Human biodistribution and radiation dosimetry of 82Rb

Prior estimates of radiation-absorbed doses from (82)Rb, a frequently used PET perfusion tracer, yielded discrepant results. We reevaluated (82)Rb dosimetry using human in vivo biokinetic measurements.

METHODS

Ten healthy volunteers underwent dynamic PET/CT (6 contiguous table positions, each with separate (82)Rb infusion). Source organ volumes of interest were delineated on the CT images and transferred to the PET images to obtain time-integrated activity coefficients. Radiation doses were estimated using OLINDA/EXM 1.0.

RESULTS

The highest mean absorbed organ doses (μGy/MBq) were observed for the kidneys (5.81), heart wall (3.86), and lungs (2.96). Mean effective doses were 1.11 ± 0.22 and 1.26 ± 0.20 μSv/MBq using the tissue-weighting factors of the International Commission on Radiological Protection (ICRP), publications 60 and 103, respectively.

CONCLUSION

Our current (82)Rb dosimetry suggests reasonably low radiation exposure. On the basis of this study, a clinical (82)Rb injection of 2 × 1,480 MBq (80 mCi) would result in a mean effective dose of 3.7 mSv using the weighting factors of the ICRP 103-only slightly above the average annual natural background exposure in the United States (3.1 mSv).

Radiation dose estimation using preclinical imaging with 124I-metaiodobenzylguanidine (MIBG) PET

PURPOSE

A pretherapy 124I-metaiodobenzylguanidine (MIBG) positron emission tomography (PET)/computed tomography (CT) provides a potential method to estimate radiation dose to normal organs, as well as tumors prior to 131I-MIBG treatment of neuroblastoma or pheochromocytoma. The aim of this work was to estimate human-equivalent internal radiation dose of 124I-MIBG using PET/CT data in a murine xenograft model.

METHODS

Athymic mice subcutaneously implanted with NB1691 cells that express high levels of human norepinephrine transporter (n = 4) were imaged using small animal microPET/CT over 96 h (approximate imaging time points: 0.5, 2, 24, 52, and 96 h) after intravenous administration of 3.07-4.84 MBq of 124I-MIBG via tail vein. The tumors did not accumulate 124I-MIBG to a detectable level. All four animals were considered as control and organ radiation dosimetry was performed. Volumes of interest were drawn on the coregistered CT images for thyroid, heart, lung, liver, kidney, and bladder, and transferred to PET images to obtain pharmacokinetic data. Based on tabulated organ mass distributions for both mice and adult male human, preclinical pharmacokinetic data were extrapolated to their human-equivalent values. Radiation dose estimations for different age groups were performed using the OLINDA/EXM software with modified tissue weighting factors in the recent International Commission on Radiological Protection (ICRP) Publication 103.

RESULTS

The mean effective dose from 124I-MIBG using weighting factors from ICRP 103 to the adult male was estimated at 0.25 mSv/MBq. In different age groups, effective doses using values from ICRP 103 were estimated as follows: Adult female: 0.34, 15-yr-old: 0.39 mSv/MBq, 10-yr-old: 0.58 mSv/MBq, 5-yr-old: 1.03 mSv/MBq, 1-yr-old: 1.92 mSv/MBq, and newborn: 3.75 mSv/ MBq. For comparison, the reported effective dose equivalent of 124I-NaI for adult male (25% thyroid uptake, MIRD Dose Estimate Report No. 5) was 6.5 mSv/MBq.

CONCLUSIONS

The authors estimated human-equivalent internal radiation dose of 124I-MIBG using preclinical imaging data. As a reference, the effective dose estimation showed that 124I-MIBG would deliver less radiation dose than 124I-NaI, a radiotracer already being used in patients with thyroid cancer.

How tissue optics affect dosimetry of photodynamic therapy

We describe three lessons learned about how tissue optics affect the dosimetry of red to near-infrared treatment light during PDT, based on working with Dr. Tayyaba Hasan. Lesson 1-The optical fluence rate φ near the tissue surface exceeds the delivered irradiance (E). A broad beam penetrates into tissue to a depth (z) as φ=Eke(-μz), with an attenuation constant μ and a backscatter term k. In tissues, k is typically in the range 3-5, and 1∕μ equals δ, the 1∕e optical penetration depth. Lesson 2-Edge losses at the periphery of a uniform treatment beam extend about 3δ from the beam edge. If the beam diameter exceeds 6δ, then there is a central zone of uniform fluence rate in the tissue. Lesson 3-The depth of treatment is linearly proportional to δ (and the melanin content of pigmented epidermis in skin) while proportional to the logarithm of all other factors, such as irradiance, exposure time, or the photosensitizer properties (concentration, extinction coefficient, quantum yield for oxidizing species). The lessons illustrate how tissue optics play a dominant role in specifying the treatment zone during PDT.

Evaluation of interpolation methods for TG-43 dosimetric parameters based on comparison with Monte Carlo data for high-energy brachytherapy sources

Purpose

The aim of this work was to determine dose distributions for high-energy brachytherapy sources at spatial locations not included in the radial dose function g_L(r) and 2D anisotropy function F(r,θ) table entries for radial distance r and polar angle θ. The objectives of this study are as follows: 1) to evaluate interpolation methods in order to accurately derive g_L(r) and F(r,θ) from the reported data; 2) to determine the minimum number of entries in g_L(r) and F(r,θ) that allow reproduction of dose distributions with sufficient accuracy.

Material and methods

Four high-energy photon-emitting brachytherapy sources were studied: ⁶⁰Co model Co0.A86, ¹³⁷Cs model CSM-3, ¹⁹²Ir model Ir2.A85-2, and ¹⁶⁹Yb hypothetical model. The mesh used for r was: 0.25, 0.5, 0.75, 1, 1.5, 2–8 (integer steps) and 10 cm. Four different angular steps were evaluated for F(r,θ): 1°, 2°, 5° and 10°. Linear-linear and logarithmic-linear interpolation was evaluated for g_L(r). Linear-linear interpolation was used to obtain F(r,θ) with resolution of 0.05 cm and 1°. Results were compared with values obtained from the Monte Carlo (MC) calculations for the four sources with the same grid.

Results

Linear interpolation of g_L(r) provided differences ≤ 0.5% compared to MC for all four sources. Bilinear interpolation of F(r,θ) using 1° and 2° angular steps resulted in agreement ≤ 0.5% with MC for ⁶⁰Co, ¹⁹²Ir, and ¹⁶⁹Yb, while ¹³⁷Cs agreement was ≤ 1.5% for θ < 15°.

Conclusions

The radial mesh studied was adequate for interpolating g_L(r) for high-energy brachytherapy sources, and was similar to commonly found examples in the published literature. For F(r,θ) close to the source longitudinal-axis, polar angle step sizes of 1°-2° were sufficient to provide 2% accuracy for all sources.

Biodosimetry on small blood volume using gene expression assay

This paper reports the development of a biodosimetry device suitable for rapidly measuring expression levels of a low-density gene set that can define radiation exposure, dose and injury in a public health emergency. The platform comprises a set of 14 genes selected on the basis of their abundance and differential expression level in response to radiation from an expression profiling series measuring 41,000 transcripts. Gene expression is analyzed through direct signal amplification using a quantitative Nuclease Protection Assay (qNPA). This assay can be configured as either a high-throughput microplate assay or as a handheld detection device for individual point-of-care assays. Recently, we were able to successfully develop the qNPA platform to measure gene expression levels directly from human whole blood samples. The assay can be performed with volumes as small as 30 microL of whole blood, which is compatible with collection from a fingerstick. We analyzed in vitro irradiated blood samples with qNPA. The results revealed statistically significant discrimination between irradiated and non-irradiated samples. These results indicate that the qNPA platform combined with a gene profile based on a small number of genes is a valid test to measure biological radiation exposure. The scalability characteristics of the assay make it appropriate for population triage. This biodosimetry platform could also be used for personalized monitoring of radiotherapy treatments received by patients.

The view from the trenches: part 1-emergency medical response plans and the need for EPR screening

Few natural disasters or intentional acts of war or terrorism have the potential for such severe impact upon a population and infrastructure as the intentional detonation of a nuclear device within a major U.S. city. In stark contrast to other disasters or even a "dirty bomb," hundreds of thousands will be affected and potentially exposed to a clinically significant dose of ionizing radiation. This will result in immediate deaths and injuries and subsequently the development of Acute Radiation Syndrome (ARS). Additionally, millions more who are unlikely to develop ARS will seek medical evaluation and treatment, overwhelming the capacity of an already compromised medical system. In this paper, the authors propose that in vivo electron paramagnetic resonance (EPR) dosimetry be utilized to screen large numbers of potentially exposed victims, and that this screening process be incorporated into the medical-surge framework that is currently being implemented across the nation for other catastrophic public health emergencies. The National Incident Management System (NIMS), the National Response Framework (NRF), the Target Capabilities List (TCL), Homeland Security Presidential Directives (HSPD), as well as additional guidance from multiple federal agencies provide a solid framework for this response. The effective screening of potentially-exposed victims directly following a nuclear attack could decrease the number of patients seeking immediate medical care by greater than 90%.

Dosimetry based on EPR spectral analysis of fingernail clippings

Exposure of fingernails and toenails to ionizing radiation creates radicals that are stable over a relatively long period (days to weeks) and characterized by an isotropic EPR signal at g = 2.003 (so-called radiation-induced signal, RIS). This signal in readily obtained fingernail parings has the potential to be used in screening a population for exposure to radiation and determining individual dose to guide medical treatment. However, the mechanical harvesting of fingernail parings also creates radicals, and their EPR signals (so-called mechanically-induced signals, MIS) overlap the g approximately 2.0 region, interfering with efforts to quantify the RIS and, therefore, the radiation dose. Careful analysis of the time evolution and power-dependence of the EPR spectra of freshly cut fingernail parings has now resolved the MIS into three major components, including one that is described for the first time. It dominates the MIS soon after cutting, but decays within the first hour and consists of a unique doublet that can be resolved from the RIS. The MIS obtained within the first few minutes after cutting is consistent among fingernail samples and provides an opportunity to achieve the two important dosimetry objectives. First, perturbation of the initial MIS by the presence of RIS in fingernails that have received a threshold dose of radiation leads to spectral signatures that can be used for rapid screening. Second, decomposition of the EPR spectra from irradiated fingernails into MIS and RIS components can be used to isolate and thus quantify the RIS for determining individual exposure dose.

Surface loop resonator design for in vivo EPR tooth dosimetry using finite element analysis

Finite element analysis is used to evaluate and design L-band surface loop resonators for in vivo electron paramagnetic resonance (EPR) tooth dosimetry. This approach appears to be practical and useful for the systematic examination and evaluation of resonator configurations to enhance the precision of dose estimates. The effects of loop positioning in the mouth are examined, and it is shown that the sensitivity to loop position along a row of molars is decreased as the loop is moved away from the teeth.

A quantitative analysis of the structure of human sternum

An extensive study of the human sternum has been carried out to obtain estimates of the omnidirectional path-length distributions and structural parameters for trabeculation and marrow spaces. Data for sternum samples have been collected, using an object plane scanning microscope. These data have been used to produce the omnidirectional path-length distributions and values of structural parameters for the whole sternum. For a typical adult man the mean trabecular and marrow space path lengths are 224 and 1364 μm, respectively. The percentage bone volume is 13.8 and the surface to volume ratio is 190 cm. Data on the structural variations within the whole sternum are presented. They show a percentage difference in bone volume between the manubrium and the body of sternum of about 36%.

A study of the shielding used to reduce leakage and scattered radiation to the fetus in a pregnant patient treated with a 6-MV external X-ray beam

A Monte Carlo-based procedure has been developed to assess the shielded fetal doses from 6 MV external photon beam radiation treatments and improve upon existing techniques that are based on AAPM Task Group Report 36 (TG-36). Anatomically realistic models of the pregnant patient representing 3- and 6-mo gestational stages were implemented into the MCNPX code together with a detailed accelerator model that is capable of simulating scattered and leakage radiation from the accelerator head. The phantom was shielded using suggested lead and Cerrobend in different locations and with different thicknesses. Absorbed doses to the fetus both with and without shielding were calculated considering typical mantle, head and neck, and brain treatment plans. The unshielded fetal doses tended to increase with decreasing distance from the field edge to the nearest fetal point and increasing of the field size. The unshielded absorbed doses to the fetus for all treatment plans ranged from a maximum of 4.08 microGy/MU (monitor unit) to a minimum 0.09 microGy/MU. The use of lead or Cerrobend shielding reduced the fetal doses by factors of up to 4. For an optimal shield half-value layer, the dose reduction between lead and Cerrobend was statistically insignificant. The maximum permitted MUs for the mantle treatments with shielding were calculated based on 5 cGy dose limits suggested by TG-36. The study demonstrates an accurate assessing tool that can be used to determine the absorbed dose to the fetus and to design the shielding as part of the treatment planning and risk management.

The impact of dosimetry uncertainties on dose-response analyses

Radiation dose estimates used in epidemiological studies are subject to many sources of uncertainty, and the error structure may be a complicated mixture of different types of error. Increasingly, efforts are being made to evaluate dosimetry uncertainties and to take account of them in statistical analyses. The impact of these uncertainties on dose-response analyses depends on the magnitude and type of error. Errors that are independent from subject to subject (random errors) reduce statistical power for detecting a dose-response relationship, increase uncertainties in estimated risk coefficients, and may lead to underestimation of risk coefficients. The specific effects of random errors depend on whether the errors are "classical" or "Berkson." Classical error can be thought of as error that arises from an imprecise measuring device, whereas Berkson error occurs when a single dose is used to represent a group of subjects (with varying true doses). Uncertainties in quantities that are common to some or all subjects are "shared" uncertainties. Such uncertainties increase the possibility of bias, and accounting for this possibility increases the length of confidence intervals. In studies that provide a direct evaluation of risk at low doses and dose rates, dosimetry errors are more likely to mask a true effect than to create a spurious one. In addition, classical errors and shared dosimetry uncertainties increase the potential for bias in estimated risks coefficients, but this potential may already be large due to the extreme vulnerability to confounding in studies involving very small relative risk.

Reconstruction of hemodynamics and sensitizer distributions during interstitial PDT using spectroscopy with linear light sources

Light dosimetry for photodynamic therapy requires a knowledge of the optical absorption spectrum of the tissue being treated Here, we present a theoretical and experimental analysis of the capabilities of a system using interstitial linear light sources ranging in length from 2 to 5 cm to illuminate the tissue interstitially, and isotropic point-like detectors to measure the resulting diffusely transmitted light. The sources and detectors are translated in transparent plastic catheters under the control of a motorized positioning system designed for interstitial measurements in the prostate. The light source is a quartz-tungsten-halogen (QTH), and the spectrally resolved detection is accomplished using a CCD-based grating spectrometer. The data are analyzed using an approximation to the radiative transport equation, assuming homogeneous scattering and heterogeneous absorption spectra Absorption spectra are reconstructed independently for individual source-detector channel pairs. Sequential reconstruction can then be used to create a 3-dimensional reconstruction. The results of simulated data, measurements made in multi-component phantoms, and synthetic data reconstructed from in vivo measurements made with point sources demonstrate the feasibility of this method.

The design of a robotic multichannel platform for photodynamic therapy

A compact robotic platform is designed for simultaneous multichannel motion control for light delivery and dosimetry during interstitial photodynamic therapy (PDT). Movements of light sources and isotropic detectors are controlled by individual motors along different catheters for interstitial PDT. The robotic multichannel platform adds feedback control of positioning for up to 16 channels compared to the existing dual-motor system, which did not have positioning encoders. A 16-channel servo motion controller and micro DC motors, each with high resolution optical encoder, are adopted to control the motions of up to 16 channels independently. Each channel has a resolution of 0.1mm and a speed of 5cm/s. The robotic platform can perform light delivery and dosimetry independently, allowing arbitrary positioning of light sources and detectors in each catheter. Up to 16 compact translational channels can be combined according to different operational scheme with real-time optimal motion planning. The characteristic of high speed and coordinating motion will make it possible to use short linear sources (e.g., 1- cm) to deliver uniform PDT treatment to a bulk tumor within reasonable time by source stepping optimization of multiple sources simultaneously. Advanced robotic control algorithm handles the various unexpected circumstance in clinical procedure, e.g., positiontorque/current control will be applied to prevent excessive force in the case of resistance in the fiber or motorized mechanism. The robotic platform is fully compatible with operation room (OR) environment and improves the light delivery and dosimetry in PDT. It can be adopted for diffusing optical tomography (DOT), spectroscopic DOT and fluorescent spectroscopy.

Interstitial brachytherapy guided intensity modulated radiation therapy (IBGIMRT) in cervical cancer: a dosimetric study

PURPOSE

Interstitial brachytherapy (IBT) is used as an alternative to intracavitary radiotherapy in the management of cervical carcinoma. We have devised a new technique called interstitial brachytherapy guided intensity modulated radiotherapy (IBGIMRT) which can potentially reduce doses to organs at risk (OaRs). It utilizes IMRT planning on the target volume (TV) defined by implantation of IBT needles. This study compares the dosimetry of IBT and IBGIMRT.

MATERIAL AND METHODS

CT scan images of 18 patients with cervical cancer, who have been already treated by HDR-BT, were used to generate two rival plans, IBT and IBGIMRT, for a prescription dose of 10 Gy. Following dosimetric factors were used for comparison: volume receiving 95% of prescription dose (V₉₅), conformity index (COIN) and external volume index (EI) for target and for OaR, dose received by volume of 1 cm³ (D_1cc), 2 cm³ (D_2cc), 5 cm³ (D_5cc) and also volume receiving 50% and 75% of prescription dose (V₅₀ and V₇₅).

RESULTS

The two plans resulted in COIN difference of 49.8% (p < 0.0001) and EI difference of 36.4% (p < 0.0028) in favor of IBGIMRT. Mean D_1cc, D_2cc and D_5cc values for bladder were 8.3 Gy, 7.6 Gy and 6.4 Gy; and 7.8 Gy, 7.3 Gy and 5.8 Gy with IBT and IBGIMRT, respectively (p > 0.05). Similar figures for rectum with IBT vs. IBGIMRT were 11.2 Gy vs. 7.02 Gy, 10.5 Gy vs. 6.4 Gy and 9.1 Gy vs. 4.8 Gy respectively (p < 0.01).

CONCLUSIONS

Our novel technique, IBGIMRT, has shown its dosimetric superiority and therefore needs to be studied in clinical set up.

A study of predicted bone marrow distribution on calculated marrow dose from external radiation exposures using two sets of image data for the same individual

Red bone marrow is among the tissues of the human body that are most sensitive to ionizing radiation, but red bone marrow cannot be distinguished from yellow bone marrow by normal radiographic means. When using a computational model of the body constructed from computed tomography (CT) images for radiation dose, assumptions must be applied to calculate the dose to the red bone marrow. This paper presents an analysis of two methods of calculating red bone marrow distribution: 1) a homogeneous mixture of red and yellow bone marrow throughout the skeleton, and 2) International Commission on Radiological Protection cellularity factors applied to each bone segment. A computational dose model was constructed from the CT image set of the Visible Human Project and compared to the VIP-Man model, which was derived from color photographs of the same individual. These two data sets for the same individual provide the unique opportunity to compare the methods applied to the CT-based model against the observed distribution of red bone marrow for that individual. The mass of red bone marrow in each bone segment was calculated using both methods. The effect of the different red bone marrow distributions was analyzed by calculating the red bone marrow dose using the EGS4 Monte Carlo code for parallel beams of monoenergetic photons over an energy range of 30 keV to 6 MeV, cylindrical (simplified CT) sources centered about the head and abdomen over an energy range of 30 keV to 1 MeV, and a whole-body electron irradiation treatment protocol for 3.9 MeV electrons. Applying the method with cellularity factors improves the average difference in the estimation of mass in each bone segment as compared to the mass in VIP-Man by 45% over the homogenous mixture method. Red bone marrow doses calculated by the two methods are similar for parallel photon beams at high energy (above about 200 keV), but differ by as much as 40% at lower energies. The calculated red bone marrow doses differ significantly for simplified CT and electron beam irradiation, since the computed red bone marrow dose is a strong function of the cellularity factor applied to bone segments within the primary radiation beam. These results demonstrate the importance of properly applying realistic cellularity factors to computation dose models of the human body.

Liquid scintillator for 2D dosimetry for high-energy photon beams

Complex radiation therapy techniques require dosimetric verification of treatment planning and delivery. The authors investigated a liquid scintillator (LS) system for application for real-time high-energy photon beam dosimetry. The system was comprised of a transparent acrylic tank filled with liquid scintillating material, an opaque outer tank, and a CCD camera. A series of images was acquired when the tank with liquid scintillator was irradiated with a 6 MV photon beam, and the light data measured with the CCD camera were filtered to correct for scattering of the optical light inside the liquid scintillator. Depth-dose and lateral profiles as well as two-dimensional (2D) dose distributions were found to agree with results from the treatment planning system. Further, the corrected light output was found to be linear with dose, dose rate independent, and is robust for single or multiple acquisitions. The short time needed for image acquisition and processing could make this system ideal for fast verification of the beam characteristics of the treatment machine. This new detector system shows a potential usefulness of the LS for 2D QA.

Precise dielectric property measurements and E-field probe calibration for specific absorption rate measurements using a rectangular waveguide

Specific absorption rate (SAR) measurements require accurate calculations of the dielectric properties of tissue-equivalent liquids and associated calibration of E-field probes. We developed a precise tissue-equivalent dielectric measurement and E-field probe calibration system. The system consists of a rectangular waveguide, electric field probe, and data control and acquisition system. Dielectric properties are calculated using the field attenuation factor inside the tissue-equivalent liquid and power reflectance inside the waveguide at the air/dielectric-slab interface. Calibration factors were calculated using isotropicity measurements of the E-field probe. The frequencies used are 900 MHz and 1800 MHz. The uncertainties of the measured values are within ±3%, at the 95% confidence level. Using the same waveguide for dielectric measurements as well as calibrating E-field probes used in SAR assessments eliminates a source of uncertainty. Moreover, we clearly identified the system parameters that affect the overall uncertainty of the measurement system.

Particulate matter (PM) research centers (1999-2005) and the role of interdisciplinary center-based research

OBJECTIVE

The U.S. Environmental Protection Agency funded five academic centers in 1999 to address the uncertainties in exposure, toxicity, and health effects of airborne particulate matter (PM) identified in the "Research Priorities for Airborne Particulate Matter" of the National Research Council (NRC). The centers were structured to promote interdisciplinary approaches to address research priorities of the NRC. In this report, we present selected accomplishments from the first 6 years of the PM Centers, with a focus on the advantages afforded by the interdisciplinary, center-based research approach. The review highlights advances in the area of ultrafine particles and traffic-related health effects as well as cardiovascular and respiratory effects, mechanisms, susceptibility, and PM exposure and characterization issues.

DATA SOURCES AND SYNTHESIS

The collective publications of the centers served as the data source. To provide a concise synthesis of overall findings, authors representing each of the five centers identified a limited number of topic areas that serve to illustrate the key accomplishments of the PM Centers program, and a consensus statement was developed.

CONCLUSIONS

The PM Centers program has effectively applied interdisciplinary research approaches to advance PM science.

Nanotechnology-based electrochemical sensors for biomonitoring chemical exposures

The coupling of dosimetry measurements and modeling represents a promising strategy for deciphering the relationship between chemical exposure and disease outcome. To support the development and implementation of biological monitoring programs, quantitative technologies for measuring xenobiotic exposure are needed. The development of portable nanotechnology-based electrochemical (EC) sensors has the potential to meet the needs for low cost, rapid, high-throughput, and ultrasensitive detectors for biomonitoring an array of chemical markers. Highly selective EC sensors capable of pM sensitivity, high-throughput and low sample requirements (<50 microl) are discussed. These portable analytical systems have many advantages over currently available technologies, thus potentially representing the next generation of biomonitoring analyzers. This paper highlights research focused on the development of field-deployable analytical instruments based on EC detection. Background information and a general overview of EC detection methods and integrated use of nanomaterials in the development of these sensors are provided. New developments in EC sensors using various types of screen-printed electrodes, integrated nanomaterials, and immunoassays are presented. Recent applications of EC sensors for assessing exposure to pesticides or detecting biomarkers of disease are highlighted to demonstrate the ability to monitor chemical metabolites, enzyme activity, or protein biomarkers of disease. In addition, future considerations and opportunities for advancing the use of EC platforms for dosimetric studies are discussed.

Transient noise characterization and filtration in CCD cameras exposed to stray radiation from a medical linear accelerator

Charge coupled devices (CCDs) are being increasingly used in radiation therapy for dosimetric purposes. However, CCDs are sensitive to stray radiation. This effect induces transient noise. Radiation-induced noise strongly alters the image and therefore limits its quantitative analysis. The purpose of this work is to characterize the radiation-induced noise and to develop filtration algorithms to restore image quality. Two models of CCD were used for measurements close to a medical linac. The structure of the transient noise was first characterized. Then, four methods of noise filtration were compared: median filtering of a time series of identical images, uniform median filtering of single images, an adaptive filter with switching mechanism, and a modified version of the adaptive switch filter. The intensity distribution of noisy pixels was similar in both cameras. However, the spatial distribution of the noise was different: The average noise cluster size was 1.2 +/- 0.6 and 3.2 +/- 2.7 pixels for the U2000 and the Luca, respectively. The median of a time series of images resulted in the best filtration and minimal image distortion. For applications where time series is impractical, the adaptive switch filter must be used to reduce image distortion. Our modified version of the switch filter can be used in order to handle nonisolated groups of noisy pixels.

Clinical performance and radiation dosimetry of no-carrier-added vs carrier-added 123I-metaiodobenzylguanidine (MIBG) for the assessment of cardiac sympathetic nerve activity

PURPOSE

We hypothesized that assessment of myocardial sympathetic activity with no-carrier-added (nca) (123)I-meta-iodobenzylguanidine (MIBG) compared to carrier-added (ca) (123)I-MIBG would lead to an improvement of clinical performance without major differences in radiation dosimetry.

METHODS

In nine healthy volunteers, 15 min and 4 h planar thoracic scintigrams and conjugate whole-body scans were performed up to 48 h following intravenous injection of 185 MBq (123)I-MIBG. The subjects were given both nca and ca (123)I-MIBG. Early heart/mediastinal ratios (H/M), late H/M ratios and myocardial washout were calculated. The fraction of administered activity in ten source organs was quantified from the attenuation-corrected geometric mean counts in conjugate views. Radiation-absorbed doses were estimated with OLINDA/EXM software.

RESULTS

Both early and late H/M were higher for nca (123)I-MIBG (ca (123)I-MIBG early H/M 2.46 +/- 0.15 vs nca (123)I-MIBG 2.84 +/- 0.15, p = 0.001 and ca (123)I-MIBG late H/M 2.69 +/- 0.14 vs nca (123)I-MIBG 3.34 +/- 0.18, p = 0.002). Myocardial washout showed a longer retention time for nca (123)I-MIBG (p < 0.001). The effective dose equivalent (adult male model) for nca (123)I-MIBG was similar to that for ca (123)I-MIBG (0.025 +/- 0.002 mSv/MBq vs 0.026 +/- 0.002 mSv/MBq, p = 0.055, respectively).

CONCLUSION

No-carrier-added (123)I-MIBG yields a higher relative myocardial uptake and is associated with a higher myocardial retention. This difference between nca (123)I-MIBG and ca (123)I-MIBG in myocardial uptake did not result in major differences in estimated absorbed doses. Therefore, nca (123)I-MIBG is to be preferred over ca (123)I-MIBG for the assessment of cardiac sympathetic activity.

An Absorbed-Dose/Dose-Rate Dependence for the Alanine-EPR Dosimetry System and Its Implications in High-Dose Ionizing Radiation Metrology

NIST developed the alanine dosimetry system in the early 1990s to replace radiochromic dye film dosimeters. Later in the decade the alanine system was firmly established as a transfer service for high-dose radiation dosimetry and an integral part of the internal calibration scheme supporting these services. Over the course of the last decade, routine monitoring of the system revealed a small but significant observation that, after examination, led to the characterization of a previously unknown absorbed-dose-dependent, dose-rate effect for the alanine system. Though the potential impact of this effect is anticipated to be extremely limited for NIST's customer-based transfer dosimetry service, much greater implications may be realized for international measurement comparisons between National Measurement Institutes.

Monte Carlo simulations of absorbed dose in a mouse phantom from 18-fluorine compounds

The purpose of this study was to calculate internal absorbed dose distribution in mice from pre-clinical small animal PET imaging procedures with fluorine-18 labeled compounds (18FDG, 18FLT, and fluoride ion). The GATE Monte Carlo software and a realistic, voxel-based mouse phantom that included a subcutaneous tumor were used to perform simulations. Discretized time-activity curves obtained from dynamic in vivo studies with each of the compounds were used to set the activity concentration in the simulations. For 18FDG, a realistic range of uptake ratios was considered for the heart and tumor. For each simulated time frame, the biodistribution of the radionuclide in the phantom was considered constant, and a sufficient number of decays were simulated to achieve low statistical uncertainty. Absorbed dose, which was scaled to take into account radioactive decay, integration with time, and changes in biological distribution was reported in mGy per MBq of administered activity for several organs and uptake scenarios. The mean absorbed dose ranged from a few mGy/MBq to hundreds of mGy/MBq. Major organs receive an absorbed dose in a range for which biological effects have been reported. The effects on a given investigation are hard to predict; however, investigators should be aware of potential perturbations especially when the studied organ receives high absorbed dose and when longitudinal imaging protocols are considered.

Method for Fast CT/SPECT-Based 3D Monte Carlo Absorbed Dose Computations in Internal Emitter Therapy

The DPM (Dose Planning Method) Monte Carlo electron and photon transport program, designed for fast computation of radiation absorbed dose in external beam radiotherapy, has been adapted to the calculation of absorbed dose in patient-specific internal emitter therapy. Because both its photon and electron transport mechanics algorithms have been optimized for fast computation in 3D voxelized geometries (in particular, those derived from CT scans), DPM is perfectly suited for performing patient-specific absorbed dose calculations in internal emitter therapy. In the updated version of DPM developed for the current work, the necessary inputs are a patient CT image, a registered SPECT image, and any number of registered masks defining regions of interest. DPM has been benchmarked for internal emitter therapy applications by comparing computed absorption fractions for a variety of organs using a Zubal phantom with reference results from the Medical Internal Radionuclide Dose (MIRD) Committee standards. In addition, the β decay source algorithm and the photon tracking algorithm of DPM have been further benchmarked by comparison to experimental data. This paper presents a description of the program, the results of the benchmark studies, and some sample computations using patient data from radioimmunotherapy studies using (131)I.

Therapeutic potential of 90Y- and 131I-labeled anti-CD20 monoclonal antibody in treating non-Hodgkin's lymphoma with pulmonary involvement: a Monte Carlo-based dosimetric analysis

Pulmonary involvement is common in patients with non-Hodgkin's lymphoma (NHL). (90)Y- and (131)I-anti-CD20 antibodies (ibritumomab tiuxetan and tositumomab, respectively) have been approved for the treatment of refractory low-grade follicular NHL. In this work, we used Monte Carlo-based dosimetry to compare the potential of (90)Y and (131)I, based purely on their emission properties, in targeted therapy for NHL lung metastases of various nodule sizes and tumor burdens.

METHODS

Lung metastases were simulated as spheres, with radii ranging from 0.2 to 5.0 cm, which were randomly distributed in a voxelized adult male lung phantom. Total tumor burden was varied from 0.2 to 1,641 g. Tumor uptake and retention kinetics of the 2 radionuclides were assumed equivalent; a uniform distribution of activity within tumors was assumed. Absorbed dose to tumors and lung parenchyma per unit activity in lung tumors was calculated by a Monte Carlo-based system using the MCNP4B package. Therapeutic efficacy was defined as the ratio of mean absorbed dose in the tumor to that in normal lung. Dosimetric analysis was also performed for a lung-surface distribution of tumor nodules mimicking pleural metastatic disease.

RESULTS

The therapeutic efficacy of both (90)Y and (131)I declined with increasing tumor burden. In treating tumors with radii less than 2.0 cm, (131)I targeting was more efficacious than (90)Y targeting. (90)Y yielded a broader distribution of tumor absorbed doses, with the minimum 54.1% lower than the average dose; for (131)I, the minimum absorbed dose was 33.3% lower than the average. The absorbed dose to normal lungs was reduced when the tumors were distributed on the lung surface. For surface tumors, the reductions in normal-lung absorbed dose were greater for (90)Y than for (131)I, but (131)I continued to provide a greater therapeutic ratio across different tumor burdens and sizes.

CONCLUSION

Monte Carlo-based dosimetry was performed to compare the therapeutic potential of (90)Y and (131)I targeting of lung metastases in NHL patients. (131)I provided a therapeutic advantage over (90)Y, especially in tumors with radii less than 2.0 cm and at lower tumor burdens. For both (90)Y- and (131)I-labeled antibodies, treatment is more efficacious when applied to metastatic NHL cases with lower tumor burdens. (131)I has advantages over (90)Y in treating smaller lung metastases.

Incorporating PET information in radiation therapy planning

PET scanning, because of its impressive sensitivity and accuracy, is being incorporated into the standard staging workup for many cancers. These include lung cancer, lymphomas, head and neck cancers, and oesophageal cancers. PET often provides incremental information about the patient’s disease status, adding to the data obtained from structural imaging methods, such as, CT scan or MRI. PET commonly upstages patients into more advanced disease categories. Incorporation of PET information into the radiotherapy planning process has the potential to reduce the risks of geographic miss and can help minimise unnecessary irradiation of normal tissues. The best means of incorporating PET information into radiotherapy planning is uncertain, and considerable effort is being expended in this area of research.

Whole body radiotherapy: A TBI-guideline

Total Body Irradiation (TBI) is one main component in the interdisciplinary treatment of widely disseminated malignancies predominantly of haematopoietic diseases. Combined with intensive chemotherapy, TBI enables myeloablative high dose therapy and immuno-ablative conditioning treatment prior to subsequent transplantation of haematopoietic stem cells: bone marrow stem cells or peripheral blood progenitor stem cells. Jointly prepared by DEGRO and DGMP, the German Society of Radio-Oncology, and the German Association of Medical Physicists, this DEGRO/DGMP-Leitlinie Ganzkoerper-Strahlenbehandlung - DEGRO/DGMP Guideline Whole Body Radiotherapy, summarises the concepts, principles, facts and common methods of Total Body Irradiation and poses a set of recommendations for reliable and successful application of high dose large-field radiotherapy as essential part of this interdisciplinary, multi-modality treatment concept. The guideline is geared towards radio-oncologists, medical physicists, haematooncolo-gists, and all contributing to Whole Body Radiotherapy. To guide centres intending to start or actualise TBI criteria are included. The relevant treatment parameters are defined and a sample of a form is given for reporting TBI to international registries.

X-ray microbeams: Tumor therapy and central nervous system research

Irradiation with parallel arrays of thin, planar slices of X-ray beams (microplanar beams, or microbeams) spares normal tissue, including the central nervous system (CNS), and preferentially damages tumors. The effects are mediated, at least in part, by the tissue's microvasculature that seems to effectively repair itself in normal tissue but fails to do so in tumors. Consequently, the therapeutic index of single-fraction unidirectional microbeam irradiations has been shown to be larger than that of single-fraction unidirectional unsegmented beams in treating the intracranial rat 9L gliosarcoma tumor model (9LGS) and the subcutaneous murine mammary carcinoma EMT-6. This paper presents results demonstrating that individual microbeams, or arrays of parallel ones, can also be used for targeted, selective cell ablation in the CNS, and also to induce demyelination. The results highlight the value of the method as a powerful tool for studying the CNS through selective cell ablation, besides its potential as a treatment modality in clinical oncology.

Permanent prostate brachytherapy: the significance of postimplant dosimetry

Recent developments in imaging science and treatment-planning software allow for accurate postimplant dosimetric assessment in all patients after prostate brachytherapy. This article reviews the available data correlating cancer control and morbidity with dosimetric quantifiers obtained from postimplant dosimetric assessment after prostate brachytherapy. Continued collection of dosimetric data in patients treated with prostate brachytherapy will allow for further refinements in the technique, leading to continued high rates of cure with increasingly lower rates of morbidity.

When may a nonuniform distribution of 131I be considered uniform? An experimental basis for multicellular dosimetry

To varying degrees, radiopharmaceuticals are distributed nonuniformly in tissue. At a macroscopic level, the radiopharmaceutical may appear to be uniformly distributed throughout the tissue. However, on closer inspection, not all cells in the tissue may be labeled with the radiopharmaceutical. Furthermore, the radioactivity in the cells may be localized only in certain compartments within the cell. This work uses a cell culture model to examine the impact of nonuniformity at the multicellular level on the lethal effects of (131)I.

METHODS

A 3-dimensional tissue culture model was used to investigate the biologic effects of nonuniform distributions of (131)I in a large population of mammalian cells. Chinese hamster V79 cells were labeled with (131)I-iododeoxyuridine ((131)IdU), mixed with unlabeled cells, and multicellular clusters (4 x 10(6) cells) were formed by gentle centrifugation. Thus, the labeled cells were randomly located in the cluster to achieve a uniform distribution of radioactivity at the macroscopic level, yet nonuniform at the multicellular level. The clusters were assembled as described and then maintained at 10.5 degrees C for 72 h to allow (131)I decays to accumulate. The clusters were then dismantled and the cells were plated for colony formation.

RESULTS

When 100% of the cells were labeled, the surviving fraction of cells in the cluster was exponentially dependent on the cluster activity down to 0.1% survival. In contrast, when 10% of the cells were labeled, it was observed that the survival fraction begins to saturate at about 1% survival. Absorbed-dose estimates reveal that the mean lethal cluster dose is 4.5, 5.7, and 6.4 Gy for 100%, 10%, and 1% labeling, respectively.

CONCLUSION

These data indicate that when the distribution of (131)I is uniform at the macroscopic level, but nonuniform at the multicellular level, the mean absorbed dose to a tissue element may not be a suitable quantity for use in predicting biologic effect. Rather, cellular and multicellular dosimetry approaches may be necessary to predict the biologic effects of incorporated (131)I.

Radiation exposure benefit of a lead cap in invasive cardiology

BACKGROUND

Occupational head exposure to radiation in cardiologists may cause radiation induced cataracts and an increased risk of brain cancer.

OBJECTIVE

To determine the effectiveness of 0.5 mm lead equivalent caps, not previously used in invasive cardiology, in comparison with a 1.0 mm lead equivalent ceiling mounted lead glass screen.

DESIGN

An anthropomorphic Alderson-Rando phantom was used to represent the patient. Scatter entrance skin air kerma to the operator position (S-ESAK-O) was measured during fluoroscopy for all standard angulations and the S-ESAK-O per dose-area product (DAP) calculated, as applied to the phantom.

RESULTS

Measured mean (SD) left/right anterior oblique angulation ratios of S-ESAK-O without lead devices were 23.1 (10.1), and varied as a function of tube angulation, body height, and angle of incidence. S-ESAK-O/DAP decreased with incremental operator body height by 10 (3)% per 10 cm. A 1.0 mm lead glass shield reduced mean S-ESAK-O/DAP originating from coronary angiography from 1089 (764) to 54 (29) nSv/Gy x cm2. A 0.5 mm lead cap was effective in lowering measured levels to 1.8 (1.1) nSv/Gy x cm2. Both devices together enabled attenuation to 0.5 (0.1) nSv/Gy x cm2. The most advantageous line of vision for protection of the operator's eyes was > or = 60 degrees rightward.

CONCLUSIONS

Use of 0.5 mm lead caps proved highly effective, attenuating S-ESAK-O to 2.7 (2.0) x 10(-3) of baseline, and to 1.2 (1.4) x 10(-3) of baseline where there was an additional 1.0 mm lead glass shield. These results could vary according to the x ray systems used, catheterisation protocols, and correct use of radiation protection devices.

Radiation dose, chemotherapy, hormonal treatment and risk of second cancer after breast cancer treatment

In total, 281 of the 7711 women who were initially treated for breast cancer between 1954 and 1983 at the Gustave Roussy Institute developed a second malignant neoplasm (SMN) other than second primary breast cancer and nonmelanoma skin cancer at least 1 year after breast cancer treatment. We carried out a nested case-control study to determine the overall relationship between the dose of radiotherapy received at a given anatomical site and the risk of SMN at the same site. In total, 75% of the cases of SMN were previously treated by radiotherapy, as compared to 73% of the controls. In the irradiated patients, the median local dose was higher among cases (3.1 Gy) than among controls (1.3 Gy). More than 40% of the irradiated patients received a local dose of less than 1 Gy. A purely quadratic relationship was observed between the dose of radiation received at an anatomical site and the risk of SMN at this site. According to the quadratic model, the excess risk of SMN was 0.2% (95% CI 0.05-0.5%) when the target organ received 1 Gy. This risk did not differ significantly according to age at the time of radiotherapy (<40 vs >or=40 years). The risk of SMN was 6.7-fold higher for doses of 25 Gy or more than in the absence of radiotherapy. No carcinogenic effect of chemotherapy was observed and a dose-effect relationship between the length of tamoxifen treatment and SMN occurrence was found. This relationship was limited to endometrial cancers and did not modify the relationship with radiation dose. Our results suggest that high radiation doses slightly increase the risk of second malignancies after breast cancer.

Differences in electron beam dosimetry using two commercial ionization chambers and the TG-21 protocol: another reason to switch to TG-51

Two of the most popular dosimetry systems used for calibration of megavoltage photon and electron beams in radiation therapy are (i) cylindrical Farmer-type chambers in liquid water and (ii) Holt Memorial parallel-plate chambers in clear polystyrene. Since implementation of the AAPM TG-21 calibration protocol, the Radiological Physics Center (which uses the Farmer in-water system) has compared machine calibrations on two occasions with those of Memorial Sloan-Kettering Cancer Center (which uses the Holt in-polystyrene system). Two years post publication of the TG-51 protocol, 70% of the clinics monitored by the RPC still use TG-21. Seventeen photon beams from cobalt-60 to 18 MV and 31 electron beams from 6 to 20 MeV were compared using the TG-21 protocol. These data represent the most comprehensive comparison of the two most popular systems in use. Based on the average percent difference, the two systems yielded the same absorbed dose to water at the reference point in phantom to within 1.5% for both modalities. No energy dependence was evident in the results; however, a systematic average percent difference between photons and electrons was seen, with the Farmer in-water system consistently predicting a dose 1.3% lower for electrons than the Holt in-polystyrene system. For photons both systems predicted the same dose to within 0.3% on average. When a physicist converts from TG-21 to TG-51, these data may be of assistance in explaining unexpected changes in output that are different from previously published values. Implementation of the TG-51 protocol should eliminate any of the observed differences in electron beam dosimetry between the two dosimetry systems because the Holt system cannot be used with TG-51.

TG-51: experience from 150 institutions, common errors, and helpful hints

The Radiological Physics Center (RPC) is a resource to the medical physics community for assistance regarding dosimetry procedures. Since the publication of the AAPM TG-51 calibration protocol, the RPC has responded to numerous phone calls raising questions and describing areas in the protocol where physicists have had problems. At the beginning of the year 2000, the RPC requested that institutions participating in national clinical trials provide the change in measured beam output resulting from the conversion from the TG-21 protocol to TG-51. So far, the RPC has received the requested data from approximately 150 of the approximately 1300 institutions in the RPC program. The RPC also undertook a comparison of TG-21 and TG-51 and determined the expected change in beam calibration for ion chambers in common use, and for the range of photon and electron beam energies used clinically. Analysis of these data revealed two significant outcomes: (i) a large number (approximately 1/2) of the reported calibration changes for photon and electron beams were outside the RPC's expected values, and (ii) the discrepancies in the reported versus the expected dose changes were as large as 8%. Numerous factors were determined to have contributed to these deviations. The most significant factors involved the use of plane-parallel chambers, the mixing of phantom materials and chambers between the two protocols, and the inconsistent use of depth-dose factors for transfer of dose from the measurement depth to the depth of dose maximum. In response to these observations, the RPC has identified a number of circumstances in which physicists might have difficulty with the protocol, including concerns related to electron calibration at low energies (R50<2 cm), and the use of a cylindrical chamber at 6 MeV electrons. In addition, helpful quantitative hints are presented, including the effect of the prescribed lead filter for photon energy measurements, the impact of shifting the chamber depth for photon depth-dose measurements, and the impact of updated stopping-power data used in TG-51 versus that used in TG-21, particularly for electron calibrations.

Patient specific quality assurance for the delivery of intensity modulated radiotherapy

A patient specific quality assurance program has been developed to facilitate the clinical implementation of intensity modulated radiotherapy (IMRT) delivered using a micro-multileaf collimator. The methodology includes several dosimetric tasks that are performed prior to the treatment of each patient. Film dosimetry is performed for each individual field and for the multifield composite plan. Individual field measurements are performed at a depth of 5 cm in a water equivalent slab phantom; export of dose calculations from the treatment planning system is similarly specified. For the composite distribution, parameters from the patient plan are applied to an IMRT phantom, and film is exposed in an axial orientation. Distributions are compared with the aid of software developed for the specific tasks. The measured and calculated dose distributions can be superimposed and positioned graphically using move, rotate, and mirror tools, as well as by specifying isocenter coordinates and using fiducial marks. Horizontal and vertical profiles are available for analysis. Dose difference, distance-to-agreement, and gamma index, the minimum scaled multidimensional distance between a measurement and a calculation point determined in combined dose and physical distance space, are calculated along a specified isodose line and displayed. gamma provides an excellent measure of disagreement between measurement and calculation for complex intensity distributions. We specify 3% dose difference and 3 mm distance as our scaling acceptability criteria. Absolute dosimetry for each composite plan is performed using an ionization chamber. To date, excellent agreement between measurements and calculations has been observed.

Monitor unit calculation for tangential breast treatments: verification in an anthropomorphic phantom

This paper presents an anthropomorphic phantom study of dose delivered to a specific point during tangential breast irradiation to verify monitor unit calculations. Measurements were made using a 0.6 cc Farmer type cylindrical ionization chamber in the phantom and compared to calculations made on a three-dimensional radiotherapy treatment planning system using single digitized contour through to multi slice CT data. A large breast phantom was used for a single field size with a combination of open and wedged fields for three different energies (4, 6, and 18 MV). Solid flat phantom measurements were also performed for comparison. Results showed a lower calculated dose than the dose measured for a fixed number of monitor units where the variations were within a range of 0.8% to 4.5%. Differences were larger for the anthropomorphic phantom than the flat phantom. We conclude that little accuracy is gained from CT based monitor unit calculations compared to those based on digitised contours for this breast treatment but that the dose distributions will be affected. This type of test is recommended as one of a large set, in the commissioning and testing procedures for treatment planning systems.

Verification of dose calculations with a clinical treatment planning system based on a point kernel dose engine

Dose calculations with a collapsed cone algorithm implemented in a clinical treatment planning system have been studied. The algorithm has been evaluated in homogeneous as well as in heterogeneous media, and the results have been compared to measurements and Monte Carlo simulations. Commonly encountered clinical beam configurations as well as more complex geometries have been pursued to test the limitations of the model. The results show that the accuracy level reached allows for clinical use. Some situations, e.g., large wedge beams and dose calculations in the build up region, not specific to the collapsed cone model, show deviations (outside +/- 3%) compared to measurements.

Comparison between TG-51 and TG-21: Calibration of photon and electron beams in water using cylindrical chambers

A new calibration protocol, developed by the AAPM Task Group 51 (TG-51) to replace the TG-21 protocol, is based on an absorbed-dose to water standard and calibration factor (N(D,w)), while the TG-21 protocol is based on an exposure (or air-kerma) standard and calibration factor (N(x)). Because of differences between these standards and the two protocols, the results of clinical reference dosimetry based on TG-51 may be somewhat different from those based on TG-21. The Radiological Physics Center has conducted a systematic comparison between the two protocols, in which photon and electron beam outputs following both protocols were compared under identical conditions. Cylindrical chambers used in this study were selected from the list given in the TG-51 report, covering the majority of current manufacturers. Measured ratios between absorbed-dose and air-kerma calibration factors, derived from the standards traceable to the NIST, were compared with calculated values using the TG-21 protocol. The comparison suggests that there is roughly a 1% discrepancy between measured and calculated ratios. This discrepancy may provide a reasonable measure of possible changes between the absorbed-dose to water determined by TG-51 and that determined by TG-21 for photon beam calibrations. The typical change in a 6 MV photon beam calibration following the implementation of the TG-51 protocol was about 1%, regardless of the chamber used, and the change was somewhat smaller for an 18 MV photon beam. On the other hand, the results for 9 and 16 MeV electron beams show larger changes up to 2%, perhaps because of the updated electron stopping power data used for the TG-51 protocol, in addition to the inherent 1% discrepancy presented in the calibration factors. The results also indicate that the changes may be dependent on the electron energy.

An investigation of eye lens dose for gamma knife treatments of trigeminal neuralgia

Stereotactic Gamma Knife radiosurgery has been widely used for treating trigeminal neuralgia (TN). A single large fractional dose of 7000 to 9000 cGy is commonly prescribed as the maximum dose for these treatments. For this reason, if a small percentage of the prescribed dose such as 2-3% scattered to the eye, it could reach or even exceed the tolerance dose of the lens. For several TN cases, we found that the Leksell Gamma Plan system calculates the lens dose about 0.5-2% of the maximum dose independent of the use of eye shielding. These dose values are significantly high and it motivated us to investigate the lens dose for the TN patients treated with stereotactic Gamma Knife radiosurgery. Phantom studies and in vivo dosimetry measurements were carried out for six patients treated at our institution. The average dose to the lens ipsilateral to the treated nerve was measured to be 7.7+/-0.6 cGy. Based on the biological model of Lyman and Emami [Int. J. Radiat. Oncol. Biol. Phys. 21, 109-122 (1991)], the probability of the lens complication (cataract) was determined to be 0.1%. Our findings suggest that few TN patients would develop cataracts after receiving Gamma Knife radiosurgery.

Dosimetric evaluation and radioimmunotherapy of anti-tumour multivalent Fab' fragments

We have been investigating the use of cross-linked divalent (DFM) and trivalent (TFM) versions of the anti-carcinoembryonic antigen (CEA) monoclonal antibody A5B7 as possible alternatives to the parent forms (IgG and F(ab')2) which have been used previously in clinical radioimmunotherapy (RIT) studies in colorectal carcinoma. Comparative biodistribution studies of similar sized DFM and F(ab')2 and TFM and IgG, radiolabelled with both 131I and 90Y have been described previously using the human colorectal tumour LS174T nude mouse xenograft model (Casey et al (1996) Br J Cancer 74: 1397-1405). In this study quantitative estimates of radiation distribution and RIT in the xenograft model provided more insight into selecting the most suitable combination for future RIT. Radiation doses were significantly higher in all tissues when antibodies were labelled with 90Y. Major contributing organs were the kidneys, liver and spleen. The extremely high absorbed dose to the kidneys on injection of 90Y-labelled DFM and F(ab')2 as a result of accumulation of the radiometal would result in extremely high toxicity. These combinations are clearly unsuitable for RIT. Cumulative dose of 90Y-TFM to the kidney was 3 times lower than the divalent forms but still twice as high as for 90Y-IgG. TFM clears faster from the blood than IgG, producing higher tumour to blood ratios. Therefore when considering only the tumour to blood ratios of the total absorbed dose, the data suggests that TFM would be the most suitable candidate. However, when corrected for equitoxic blood levels, doses to normal tissues for TFM were approximately twice the level of IgG, producing a two-fold increase in the overall tumour to normal tissue ratio. In addition RIT revealed that for a similar level of toxicity and half the administered activity, 90Y-IgG produced a greater therapeutic response. This suggests that the most promising A5B7 antibody form with the radionuclide 90Y may be IgG. Dosimetry analysis revealed that the tumour to normal tissue ratios were greater for all 131I-labelled antibodies. This suggests that 131I may be a more suitable radionuclide for RIT, in terms of lower toxicity to normal tissues. The highest tumour to blood dose and tumour to normal tissue ratio at equitoxic blood levels was 131I-labelled DFM, suggesting that 131I-DFM may be best combination of antibody and radionuclide for A5B7. The dosimetry estimates were in agreement with RIT results in that twice the activity of 131I-DFM must be administered to produce a similar therapeutic effect as 131I-TFM. The toxicity in this therapy experiment was minimal and further experiments at higher doses are required to observe if there would be any advantage of a higher initial dose rate for 131I-DFM.

Panoramic dental radiography using a charge-coupled device receptor

Panoramic radiography using a slit beam and film/screen receptor is standard for the emergency room evaluation of mandibular fractures and also in dentistry. This study compared the spatial resolution, area distortion factors, and the dosage considerations for a panoramic system where standard film/screen and a charge-coupled device were alternatively employed as the image receptor. Resolution and image contours were determined using a lead resolution grid positioned at selected beam projection angulations. Exposure measurements were carried out using a RANDO average man phantom and a 3 cc beryllium-windowed ionization chamber. The maximum spatial resolution with film approached 5 lp mm-1 whereas with the CCD the maximum resolution was just above 4 lp mm-1. Consequently, the image layer was reduced slightly in width when using the CCD receptor. The use of the CCD resulted in skin exposure reduction exceeding 70%.

Rhenium-186-mercaptoacetyltriglycine-labeled monoclonal antibody for radioimmunotherapy: in vitro assessment, in vivo kinetics and dosimetry in tumor-bearing nude mice

Stability and immunoreactivity of 186Re-labeled monoclonal antibody were examined, and its in vivo kinetics was investigated in tumor-bearing Balb/c nu/nu female mice to assess the feasibility of using it in radioimmunotherapy (RIT). A murine IgG1, A7, against a 45 kD glycoprotein in human colon cancer was radiolabeled with 186Re by using a chelating method with a mercaptoacetyltriglycine (MAG3). 186Re-MAG3 complex was conjugated to A7 after esterification of 186Re-MAG3 with tetrafluorophenol (TFP). The efficiency of 186Re-MAG3-TFP production and the labeling efficiency of A7 were 51-59% and 57-60%, respectively. Immunoreactivity of purified 186Re-MAG3-A7 was 68.2% at infinite antigen excess. In 0.9% NaCl at 4 degrees C, the radioactivity (12.7 MBq/mg, 3.55 MBq/ml) dissociated with time from 186Re-MAG3-A7 as a small molecular weight moiety because of autoradiolysis. The addition of ascorbic acid, 5 mg/ml, as a radioprotectant or storage at -80 degrees C could effectively prevent the radiolysis of 186Re-MAG3-A7 for 7 days. Immunoreactivity of 186Re-MAG3-A7, 6.70 MBq/mg (6.66 MBq/ml), stored in the presence of ascorbic acid was well retained up to 8 days after the preparation. In colon cancer xenografted mice, 31.0% of the injected dose/g of 186Re-MAG3-A7 had accumulated in the tumors at 24 h postinjection. Estimated radiation dose to tumors was 14.9 cGy/37 kBq up to 8 days postinjection which was 12-fold greater than the whole-body radiation dose. These in vivo characteristics were superior to those of A7 labeled with radioiodine, affording greater therapeutic ratios than 131I-A7. Because of the better image quality of 186Re-MAG3-A7 as well as more favorable dosimetry, 186Re-MAG3-A7 would be a better choice for RIT of colon cancer than 131I-A7. These results indicated the feasibility of RIT with 186Re-MAG3-A7, though the prevention of radiolysis of the labeled antibody should be considered.

Proliferation and the advantage of longer-lived radionuclides in radioimmunotherapy

In our previous study we used the linear-quadratic model [J. Nucl. Med. 35, 1861 (1994)] to confirm our initial finding, based on the time-dose-fractionation model [J. Nucl. Med. 34, 1801 (1993)], that longer-lived radionuclides (e.g., 32P, 91Y) can offer a substantial therapeutic advantage over the shorter-lived radionuclides presently used in radioimmunotherapy (e.g., 90Y). The original calculations using the linear-quadratic (LQ) model did not account for proliferation of the tumor and critical bone marrow tissues. It has been suggested that inclusion of a proliferation term in the LQ model can have a substantial impact on the biologically effective dose (BED). With this in mind, we have reexamined the therapeutic efficacy of longer versus short-lived radionuclides using the LQ model replete with proliferation terms for tumor and bone marrow. Relative advantage factors (RAF), which quantify the overall therapeutic advantage of a long-lived compared to short-lived radionuclide, were calculated accordingly. While the extrapolated initial dose rate required to achieve a given BED can be affected by the inclusion of proliferation terms for both the tumor and marrow, the relative advantage factors for the longer-lived radionuclides were not significantly affected. Longer-lived radionuclides such as (114m)In and 91Y are about three times more therapeutically effective than the shorter-lived 90Y which is currently used in RIT. In other words, for a given therapeutic effect in the tumor, a longer-lived radionuclide can result in a lower deleterious effect to the bone marrow than a short-lived radionuclide. Given that bone marrow is generally considered to be the dose-limiting organ, these results have important implications for radioimmunotherapy.

Water Calorimetry: The Heat Defect

Domen developed a sealed water calorimeter at NIST to measure absorbed dose to water from ionizing radiation. This calorimeter exhibited anomalous behavior using water saturated with gas mixtures of H2 and O2. Using computer simulations of the radiolysis of water, we show that the observed behavior can be explained if, in the gas mixtures, the amount-of-substance of H2 and of O2 differed significantly from 50 %. We also report the results of simulations for other dilute aqueous solutions that are used for water calorimetry-pure water, air-saturated water, and H2-saturated water. The production of H2O2 was measured for these aqueous solutions and compared to simulations. The results indicate that water saturated with a gas mixture containing an amount-of-substance of H2 of 50 % and of O2 of 50 % is suitable for water calorimetry if the water is stirred and is in contact with a gas space of similar volume. H2-saturated water does not require a gas space but O2 contamination must be guarded against. The lack of a scavenger for OH radicals in "pure" water means that, depending on the water purity, some "pure" water might require a large priming dose to remove reactive impurities. The experimental and theoretical problems associated with air-saturated water and O2-saturated water in water calorimeters are discussed.

Macroscopic dosimetry for radioimmunotherapy: nonuniform activity distributions in solid tumors

In the context of radioimmunotherapy of cancer, there is a need for continued improvement of dosimetry of radionuclides localized in tumors. Current methods assume uniform distribution of radionuclides in the tumor despite experimental evidence indicating nonuniormity. We have developed a model in which nonuniform distribution of radioactivity in the tumor is taken into account. Spherically symmetric radionuclide distributions, depending linearly and exponentially on the radial position, are considered. Dose rate profiles in the tumor are calculated for potentially useful beta-emitting radionuclides, including 32P, 67Cu, 90Y, 111Ag, 131I, and 188Re, and for 193mPt, an emitter of conversion electrons and low-energy Auger electrons. For the radionuclide distributions investigated, high-energy beta emitters, such as 90Y, are most effective in treating large tumors (diameter, d greater than approximately 1 cm), whereas for small tumors (d approximately 1 mm), medium energy beta emitters such as 67Cu are better suited. Very small tumors (d less than 1 mm), and micrometastases are best handled with low-energy electron emitters such as 193mPt.

Steady-State Response of Silicon Radiation Detectors of the Diffused P-N Junction Type to X rays. II: Photodiode Mode of Operation

The response to x rays of silicon radiation detectors of the p-n junction type was investigated with special consideration of their dependence on the applied voltage. In agreement with theory, the photocurrent, I g , was found to consist of a voltage-independent part mainly determined by the average diffusion length of minority carriers in the base layer, and a voltage-dependent part which is proportional to the width of the depletion region, w. Due to the voltage dependence of w, I g increases with increasing voltage applied, but its relative change produced by different voltages is independent of exposure rate and quality of radiation. Exposure rate and energy dependence of I g expressed in relative values are thus independent of applied voltage. Silicon radiation detectors, used as photodiodes can therefore be useful for monitoring of radiations at exposure rates larger than 1 R/min, taking advantage of the possibility to increase current sensitivity by increasing the voltage V and to increase the voltage signal by increasing the load resistance R L . There are however limitations in increasing V and R L . because of the increasing noise with increasing dark current and some dependence of measured current signals on R L . The temperature coefficient of I g is positive and independent of R L , but shows some small voltage dependence. In the temperature range between 25 and 50 °C, the average temperature coefficient is approximately 0.35 percent per degree centigrade. A value of the average diffusion length of minority carriers in the base layer has been derived from the measured voltage dependence of I g .