Linear energy transfer characterization of five gel dosimeter formulations for electron and proton therapeutic beams

Gel dosimeters, including radiochromic types like Fricke, as well as polymer formulations, are considered to be the only reliable option for accurate 3D dosimetry. Nevertheless, their implementation in daily clinical quality assurance still remains strongly limited for a few high specialized radiotherapy centres. Although gel dosimeters present very good water-equivalence due to their inherent chemical and isotopic compositions, addressing the corresponding dosimetry outputs is highly challenging, needing careful assessment in terms of the different radiation qualities involved in the mixed field. Accurate estimations of the linear energy transfer for each gel dosimeter formulation stands as a baseline for further accurate dose deconvolution in mixed radiation fields. The present study reports on the linear energy transfer characterization of five different gel dosimeter formulations, Fricke, Itabis, Magic, Nipam, and Pagat, for electron and proton therapeutic beams as obtained by Monte Carlo approaches, along with experimental results for validation purposes. The linear energy transfer, as a function of beam quality and penetration depth, is obtained for electron and proton therapeutic beams remarking the presence of non-negligible variations, which need to be accounted for a further accurate implementation of gel dosimetry as well as for precise dose deconvolution in mixed radiation fields.

Dose-response of Fricke- and PAGAT-dosimetry gels in kilovoltage and megavoltage photon beams: Impact of LET on sensitivity

PURPOSE

Dosimetry of ionizing radiation quantifies the energy deposited by an incident beam to the medium. This study presents the relative response of two types of gel dosimeters describing their differences by estimating radiation chemical yields produced in water radiolysis.

METHODS

Two types of gel dosimeter were used, namely an acid ferrous ion solution infused with xylenol orange known as Fricke gel and a polymer gel based on acrylamide and N,N'-methylenebis(acrylamide) known as PAGAT. Samples were irradiated using two photon beam energies, one from a conventional X-ray tube operated at 44 kV and the other one from a LINAC operated at 6 MV. The dosimeters were analyzed by optical absorbance and magnetic resonance imaging. Additionally, the linear energy transfer of each beam was calculated using Monte Carlo simulations for further estimation of the radiation chemical yields produced during water radiolysis.

RESULTS

Obtained results for both gel dosimeters indicate that their response at 44 kV and 6 MV are different, regardless of the read-out technique. On average, the sensitivity at 44 kV was found to be 65 % of the response at 6 MV. The calculated radiation chemical yields are in agreement with the observed experimental results.

CONCLUSIONS

The main reason for the difference in the response of the dosimeters may be related to the linear energy transfer of each photon beam, which varies the production of primary chemical species during water radiolysis.

Raman spectroscopy as a tool to evaluate oxygen effects on the response of polymer gel dosimetry

Currently, advanced dosimeters like polymer gels are capable of obtaining reliable and accurate 3D dose distributions from correlations with the different polymerization degrees induced by incident radiation. Samples of polymer gel dosimeters are commonly read out using magnetic resonance imaging or optical methods like visible light transmission or laser computed tomography. Alternatively, this work proposes and evaluates the implementation of Raman spectroscopy to provide direct information on the effect of oxygen permeating through the walls of phantoms on the polymerization initiated by irradiation in three types of polymer gel dosimeters, namely NIPAM, ITABIS and PAGAT. The aim of the present study is to provide better and complete interpretations using three different containers, adequate for integral, 2D and 3D dose mapping. Moreover, Raman spectroscopy has been used to analyze the well-known effect of oxygen inhibition on the different polymer gel dosimeters remarking the importance of avoiding air exposition during sample storage and readout. Dose-response curves for different polymer gels were obtained in terms of measurements with a calibrated ionization chamber. Additionally, dedicated Monte Carlo simulations were performed aimed at characterizing dose for different X-ray irradiation setups, providing also suitable information to evaluate oxygen diffusion through the sample wall. The obtained results were contrasted with optical transmission readout as well as Monte Carlo simulations attaining very good agreements for all dosimeter types.

Water-equivalence of gel dosimeters for radiology medical imaging

International dosimetry protocols are based on determinations of absorbed dose to water. Ideally, the phantom material should be water equivalent; that is, it should have the same absorption and scatter properties as water. This study presents theoretical, experimental and Monte Carlo modeling of water-equivalence of Fricke and polymer (NIPAM, PAGAT and itaconic acid ITABIS) gel dosimeters. Mass and electronic densities along with effective atomic number were calculated by means of theoretical approaches. Samples were scanned by standard computed tomography. Photon mass attenuation coefficients and electron stopping powers were examined. Theoretical, Monte Carlo and experimental results confirmed good water-equivalence for all gel dosimeters. Overall variations with respect to water in the low energy radiology range (up to 130 kVp) were found to be less than 3% in average.

Polymer gel dosimeter based on itaconic acid

A new polymeric dosimeter based on itaconic acid and N, N'-methylenebisacrylamide was studied. The preparation method, compositions of monomer and crosslinking agent and the presence of oxygen in the dosimetric system were analyzed. The resulting materials were irradiated with an X-ray tube at 158cGy/min, 226cGymin and 298cGy/min with doses up to 1000Gy. The dosimeters presented a linear response in the dose range 75-1000Gy, sensitivities of 0.037 1/Gyat 298cGy/min and an increase in the sensitivity with lower dose rates. One of the most relevant outcomes in this study was obtaining different monomer to crosslinker inclusion in the formed gel for the dosimeters where oxygen was purged during the preparation method. This effect has not been reported in other typical dosimeters and could be attributed to the large differences in the reactivity among these species.

A procedure for the efficient selection of piezoelectric ceramics constituting high-power ultrasonic transducers

The most characteristic narrow-band transducer structure for high-power ultrasonic applications is the well known piezoelectric sandwich which is reminiscent of the Langevin transducer. Such structure is generally used jointly with other components in the construction of industrial high-power transducers. One of the main objectives in the design and construction of such high-power transducers is to minimize energy losses. To that purpose the selection of the piezoelectric ceramic rings forming the sandwich requires clear and specific criteria. This paper deals with a numerical and experimental procedure for the accurate selection of the piezoelectric rings constituting high-power transducers, based on the analysis of the mechanical Q, the frequency and the resonance curve. The procedure was experimentally checked by constructing and characterizing several transducer structures.

Detection of respiratory syncytial virus in nasopharyngeal secretions by 24-well plate precentrifugation assay using a monoclonal antibody against F protein

BACKGROUND

Respiratory syncytial virus (RSV) is responsible for 50% of all bronchiolitis and 25% of pneumonia cases during the first month of life. Detection of the RSV antigen by immunofluorescence in exfoliated nasal epithelium or by other methods in nasopharyngeal swabs is useful in the potentially infected patient because results are available within a few hours. In contrast, RSV antigen detection in cell culture may require as much as 3 weeks.

METHODS

Three methods for detection of respiratory syncytial virus in 131 clinical respiratory specimens from patients with acute respiratory disease and bronchiolitis were compared utilizing the following: a precentrifugation immunofluorescence assay using Hep-2 cells, indirect immunofluorescence assay, and conventional tube cell culture using Hep-2 cells.

RESULTS

Respiratory syncytial virus was identified in 36 specimens by the three methods previously described. The virus was recovered in 41 (31.3%) samples by precentrifugation immunofluorescence assay, 40 (30.5%) were identified by the immunofluorescence technique, and 38 (29.0%) cases were positive by conventional cell culture. The sensitivity of the precentrifugation assay in relation to the immunofluorescence technique was 90%, the specificity 94.5%, and the agreement, 96.2%. A positive predictive value of 90.2% was obtained. Sensitivity, specificity, agreement, and positive predictive values obtained by the precentrifugation assay variant compared to the conventional cell were 90.8%, 94.5%, 93.1%, and 87.8%, respectively.

CONCLUSIONS

The precentrifugation immunofluorescence assay method was as sensitive as the remainder of the methods used in our study and represents a valid alternative for rapid detection of respiratory syncytial virus in clinical samples.

[Fast detection and characterization of influenza A and B viruses in nasopharyngeal secretions by the immunoperoxidase method]

One hundred and fourty eight samples from patients with a symptomatology compatible with the influenza virus were studied aimed at identifying in a fast way these viruses. A rapid MDCK-L cell culture was developed on 96 well plates, where nasopharingeal exudates or gargarisms were inoculated and incubated all night long at 37 degrees C. The medium was removed and cells were washed with PBS and fixed with methanol. Viral antigens were detected through the immunoperoxidase staining by using two monoclonal antibody pools for the identification of influenza A and influenza B viruses. The HA1-71 monoclonal antibody, specific for influenza A (H3N2) and the HA2-76, which react with both A (H3N2) and A (H1N1) were used for subtyping. Of all the positive samples (136), 72% corresponded to type A while 34.6% and 37.5% corresponded to subtypes H1 and H3, respectively. Influenza B was detected in 27.9% of the 148 samples studied. Only 12 were negative (8.1%). The use of this technique is recommended as a rapid, convenient and sensitive method that is easy to carry put and to interpretate for the detection and characterization in type and subtype of the influenza viruses starting from the nasopharyngeal exudates or gargarisms.

[Subgroups classification of strains of the respiratory syncytial virus isolated in an outbreak in Ciudad de La Havana]

A high number of acute respiratory diseases was detected among children under one year admitted in a hospital of Havana City. 25 respiratory syncytial virus strains were obtained from 93 patients studied. Viral isolations were multiplied in HEP-2 cells and after observing a cytopathic effect of 80%, they were classified into subgroups by the indirect immunofluorescence technique, using anti-protein G antibodies from the respiratory syncytial virus. All the samples studied were classified within subgroup A. It is the first time a study like this is conducted in our country, which allowed us to deepen into the viral cause of these diseases and to know that the subgroup A of the respiratory syncytial virus circulated during the outbreak.

[Evaluation of a mouse anti-IgG-fluorescein conjugate using indirect immunofluorescence and flow cytometry techniques]

An immunoglobulin G of mouse was purified from sera by affinity chromatography in protein A. The rabbits whose sera were able to recognize the antigen injected by double immunodiffusion were immunized with this preparation. The antibodies were precipitated from the rabbit's serum and purified by ion exchange chromatography. This preparation was conjugated to fluorescin isothiocyanate according to the conventional technique. The conjugated obtained was evaluated with the reference strains of Parainfluenza virus 1, 2, 3; Adenovirus; respiratory syncytial virus; and influenza virus A and B, by an indirect immunofluorescence technique and HIV positive samples by flow citometry. Specific monoclonal antibodies were used in both cases. Clinical specimens of patients with acute respiratory infection were evaluated.

[Application of the polymerase chain reaction for detecting respiratory syncytial virus]

The polymerase chain reaction (PCR) was developed in order to identify the respiratory syncytial virus by using the reference strain. The high sensitivity and specificity obtained show the PCR utility for detecting the RSV genoma and its application on the diagnosis.

Molecular characterization of an outbreak of respiratory syncytial virus (subgroup A) in Havana, Cuba, by monoclonal antibodies and restriction mapping (N gene)

Twenty-one respiratory syncytial virus (RSV) strains isolated from one outbreak in Havana, Cuba (1994 to 1995), were analyzed to determine their relatedness. All isolated strains were classified as subgroup A by monoclonal antibodies. Of 21 RSV strains examined, 20 were classified as having restriction pattern NP4 and only 1 was classified as having restriction pattern NP5.

Analysis of respiratory syncytial virus in clinical samples by reverse transcriptase-polymerase chain reaction restriction mapping

The aim of this study was to develop a polymerase chain reaction (PCR) for the detection of respiratory syncytial virus (RSV) genomes. The primers were designed from published sequences and selected from conserved regions of the genome encoding for the N protein of subgroups A and B of RSV. PCR was applied to 20 specimens from children admitted to the respiratory ward of "William Soler" Pediatric Hospital in Havana City with a clinical diagnosis of bronchiolitis. The PCR was compared with viral isolation and with an indirect immunofluorescence technique that employs monoclonal antibodies of subgroups A and B. Of 20 nasopharyngeal exudates, 10 were found positive by the three assayed methods. In only two cases, samples that yielded positive RNA-PCR were found negative by indirect immunofluorescence and cell culture. Considering viral isolation as the "gold standard" technique, RNA-PCR had 100% sensitivity and 80% specificity. RNA-PCR is a specific and sensitive technique for the detection of the RSV genome. Technical advantages are discussed.

Reproduction and conservation of the leatherback turtle Dermochelys coriacea (Testudines: Dermochelyidae) in Gandoca, Costa Rica

The leatherback turtle was studied in Gandoca, an important nesting beach on the southeastern Caribbean coast of Costa Rica (82 degrees 37' W; 09 degrees 37' N). In 1994, a total of 530 nests was recorded during the nesting season (February/July) and 160 leatherbacks were tagged; five were remigrants from the 1992 season and 15 carried tags from elsewhere. Eighty eight females only nested once. Mean curve carapace measurements were length 153.8 cm and width 112.0 cm. A hatchery received 82 clutches, with 6277 normal eggs. Their mean incubation period was 62.24 days (range: 56-68 days). Average hatching rate was 55.10% (S.D.: 25.04, range 15-96%). Extensive erosion, beach debris and poaching activity represent the main hazards for nesting in Gandoca.