Dual-Responsive and ROS-Augmented Nanoplatform for Chemo/Photodynamic/Chemodynamic Combination Therapy of Triple Negative Breast Cancer

Integrating chemodynamic therapy (CDT) and photodynamic therapy (PDT) into one nanoplatform can produce much more reactive oxygen species (ROS) for tumor therapy. Nevertheless, it is still a great challenge to selectively generate sufficient ROS in tumor regions. Meanwhile, CDT and PDT are restricted by insufficient H2O2 content in the tumor as well as by the limited tumor tissue penetration of the light source. In this study, a smart pH/ROS-responsive nanoplatform, Fe2+@UCM-BBD, is rationally designed for tumor combination therapy. The acidic microenvironment can induce the pH-responsive release of doxorubicin (DOX), which can induce tumor apoptosis through DNA damage. Beyond that, DOX can promote the production of H2O2, providing sufficient materials for CDT. Of note, upconversion nanoparticles at the core can convert the 980 nm light to red and green light, which are used to activate Ce6 to produce singlet oxygen (1O2) and achieve upconversion luminescence imaging, respectively. Then, the ROS-responsive linker bis-(alkylthio)alkene is cleaved by 1O2, resulting in the release of Fenton reagent (Fe2+) to realize CDT. Taken together, Fe2+@UCM-BBD exhibits on-demand therapeutic reagent release capability, excellent biocompatibility, and remarkable tumor inhibition ability via synergistic chemo/photodynamic/chemodynamic combination therapy.

Yb3+, Er3+ Codoped Cerium Oxide Upconversion Nanoparticles Enhanced the Enzymelike Catalytic Activity and Antioxidative Activity for Parkinson's Disease Treatment

Oxidative stress plays an important role in Parkinson's disease (PD) and is considered a therapeutic target for PD. However, most therapeutic antioxidants show limitations due to their low reactive oxygen species (ROS) catalytic properties and low crossing of blood-brain barrier. Herein, the antioxidative activity of Yb3+ and Er3+ double-doped CeO2-x (Yb/Er/CeO2-x) upconversion nanoparticles (UCNPs) is obtained for PD treatment. Doping of Yb3+ and Er3+ ions increases oxygen vacancies, which leads to higher enzymelike catalytic activities compared to CeO2-x nanoparticles alone. Tyrosine hydroxylase protein and glial fibrillary acidic protein expression in substantia nigra and striatum as well as the open-field activity test indicates that Yb/Er/CeO2-x is effective for treatment of PD. The activities of glutathione peroxidase and total antioxidant capacity increase and the production of ROS decreases with Yb/Er/CeO2-x UCNP treatment compared with MPTP-induced injury. This indicates that the mechanism of PD treatment is to catalyze ROS products. There have been no reports to date on the usage of Yb/Er/CeO2-x as an antioxidant for PD treatment. Yb/Er/CeO2-x UCNPs cross the blood-brain barrier and exhibit biocompatibility and antioxidant catalytic properties, which decrease the ROS and effectively help in treating PD.

MONTE CARLO-BASED INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTION OF HIGH ENERGY BRACHYTHERAPY SOURCES

FLUKA-based Monte Carlo calculations were carried out to study microdosimetric distributions in air and in water for encapsulated high energy brachytherapy sources (60Co, 137Cs, 192Ir and 169Yb) by simulating a Tissue Equivalent Proportional Counter (Model LET1/2) having sensitive diameter of 1. 27 cm for a site size of 1 μm. The study also included microdosimetric distributions of bare sources. When the sources are in air, for a given source, the source geometry does not affect the y¯F and y¯D values significantly. When the encapsulated 192Ir, 137Cs and 60Co sources are in water, y¯F and y¯D values increase with distance in water which is due to degradation in the energy of photons. Using the calculated values of y¯D, relative biological effectiveness (RBE) was obtained for the investigated sources. When 60Co, 137Cs and 192Ir sources are in water, RBE increases from 1.03 ± 0.01 to 1.17 ± 0.01, 1.24 ± 0.01 to 1.46 ± 0.02 and 1.50 ± 0.01 to 1.75 ± 0.03, respectively, when the distance was increased from 3-15 cm, whereas for 169Yb, RBE is about 2, independent of distance in water.

Dosimetric characterization of the GammaClip™ 169Yb low dose rate permanent implant brachytherapy source for the treatment of nonsmall cell lung cancer postwedge resection

PURPOSE

A novel (169)Yb low dose rate permanent implant brachytherapy source, the GammaClip™, was developed by Source Production & Equipment Co. (New Orleans, LA) which is designed similar to a surgical staple while delivering therapeutic radiation. In this report, the brachytherapy source was characterized in terms of "Dose calculation for photon-emitting brachytherapy sources with average energy higher than 50 keV: Report of the AAPM and ESTRO" by Perez-Calatayud et al. [Med. Phys. 39, 2904-2929 (2012)] using the updated AAPM Task Group Report No. 43 formalism.

METHODS

Monte Carlo calculations were performed using Monte Carlo N-Particle 5, version 1.6 in water and air, the in-air photon spectrum filtered to remove photon energies below 10 keV in accordance with TG-43U1 recommendations and previously reviewed (169)Yb energy cutoff levels [D. C. Medich, M. A. Tries, and J. M. Munro, "Monte Carlo characterization of an Ytterbium-169 high dose rate brachytherapy source with analysis of statistical uncertainty," Med. Phys. 33, 163-172 (2006)]. TG-43U1 dosimetric data, including SK, Ḋ(r,θ), Λ, gL(r), F(r, θ), φan(r), and φan were calculated along with their statistical uncertainties. Since the source is not axially symmetric, an additional set of calculations were performed to assess the resulting axial anisotropy.

RESULTS

The brachytherapy source's dose rate constant was calculated to be (1.22±0.03) cGy h(-1) U(-1). The uncertainty in the dose to water calculations, Ḋ(r,θ), was determined to be 2.5%, dominated by the uncertainties in the cross sections. The anisotropy constant, φan, was calculated to be 0.960±0.011 and was obtained by integrating the anisotropy factor between 1 and 10 cm using a weighting factor proportional to r(-2). The radial dose function was calculated at distances between 0.5 and 12 cm, with a maximum value of 1.20 at 5.15±0.03 cm. Radial dose values were fit to a fifth order polynomial and dual exponential regression. Since the source is not axially symmetric, angular Monte Carlo calculations were performed at 1 cm which determined that the maximum azimuthal anisotropy was less than 8%.

CONCLUSIONS

With a higher photon energy, shorter half-life and higher initial dose rate 169Yb is an interesting alternative to 125I for the treatment of nonsmall cell lung cancer.

The dawn of computer-assisted robotic osteotomy with ytterbium-doped fiber laser

Currently, laser radiation is used routinely in medical applications. For infrared lasers, bone ablation and the healing process have been reported, but no laser systems are established and applied in clinical bone surgery. Furthermore, industrial laser applications utilize computer and robot assistance; medical laser radiations are still mostly conducted manually nowadays. The purpose of this study was to compare the histological appearance of bone ablation and healing response in rabbit radial bone osteotomy created by surgical saw and ytterbium-doped fiber laser controlled by a computer with use of nitrogen surface cooling spray. An Ytterbium (Yb)-doped fiber laser at a wavelength of 1,070 nm was guided by a computer-aided robotic system, with a spot size of 100 μm at a distance of approximately 80 mm from the surface. The output power of the laser was 60 W at the scanning speed of 20 mm/s scan using continuous wave system with nitrogen spray level 0.5 MPa (energy density, 3.8 × 10(4) W/cm(2)). Rabbits radial bone osteotomy was performed by an Yb-doped fiber laser and a surgical saw. Additionally, histological analyses of the osteotomy site were performed on day 0 and day 21. Yb-doped fiber laser osteotomy revealed a remarkable cutting efficiency. There were little signs of tissue damage to the muscle. Lased specimens have shown no delayed healing compared with the saw osteotomies. Computer-assisted robotic osteotomy with Yb-doped fiber laser was able to perform. In rabbit model, laser-induced osteotomy defects, compared to those by surgical saw, exhibited no delayed healing response.

A dosimetric comparison of Yb169 and Ir192 for HDR brachytherapy of the breast, accounting for the effect of finite patient dimensions and tissue inhomogeneities

Monte Carlo simulation dosimetry is used to compare 169Yb to 192Ir for breast high dose rate (HDR) brachytherapy applications using multiple catheter implants. Results for bare point sources show that while 169Yb delivers a greater dose rate per unit air kerma strength at the radial distance range of interest to brachytherapy in homogeneous water phantoms, it suffers a greater dose rate deficit in missing scatter conditions relative to 192Ir. As a result of these two opposing factors, in the scatter conditions defined by the presence of the lung and the finite patient dimensions in breast brachytherapy the dose distributions calculated in a patient equivalent mathematical phantom by Monte Carlo simulations for the same implant of either 169Yb or 1921r commercially available sources are found comparable. Dose volume histogram results support that 169Yb could be at least as effective as 192Ir delivering the same dose to the lung and slightly reduced dose to the breast skin. The current treatment planning systems' approach of employing dosimetry data precalculated in a homogeneous water phantom of given shape and dimensions, however, is shown to notably overestimate the delivered dose distribution for 169Yb. Especially at the skin and the lung, the treatment planning system dose overestimation is on the order of 15%-30%. These findings do not undermine the potential of 169Yb HDR sources for breast brachytherapy relative to the most commonly used 192Ir HDR sources. They imply, however, that there could be a need for the amendment of dose calculation algorithms employed in clinical treatment planning of particular brachytherapy applications, especially for intermediate photon energy sources such as 169Yb.

Comparison of radiation shielding requirements for HDR brachytherapy using Yb169 and Ir192 sources

169Yb has received a renewed focus lately as an alternative to 192Ir sources for high dose rate (HDR) brachytherapy. Following the results of a recent work by our group which proved 169Yb to be a good candidate for HDR prostate brachytherapy, this work seeks to quantify the radiation shielding requirements for 169Yb HDR brachytherapy applications in comparison to the corresponding requirements for the current 192Ir HDR brachytherapy standard. Monte Carlo simulation (MC) is used to obtain 169Yb and 192Ir broad beam transmission data through lead and concrete. Results are fitted to an analytical equation which can be used to readily calculate the barrier thickness required to achieve a given dose rate reduction. Shielding requirements for a HDR brachytherapy treatment room facility are presented as a function of distance, occupancy, dose limit, and facility workload, using analytical calculations for both 169Yb and 192Ir HDR sources. The barrier thickness required for 169Yb is lower than that for 192Ir by a factor of 4-5 for lead and 1.5-2 for concrete. Regarding 169Yb HDR brachytherapy applications, the lead shielding requirements do not exceed 15 mm, even in highly conservative case scenarios. This allows for the construction of a lead door in most cases, thus avoiding the construction of a space consuming, specially designed maze. The effects of source structure, attenuation by the patient, and scatter conditions within an actual treatment room on the above-noted findings are also discussed using corresponding MC simulation results.

A genetically engineered anti-CD45 single-chain antibody-streptavidin fusion protein for pretargeted radioimmunotherapy of hematologic malignancies

Acute myelogenous leukemia (AML) currently kills the majority of afflicted patients despite combination chemotherapy and hematopoietic cell transplantation (HCT). Our group has documented the promise of radiolabeled anti-CD45 monoclonal antibodies (Ab) administered in the setting of allogeneic HCT for AML, but toxicity remains high, and cure rates are only 25% to 30% for relapsed AML. We now show the superiority of pretargeted radioimmunotherapy (PRIT) compared with conventional radioimmunotherapy using a recombinant tetravalent single-chain Ab-streptavidin (SA) fusion protein (scFv(4)SA) directed against human CD45, administered sequentially with a dendrimeric N-acetylgalactosamine-containing clearing agent and radiolabeled 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic (DOTA)-biotin. The scFv(4)SA construct was genetically engineered by fusing Fv fragments of the human CD45-specific BC8 Ab to a full-length genomic SA gene and was expressed as a soluble tetramer in the periplasmic space of Escherichia coli. The fusion protein was purified to >95% homogeneity at an overall yield of approximately 50% using iminobiotin affinity chromatography. The immunoreactivity and avidity of the fusion protein were comparable with those of the intact BC8 Ab, and the scFv(4)SA construct bound an average of 3.9 biotin molecules out of four theoretically possible. Mouse lymphoma xenograft experiments showed minimal toxicity, excellent tumor-specific targeting of the fusion protein and radiolabeled DOTA-biotin in vivo, marked inhibition of tumor growth, and cured 100% of mice bearing CD45-expressing tumors. These promising results have prompted large-scale cGMP production of the BC8 fusion protein for clinical trials to be conducted in patients with hematologic malignancies.

175Yb-labeled hydroxyapatite: a potential agent for use in radiation synovectomy of small joints

The preparation of 175Yb-labeled hydroxyapatite (HA) particle is described for possible use as an agent for radiation synovectomy (RS) of small-sized joints. 175Yb was produced by thermal neutron irradiation of enriched (98.6% in 174Yb) ytterbium target at a flux of approximately 3 x 10(13) n/cm(2)/s for 7 days. Specific activity of 5.5-6.0 GBq/mg and a very high radionuclidic purity to the extent of approximately 100% were obtained. In the work reported herein, HA could be labeled with 175Yb in very high radiochemical purity (>99%) using 10 mg of HA particle at pH approximately 7. The radiolabeled particulates showed excellent in vitro stability at room temperature. Serial scintigraphic images of normal as well as arthritis-bearing Wistar rats following intra-articular injection of 175Yb-HA particles in the knee joint showed complete retention of activity within the synovial cavity with no measurable activity leaching out from the joint till 144 h post-injection.

Broad-beam transmission data for new brachytherapy sources, Tm-170 and Yb-169

The characteristics of the radionuclides (170)Tm and (169)Yb are highly interesting for their use as high dose-rate brachytherapy sources. The introduction of brachytherapy equipment containing these sources will lead to smaller required thicknesses of the materials used in radiation protection barriers compared with the use of conventional sources such as (192)Ir and (137)Cs. The purpose of this study is to determine the required thicknesses of protection material for the design of the protecting walls. Using the Monte Carlo method, transmission data were derived for broad-beam geometries through lead and concrete barriers, from which the first half value layer and tenth value layer are obtained. In addition, the dose reduction in a simulated patient was studied to determine whether transmission in the patient is a relevant factor in radiation protection calculations.

A dosimetric comparison of Yb169 versus Ir192 for HDR prostate brachytherapy

For the purpose of evaluating the use of 169Yb for prostate High Dose Rate brachytherapy (HDR), a hypothetical 169Yb source is assumed with the exact same design of the new microSelectron source replacing the 192Ir active core by pure 169Yb metal. Monte Carlo simulation is employed for the full dosimetric characterization of both sources and results are compared following the AAPM TG-43 dosimetric formalism. Monte Carlo calculated dosimetry results are incorporated in a commercially available treatment planning system (SWIFT), which features an inverse treatment planning option based on a multiobjective dose optimization engine. The quality of prostate HDR brachytherapy using the real 192Ir and hypothetical 169Yb source is compared in a comprehensive analysis of different prostate implants in terms of the multiobjective dose optimization solutions as well as treatment quality indices such as Dose Volume Histograms (DVH) and the Conformal Index (COIN). Given that scattering overcompensates for absorption in intermediate photon energies and distances in the range of interest to prostate HDR brachytherapy, 169Yb proves at least equivalent to 192Ir irrespective of prostate volume. This has to be evaluated in view of the shielding requirements for the 169Yb energies that are minimal relative to that for 192Ir.

Cross-section measurement of the 169Tm(p,n) reaction for the production of the therapeutic radionuclide 169Yb and comparison with its reactor-based generation

The radionuclide (169)Yb (T(1/2)=32.0 d) is potentially important for internal radiotherapy. It is generally produced using a nuclear reactor. In this work the possibility of its production at a cyclotron was investigated. A detailed determination of the excitation function of the (169)Tm(p,n)(169)Yb reaction was done over the proton energy range up to 45 MeV using the stacked-foil technique and high-resolution gamma-ray spectrometry. The integral yield of (169)Yb was calculated. Over the optimum energy range E(P)=16-->7 MeV the yield amounts to 1.5 MBq/micro Ah and is thus rather low. A comparison of this production route with the established (168)Yb(n,gamma)(169)Yb reaction at a nuclear reactor is given. The (169)Yb yield via the reactor route is by several orders of magnitude higher than by the cyclotron method. The latter procedure, however, leads to "no-carrier-added" product.

Targeting Tomoregulin for Radioimmunotherapy of Prostate Cancer

Radiotherapy is an effective approach for the treatment of local prostate cancer. However, once prostate cancer metastasizes, radiotherapy cannot be used due to the distribution of multiple metastases to lymph nodes and bones. In contrast, radioimmunotherapy should still be efficacious in metastatic prostate cancer as radioisotopes are brought to tumor cells by targeting antibodies. Here we identify and validate a prostate-expressed molecule, tomoregulin, as a target for radioimmunotherapy of prostate cancer. Tomoregulin is a transmembrane protein selectively expressed in the brain, prostate, and prostate cancer, but not expressed in other normal tissues. Immunohistochemical studies of tomoregulin protein in clinical samples show its location in the luminal epithelium of normal prostate, benign prostatic hyperplasia, and prostatic intraepithelial neoplasia. More importantly, the tomoregulin protein is expressed in primary prostate tumors and in their lymph node and bone metastases. The nature of tomoregulin as a transmembrane protein and its tissue-specific expression make tomoregulin an attractive target for radioimmunotherapy, in which tomoregulin-specific antibodies will deliver a radioisotope to prostate tumor cells and metastases. Indeed, biodistribution studies using a prostate tumor xenograft model showed that the (111)In-labeled anti-tomoregulin antibody 2H8 specifically recognizes tomoregulin protein in vivo, leading to a strong tumor-specific accumulation of the antibody. In efficacy studies, a single i.p. dose of 150 microCi (163 microg) (90)Y-labeled 2H8 substantially inhibits the growth rate of established LNCaP human prostate tumor xenograft in nude mice but produces no overt toxicity despite cross-reactivity of 2H8 with mouse tomoregulin. Our data clearly validate tomoregulin as a target for radioimmunotherapy of prostate cancer.

Accounting for high Z shields in brachytherapy using collapsed cone superposition for scatter dose calculation

Common clinical brachytherapy treatment planning algorithms perform at best one-dimensional corrections for high Z heterogeneities that will be inaccurate for intermediate energies (60-100 keV). The development of fast methods for a three-dimensional dose calculation to account for high Z materials in this energy range is important, e.g., to fully utilize the potential of patient individualized shields using isotopes such as 241Am and 169Yb. In this work we use the collapsed cone superposition algorithm to calculate the scatter dose contribution around partly lead-shielded point sources at 60, 100, and 350 keV. Methods to scale point kernels for water into kernels for high Z materials are derived. The scaling accounts for scattered photon spectral differences between materials and thus goes beyond the common density scaling approach. Compared to Monte Carlo simulations, the results of our algorithm yield agreements on the unshielded side to within 3% at 350 and 60 keV and to within 7% at 100 keV out to distances of 8 cm from the source. The effect of the shield in the center of the unshielded region is small at 350 keV but significant and occurs at short distances at 100 and 60 keV. At 60 keV, the shield causes a dose reduction of around 10%, 1 cm from the source on the unshielded side. At 100 keV, the reverse effect is seen, the insertion of shields leading to the total dose being increased by about 10% at 1 cm. That one-dimensional algorithms are incapable of predicting these changes shows the importance of accounting for the full three-dimensional geometry in correctly determining the scatter dose contribution.

Monte Carlo aided room scatter corrections in the air-kerma strength standardization of 169Yb and 60Co brachytherapy sources

For accurate evaluation of air-kerma strength, S(k), of 169Yb and 60Co brachytherapy sources, the present study reports Monte Carlo (MC) based corrections for (1) room scatter, and (2) departure from constant room scatter for rooms of various sizes. Correction for exponential attenuation of effective primary in air is also reported for the above sources. Values of S(k) per contained mCi, S(k)/A(c) predicted by MC calculations for 169Yb source (model X1267) with and without Ti K x-rays are 1.302 +/- 0.03% (this value is in excellent agreement with the published value reported by Piermattei et al) and 1.260 +/- 0.03% cGy cm2 h(-1) mCi(-1) respectively, and in the case of Cathetron 60Co source the value of S(k)/A(c) is 11.015 +/- 0.01% cGy cm2 h(-1) mCi(-1). It is observed that depending upon the position of the source with respect to the surrounding concrete scattering surfaces and set of d values, the assumption of constant room scatter has resulted in overestimation of S(k) that varied between 0.30% and 1.5% for the 169Yb source and only between 0.10% and 0.20% for the 60Co source.

Radioimmunotherapy of A431 xenografted mice with pretargeted B3 antibody-streptavidin and (90)Y-labeled 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA)-biotin

We investigated the biodistribution of (88)Y/(111)In-labeled 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA)-biotin and therapy with (90)Y-labeled DOTA-biotin in tumor-bearing mice after B3-streptavidin antibody conjugate (B3-SA) pretargeting. B3 antibody, recognizing Lewis(y) antigen, was conjugated to streptavidin (B3-SA). For pretargeting, 400 micro g of the B3-SA was injected i.v. into mice bearing A431 tumor xenografts. After tumor localization of B3-SA, 100 micro g of synthetic clearing agent was injected i.v. to clear the unbound B3-SA from the circulation. Four h later, 1 micro g of radiolabeled DOTA-biotin was injected i.v. Radioimmunotherapy was performed with doses of 9.25 to 37 MBq of (90)Y-labeled DOTA-biotin. As a result, radiolabeled DOTA-biotin cleared rapidly. All of the normal tissues had <2.6% of the injected dose per gram, whereas tumor uptake reached approximately 15% ID/g. The total tumor uptake of radioactivity remained similar for 96 h or longer. In the first study, the median survival of the control group was 8 days, whereas it increased to >163 days in the 37 MBq (90)Y group (P < 0.005). In a second therapy group, 7 of 10 mice receiving 37 MBq of (90)Y and B3-SA were cured, and remained healthy for >180 days after therapy, compared with control groups, with </=29.2 days mean survival time (P < 0.001). Tumor pretargeting with B3-SA and radiolabeled DOTA-biotin has shown favorable, specific, and fast targeting that has resulted in good tumor responses and, thus, serves as a rationale for human studies with the B3-SA pretargeting approach.

Feasibility study for production of 175Yb: a promising therapeutic radionuclide

Owing to its favourable decay characteristics 175Yb (T1/2 = 4.2 d, E beta(max) = 480 keV) can be regarded as a potential radionuclide for therapeutic applications. Production of 175Yb using (174Yb(n, gamma)175Yb) reaction by thermal neutron bombardment on natural ytterbium target is described. The activity of 175Yb produced as well as its radionuclidic purity under different irradiation conditions were determined by gamma-ray spectrometry using an HPGe 4 K MCA system and compared with theoretically calculated values. The radiochemical purity after chemical processing was determined by paper chromatography as well as paper electrophoresis techniques. It is found that 31 Ci/g (1145 GBq/g) of 175Yb can be produced with > 95% radionuclidic purity (with approximately 3% of 169Yb and approximately 2% of 177Lu) by irradiating natural Yb2O3 target at a thermal neutron flux of 3 x 10(13) n/cm2/s for a period of 5 d.

Dose distributions for 90Y intravascular brachytherapy sources used with balloon catheters

The dose distribution around an intravascular brachytherapy 90Y line source with centering balloon catheters was measured with a plastic scintillator, TLD and radiochromic film. The absolute dose rates measured with the three detectors in a solid water phantom at 1, 2 and 3 mm distance from the centering balloon surfaces are in agreement within 3.5%, when the detectors are calibrated with the same 90Sr/90Y source. The dose rates measured with the plastic scintillator in the solid water phantom are in agreement with those directly measured in water. The measured relative dose distributions can be reproduced by Monte Carlo calculations. Also, the influence of the balloon diameter on the dose rate can be reproduced by the calculations. The dose rate calibration routinely performed with the plastic scintillator was checked for fifty-one sources with a well chamber and with another dedicated dose rate checking device. These measurements show that the consistency of the calibration of these sources was better than 10%. In a previous paper absolute dose rates for five other 90Y sources measured with TLD and radiochromic film in a solid water phantom were compared with those obtained with the plastic scintillator in water [Piessens and Reynaert, "Verification of absolute dose rates for intravascular beta sources," Phys. Med. Biol. 45, 2219-2231 (2000)]. Differences of 25 to 41%, depending on the balloon diameter, were reported. In this paper we show the evidence for three main reasons for these previously observed discrepancies: an inconsistency between a detector calibration performed with a 6 MeV electron beam and with a calibrated 90Sr/90Y source from NIST (16%), inaccuracies of the measuring distances in the solid water phantom (maximum 7.5%) and a time instability of the plastic scintillator, probably due to radiation damage (6%).

Anisotropy functions for 169Yb brachytherapy seed models 5, 8 and X1267. An EGS4 Monte Carlo study

Anisotropy functions for 169Yb sources used in interstitial brachytherapy are investigated. A comprehensive study of several factors affecting the angular dose distribution around four 169Yb seed models (Amersham International) has been undertaken. Absolute dose rates around 169Yb seed models 5, 8a, 8b and X1267 have been estimated by means of the EGS4 Monte Carlo Simulation System. An updated cross section library (DLC-136/PHOTX), binding corrections for Compton scattering and water molecular form factors were included in the calculations. Following the formalism developed by the Interstitial Brachytherapy Collaborative Working Group, anisotropy functions, F(r, theta), have been calculated and compared with other Monte Carlo results and whenever possible with experimental data. Excellent agreement is found with other Monte Carlo calculations. Considering the large experimental errors reported, a fairly good coincidence has been achieved between experimental and Monte Carlo data for models 8a and 8b. For model X1267 large discrepancies with experiment are obtained. Monte Carlo calculations for all seed models showed model 5 to be the least anisotropic and models 8b and X1267 to be almost identical. Statistical fluctuations can be drastically reduced computationally, offering an efficient alternative to measured data. Our results have estimated uncertainties of 0.5%-1.0% within one standard deviation everywhere excluding the longitudinal source axis, where uncertainties are below 3% up to 5 cm, this accuracy being excellent for clinical calculations.

A Monte Carlo calculation of dosimetric parameters of 90Sr/90Y and 192Ir SS sources for intravascular brachytherapy

The AAPM TG-60 report has proposed various dose calculation parameters for intravascular brachytherapy (IVBT). These parameters include the dose rate constant (or the dose rate at a reference position for a beta-particle emitting source), the radial dose function, and the anisotropy function. In this work, we have used a modified EGS4 Monte Carlo system to calculate these parameters for the two most commonly used IVBT sources (the beta-particle emitting 90Sr/90Y source and the photon emitting 192Ir SS source). To ensure the calculation accuracy, the present calculation was compared with several measurements and calculations reported by other authors. Excellent agreement was found for the results with the photon source. For beta-particle source calculation, the present results for a variety of point sources agree very well with a previous work. The presently calculated radial dose functions for the 90Sr/90Y source are consistent with those of a published work for intermediate radial distances. The dose uniformity in the axial direction was also studied. The contributions of bremsstrahlung photons to total doses for the 90Sr/90Y beta source, and the influence of ignoring electron transport on calculated doses for the 192Ir SS photon source are discussed.

Phase I study of 90Y-labeled B72.3 intraperitoneal administration in patients with ovarian cancer: effect of dose and EDTA coadministration on pharmacokinetics and toxicity

The tumor-associated glycoprotein 72 (TAG-72) antigen is present on a high percentage of tumor types including ovarian carcinomas. Antibody B72.3 is a murine monoclonal recognizing the surface domain of the TAG-72 antigen and has been widely used in human clinical trials. After our initial encouraging studies (M. G. Rosenblum et al., J. Natl. Cancer Inst., 83: 1629-1636, 1991) of tissue disposition, metabolism, and pharmacokinetics in 9 patients with ovarian cancer, we designed an escalating dose, multi-arm Phase I study of 90Y-labeled B72.3 i.p. administration. In the first arm of the study, patients (3 pts/dose level) received an i.p. infusion of either 2 or 10 mg of B72.3 labeled with either 1, 10, 15, or 25 mCi of 90Y. Pharmacokinetic studies demonstrated that concentrations of 90Y-labeled B72.3 persist in peritoneal fluid with half-lives >24 h after i.p. administration. In addition, 90Y-labeled B72.3 was absorbed rapidly into the plasma with peak levels achieved within 48 h, and levels declined slowly thereafter. Cumulative urinary excretion of the 90Y label was 10-20% of the administered dose which suggests significant whole-body retention of the radiolabel. Biopsy specimens of bone and marrow obtained at 72 h after administration demonstrated significant content of the label in bone (0.015% of the dose/g) with relatively little in marrow (0.005% of the dose/g). The maximal tolerated dose was determined to be 10 mCi because of hematological toxicity and platelet suppression. This typically occurred on the 29th day after administration and was thought to be a consequence of the irradiation of the marrow from the bony deposition of the radiolabel. In an effort to suppress the bone uptake of 90Y, patients were treated with a continuous i.v. infusion of EDTA (25 mg/kg/12 h x 6) infused immediately before i.p. administration of the radiolabeled antibody. Patients (3 pts/dose level) were treated with doses of 10, 15, 20, 25, 30, 35, 40, or 45 mCi of 90Y-labeled B72.3 for a total of 38 patients. EDTA administration resulted in significant myeloprotection, which allowed escalation to the maximal tolerated dose of 40 mCi. Dose-limiting toxicity was thrombocytopenia and neutropenia. Studies of plasma and peritoneal fluid pharmacokinetics demonstrate no changes compared with patients without EDTA pretreatment. Cumulative urinary excretion of the radiolabel was not increased in patients pretreated with EDTA compared with the untreated group. However, analysis of biopsy specimens of bone and marrow demonstrated that bone and marrow content of the 90Y label was 15-fold lower (<0.001% injected dose/g) than a companion group without EDTA. Four responses were noted in patients who received 15-30 mCi of 90Y-labeled B72.3 with response durations of 1-12 months. These results demonstrate the myeloprotective ability of EDTA, which allows safe i.p. administration of higher doses of 90Y-labeled B72.3 and, therefore, clearly warrant an expanded Phase II trial in patients with minimal residual disease after standard chemotherapy or for the palliation of refractory ascites.

Locoregional regulatory peptide receptor targeting with the diffusible somatostatin analogue 90Y-labeled DOTA0-D-Phe1-Tyr3-octreotide (DOTATOC): a pilot study in human gliomas

Human gliomas, especially of low-grade type, have been shown to express high-affinity somatostatin receptor type 2 (J-C. Reubi et al., Am. J. Pathol, 134: 337-344, 1989). We enrolled seven low-grade and four anaplastic glioma patients in a pilot study using the diffusible peptidic vector 90Y-labeled DOTA0-D-Phe1-Tyr3-octreotide (DOTATOC) for receptor targeting. The radiopharmakon was locoregionally injected into a stereotactically inserted Port-a-cath. DOTATOC competes specifically with somatostatin binding to somatostatin receptor type 2 in the low nanomolar range as shown by a displacement curve of 125I-[Tyr3]-octreotide in tumor tissue sections. Diagnostic (111)In-labeled DOTATOC-scintigraphy following local injection displayed homogeneous to nodular intratumoral vector distribution. The cumulative activity of regionally injected peptide-bound 90Y amounted to 370-3300 MBq, which is equivalent to an effective dose range between 60 +/- 15 and 550 +/- 110 Gy. Activity was injected in one to four fractions according to tumor volumes; 1110 MBq of 90Y-labeled DOTATOC was the maximum activity per single injection. We obtained six disease stabilizations and shrinking of a cystic low-grade astrocytoma component. The only toxicity observed was secondary perifocal edema. The activity:dose ratio (MBq:Gy) represents a measure for the stability of peptide retention in receptor-positive tissue and might predict the clinical course. We conclude that SR-positive human gliomas, especially of low-grade type, can be successfully targeted by intratumoral injection of the metabolically stable small regulatory peptide DOTATOC.

Dose rate constants for 125I, 103Pd, 192Ir and 169Yb brachytherapy sources: an EGS4 Monte Carlo study

An exhaustive revision of dosimetry data for 192Ir, 125I, 103Pd and 169Yb brachytherapy sources has been performed by means of the EGS4 simulation system. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. The absorbed dose rate per unit contained activity in a medium at 1 cm in water and air-kerma strength per unit contained activity for each seed model were calculated, allowing the dose rate constant (DRC) A to be estimated. The influence of the calibration procedure on source strength for low-energy brachytherapy seeds is discussed. Conversion factors for 125I and 103Pd seeds to obtain the dose rate in liquid water from the dose rate measured in a solid water phantom with a detector calibrated for dose to water were calculated. A theoretical estimate of the DRC for a 103Pd model 200 seed equal to 0.669 +/- 0.002 cGy h(-1) U(-1) is obtained. Comparison of obtained DRCs with measured and calculated published results shows agreement within 1.5% for 192Ir, 169Yb and 125I sources.

Radioactivity measurements of ytterbium-169 brachytherapy sources

Ytterbium-169 is being considered as a new radiation source for brachytherapy applications. This radioisotope emits photons with energies ranging from 50 to 308 keV (average energy 93 keV) and decays with a half life of 32 days. For these reasons, it is believed to offer radiological and radiobiological advantages over some other isotopes currently in use. One impediment to widespread clinical use of this isotope is the determination of source strength in units of air kerma rate [cGy h-1]. The source strength can be measured directly with an ion chamber or calculated indirectly from the source radioactivity [Bq] with corrections for encapsulation. Our attempts to reconcile these two approaches have led to the development of a spectrometric technique for determining the radioactivity of ytterbium-169 brachytherapy seeds. A High Purity Germanium (HPGe) spectrometer is used to count the 307.7 keV photon emitted within a defined solid angle. The intrinsic photopeak efficiency of the detector was determined by Monte Carlo simulation followed by experimental verification with an activity-calibrated europium-152 source. Finally, the HPGe system has been used to calibrate a re-entrant ionisation chamber, allowing routine of Ytterbium source activity for clinical applications.

Analysis of the radiobiology of ytterbium-169 and iodine-125 permanent brachytherapy implants

Recently, Yb-169 has been considered as a potential replacement for I-125 and Pd-103 in permanent implants. In spite of the uncertainties in the parameters necessary for an accurate radiobiological modelling, the linear quadratic model can be useful in the comparative evaluation of the radiotherapeutic merit of similar implants. In order to find out if a Yb-169 permanent implant can be made biologically 'equivalent' to an I-125 implant, we studied the dependence of local control on the tumour cell radiosensitivity and on the balance between the rate of tumour cell killing and tumour cell proliferation, for rapidly and slowly proliferating tumours. The extrapolated response dose (ERD) has been calculated for tumour and late reacting normal tissue for both types of implants and the possible biological restrictions due to the normal tissue tolerance have been discussed. Our theoretical analysis is consistent with the clinical results published for I-125 permanent implants in prostate tumours and meningiomas. It predicts that Yb-169, which has only recently been used in human tumours, can provide comparable tumour control for permanent implants in slowly proliferating tumours with an initial dose rate of 13 cGy h-1. Control might be extended to rapidly proliferating tumours by increasing the initial dose rate within a range consistent with an acceptable level of normal tissue late reaction.

The Sievert integral revisited: evaluation and extension to 125I, 169Yb, and 192Ir brachytherapy sources

PURPOSE

The goal of this study is to assess the accuracy of the Sievert integral dose calculation model for medium and low-energy brachytherapy sources (photon energy: 25-500 keV). A simple modification of the basic model, the isotropic scattering correction, is proposed that significantly improves its accuracy.

METHODS AND MATERIALS

Both the classical model and revised Sievert algorithms were tested against 2D dose distributions derived from Monte Carlo photon transport (MCPT) calculations for the following sources: a 169Yb interstitial source, pulsed and high dose rate 192Ir sources and the model 6702 125I source. The Sievert model was implemented as a 3D numerical integral over the radioactivity distribution and included photon attenuation and scattering by the surrounding medium. The Sievert filtration coefficients were approximated by linear energy absorption coefficients, parameters of best fit, and curve fits to simulated open-air transmission measurements. The revised model consists of using the Sievert integral only to calculate the primary dose distribution using contact absorber filtration coefficients. The dose component due to photon scattering in the medium is assumed to be isotropically distributed and is modeled by point-source scatter-to-primary dose ratios.

RESULTS

The classical Sievert integral produces maximum and RMS average dose-calculation errors ranging from -53 to -20% and 3 to 19%, respectively. In contrast, the revised model reproduces the MCPT dose distribution with maximum and RMS mean errors ranging from 5 to 13% and 1 to 6%, respectively.

CONCLUSIONS

The classical Sievert model fails to accurately describe brachytherapy dose distributions around heavily filtered sources emitting photons with average energies of 28 to 400 keV. The revised Sievert model accurately models single-source dose distributions for a wide range of sources, using well-defined filtration coefficients and scatter ratios that can be measured or calculated without knowledge of the final dose distribution. The model is potentially useful as a single-source dose-array generator for clinical treatment planning in the low energy domain.

Implantation guidelines for 169 Yb seed interstitial treatments

The adequacy of an interstitial implant carried out with a new radioactive source, the 169 Yb seed model X1267, has been examined by computing volumetric indices based on dose-volume histograms. The comparison of these indices with the ones computed for 125I seed implantations shows that the use of ytterbium seeds presents an improvement of the dose homogeneity in interstitial implants. This is due to the significant build-up associated with 169 Yb photons that reduces the rapid dose fall-off with the distance from the source. Moreover, relative to 192Ir, the lower photon energy gives 169 Yb the advantage in clinical use of reduced radiation exposure (i) to health care workers, (ii) to relatives of treated patients and (iii) to healthy neighbouring tissues of the patients if appropriate thin shielding is used.

Dosimetry of 169 Yb seed model X1267

Unlike previous brachytherapy sources a number of published studies have been addressed to the dosimetry of 169 Yb seeds, manufactured in several prototypes, before widespread clinical use has been made. Discrepancies seen in the dosimetry obtained for ytterbium seed prototypes appear to be related to inconsistency and non-reproducibility in the vendor's calibration procedure to determine contained activity. Av. The comparison of 169 Yb seed dosimetries demonstrates a need for more accurate implementation of calibration procedures to determine the air kerma rate for the definitive 169 Yb seed design. This paper reports an experimental procedure to determine the reference air kerma rate, Kr (mu Gy h-1), defined as the kerma rate at 1 m along the source transverse axis in free space for the new 169 Yb seed, model X1267. A mean value of the ratio Kr/Av = 1.53 mu Gy h-1 mCi-1 was obtained from determining the Kr value of eleven seeds. Since this ratio is only 3% less than the air kerma rate constant for the 169 Yb point source, (gamma delta)k = 1.58 mu Gy h-1 m2 mCi-1, this means that the Av is closer to an apparent activity than a contained activity, Ac. A Monte Carlo simulation to determine the ratio between reference air kerma rate and the contained activity gave Kr/Ac = 1.33 mu Gy h-1 mCi-1. For the dose rate constant in water we obtained DKr (1, pi/2) = 1.20 +/- 0.05 cGy h-1 (mu Gy h-1)-1, using calibrated thermoluminescent dosimeters (TLDs) and DKr (1, pi/2) = 1.21 +/- 0.03 cGy h-1 (mu Gy h-1)-1 by Monte Carlo simulation. TLDs were used both to determine the radial dose distribution along the seed transverse axis and to calibrate GAFChromic films to obtain the two-dimensional dose distribution around the seed.

Dose distributions and dose rate constants for new ytterbium-169 brachytherapy seeds

Relative dose distributions in the vicinities of two new prototypes of ytterbium-169 brachytherapy sources have been measured using lithium fluoride thermoluminescent dosimeters (TLD) placed in a solid water phantom. The type 6 seed consists of four Yb2O3 spheres contained in a 0.075-mm thick titanium tube, with overall dimensions of 4.5 mm in length and 0.8 mm in diameter. The type 8 seed contains a cylindrical segment of Yb wire, 2.5 mm long, sealed in a 0.075-mm thick titanium tube 4 mm in length and 0.5 mm in diameter. The relative dose distributions, measured with TLD, were compared with those predicted by Monte Carlo simulations (MCNP code). Experimental and theoretical dose distributions were generally in agreement within 5% of the local dose value. Source strength was determined from activity measurements using a high purity germanium (HPGe) spectrometer. With source strengths established, thermoluminescence was then used to measure the dose rate constant, lambda 0, for the type 8 seed. The measured lambda 0 of 1.34 +/- 0.10 cGy U-1 for the type 8 seed agrees, within statistical uncertainty, with the value of 1.25 +/- 0.05 cGy U-1 predicted through Monte Carlo simulation. Comparisons are made with experimental data reported by other investigators.

Effect of tumour shrinkage on the biological effectiveness of permanent brachytherapy implants

A tumour shrinkage factor is incorporated into previously derived linear-quadratic (LQ) formulae which allowed radiobiological assessment of the efficacy of permanently implanted radionuclides. The new formulations relate the biologically effective dose (BED) to radionuclide half-life, recovery half-life, tumour radiosensitivity, potential doubling time and linear shrinkage rate. Specific attention has been given to the following radionuclides: gold-198 (half-life, 2.7 days), palladium-103 (half-life, 17 days), ytterbium-169 (half-life, 32 days) and iodine-125 (half-life, 60 days). For each nuclide the log cell kill resulting from typically prescribed doses was calculated for a range of tumour clonogen doubling times at various radiosensitivities and linear shrinkage rates. It is shown that even relatively modest shrinkage rates are capable of enhancing the clinical potential of the longer-lived nuclides. However, even though the effect of tumour shrinkage is minimal in the case of gold-198, for fast growing and/or insensitive tumours there are fewer radiobiological uncertainties associated with the use of this nuclide. The revised equations may also have applications in certain types of biologically targeted radiotherapy.

Dosimetric characteristics, air-kerma strength calibration and verification of Monte Carlo simulation for a new Ytterbium-169 brachytherapy source

PURPOSE

Ytterbium-169 (169Yb) is a promising new isotope for brachytherapy with a half life of 32 days and an average photon energy of 93 KeV. It has an Ir-192-equivalent dose distribution in water but a much smaller half-value layer in lead (0.2 mm), affording improved radiation protection and customized shielding of dose-limiting anatomic structures. The goals of this study are to: (a) experimentally validate Monte Carlo photon transport dose-rate calculations for this energy range, (b) to develop a secondary air-kerma strength standard for 169Yb, and (c) to present essential treatment planning data including the transverse-axis dose-rate distribution and dose correction factors for a number of local shielding materials.

METHODS AND MATERIALS

Several interstitial 169Yb sources (type 6) and an experimental high dose-rate source were made available for this study. Monte-Carlo photon-transport (MCPT) simulations, based upon validated geometric models of source structure, were used to calculate dose rates in water. To verify MCPT predictions, the transverse-axis dose distribution in homogeneous water medium was measured using a silicon-diode detector. For use in designing shielded applicators, heterogeneity correction factors (HCF) arising from small cylindrical heterogeneities of lead, aluminum, titanium, steel and air were measured in a water medium. Finally, to provide a sound experimental basis for comparing experimental and theoretical dose-rate distributions, the air-kerma strength of the sources was measured using a calibrated ion chamber. To eliminate the influence of measurement artifacts on the comparison of theory and measurement, simulated detector readings were compared directly to measured diode readings. The final data are presented in the format endorsed by the Interstitial Collaborative Working Group.

RESULTS

The in-air calibration revealed that the air-kerma strength per unit activity (mCi), as quoted by the vendor, varied from 1.30 to 1.57 cGy.cm2/mCi.h depending on seed design. The maximum difference between measured and MCPT-simulated absolute diode readings on the transverse axis was less than 2%, indicating that MCPT accurately predicts dose rate in medium for brachytherapy sources in this energy range. Comparison of measured and simulated HCFs for each of the 16 different cylindrical heterogeneities demonstrated 1-3% agreement. The HCFs vary by as much as 200% with respect to distance and by as much as 48% as a function of disk diameter, showing that HCF is strongly dependent on heterogeneity location and lateral dimensions as well as thickness. The dose-rate constant for water medium was found to be 1.225 cGy/h per kerma unit air-strength and 1.962 cGy/h per unit mCi as measured by the vendor.

CONCLUSION

Monte Carlo simulation is an accurate and powerful tool for dosimetric characterization of brachytherapy sources in this energy range. Thin lead foils produce shielding factors comparable to standard shielded applicators for 137Cs. Meaningful theoretical absolute dose calculations in brachytherapy require accurately implemented air-kerma strength standards.

The relative biological effectiveness of ytterbium-169 for low dose rate irradiation of cultured mammalian cells

PURPOSE

An important step in the development of 169Yb as a new brachytherapy source is to determine its biological effectiveness relative to other commonly used radioisotopes. The purpose of this paper is to determine the relative biological effectiveness of 169Yb, with respect to 60Co, for a range of low dose rates.

METHOD AND MATERIALS

The relative biological effectiveness of photon radiation from encapsulated 169Yb was determined by exposing Chinese hamster ovary cells, in exponential growth, to graded doses of radiation from either 169Yb or 60Co. Clonogenic cell survival was determined for continuous low dose rates ranging from 6.5 cGy/hr to 52 cGy/hr.

RESULTS

The relative biological effectiveness of 169Yb, with respect to 60Co, was determined to be 1.2 +/- 0.3 and did not vary significantly over the dose-rate range from 13 cGy/hr to 50 cGy/hr. An inverse dose-rate effect was observed, but only for 60Co irradiation at 8.9 cGy/hr. Therefore, relative biological effectiveness values could not be determined reliably for dose rates less than 13 cGy/hr.

CONCLUSIONS

We have established that 169Yb is approximately 20% more effective than 60Co in vitro. It is hoped that this study will guide the introduction of 169Yb into clinical brachytherapy practice.

Prostate brachytherapy. An overview

BACKGROUND

Prostate brachytherapy represents one of the oldest techniques of using radiation therapy to treat prostate cancer. Over the past 10 years, there have been major changes in the types of prostate brachytherapy that can be performed with the introduction of new radioactive isotopes, new afterloading techniques, and an improved understanding of the radiobiology associated with differing dose rates.

METHODS

Prostate brachytherapy can be divided into temporary implantation using high activity sources such as iridium-192, or permanent brachytherapy using the interstitial implantation of iodine-125 or palladium-103 sources. There are various techniques that can be used to actually insert the radioactive material into the prostate. This can be done as an open or closed procedure and can be performed via a suprapubic or a perineal retropubic approach. The use of remote afterloading has substantially reduced the radiation protection problems associated with manually loaded radioactive sources.

RESULTS

Results using brachytherapeutic isotopes in the treatment of prostate cancer have been variable, but it appears that using the higher dose rate sources and delivering a high relative integral dose to the prostate can result in improved histologic control of prostate cancer. In patients with aggressive prostate cancer, the use of iodine-125 permanent implantation has not been successful.

CONCLUSIONS

The role of brachytherapy in the treatment of prostate cancer remains an exciting alternative in the management of prostate cancer. Its role is becoming more defined in the treatment of large, bulky prostatic neoplasms as a way of improving the dose distribution achieved between normal and tumor tissue.

[The effect of ethylenediaminetetramethylenephosphonate on the biodistribution of tumor-seeking radionuclides]

It is well known that after application of radioactive complexes for tumour diagnosis or therapy, such as 67Ga-citrate or radiolanthanide complexes (167Tm- or 169Yb-nitrilotriacetate, -citrate, -alpha-hydroxyisobutyrate, 90Y-citrate, etc.) activity is accumulated not only in the tumour but also in other organs, above all liver and bone. This is the main obstacle to their medical use. Recently published results encouraged us to use ethylenediaminetetramethylene phosphonate (EDTMP) for the reduction of extratumoural liver activity. The results show that even small amounts of EDTMP (1-2 mg/kg BW) reduce the activity deposition in the liver by about one order of magnitude. EDTMP provoked elimination of activity from tumour, skeleton and other tissues but not to the same extent as from the liver. Tumor/liver activity ratios > 5 are achievable in this manner.

[The effect of bone-seeking metal salts on the biodistribution of tumor-seeking heavy metal complexes]

This work attempted to overcome the problem of unwanted bone radioactivity after injection of tumour-affine heavy-metal compounds (prototype 169Yb-citrate) by pre-application of stable yttrium- and calcium-compounds into tumour-bearing mice in doses of 1 mg metal/kg body weight. The pre-application of stable yttrium and calcium resulted in a smaller bone radioactivity. The most favourable results were achieved by injecting the metal salts simultaneously at or within 5 h before the 169Yb-citrate. On the other hand a strong radioactivity increase in the RES (liver and spleen) by a factor of 2 to 4 was observed after yttrium-preapplication.

Ytterbium-169: calculated physical properties of a new radiation source for brachytherapy

Seeds containing radioactive Ytterbium-169 (169Yb) have recently been manufactured for possible application to brachytherapy. Ytterbium-169 emits photons with an average energy of 93 keV (excluding energies less than 10 keV), and decays with a half-life of 32 days. Analytic and Monte Carlo computations have been used to predict physical quantities useful in treatment planning and radiation protection. Analytic calculations based on the primary photon spectrum of 169Yb (excluding energies less than 10 keV) yield an air-kerma rate constant of 0.0427 cGy cm2 h-1 MBq-1, and an exposure rate constant of 1.80 R cm2 mCi-1 h-1 for this radionuclide. Calculated fmed factors are 0.922 cGy/R for soft tissue and 2.12 cGy/R for bone. The first half-value layer in lead is 0.2 mm; the first tenth-value layer is 1.6 mm. Using Monte Carlo simulations, the relative dose distributions around 169Yb seeds (Amersham, prototypes 4 and 5) are provided, and are then compared with those around an 125I seed (3M model 6702). The 169Yb seeds produce more isotropic dose distributions, and for permanent implants, can deliver it at a greater initial dose rate. A value of 1.19 cm-2 was also calculated for the specific dose constant D0, a value which is applicable to both seed types. Radiation protection is not as easily achieved for permanent implants with 169Yb because of the higher energy emissions (vs 125I). However, for temporary implants, Ytterbium-169 may prove to be a useful substitute for 192Ir or 137Cs because of its relatively lower energy emissions. It is concluded that 169Yb merits further investigation, including dosimetry, radiobiological, and clinical studies.

The potential of ytterbium 169 in brachytherapy: a brief physical and radiobiological assessment

Ytterbium 169 (half-life 32 days; mean gamma emission 93 keV, after excluding photons of energy less than 10 keV) is a radionuclide with interesting potential for brachytherapy applications. Although not yet commercially available, its possible application as a clinical radionuclide is currently being considered by Amersham International. This article presents an assessment of some properties of the nuclide that may be clinically relevant. Use is made of some new ideas that allow quantification of the likely dose homogeneity that can be obtained in a brachytherapy distribution, and in this context ytterbium 169 is shown to be superior to some currently available brachytherapy nuclides. The assessment also uses recent extensions to the linear-quadratic model to consider the likely radiobiological implications associated with the use of the nuclide. From this it is suggested that the main potential for ytterbium 169 would be as a source that may be re-used for a number of short-term applications, rather than as a permanently implantable nuclide.

Microdosimetric single event spectra of Ytterbium-169 compared with commonly used brachytherapy sources and teletherapy beams

Ytterbium-169 has been developed as a possible replacement for Iridium-192 and Iodine-125. The Theory of Dual Radiation Action predicts that the initial slope of the cell survival curve and therefore the relative biological effect at low dose rate is proportional to dose average lineal energy, yd, which is the microscopic analog of the dose average linear energy transferred. The quality factor used in radiation protection has been shown to be a function of the frequency average lineal energy, yf. Single event microdosimetric spectra for 60Co, 137Cs, 192Ir, 125I and 169Yb were measured in air and at several depths in phantom with a Rossi proportional counter. These spectra show marked differences between sources. The microscopic analogs of the track average and dose average LET, (yd and yf, respectively) differ between isotopes by factors of two or even higher in comparison to megavoltage electron beams. These yd's and yf's for 169Yb are consistently higher when compared to 60Co or 137Cs but are approximately equal to those for 125I. Values of yf and yd for 192Ir are intermediate between 60Co and 169Yb. The Theory of Dual Radiation Action predicts a low dose rate RBE (assuming a 1 micron effective site diameter) compared to 60Co (in air) of: 1.00 for 137Cs, 1.29 for 192Ir, 1.60 for 169Yb and 1.77 for 125I.

Anti-inflammatory effects of praseodymium, gadolinium and ytterbium chlorides

Anti-inflammatory effects of chloride salts of praseodymium, gadolinium and ytterbium were investigated, using various experimental inflammatory models in rats. The lanthanide salts administered by oral route showed no significant effect, but when injected intraperitoneally they significantly inhibited the carrageenin-induced oedema, proportional to their doses ranging from 15 to 75 mg/kg. They also reduced nystatin-induced oedema and vascular permeability response to histamine and serotonin. Pronounced inhibitory effect of lanthanide salts at the dose of 50 mg/kg, i.p., was observed in histamine- and serotonin-induced changes in vascular permeability. Repeated administration of lanthanide salts in the dose of 20 mg/kg for 13 d significantly inhibited arthritis development. The same dose of these salts for a 6-d period similarly reduced granuloma formation. However, praseodymium, gadolinium and ytterbium chlorides showed no significant difference among themselves and their anti-inflammatory effects were smaller than those from phenylbutazone.