Radiosensitivity and capacity for radiation-induced sublethal damage repair of canine transitional cell carcinoma (TCC) cell lines.

Understanding the inherent radiosensitivity and repair capacity of canine transitional cell carcinoma (TCC) can aid in optimizing radiation protocols to treat this disease. The objective of this study was to evaluate the parameters surviving fraction at 2 Gy (SF(2) ), α/ß ratio and capacity for sublethal damage repair (SLDR) in response to radiation. Dose-response and split-dose studies were performed using the clonogenic assay. The mean SF(2) for three established TCC cell lines was high at 0.61. All the three cell lines exhibited a low to moderate α/ß ratio, with the mean being 3.27. Two cell lines exhibited statistically increased survival at 4 and 24 h in the dose-response assay. Overall, our results indicate that the cell lines are moderately radioresistant, have a high repair capacity and behave similarly to a late-responding normal tissue. These findings indicate that the radiation protocols utilizing higher doses with less fractionation may be more effective for treating TCC.

Organ dose and inherent uncertainty in helical CT dosimetry due to quasiperiodic dose distributions.

PURPOSE: The purpose of this study was to characterize relative magnitudes of peripheral dose modulations present in helical MDCT resulting from variation in x-ray tube starting position and phantom position relative to isocenter using a novel methodology that employs direct dosimetric measurements and knowledge of scan geometry. The magnitudes of potential dose savings to specific radiosensitive tissues related to the phase of the quasiperiodic dose distribution are also quantified and compared to similar Monte Carlo based studies. METHODS: For this study, a Siemens SOMATOM Sensation 16 helical MDCT scanner and a tomographic adult anthropomorphic phantom from the University of Florida phantom series were used for all scans. In addition, a plastic scintillator-based fiber-optic-coupled dosimetry system was used to record real-time axial dosimetric measurements. These direct measurements were used to derive cumulative point doses and tissue doses for helical MDCT using different pitch values and surface to isocenter distances. RESULTS: Cumulative point doses and doses for the lens of the eye and thyroid showed strong variation with both the phase of the dose distribution and phantom positioning relative to isocenter. Depending on the phantom positioning relative to isocenter, individual in-phantom cumulative point dose values were shown to vary anywhere from 0 to 25% lower than the maximum value for scans of pitch 1, and greater than 60% for scans of pitch 1.5. Reduction in total tissue dose to the lens of the eye (thyroid) varied from 0 to 20% (4%) lower than the maximum value for pitch 1, and from 59 (14%) to 71% (19%) lower than the maximum value for scans using pitch 1.5. These values are similar to those found in previous Monte Carlo based studies. The reduction in average total tissue dose between the extremes (+/- 3 cm from nominal) of phantom positioning relative to isocenter for the eye (thyroid) was 16% (13%) for pitch 1, and 14% (12%) for pitch 1.5. CONCLUSIONS: As recent Monte Carlo simulations have shown, there exists an inherent uncertainty when performing dose measurements within a phantom during helical MDCT scans. The periodic dose distributions in helical MDCT means that low resolution sampling of local phantom doses could result in dose measurement aliasing. For reliable results, these considerations should be accounted for in helical MDCT phantom dosimetry studies. The variability in surface dose has a strong dependence on phantom positioning relative to isocenter. Dose to tissues such as the lens of the eye and thyroid can be minimized by positioning patients, so these tissues are closer to isocenter because the decrease in x-ray intensity due to beam divergence dominates the increases resulting from increased primary beam exposure overlap. Of course, this dose decrease would have to be balanced against any diminished image quality resulting from misalignment of the patient with the bowtie filter. Additionally, significantly reduced dose to small radiosensitive tissues such as the lens of the eye could occur if it were possible to shift the phase of the periodic dose distribution present in helical MDCT. These dose reductions would come at no cost to image quality.

Tomographic physical phantom of the newborn child with real-time dosimetry I. Methods and techniques for construction.

A tomographic phantom representing a newborn female patient was constructed using tissue-equivalent materials previously developed at the University of Florida. This phantom was constructed using contoured images from an actual patient data set, a whole-body computed tomography of a newborn cadaver previously described by Nipper et al. [Phys. Med. Biol. 47, 3143-1364 (2002)]. Four types of material are incorporated in the phantom: soft tissue, bone tissue, lung tissue, and air. The phantom was constructed on a slice-by-slice basis with a z-axis resolution of 5 mm, channels for dosimeters (thermoluminescent dosimeter (TLD), metal-oxide-semiconductor field-effect transistor, or gated fiber-optic-coupled dosimeter (GFOC)) were machined into slices prior to assembly, and the slices were then fixed together to form the complete phantom. The phantom will be used in conjunction with an incorporated dosimetry system to calculate individual organ and effective doses delivered to newborn patients during various diagnostic procedures, including, but not limited to, projection radiography and computed tomography. Included in this paper are images detailing the construction process, and images of the completed phantom.

MOSFET dosimeter depth-dose measurements in heterogeneous tissue-equivalent phantoms at diagnostic x-ray energies.

The objective of the present study was to explore the use of the TN-1002RD metal-oxide-semiconductor field effect transistor (MOSFET) dosimeter for measuring tissue depth dose at diagnostic photon energies in both homogeneous and heterogeneous tissue-equivalent materials. Three cylindrical phantoms were constructed and utilized as a prelude to more complex measurements within tomographic physical phantoms of pediatric patients. Each cylindrical phantom was constructed as a stack of seven 5-cm-diameter and 1-cm-thick discs of materials radiographically representative of either soft tissue (S), bone (B), or lung tissue (L) at diagnostic photon energies. In addition to a homogeneous phantom of soft tissue (SSSSSSS), two heterogeneous phantoms were constructed: SSBBSSS and SBLLBSS. MOSFET dosimeters were then positioned at the interface of each disc, and the phantoms were then irradiated at 66 kVp and 200 mAs. Measured values of absorbed dose at depth were then compared to predicated values of point tissue dose as determined via Monte Carlo radiation transport modeling. At depths exceeding 2 cm, experimental results matched the computed values of dose with high accuracy regardless of the dosimeter orientation (epoxy bubble facing toward or away from the x-ray beam). Discrepancies were noted, however, between measured and calculated point doses near the surface of the phantom (surface to 2 cm depth) when the dosimeters were oriented with the epoxy bubble facing the x-ray beam. These discrepancies were largely eliminated when the dosimeters were placed with the flat side facing the x-ray beam. It is therefore recommended that the MOSFET dosimeters be oriented with their flat sides facing the beam when they are used at shallow depths or on the surface of either phantoms or patients.

Tissue-equivalent materials for construction of tomographic dosimetry phantoms in pediatric radiology.

Tissue equivalent materials have a variety of uses, including routine quality assurance and quality control in both diagnostic and therapeutic physics. They are frequently used in a research capacity to measure doses delivered to patients undergoing various therapeutic procedures. However, very few tissue equivalent materials have been developed for research use at the low photon energies encountered in diagnostic radiology. In this paper, we present a series of tissue-equivalent (TE) materials designed to radiographically mimic human tissue at diagnostic photon energies. These tissue equivalent materials include STES-NB (newborn soft tissue substitute), BTES-NB (newborn bone tissue substitute), LTES (newborn as well as a child/adult lung tissue substitute), STES (child/adult soft tissue substitute), and BTES (child/adult bone tissue substitute). In all cases, targeted reference elemental compositions are taken from those specified in the ORNL stylized computational model series. For each material, reference values of mass density, mass attenuation coefficients (10-150 keV), and mass energy-absorption coefficients (10-150 keV) were matched as closely as permitted by material selection and manufacturing constraints. Values of mu/rho and mu(en)/rho for the newborn TE materials are noted to have maximum deviations from their ORNL reference values of from 0 to -3% and from +2% to -3%, respectively, over the diagnostic energy range 10-150 keV. For the child/adult TE materials, these same maximal deviations of mu/rho and mu(en)/rho are from +1.5% to -3% and from +3% to -3%, respectively. Simple calculations of x-ray fluence attenuation under narrow-beam geometry using a 66 kVp spectrum typical of newborn CR radiographs indicate that the tissue-equivalent materials presented here yield estimates of absorbed dose at depth to within 3.6% for STES-NB, 3.2% for BTES-NB, and 1.2% for LTES of the doses assigned to reference newborn soft, bone, and lung tissue, respectively.

Comparisons of point and average organ dose within an anthropomorphic physical phantom and a computational model of the newborn patient.

Pediatric radiographic examinations yield medical benefits and/or diagnostic information that must be balanced against potential risk from patient radiation exposure. Consequently, clinical tools for measuring internal organ dose are needed for medical risk assessment. In this study, a physical phantom and Monte Carlo simulation model of the newborn patient were developed based upon their stylized mathematical expressions (ORNL and MIRD model series). The physical phantom was constructed using tissue equivalent substitutes for soft tissue, lung, and skeleton. Twenty metal-oxide-semiconductor field effect transistor (MOSFET) dosimeters were then inserted at three-dimensional positions representing the centroids of organs assigned in the ICRP's definition of the effective dose. MOSFET-derived point estimates of organ dose were shown to be in reasonable agreement with Monte Carlo estimates for representative newborn head, chest, and abdomen radiographic exams. Ratios of average organ dose assessed via MCNP simulations to the MOSFET-derived point doses (point-to-organ dose scaling factors, SF(POD)) are tabulated for subsequent use in clinical irradiations of the newborn phantom/MOSFET system. Values of SF(POD) indicate that MOSFET measurements of point dose for in-field exposures need to be adjusted only to within 10% to report volume-averaged organ dose. Larger adjustments to point doses are noted for organs out-of-field. For walled organs, point estimates of organ dose at the content centroid are shown to underestimate the average wall dose when the organ is within the primary field: SF(POD) of 1.19 for the stomach (AP chest exam), and SF(POD) of 1.15 for the urinary bladder (AP abdomen exam).

Comparison of angular free-in-air and tissue-equivalent phantom response measurements in p-MOSFET dosimeters.

Metal Oxide Semiconductor Field Effect Transistor (MOSFET) radiation dosimeters have found recent application in providing real-time measurement in diagnostic radiology as well as in radiotherapy. Due to the design of the MOSFET dosimeter, the response is dependent on both energy and angulation with respect to the direction of primary radiation. The axial angular dependence has been characterized for both free-in-air and for tissue-equivalent phantoms. However, neither the angular dependence normal (90-degree) to the axial rotation, nor the effects of various tissue compositions on angular dependence, have been investigated for radiation energies in the diagnostic range. To characterize the angular dependence normal to the axial rotation, we exposed three "high sensitivity" MOSFET dosimeters simultaneously to x-rays from a medical diagnostic x-ray unit over a 360-degree rotation, at 22.5-degree increments, for both free-in-air and in lung, skeletal, and soft tissue-equivalent phantoms. The MOSFET dosimeters clearly showed an angular dependence in the orientation normal-to-axial as well as in the axial rotation, both for free-in-air and in tissue-equivalent phantoms. Significant variations in response occurred when the MOSFETs were exposed at incident angles between 90 degrees and 180 degrees normal-to-axial, as compared to the normal position (i.e., the zero-degree position with the bubble-side of the MOSFETs facing the radiation source). A maximum decrease in response to 32% of normal was observed when the distal ends (end opposite the wire lead) of the dosimeters were pointing directly away from the x-ray source (270-degree position). To avoid significant errors in MOSFET dosimeter readings, placement of the dosimeters should be consistent, and care should be taken to avoid orienting the dosimeter with its sensitive region (bubble side) facing away from the source of primary radiation at particular angles.

The characterization of a commercial MOSFET dosimeter system for use in diagnostic x ray.

A commercial patient dose verification system utilizing non-invasive metal oxide semiconductor field effect transistor (MOSFET) dosimeters originally designed for radiotherapy applications has been evaluated for use at diagnostic energy levels. The system features multiple dosimeters that may be used to monitor entrance or exit skin dose and intracavity doses in phantoms in real time. We have characterized both the standard MOSFET dosimeter designed for radiotherapy dose verification and a newly developed "high sensitivity" MOSFET dosimeter designed for lower dose measurements. The sensitivity, linearity, angular response, post-exposure response, and physical characteristics were evaluated. The average sensitivity (free in air, including backscatter) of the radiotherapy MOSFET dosimeters ranged from 3.55 x 10(4) mV per C kg(-1) (9.2 mV R(-1)) to 4.87 x 10(4) mV per C kg(-1) (12.6 mV R(-1)) depending on the energy of the x-ray field. The sensitivity of the "high sensitivity" MOSFET dosimeters ranged from 1.15 x 10(5) mV per C kg(-1) (29.7 mV R(-1)) to 1.38 x 10(5) mV per C kg(-1) (35.7 mV R(-1)) depending on the energy of the x-ray field. The high sensitivity dosimeters demonstrated excellent linearity at high energies (90 and 120 kVp) and acceptable linearity at lower energies (60 kVp). The angular response was significant for free-in-air exposures, as illustrated by the sensitivity differences between the two sides of the dosimeter, but was excellent for measurements within a tissue equivalent cylinder. The post-exposure drift response is a complicated but reproducible function of time. Real-time monitoring requires little if any corrections for the post-exposure drift response. The MOSFET dosimeter system brings some unique capabilities to diagnostic radiology dosimetry including small size, real-time capabilities, nondestructive measurement, good linearity, and a predictable angular response.

Influence of ventilation strategies on indoor radon concentrations based on a semiempirical model for Florida-style houses.

Measurements in a full-scale experimental facility are used to benchmark a semiempirical model for predicting indoor radon concentrations for Florida-style houses built using slab-on-grade construction. The model is developed to provide time-averaged indoor radon concentrations from quantitative relationships between the time-dependent radon entry and elimination mechanisms that have been demonstrated to be important for this style of residential construction. The model successfully predicts indoor radon concentrations in the research structure for several pressure and ventilation conditions. Parametric studies using the model illustrate how different ventilation strategies affect indoor radon concentrations. It is demonstrated that increasing house ventilation rates by increasing the effective leakage area of the house shell does not reduce indoor radon concentrations as effectively as increasing house ventilation rates by controlled duct ventilation associated with the heating, ventilating, and air conditioning system. The latter strategy provides the potential to minimize indoor radon concentrations while providing positive control over the quality of infiltration air.

Synergistic effects of ionizing radiation and 60 Hz magnetic fields.

Experiments designed to evaluate the synergistic production of clastogenic effects by ionizing radiation and 60 Hz magnetic fields were performed using human lymphocytes from peripheral blood. Following exposure to ionizing radiation, cells were cultured in 60 Hz magnetic fields having field strengths up to 1.4 mT. Cells exposed to both ionizing radiation and 60 Hz magnetic fields demonstrated an enhanced frequency of near tetraploid chromosome complements, a feature not observed following exposure to only ionizing radiation. The results are discussed in the context of a multiple-stage model of cellular transformation, employing both initiating and promoting agents.