables into the regression resulted in an improvement of the prediction of the DXA values from the DPA values. However, the residual standard error of the estimate was still higher than the combined precision errors of the two methods, so that no simple relation allows a conversion of BMDDPA into BMDDXA. Our results confirm that BMD is positively correlated with weight in postmenopausal women; the influence of weight on BMD is blunted when the Novo Lab 22a DPA machine is used for measuring bone mineral.

Validation of body composition by dual energy X‐ray absorptiometry (DEXA)

The present study validates the use of dual energy X-ray absorptiometry (DEXA) for measurement of body composition. The precision error was expressed as the SD (CV%) for fat mass, FAT%, lean tissue mass, and total body bone mineral: 1.1 kg (6.4%), 1.6% (5.7%), 1.4 kg (3.1%), and 0.03 kg (1.2%), respectively. The accuracy study in vitro used (1) mixtures of water and alcohol, (2) mixtures of ox muscle and lard, and (3) dried bones. In the clinically relevant range of values there were only small influences on DEXA measurements of variations in amount and composition of the soft tissue equivalents. The accuracy study in vivo compared the components of body composition measured recently by DEXA and earlier by dual photon absorptiometry, counting of naturally occurring total body 40K, and body density by underwater weighing in 25 healthy adult subjects. We found agreement between fat percentage (and lean body mass) by DEXA and the three established measurements modalities; mean differences were (-5.3 to -0.4%) and (-0.7 to 2.5 kg) for fat percentage and lean body mass, respectively. We conclude that DEXA provides a new method of measuring body composition with precision and accuracy errors, which are compatible with the application of DEXA in group research studies and probably also in clinical measurements of the single subject.

Lead as surface bolus for high‐energy photon and electron therapy

Dose distributions were evaluated under thin sheet lead used as surface bolus for 4- and 10-MV photons and 6- and 9-MeV electrons using a parallel-plate ion chamber and film. A narrow, significantly low dose region (-17%) was noted for 4-MV photons, whereas a 6% increase in dose was present for 10-MV photons. The dose was elevated 15%-22% near the surface of electron fields with lead bolus, but depth dose relationships were similar to soft-tissue-equivalent (STE) bolus. Investigation of partial-field bolus (2-cm-diam circle) documented reduced doses due to lack of lateral electron equilibrium for 10-MV photons, which was less evident using lead, and large edge effects (up to 30%) for electrons using either lead or STE bolus. Dose distributions on sloped surfaces with electron fields were similar for lead and STE; both require thickness adjustment to achieve a desired effective thickness normal to the surface. Lead bolus has been used successfully in clinical practice for photons.