[en] Purpose: To verify the accuracy of total body irradiation (TBI) measurement commissioning data using the treatment planning system (TPS) for a wide range of patient separations. Methods: Our institution conducts TBI treatments with an 18MV photon beam at 380cm extended SSD using an AP/PA technique. Currently, the monitor units (MU) per field for patient treatments are determined using a lookup table generated from TMR measurements in a water phantom (75 × 41 × 30.5 cm3). The dose prescribed to an umbilicus midline point at spine level is determined based on patient separation, dose/ field and dose rate/MU. One-dimensional heterogeneous dose calculations from Pinnacle TPS were validated with thermoluminescent dosimeters (TLD) placed in an average adult anthropomorphic phantom and also in-vivo on four patients with large separations. Subsequently, twelve patients with various separations (17–47cm) were retrospectively analyzed. Computed tomography (CT) scans were acquired in the left and right decubitus positions from vertex to knee. A treatment plan for each patient was generated. The ratio of the lookup table MU to the heterogeneous TPS MU was compared. Results: TLD Measurements in the anthropomorphic phantom and large TBI patients agreed with Pinnacle calculated dose within 2.8% and 2%, respectively. The heterogeneous calculation compared to the lookup table agreed within 8.1% (ratio range: 1.014–1.081). A trend of reduced accuracy was observed when patient separation increases. Conclusion: The TPS dose calculation accuracy was confirmed by TLD measurements, showing that Pinnacle can model the extended SSD dose without commissioning a special beam model for the extended SSD geometry. The difference between the lookup table and TPS calculation potentially comes from lack of scatter during commissioning when compared to extreme patient sizes. The observed trend suggests the need for development of a correction factor between the lookup table and TPS dose calculations.

[en] Full text: This paper will summarize the experience at the University of Texas M.D. Anderson Cancer Center (UTMDACC) with thermoluminescent dosimetry as a quality assurance tool for output and energy monitoring of radiation therapy beams. UTMDACC has two sections, the Radiological Physics Center (RPC) and the Radiation Dosimetry Services (RDS) which offer periodic verifications of machine output to some 1,500 institutions in the US, Canada and some other parts of the world. The two centers process TLD measurements for approximately 8,000 x-ray beams and 7,000 electron beams per year. Throughout the past 25 years the results from monitoring institutions, and the data for commissioning TLD readers, characterization of lithium fluoride TLD-100 powder and the records of a quality assurance program of the system have been accumulated. The precision limits of the system as well as the disagreement with the institutions will be summarized. The methodology of the TLD reading process and the quality assurance program will be discussed in detail. The accuracy achieved with the system will be described as well as the degree of reliability of the entire operation. Over the past 8 years the RPC has used manually operated TLD readers to read powder. RDS uses manual readers from a different manufacturer. Each TLD sample is weighed and the measurement is reported as TL per unit mass. Nitrogen gas flows through the system during each reading session, beginning 30 minutes before readings are taken, to reduce spurious signal. The powder is produced in large batches that are then dispensed into capsules that hold about 25 mg per capsule. A batch of powder is tested for reproducibility, and then characterized for fading, sensitivity, energy dependence and dose-response linearity. The powder is irradiated and analyzed once and is then discarded. TLD powder is irradiated to a known dose with a cobalt unit at UTMDACC under very tightly controlled conditions to provide standards that are used to derive the relationship between TLD signal and dose. Additional powder is irradiated to a known dose with a second cobalt unit at UTMDACC to provide control samples used for monitoring the reader stability during a session and to test the TLD dose prediction. Sets of standards are read at the beginning and end of each session while sets of controls are interspersed evenly with experimental readings. A typical reading session includes 12 sets of experimental TLD irradiated at participating institutions. Each set consists of three samples for photons and six for electrons. To maintain high quality results, a comprehensive QA program is in place that includes acceptance testing and commissioning of each instrument and each batch of powder. The program also provides QA verifications for each result, each session, each month of operation and each year of operation. A maintenance and repair program is conducted and carefully documented. Training of technical personnel is geared to the passing on of a uniform method of reading samples with emphasis on repetitivity of actions within each reading cycle. Each cycle is timed and maintained at 2 minutes per sample. Analysis of the data for several years of operation shows that the system predicts the dose to TLDs irradiated under very controlled conditions (controls) with very high precision (SD=0.9%). Analysis of the results for beams at different institutions shows a spread of 1.9% for photons and 2.2% for electrons. This spread is the combined result of the variability of the beam energies, the different makes and models of machines, the institutional performance that includes beam calibration, TLD set- up errors and beam drifts. The data also show that TLD powder, independent of the batch, has fading characteristics that can be predicted using a double exponential equation. Energy correction factors remain very constant from batch to batch and the dose response is also a very predictable value. The system is used to monitor institutions and is designed to pursue large discrepancies in an expeditious manner that aims at the resolution of the discrepancy either through discussions, more TLD or site visits. A system has been developed that provides results with a level of confidence about 2%. (author)

[en] Inelastic scattering of 25.7-MeV neutrons to unresolved final states with excitation energies up to approx. =13 MeV were measured for monoisotopic samples of ⁵¹V, ⁵⁶Fe, ⁶⁵Cu, ⁹³Nb, and ²⁰⁹Bi. Neutrons were produced via T(d,n)⁴ He reaction in a gas cell that provides a background-free source spectrum above E /SUB n/ = 12 MeV. Time-of-flight spectra were taken at several angles between 25 and 145 deg using the beam-swinger spectrometer. The technique of dynamic biasing proved valuable in providing maximum detector efficiency and low background throughout the broad range of neutron energies. Data were converted to energy spectra, corrected for detector efficiency, averaged over 1-MeV bins, and corrected for sample attenuation and multiple scattering

[en] Neptunium (IV) is oxidized to Np(V) with nitric acid in the presence of U(VI) under conditions of low acidity (<∼0.1 M). The reaction rate is described by the equation d[Np(V)]/dt = k₁[Np(IV)]/[H⁺]² + k₂[Np(IV)]²[U(VI)]/[H⁺]³, in which k₁ = (2.0 ± 0.3) · 10^-5 mol² · l^-2 · min^-1 and k₂ = (5.50 ± 0.47) · 10^-2 mol · l^-1 · min^-1 at 50 deg C and solution ionic strength μ = 0.5. The activation energies of the two pathways are 148 ± 31 and 122 ± 12 kJ mol^-1. The reaction along the main pathway (with the rate constant k₂) is limited by disproportionation of Np(IV) involving NpOH³⁺ and Np(OH)₂UO₂⁴⁺ complex ions

[en] Purpose: To determine the effect of a strong magnetic field on TLD-100, OSLD (Al₂O₂:C), and PRESAGE dosimetry devices. This study will help to determine which types of dosimeters can be used for quality assurance and in-vivo dosimetry measurements in a magnetic resonance imaginglinear accelerator (MRI-linac) system. Methods: The dosimeters were separated into two categories which were either exposed or not exposed to a strong magnetic field. In each category a set of dosimeters was irradiated with 0, 2, or 6 Gy. To expose the dosimeters to a magnetic field the samples in that category were place in a Bruker small animal magnetic resonance scanner at a field strength slightly greater than 2.5 T for at least 1 hour preirradiation and at least 1 hour post-irradiation. Irradiations were performed with a 6 MV x-ray beam from a Varian TrueBeam linac with 10×10 cm² field at a 600 MU/min dose rate. The samples that received no radiation dose were used as control detectors. Results: The readouts of the dosimeters which were not exposed to a strong magnetic field were compared with the measurements of the dosimetry devices which were exposed to a magnetic field. No significant differences (less than 2% difference) in the performance of TLD, OSLD, or PRESAGE dosimeters due to exposure to a strong magnetic field were observed. Conclusion: Exposure to a strong magnetic field before and after irradiation does not appear to change the dosimetric properties of TLD, OSLD, or PRESAGE which indicates that these dosimeters have potential for use in quality assurance and in-vivo dosimetry in a MRI-linac. We plan to further test the effect of magnetic fields on these devices by irradiating them in the presence of a magnetic fields similar to those produced by a MRI-linac system. Elekta-MD Anderson Cancer Center Research Agreement

[en] Purpose: To simulate and measure magnetic-field-induced radiation dose effects in a mouse lung phantom. This data will be used to support pre-clinical experiments related to MRI-guided radiation therapy systems. Methods: A mouse lung phantom was constructed out of 1.5×1.5×2.0-cm³ lung-equivalent material (0.3 g/cm³) surrounded by a 0.6-cm solid water shell. EBT3 film was inserted into the phantom and the phantom was placed between the poles of an H-frame electromagnet. The phantom was irradiated with a cobalt-60 beam (1.25 MeV) with the electromagnet set to various magnetic field strengths (0T, 0.35T, 0.9T, and 1.5T). These magnetic field strengths correspond to the range of field strengths seen in MRI-guided radiation therapy systems. Dose increases at the solid-water-to-lung-interface and dose decreases at the lung-to-solid-water interface were compared with results of Monte Carlo simulations performed with MCNP6. Results: The measured dose to lung at the solid-water-to-lung interface increased by 0%, 16%, and 29% with application of the 0.35T, 0.9T, and 1.5T magnetic fields, respectively. The dose to lung at the lung-to-solid-water interface decreased by 4%, 18%, and 24% with application of the 0.35T, 0.9T, and 1.5T magnetic fields, respectively. Monte Carlo simulations showed dose increases of 0%, 16%, and 31% and dose decreases of 4%, 16%, and 25%. Conclusion: Only small dose perturbations were observed at the lung-solid-water interfaces for the 0.35T case, while more substantial dose perturbations were observed for the 0.9T and 1.5T cases. There is good agreement between the Monte Carlo calculations and the experimental measurements (within 2%). These measurements will aid in designing pre-clinical studies which investigate the potential biological effects of radiation therapy in the presence of a strong magnetic field. This work was partially funded by Elekta.

[en] Purpose: To evaluate the use of post-irradiation changes in respiratory rate and CBCT-based morphology as predictors of survival in mice. Methods: C57L/J mice underwent whole-thorax irradiation with a Co-60 beam to four different doses [0Gy (n=3), 9Gy (n=5), 11Gy (n=7), and 13Gy (n=5)] in order to induce varying levels of pneumonitis. Respiratory rate measurements, breath-hold CBCTs, and free-breathing CBCTs were acquired pre-irradiation and at six time points between two and seven months post-irradiation. For respiratory rate measurements, we developed a novel computer-vision-based technique. We recorded mice sleeping in standard laboratory cages with a 30 fps, 1080p webcam (Logitech C920). We calculated respiratory rate using corner detection and optical flow to track cyclical motion in the fur in the recorded video. Breath-hold and free-breathing CBCTs were acquired on the X-RAD225Cx system. For breathhold imaging, the mice were intubated and their breath was held at full-inhale for 20 seconds. Healthy lung tissue was delineated in the scans using auto-threshold contouring (0–0.7 g/cm"3). The volume of healthy lung was measured in each of the scans. Next, lung density was measured in a 6-mm"2 ROI in a fixed anatomic location in each of the scans. Results: Day-to-day variability in respiratory rate with our technique was 13%. All metrics except for breath-hold lung volume were correlated with survival: lung density on free-breathing (r=−0.7482,p<0.01) and breath-hold images (r=−0.5864,p<0.01), free-breathing lung volume (r=0.7179,p<0.01), and respiratory rate (r= 0.6953,p<0.01). Lung density on free-breathing scans was correlated with respiratory rate (r=0.7142,p<0.01) and lung density on breath-hold scans (r=0.5543,p<0.01). One significant practical hurdle in the CBCT measurements was that at least one lobe of the lung was collapsed in 36% of free-breathing scans and 45% of breath-hold scans. Conclusion: Lung density and lung volume on free-breathing CBCTs and respiratory rate outperform breath-hold CBCT measurements as indicators for survival from radiation-induced pneumonitis. This work was partially funded by Elekta.

[en] A new approach to intraoperative radiation therapy led to the development of mobile linear electron accelerators that provide lower electron energy beams than the usual conventional accelerators commonly encountered in radiotherapy. Such mobile electron accelerators produce electron beams that have nominal energies of 4, 6, 9 and 12 MeV. This work compares the absorbed dose output calibrations using both the AAPM TG-51 and TG-21 dose calibration protocols for two types of ion chambers: a plane-parallel (PP) ionization chamber and a cylindrical ionization chamber. Our results indicate that the use of a 'Markus' PP chamber causes 2-3% overestimation in dose-output determination if accredited dosimetry-calibration laboratory based chamber factors are used. However, if the ionization chamber factors are derived using a cross-comparison at a high-energy electron beam, then a good agreement is obtained (within 1%) with a calibrated cylindrical chamber over the entire energy range down to 4 MeV. Furthermore, even though the TG-51 does not recommend using cylindrical chambers at the low energies, our results show that the cylindrical chamber has a good agreement with the PP chamber not only at 6 MeV but also down to 4 MeV electron beams. (note)

[en] Purpose: Investigation of dose response curves of methacrylic acid-based “MAGAT” gel at different effective energies to verify an energy dependence of polymer-gel dosimeters for orthovoltage energy x-rays. Methods: Six small cylindrical MAGAT gel phantoms were exposed to different dose levels; one phantom was unirradiated for background subtraction. This experiment was repeated for three different effective beam energies.24 h post irradiation the spin-spin relaxation times (T2) were measured with a 4.7 T Bruker MR scanner at 2 cm depth inside the gel. The T2 values were converted to relaxation rates (R2) and plotted against the respective dose levels corresponding to the different effective energies. The resulting dose response curves were compared for a 250 kVp beam, the 250 kVp beam filtered by 6 cm of water, and a 125 kVp beam. Results: The passage of the 250 kVp beam through water resulted in a half-value-layer (HVL) change from 1.05 mm Cu to 1.32 mm Cu at 6 cm depth with a change in effective energy from 81.3 keV to 89.5 keV, respectively. The dose response curves showed a shift to higher relaxation rates for the harder beam. The dose response measurements for the 125 kVp beam (HVL: 3.13 mm Al, effective energy: 33.9 keV) demonstrated even higher relaxation rates than for either of the other beams. Conclusion: The MAGAT dose response curves for three different effective energies demonstrate a complex energy dependence, with an apparent decrease in sensitivity at 89.5 keV effective energy. This energy dependence is consistent with observed discrepancies of depth dose data compared with ion-chamber data. For future investigations of larger volumes, an energy-dependent sensitivity function is needed to properly assess 3-dimensional dose distributions

[en] Purpose: Gold nanoparticle (GNP) mediated radiosensitization has gained significant attention in recent years. However, the widely used passive targeting strategy requires high concentration of GNPs to induce the desired therapeutic effect, thus dampening the enthusiasm for clinical translation. The purpose of this study is to utilize a molecular targeting strategy to minimize the concentration of GNPs injected while simultaneously enhancing the tumor specific radiosensitization for an improved therapeutic outcome. Methods: Cetuximab (antibody specific to the epidermal growth factor receptor that is over-expressed in tumors) conjugated gold nanorods (cGNRs) was used for the tumor targeting. The binding affinity, internalization, and in vitro radiosensitization were evaluated using dark field microscopy, transmission electron microscopy, and clonogenic cell survival assay, respectively. In vivo biodistribution in tumor (HCT116-colorectal cancer cells) bearing mice were quantified using inductively coupled plasma mass spectrometry. In vivo radiosensitization potential was tested using 250-kVp x-rays and clinically relevant 6-MV radiation beams. Results: cGNRs displayed excellent cell-surface binding and internalization (∼31,000 vs 12,000/cell) when compared to unconjugated GNRs (pGNRs). In vitro, the dose enhancement factor at 10% survival (DEF10) was estimated as 1.06 and 1.17, respectively for both 250-kVp and 6-MV beams. In vivo biodistribution analysis revealed enhanced uptake of cGNRs in tumor (1.3 µg/g of tumor tissue), which is ∼1000-fold less than the reported values using passive targeting strategy. Nonetheless, significant radiosensitization was observed in vivo with cGNRs when compared to pGNRs, when irradiated with 250-kVp (tumor volume doubling time 35 days vs 25 days; p=0.002) and 6 MV (17 days vs 13 days; p=0.0052) beams. Conclusion: The enhanced radiosensitization effect observed with very low intratumoral concentrations of gold and megavoltage x-rays using the active targeting strategy holds promise for clinical translation of this strategy from a toxicity and cost-effectiveness perspective and could evolve as a paradigm-changing approach in the field of radiation oncology