Nominal Dose Rate Research Articles

Dosimetric Study of Total Marrow and Lymphoid Irradiation on a Ring Gantry-Based Medical Linac with a Two-Layer Multi-Leaf Collimator

In this study, we aimed to evaluate dosimetric quality of total marrow and lymphoid irradiation (TMLI) plans for a ring gantry-based medical Linac with a two-layer multi-leaf collimator. We retrospectively retrieved treatment planning CT images, structure sets, and plan dose for four adult patients, two male and two female, who previously received TMLI treatments on helical tomotherapy (HT) at our institution. TMLI plans were optimized for a ring gantry-based medical Linac with a two-layer multi-leaf collimator (Halcyon, Varian Medical Systems, Inc., Palo Alto, CA). A prescription dose of 12 Gy in 8 fractions was prescribed to the skeletal bones from the skull to mid-thigh, spleen, spinal canal, and lymphoid volume. Five or six isocenters were placed with equal spacing along the patient's longitudinal direction in each TMLI plan with two 6-MV flattening filter-free volumetric modulated arc therapy (VMAT) fields at each isocenter. Isocenter separation ranged from 15 cm to 16.5 cm. Each VMAT field has a field size of 28 cm to 28 cm with the collimator at 90° and a full gantry rotation. The nominal dose rate was 800 MU/minute, and the maximum gantry rotation speed was 24°/sec. Institutional dosimetric constraints were used for optimization including a mean lung dose limit of less than 8 Gy. All the plans were normalized so that 85% the primary planning target volume received the prescription dose. The average mean doses to the target volumes ranged from 12.2 to 12.6 Gy in the Halcyon TMLI plans, while they ranged from 12.1 to 12.5 Gy in the HT TMLI plans. Relative to the prescription dose, the average mean dose for normal organs ranged from 21.3% to 56.6% in the Halcyon TMLI plans, while it ranged from 10.1% to 68.4% in the clinical HT plans. The difference in the average mean dose to normal organs was less than 0.5 Gy except two organs between the Halcyon and HT TMLI plans. The average median dose for normal organs ranged from 18.2% to 48.8% relative to the prescription dose in the Halcyon TMLI plans. The mean lung dose (MLD) in the Halcyon TMLI plans met the institutional limit with an average dose of 6.75±0.42 Gy (range: 6.44 - 7.36 Gy), while the average MLD was 6.54±0.77 Gy (range: 6.24 - 7.22 Gy) in the HT plans (p-value = 0.71 in the paired t-test). The average total monitor unit in the Halcyon TMLI plans was 4,425±906 MU (range: 3,470 - 5,575 MU) with an average beam-on time of 5.1±1.3 minutes (range: 4.1 - 7.0 minutes), which excludes isocenter setup time, while the average beam-on time was 22.2±3.2 minutes (range: 19.6 - 26.1 minutes) with the HT plans. Halcyon TMLI plans met our institutional dosimetric constraints with adequate normal organ sparing and target dose coverage. The beam-on time with the Halcyon plans was significantly shorter than that with the HT plans, which could lead to shorter treatment time and increased patient comfort. This study showed the feasibility of TMLI treatments on the Halcyon machine. The same method could be used for total body irradiation on Halcyon.

Calibration of the comet assay using ionising radiation

Several trials have attempted to identify sources of inter-laboratory variability in comet assay results, aiming at achieving more equal responses. Ionising radiation induces a defined level of DNA single-strand breaks (per dose/base pairs) and is used as a reference when comparing comet results but relies on accurately determined radiation doses. In this ring test we studied the significance of dose calibrations and comet assay protocol differences, with the object of identifying causes of variability and how to deal with them. Eight participating laboratories, using either x-ray or gamma radiation units, measured dose rates using alanine pellet dosimeters that were subsequently sent to a specialised laboratory for analysis. We found substantial deviations between calibrated and nominal (uncalibrated) dose rates, with up to 46% difference comparing highest and lowest values. Three additional dosimetry systems were employed in some laboratories: thermoluminescence detectors and two aqueous chemical dosimeters. Fricke’s and Benzoic Acid dosimetry solutions gave reliable quantitative dose estimations using local equipment. Mononuclear cells from fresh human blood or mammalian cell lines were irradiated locally with calibrated (alanine) radiation doses and analysed for DNA damage using a standardised comet assay protocol and a lab-specific protocol. The dose response of eight laboratories, calculated against calibrated radiation doses, was linear with slope variance CV= 29% with the lab-specific protocol, reduced to CV= 16% with the standard protocol. Variation between laboratories indicate post-irradiation repair differences. Intra-laboratory variation was very low judging from the dose response of 8 donors (CV=4%). Electrophoresis conditions were different in the lab-specific protocols explaining some dose response variations which were reduced by systematic corrections for electrophoresis conditions. The study shows that comet assay data obtained in different laboratories can be compared quantitatively using calibrated radiation doses and that systematic corrections for electrophoresis conditions are useful.

Optimization of proton therapy eye-treatment systems toward improved clinical performances

The treatment protocols of cancerous ocular diseases with proton therapy are well established, and dedicated eye-treatment systems can produce the clinical beam properties that meet the peculiar features required by eye-treatment modalities. However, for general-purpose multiroom systems comprising eye-treatment beamlines and nozzles, the design and commissioning procedures must be optimized to achieve the performances of fully dedicated systems in terms of depth-dose distal falloff, lateral penumbra, and dose rate. This paper presents a realistic start-to-end beam transport and particle-matter interactions model of the ion beam applications Proteus\textregistered{} Plus $(\mathrm{P}+)$ single-scattering eye-treatment room with Beam Delivery SIMulation (bdsim) using Geant4. The model is used to establish optimization patterns in terms of beam optics to achieve a smaller depth-dose distal falloff than the design baseline while maintaining a nominal dose rate and lateral flatness of the dose deposition profile. An alternative design is proposed to increase the dose rate further by up to a factor 3, allowing for delivering a complete hypofractionated treatment session under 60 s. It uses a beam-stopping device to complement the existing scattering features of the nozzle. An in-depth study of the system is performed using bdsim and the numerical simulations are discussed in detail.

Radiation Dose Rate, Biologically Effective Dose, and Tumor Characteristics on Local Control and Toxicity After Radiosurgery for Acoustic Neuromas

To evaluate the relationships between calibration dose rate, calculated biologically effective dose (BED), and clinical factors and tumor control after stereotactic radiosurgery (SRS) for acoustic neuroma. We performed a retrospective study of all patients with acoustic neuromas treated with frame-based cobalt-60 SRS at a single institution between 2005 and 2019. The calibration dose rate and cobalt-60 half-life were used to calculate the nominal dose rate during treatment. An SRS-specific monoexponential model accounting for treatment time per lesion was used to estimate BED. The study included 607 patients with 612 acoustic neuromas. Median follow-up was 5.0 years. There was no association between dose rate or BED with local failure (LF), radiologic edema, or symptomatic edema. Cystic tumors (adjusted hazard ratio 0.26, P= 0.028) were associated with lower LF, while use of SRS as salvage treatment for growing tumors (adjusted hazard ratio 4.9, P < 0.0001) was associated with higher LF. LF occurred more frequently in larger-diameter tumors, while radiologic or symptomatic edema occurred more frequently in larger-volume tumors. Radiosurgery dose rate and BED were not associated with tumor control or radiologic or symptomatic edema. Salvage SRS and larger tumors were associated with a higher LF rate, while cystic tumors were associated with a lower LF rate. Patients with larger tumors should be counseled appropriately about potential side effects and when to seek follow-up care.

Assessing relative biomarker responses in marine and freshwater bivalve molluscs following exposure to phosphorus 32 (32P): Application of genotoxicological and molecular biomarkers

Anthropogenic radionuclides can enter water bodies through accidental or controlled discharges. In order to assess their potential impact, understanding the link between exposure, tissue specific bioaccumulation and radiation dose rate, to biological or biomarker responses in aquatic biota is required. Adopting an integrated, multi-biomarker, multi-species approach, we have investigated potential biological responses induced by short-lived radionuclide, phosphorus-32 (32P, radiophosphorus) in two ecologically important mussel species, the freshwater Dreissena polymorpha (DP) and marine Mytilus galloprovincialis (MG). Adult individuals were exposed to 32P for 10 days, to acquire nominal whole-body average dose rates of 0.10, 1 and 10 mGy d−1, which encompass a screening value of 10 μGy h−1 (0.24 mGy d−1), in accordance with the ERICA tool. Following exposure, a suite of genotoxic biomarkers (DNA damage, γ-H2AX induction and micronucleus [MN] formation) were measured in gill and digestive gland tissues, along with transcriptional expression of selected stress-related genes in both the species (i.e. hsp70/90, sod, cat and gst). Our results demonstrate the relationship between tissue specific dosimetry, where 32P induced a dose-dependent increase, and biological responses independent of species. Gene expression analysis revealed little significant variation across species or tissues. Overall, MG appeared to be more sensitive to short-term damage (i.e. high DNA damage and γ-H2AX induction), particularly in digestive gland. This study contributes to limited knowledge on the transfer and biological impact of radionuclides within differing aquatic systems on a tissue specific level, aiding the development of adequate management and protective strategies.

P264] Time resolved dose rate distributions in radiosurgery and their impact on biological effectiveness of treatment

Purpose To obtain time-resolved dose rate distributions (TR-DRD) in radiosurgery applications, and evaluate the impact of TR-DRD on the biological effectiveness of treatment. Methods Treatment plans of 74 acoustic schwannoma patients treated with Leksell Gamma Knife® C or PERFEXION™ were meta-analyzed. The 3D dose distribution within the Planning Target Volume (PTV) for each patient was obtained from the superposition of relevant data for each delivered GK-shot. Single shot dosimetry calculations were based on the nominal dose rate at the center of each shot and corresponding relative dose profile data. TR-DRD were estimated neglecting 60Co source decay during the irradiation of each shot. A formalism including frequently used radiobiological tools that accounts for the effect of dose rate and incorporates fast and slow components of repair, was developed on the basis of the Biologically Effective Dose (BED). Patient and plan characteristics as well as dose and dose rate indexes incorporating the TR-DRD were introduced and their correlation with the corresponding radiobiological parameters was examined using the Spearman rank test. Results The nominal mean dose rate at focus (NMDR-F) presented a high correlation with the percentage of the total dose delivered with dose rates lower than 1 Gy/min, TD 1 Gy / min , ( r = - 0.837 and p = 5.32 E - 20 ) and the percentage of the PTV volume receiving dose rates lower than 1 Gy/min from all shots, V 1 Gy / min ( TR-DRD ) ( r = - 0.830 and p = 6.43 E - 20 ). A statistically significant correlation was also found between the number of iso-centers used per PTV volume (Unit Isocenter, UI) and the V 1 Gy / min ( TR-DRD ) ( r = 0.268 and p = 0.021 ) as well as the percentage of the PTV volume receiving dose values lower than 15 Gy, V 15 Gy ( r = 0.543 and p = 5.90 E - 07 ). The Equivalent Uniform BED (EUBED) was significantly correlated with the UI ( r = - 0.4318 and p = 0.0001 ), the UI/NMDR-F ( r = - 0.3955 and p = 0.0005 ), the V 1 Gy / min ( TR-DRD ) ( r = - 0.3155 and p = 0.0062 ) and the V 15 Gy ( r = - 0.569 and p = 1.25 E - 07 ). Conclusion The observed correlation between the TR-DRD based parameters and the EUBED suggests that the former should be taken into account at the planning stage of radiosurgery, either directly or indirectly via plan characteristics readily evaluated in every day clinical routine (NMDR-F, UI) that are correlated with TR-DRD based dose and dose rate parameters.

Dosimetric characteristics of a reusable 3D radiochromic dosimetry material.

PurposeTo investigate the dosimetric characteristics of PRESAGEREU dosimeters.MethodsCommercially available PRESAGEREU dosimeters (size of 10 mm × 10 mm × 45 mm) were divided into two groups, with one of the groups placed at room temperature of 22°C (RT group) and another group placed at low temperature of 10°C (LT group). A total of 3 dosimeters (set of dosimeters) were irradiated at a time, with doses of 1 Gy, 2 Gy, 4 Gy, 8 Gy, 12 Gy, 16 Gy, and 20 Gy, at a nominal dose rate of 400 MU/min at temperature of 22°C. The dosimeters were irradiated three additional times by delivering the same doses as those during the initial irradiations (4 irradiation cycles). Optical density (OD) was assessed using optical CT scanning.ResultsConsidering both linearity and sensitivity of the OD curves, R2 above 0.95 and sensitivity above 0.04 ΔOD/Gy were observed at the 1st irradiation (reading time ≤ 6 h) and 2nd irradiation (reading time = 0.5 h) for the RT group. For the LT group, those values were observed at the 1st irradiation (reading time ≤ 2 h), and the 3rd and 4th irradiations (both reading times = 0.5 h). Considering the reproducibility of signals in response to the same dose, dosimeters in the RT group showed average deviations among dosimeters less than 5% (the 1st and 2nd irradiations at the reading time of 0.5 h), while for dosimeters in the LT group showed average deviations among dosimeters less than 6% (the 3rd and 4th irradiations at the reading time of 0.5 h). For the rest, the OD curves were not linear, sensitivities of the dosimeters were lower than 0.04 ΔOD/Gy, and OD deviations at the same dose were larger than 6%.ConclusionsAt room temperature, PRESAGEREU dosimeters could be used for dose measurement only for up to two dose measurement sessions. At low temperatures, usage of PRESAGEREU dosimeters for dose measurement seems to be possible from the 3rd irradiation. When reusing PRESAGEREU dosimeters, the OD curve should be re-defined for every measurement session because the shape of this curve depends on the irradiation history.

A portal dosimetry dose prediction method based on collapsed cone algorithm using the clinical beam model.

Amorphous silicon electronical portal imaging devices (EPIDs) are widely used for dosimetric measurements in Radiation Therapy. The purpose of this work was to determine if a portal dose prediction method can be utilized for dose map calculations based on the linear accelerator model within a commercial treatment planning system (Pinnacle3 v8.0m). The method was developed for a 6MV photon beam on the Varian Clinac 21-EX, at a nominal dose rate of 400MU/min. The Varian aS1000 EPID was unmounted from the linear accelerator and scanned to acquire CT images of the EPID. The CT images were imported into Pinnacle3 and were used as a quality assurance phantom to calculate dose on the EPID setup at a source to detector distance of 105cm. The best match of the dose distributions was obtained considering the image plane located at 106cm from the source to detector plane. The EPID was calibrated according to the manufacturer procedure and corrections were made for output factors. Arm-backscattering effect, based on profile correction curves, has been introduced. Five low-modulated and three high-modulated clinical planned treatments were predicted and measured with the method presented here and with MatriXX (IBA Dosimetry, Schwarzenbruck, Germany). A portal dose prediction method based on Pinnacle3 was developed without modifying the commissioned parameters of the model in use in the clinic. CT images of the EPID were acquired and used as a quality assurance phantom. The CT images indicated a mean density of 1.16g/cm3 for the sensitive area of the EPID. Output factor measured with the EPID were lower for small fields and larger for larger fields (beyond 10×10cm2 ). Arm-backscatter correction showed a better agreement at the target side of the EPID. Analysis of Gamma index comparison (3%, 3mm) indicated a minimum of 97.4% pass rate for low modulated and 98.3% for high modulated treatments. Pass rates were similar for MatriXX measurements. The method developed here can be easily implemented into clinic, as neither additional modeling of the clinical energy nor an independent image prediction algorithm are necessary. The main advantage of this method is that portal dose prediction is calculated with the same algorithm and beam model used for patient dose distribution calculation. This method was independently validated with an ionization chamber matrix.

SU‐G‐TeP3‐02: Determination of Geometry‐Specific Backscatter Factors for Radiobiology Studies

Purpose:Radiation biology research relies on an accurate radiation dose delivered to the biological target. Large field irradiations in a cabinet irradiator may use the AAPM TG‐61 protocol. This relies on an air‐kerma measurement and conversion to absorbed dose to water (Dw) on the surface of a water phantom using provided backscatter factors. Cell or small animal studies differ significantly from this reference geometry. This study aims to determine the impact of the lack of full scatter conditions in four representative geometries that may be used in radiobiology studies.Methods:MCNP6 was used to model the Dw on the surface of a full scatter phantom in a validated orthovoltage x‐ray reference beam. Dw in a cylindrical mouse, 100 mm Petri dish, 6‐well and 96‐well cell culture dishes was simulated and compared to this full scatter geometry. A reference dose rate was measured using the TG‐61 protocol in a cabinet irradiator. This nominal dose rate was used to irradiate TLDs in each phantom to a given dose. Doses were obtained based on TLDs calibrated in a NIST‐traceable beam.Results:Compared to the full scattering conditions, the simulated dose to water in the representative geometries were found to be underestimated by 12‐26%. The discrepancy was smallest with the cylindrical mouse geometry, which most closely approximates adequate lateral‐ and backscatter. TLDs irradiated in the mouse and petri dish phantoms using the TG‐61 determined dose rate showed similarly lower values of Dw. When corrected for this discrepancy, they agreed with the predicted Dw within 5%.Conclusion:Using the TG‐61 in‐air protocol and given backscatter factors to determine a reference dose rate in a biological irradiator may not be appropriate given the difference in scattering conditions between irradiation and calibration. Without accounting for this, the dose rate is overestimated and is dependent on irradiation geometry.

SU‐F‐T‐476: Performance of the AS1200 EPID for Periodic Photon Quality Assurance

Purpose:To assess the dosimetric performance of a new amorphous silicon flat‐panel electronic portal imaging device (EPID) suitable for high‐intensity, flattening‐filter‐free delivery mode.Methods:An EPID‐based QA suite was created with automation to periodically monitor photon central‐axis output and two‐dimensional beam profile constancy as a function of gantry angle and dose‐rate. A Varian TrueBeamTM linear accelerator installed with Developer Mode was used to customize and deliver XML script routines for the QA suite using the dosimetry mode image acquisition for an aS1200 EPID. Automatic post‐processing software was developed to analyze the resulting DICOM images.Results:The EPID was used to monitor photon beam output constancy (central‐axis), flatness, and symmetry over a period of 10 months for four photon beam energies (6x, 15x, 6xFFF, and 10xFFF). EPID results were consistent to those measured with a standard daily QA check device. At the four cardinal gantry angles, the standard deviation of the EPID central‐axis output was &lt;0.5%. Likewise, EPID measurements were independent for the wide range of dose rates (including up to 2400 mu/min for 10xFFF) studied with a standard deviation of &lt;0.8% relative to the nominal dose rate for each energy. Also, profile constancy and field size measurements showed good agreement with the reference acquisition of 0° gantry angle and nominal dose rate. XML script files were also tested for MU linearity and picket‐fence delivery. Using Developer Mode, the test suite was delivered in &lt;60 minutes for all 4 photon energies with 4 dose rates per energy and 5 picket‐fence acquisitions.Conclusion:Dosimetry image acquisition using a new EPID was found to be accurate for standard and high‐intensity photon beams over a broad range of dose rates over 10 months. Developer Mode provided an efficient platform to customize the EPID acquisitions by using custom script files which significantly reduced the time.This work was funded in part by Varian Medical Systems.

SU-G-TeP2-05: Development of a Thimble Calorimeter for Absorbed Dose to Water Characterized in MV Photons

Purpose: To develop a thimble sized polystyrene calorimeter for use from kV to MV photons, as a primary reference standard for applications from diagnostic CT imaging to therapy beam dose determination. Methods: A polystyrene calorimeter about 1.5 cm diameter embedded with small thermistors was characterized in a 6 MV photon beam from a clinical accelerator at 5 nominal dose rates from 0.8 to 4 Gy/min. Irradiations were delivered with beam on/off cycles first at 60 s and then at 20 s. Two sets of phantom conditions were evaluated: 1) in a 30 cm diameter polyethylene cylinder, and 2) in 10 cm depth of a 30 cm water phantom. The temperature waveforms were recorded and analyzed for temperature rise, arriving at a dose to polystyrene. This value is compared with the result of measurements under identical conditions using an ionization chamber calibrated for absorbed dose to water. Monte Carlo simulations were performed on the measurement systems to estimate such a ratio. Results: The ratio of the dose determined by the calorimeter to the dose reported by the ionization chamber was aggregated from all 5 dose rates. The 60 s results show a much elevated response in both phantoms compared to their respective expected results based on simulation. This deviation was reduced when the on/off cycles were shortened to 20 s. This behavior was possibly due to the heat conduction effects in the small calorimeter body. Finite element modeling is being conducted to simulate this effect. Conclusion: A small solid plastic calorimeter offers the convenience of a portable absorbed dose standard based on direct measurement of energy deposition, but comes at the expense of heat transfer complications which need to be characterized. This work offers preliminary evidence of the behavior and quantitative assessment of the issues to be resolved in future investigations.

Characterisation of a two-dimensional liquid-filled ion chamber detector array using flattened and unflattened beams for small fields, small MUs and high dose-rates

In this study, the PTW 1000SRS array with Octavius 4D phantom was characterised for FF and FFF beams. MU linearity, field size, dose rate, dose per pulse (DPP) response and dynamic conformal arc treatment accuracy of the 1000SRS array were assessed for 6MV, 6FFF and 10FFF beams using a Varian TrueBeam STx linac. The measurements were compared with a pinpoint IC, microdiamond IC and EBT3 Gafchromic film. Measured dose profiles and FWHMs were compared with film measurements. Verification of FFF volumetric modulated arc therapy (VMAT) clinical plans were assessed using gamma analysis with 3%/3 mm and 2%/2 mm tolerances (10% threshold). To assess the effect of cross calibration dose rate, clinical plans with different dose rates were delivered and analysed. Output factors agreed with film measurements to within 4.5% for fields between 0.5 and 1 cm and within 2.7% for field sizes between 1.5 and 10 cm and were highly correlated with the microdiamond IC detector. Field sizes measured with the 1000SRS array were within 0.5 mm of film measurements. A drop in response of up to 1.8%, 2.4% and 5.2% for 6MV, 6FFF and 10FFF beams respectively was observed with increasing nominal dose rate. With an increase in DPP, a drop of up to 1.7%, 2.4% and 4.2% was observed in 6MV, 6FFF and 10FFF respectively. The differences in dose following dynamic conformal arc deliveries were less than 1% (all energies) from calculated. Delivered VMAT plans showed an average pass percentage of 99.5(±0.8)% and 98.4(±3.4)% with 2%/2 mm criteria for 6FFF and 10FFF respectively. A drop to 97.7(±2.2)% and 88.4(±9.6)% were observed for 6FFF and 10FFF respectively when plans were delivered at the minimum dose rate and calibrated at the maximum dose rate. Calibration using a beam with the average dose rate of the plan may be an efficient method to overcome the dose rate effects observed by the 1000SRS array.

Calibration of GafChromic EBT3 for absorbed dose measurements in 5 MeV proton beam and (60)Co γ-rays.

To study EBT3 GafChromic film in low-energy protons, and for comparison purposes, in a reference (60)Co beam in order to use it as a calibrated dosimetry system in the proton irradiation facility under construction within the framework of the Oncological Therapy with Protons (TOP)-Intensity Modulated Proton Linear Accelerator for RadioTherapy (IMPLART) Project at ENEA-Frascati, Italy. EBT3 film samples were irradiated at the Istituto Nazionale di Fisica Nucleare-Laboratori Nazionali di Legnaro, Italy, with a 5 MeV proton beam generated by a 7 MV Van de Graaff CN accelerator. The nominal dose rates used were 2.1 Gy/min and 40 Gy/min. The delivered dose was determined by measuring the particle fluence and the energy spectrum in air with silicon surface barrier detector monitors. A preliminary study of the EBT3 film beam quality dependence in low-energy protons was conducted by passively degrading the beam energy. EBT3 films were also irradiated at ENEA-National Institute of Ionizing Radiation Metrology with gamma radiation produced by a (60)Co source characterized by an absorbed dose to water rate of 0.26 Gy/min as measured by a calibrated Farmer type ionization chamber. EBT3 film calibration curves were determined by means of a set of 40 film pieces irradiated to various doses ranging from 0.5 Gy to 30 Gy absorbed dose to water. An EPSON Expression 11000XL color scanner in transmission mode was used for film analysis. Scanner response stability, intrafilm uniformity, and interfilm reproducibility were verified. Optical absorption spectra measurements were performed on unirradiated and irradiated EBT3 films to choose the most sensitive color channel to the dose range used. EBT3 GafChromic films show an under response up to about 33% for low-energy protons with respect to (60)Co gamma radiation, which is consistent with the linear energy transfer dependence already observed with higher energy protons, and a negligible dose-rate dependence in the 2-40 Gy/min range. Short- and long-term scanner stabilities were 0.5% and 1.5%, respectively; film uniformity and reproducibility were better than 0.5%. The main purpose of this study was to implement EBT3 dosimetry in the proton low-energy radiobiology line of the TOP-IMPLART accelerator, having a maximum energy of 7 MeV. Low-energy proton and (60)Co calibrated sources were used to investigate the behavior of film response vs to be written in italicum dose. The calibration in 5 MeV protons is currently used for dose assessment in the radiobiological experiments at the TOP-IMPLART accelerator carried out at that energy value.

SU‐E‐T‐606: Performance of MR‐Based 3D FXG Dosimetry for Preclinical Irradiation

Purpose:Technological advances have revolutionized preclinical radiation research to enable precise radiation delivery in preclinical models. Kilovoltage x‐rays and complex geometries in preclinical radiation studies challenge conventional dosimetry methods. Previously developed gel‐based dosimetry provides a viable means of accommodating complex geometries and accurately reporting dose at kV energies. This paper will describe the development and evaluation of gel‐based ferrous xylenol‐orange (FXG) dosimetry using a 7T preclinical imaging system.Methods:To confirm water equivalence, Zeff values were calculated for the FXG material, water and ICRU defined soft tissue. Proton T1 relaxivity response in FXG was measured using a preclinical 7T MR and a small animal irradiator for a dose range of 1–22 Gy. FXG was contained in 50 ml centrifuge tubes and irradiated with a 225 kVp x‐ray beam at a nominal dose rate of 2.3 Gy/min. Pre and post irradiation maps of the T1 relaxivity were collected using variable TR spin‐echo imaging (TE 6.65 ms; TR 500, 750, 1000, 1500, 2000, 3000 and 5000 ms) with 2 mm thick slices, 0.325 mm/pixel, 3 averages and an acquisition time of 26 minutes. A linear fit to the change in relaxation rate (1/T1) for the delivered doses reported the gel sensitivity in units of ms‐1Gy‐1. Irradiation and imaging studies were repeated using three batches of gel over 72 hrs.Results:FXG has a Zeff of 3.8 for the 225 kVp spectrum used; differing from water and ICRU defined soft tissue by 0.5% and 2.5%, respectively. The average sensitivity for the FXG dosimeter was 31.5 ± 0.7 ms‐1Gy‐1 (R2 = 0.9957) with a y‐intercept of −29.4 ± 9.0 ms‐1.Conclusion:Preliminary results for the FXG dosimeter properties, sensitivity, and dose linearity at preclinical energies is promising. Future work will explore anatomically relevant tissue inclusions to test MR performance.Student funding provided by The Terry Fox Foundation Strategic Initiative for Excellence in Radiation Research for the 21st Century at CIHR and the Gifford Ontario Student Opportunity Trust Fund.

Messungen im Festkörperphantom als Qualitätskontrolle in der Brachytherapie – Systemprüfung und Konstanzprüfung

In brachytherapy dosimetric measurements are difficult due to the inherent dose-inhomogenieties. Typically in routine clincal practice only the nominal dose rate is determined for computer controlled afterloading systems. The region of interest lies close to the source when measuring the spatial dose distribution. In this region small errors in the postioning of the detector, and its finite size, lead to large measurement uncertainties that exacerbate the routine dosimetric control of the system in the clinic.The size of the measurement chamber, its energy dependence, and the directional dependence of the measurement apparatus are the factors which have a significant influence on dosimetry. Although ionisation chambers are relatively large, they are employed since similar chambers are commonly found on clincal brachytherapy units.The dose is determined using DIN 6800 [11] since DIN 6809-2 [12], which deals with dosimetry in brachytherapy, is antiquated and is currently in the process of revision. Further information regarding dosimetry for brachytherapy can be found in textbooks [1] and [2]. The measurements for this work were performed with a HDR (High-Dose-Rate) 192Ir source, type mHDR V2, and a Microselectron Afterloader V2 both from Nucletron/Elekta.In this work two dosimetric procedures are presented which, despite the aforemention difficulties, should assist in performing checks of the proper operation of the system. The first is a system check that measures the dose distribution along a line and is to be performed when first bringing the afterloader into operation, or after significant changes to the system.The other is a dosimetric constancy check, which with little effort can be performed monhtly or weekly. It simultaneously verifies the positioning of the source at two positions, the functionality of the system clock and the automatic re-calculation of the source activity.

Fentanyl Pharmacokinetics in Pregnant Sheep after Intravenous and Transdermal Administration to the Ewe.

Fentanyl is used for pain treatment during pregnancy in human beings and animals. However, fentanyl pharmacokinetics during pregnancy has not been fully established. The aim of this study was to characterize fentanyl pharmacokinetics in pregnant sheep after intravenous and transdermal dosing during surgical procedure performed to ewe and foetus. Pharmacokinetic parameters reported for non-pregnant sheep and nominal transdermal dose rate were utilized for a priori calculation to achieve analgesic fentanyl concentration (0.5-2ng/ml) in maternal plasma. A total of 20 Aland landrace ewes at 118-127 gestational days were used. In the first protocol, 1week before surgery, 10 animals received 2μg/kg fentanyl intravenous bolus, and on the operation day, transdermal fentanyl patches at nominal dose rate of 2μg/kg/hr were applied to antebrachium, and ewes were then given a 2μg/kg intravenous bolus followed by an intra-operative 2.5μg/kg/hr infusion. In the second protocol, 10 animals received fentanyl only as transdermal patches on the operation day and oxycodone for rescue analgesia. The data were analysed with population pharmacokinetic modelling. Intra- and post-operative fentanyl concentrations were similar and slightly lower than the a priori predictions, and elimination and distribution clearances appeared slower during than before or after the surgery. Transdermal patches provided sustained fentanyl absorption for up to 5days, but the absorption rate was slower than the nominal dose rate and showed a high interindividual variability. Further research is warranted to evaluate the clinical relevance of the observations made in sheep.

Cognitive effects of partial and whole-body exposures to 16O particles

Introduction: When rats and mice are exposed to HZE particles at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory to simulate the effect of exposure to space radiation on cognitive performance, there may be differences in the amount of tissue that is irradiated: some experimenters irradiate only the head, whereas others irradiate the entire organism. Whether or not these different patterns of exposure have differential effects on cognitive performance remain to be established. Similarly, it remains to be established whether or not exposures restricted to the body can have an independent effect on cognitive performance.Radiation: Rats were exposed to 16O (1000 MeV/n; 1, 5, 10, 25 cGy) particles at the NSRL. There were three exposure conditions: head-only, body-only and whole body. Tungsten bricks were used to shield either the head or the body, as required. The bricks were removed for whole-body exposures. The non-irradiated control rats (0 cGy) were taken to the NSRL, but not exposed. Nominal dose rates were between 1 and 10 cGy/min, depending upon the total dose.Behavior: The animals were shipped to UMBC for behavioral testing. Cognitive performance was measured using the elevated-plus-maze (baseline anxiety); novel object recognition (general learning and memory); novel spatial location (spatial learning and memory); and operant responding on an ascending fixed-ratio schedule (motivation and responsiveness to environmental stimuli). The basis for the selection of the behavioral tests was that the behaviors are dopamine-mediated or show deterioration as a function of age.Results: The results of the experiments indicate that: (i) there may be differences in cognitive performance following exposure to either head-only or whole-body exposure to NASA-relevant doses of 16O particles depending on the specific task; and (ii) for some tasks, body-only exposures can disrupt neurocognitive performance. The results also suggest that exposures involving the body may influence the responsiveness of the organism to the disruptive effects of exposure to HZE particles on neurocognitive performance. This effect is seen in two ways: first, as a direct effect of body-only exposure on cognitive performance, as shown by decreased time spent in a novel location compared with non-irradiated controls, suggesting impaired spatial location memory; and second, as an interaction between head and body exposures seen with whole-body exposures.Discussion: These results indicate that body-only exposures have the potential to influence cognitive performance which is mediated by the brain. Although the mechanism remains to be fully established, it is possible that exposure of the body to HZE particles causes the release of cytokines which can affect neuronal function, either directly or through the mediation of the vagus nerve. As such, the results of studies using head-only or whole-body exposures may not be comparable. Also, astronauts may be subject to increased risk of deficits in cognitive performance during exploratory class missions.

Total skin electron beam irradiation in Greece: in-vivo dosimetry

Total skin electron beam irradiation in Greece: in-vivo dosimetry

Evaluation of an aSi‐EPID with flattening filter free beams: Applicability to the GLAaS algorithm for portal dosimetry and first experience for pretreatment QA of RapidArc

To demonstrate the feasibility of portal dosimetry with an amorphous silicon mega voltage imager for flattening filter free (FFF) photon beams by means of the GLAaS methodology and to validate it for pretreatment quality assurance of volumetric modulated arc therapy (RapidArc). The GLAaS algorithm, developed for flattened beams, was applied to FFF beams of nominal energy of 6 and 10 MV generated by a Varian TrueBeam (TB). The amorphous silicon electronic portal imager [named mega voltage imager (MVI) on TB] was used to generate integrated images that were converted into matrices of absorbed dose to water. To enable GLAaS use under the increased dose-per-pulse and dose-rate conditions of the FFF beams, new operational source-detector-distance (SDD) was identified to solve detector saturation issues. Empirical corrections were defined to account for the shape of the profiles of the FFF beams to expand the original methodology of beam profile and arm backscattering correction. GLAaS for FFF beams was validated on pretreatment verification of RapidArc plans for three different TB linacs. In addition, the first pretreatment results from clinical experience on 74 arcs were reported in terms of γ analysis. MVI saturates at 100 cm SDD for FFF beams but this can be avoided if images are acquired at 150 cm for all nominal dose rates of FFF beams. Rotational stability of the gantry-imager system was tested and resulted in a minimal apparent imager displacement during rotation of 0.2 ± 0.2 mm at SDD = 150 cm. The accuracy of this approach was tested with three different Varian TrueBeam linacs from different institutes. Data were stratified per energy and machine and showed no dependence with beam quality and MLC model. The results from clinical pretreatment quality assurance, provided a gamma agreement index (GAI) in the field area for six and ten FFF beams of (99.8 ± 0.3)% and (99.5 ± 0.6)% with distance to agreement and dose difference criteria set to 3 mm/3% with 2 mm/2% thresholds, GAI resulted (95.7.0 ± 2.3)% and (97.2 ± 2.1)%. The GLAaS methodology, introduced in clinical practice for conventional flattened photon beams for machine, IMRT, and RapidArc quality assurance, was successfully adapted for FFF beams of Varian TrueBeam Linac. The detector saturation effects could be eliminated if the portal images acquired at 150 cm for all nominal dose rates of FFF beams.

SU-E-T-153: Detector-Grade CVD Diamond for Radiotherapy Dosimetry.

To evaluate the use of commercially available detector-grade synthetic diamond films made via chemical vapor deposition (CVD) as x- ray detectors for radiotherapy dosimetry. A detector was fabricated using high-quality single crystal CVD diamond films (0.5 × 3 × 3 mm̂3) with 0.4 mm̂3 sensitive volumes, which were encapsulated with PMMA. The detector was placed in a (30 × 30 × 30 cm̂3) PTW water phantom. Six- and ten-MV photons from an Elekta Synergy linac were measured using an SSD of 90 cm and typically a 10-cm phantom depth with a 10 × 10 cm̂2 field size in the central axis of the beam. Data acquisition was performed using a PTW UNIDOS E electrometer with a 100-V bias. The detector was evaluated by measuring leakage current, priming dose, response dynamics, dose linearity, dependence on dose rate, percent depth dose (6 and 10 MV photons) and output factors. Some measurements were compared with a Si diode detector, 0.04 and/or 0.13-cc ion chamber(s). Leakage currents were negligible (∼1 pA) given the overall average sensitivity of the material (680 nC/Gy at 100 V). Detector current rise and fall times were < 2 s. Dose measurements up to 5 Gy were linear. A slight dependence on nominal dose rates up to 500 MU/min was seen with Fowler fitting parameter delta = 0.981. Percent depth dose profiles compared well to a 0.13-cc ion chamber up to 25 cm depths for 10-MV photons. Output factors measured well for field widths 5-15 cm; below 5 cm, differences are seen between all four different detectors as expected. The type of diamond tested has potential to be used for small field dosimetry due to its small sensitive volume and high sensitivity. Further experiments are ongoing and detector packaging is yet to be optimized.