Laser-Driven Transient Phase Oscillations in Individual Spin Crossover Particles

An unusual expansion dynamics of individual spin crossover nanoparticles is studied by ultrafast transmission electron microscopy. After exposure to nanosecond laser pulses, the particles exhibit considerable length oscillations during and after their expansion. The vibration period of 50-100 ns is of the same order of magnitude as the time that the particles need for a transition from the low-spin to the high-spin state. The observations are explained in Monte Carlo calculations using a model where elastic and thermal coupling between the molecules within a crystalline spin crossover particle govern the phase transition between the two spin states. The experimentally observed length oscillations are in agreement with the calculations, and it is shown that the system undergoes repeated transitions between the two spin states until relaxation in the high-spin state occurs due to energy dissipation. Spin crossover particles are therefore a unique system where a resonant transition between two phases occurs in a phase transformation of first order.

The effects of different photon beam energies in stereotactic radiosurgery with cones

BACKGROUND

Stereotactic radiosurgery (SRS) relies on small fields to ablate lesions. Currently, linac based treatment is delivered via circular cones using a 6 MV beam. There is interest in both lower energy photon beams, which can offer steeper dose fall off as well as higher energy photon beams, which have higher dose rates, thus reducing radiation delivery times. Of interest in this study is the 2.5 MV beam developed for imaging applications and both the 6 and 10 MV flattening-filter-free (FFF) beams, which can achieve dose rates up to 2400 cGy/min.

PURPOSE

This study aims to assess the benefit and feasibility among different energy beams ranging from 2.5 to 10 MV beams by evaluating the dosimetric effects of each beam and comparing the dose to organs-at-risk (OARs) for two separate patient plans. One based on a typical real patient tremor utilizing a 4 mm cone and the other a typical brain metastasis delivered with a 10 mm cone.

METHODS

The Monte Carlo codes BEAMnrc/DOSXYZnrc were used to generate beams of 2.5 MV, 6 MV-FFF, 6 MV-SRS, 6 MV, 10 MV-FFF, and 10 MV from a Varian TrueBeam except 6 MV-SRS, which is taken from a Varian TX model linear accelerator. Each beam's energy spectrum, mean energy, %dd curve, and dose profile were obtained by analyzing the simulated beams. Calculated patient dose distributions were compared among six different energy beam configurations based on a realistic treatment plan for thalamotomy and a conventional brain metastasis plan. Dose to OARs were evaluated using dose-volume histograms for the same target dose coverage.

RESULTS

The mean energies of photons within the primary beam projected area were insensitive to cone sizes and the values of percentage depth-dose curves (%dd) at d = 5 cm and SSD = 95 cm for a 4 mm (10 mm) cone ranges from 62.6 (64.4) to 82.2 (85.7) for beam energy ranging from 2.5 to 10 MV beams, respectively. Doses to OARs were evaluated among these beams based on real treatment plans delivering 15 000 and 2200 cGy to the target with a 4 and 10 mm cone, respectively. The maximum doses to the brainstem, which is 10 mm away from the isocenter, was found to be 434 (300), 632 (352), 691 (362), 733 (375), 822 (403), and 975 (441) cGy for 2.5 MV, 6 MV-FFF, 6 MV-SRS, 6 MV, 10 MV-FFF, and 10 MV beams delivering 15 000 (2200) cGy target dose, respectively.

CONCLUSION

Using the 6 MV-SRS as reference, changes of the maximum dose (691 cGy) to the brain stem are -37%, -9%, +6%, +19%, and 41% for 2.5 MV, 6 MV-FFF, 6 MV, 10 MV-FFF, and 10 MV beams, respectively, based on the thalamotomy plan, where the "-" or "+" signs indicate the percentage decrease or increase. Changes of the maximum dose (362 cGy) to brain stem, based on the brain metastasis plan are much less for respective beam energies. The sum of 21 arcs beam-on time was 39 min on our 6 MV-SRS beam with 1000 cGy/min for thalamotomy. The beam-on time can be reduced to 16 min with 10 MV-FFF.

Impact of the cavity on sinus wall dose in magnetic resonance image-guided radiation therapy

PURPOSE

This study aims to investigate the impact of the cavity on the sinus wall dose by comparing dose distributions with and without the sinus under magnetic fields using Monte Carlo calculations.

METHODS

A water phantom containing a sinus cavity (Empty) was created, and dose distributions were calculated for 1, 2, and 4 irradiation fields with 6 MV photons. The sinus in the phantom was then filled with water (Full), and the dose distributions were calculated again. The sinus was set to cubes of 2 cm and 4 cm. The magnetic field was applied to the transverse and inline direction under the magnetic flux densities of 0 T, 0.35 T, 0.5 T, 1.0 T, and 1.5 T. The dose distributions were analyzed by the dose difference, dose volume histogram, and D₂ with sinus wall thicknesses of 1 and 5 mm.

RESULTS

D₂ in the "Empty" sinus wall under transverse magnetic fields for the 1-field and 4-field cases was 51.9% higher and 3.7% lower than that in the "Full" sinus wall at 1.5 T, respectively. Meanwhile, D₂ in the Empty sinus wall under inline magnetic fields for 1-field and 4-fields was 2.3% and 2.6% lower than that in the "Full" sinus at B = 0 T, respectively, whereas D₂ was 0.9% and 0.7% larger at 1.0 T, respectively.

CONCLUSIONS

The impact of the cavity on the sinus wall dose depends on the magnetic flux density, direction of the magnetic field and irradiation beam, and number of irradiation fields.

Effect of ICRU report 90 recommendations on Monte Carlo calculated kQ for ionization chambers listed in the Addendum to AAPM's TG-51 protocol

PURPOSE

The ICRU has published new recommendations for ionizing radiation dosimetry. In this work, the effect of recommendations on the water-to-air and graphite-to-air restricted mass electronic stopping power ratios (s_{w, air} and s_{g, air} ) and the individual perturbation correction factors P_i was calculated. The effect on the beam quality conversion factors k_Q for reference dosimetry of high-energy photon beams was estimated for all ionization chambers listed in the Addendum to AAPM's TG-51 protocol.

METHODS

The s_{w, air} , s_{g, air} , individual P_i, and k_Q were calculated using EGSnrc Monte Carlo code system and key data of both ICRU report 37 and ICRU report 90. First, the P_i and k_Q were calculated using precise models of eight ionization chambers: NE2571 (Nuclear Enterprise), 30013, 31010, 31021 (PTW), Exradin A12, A12S, A1SL (Standard imaging), and FC-65P (IBA). In this simulation, the radiation sources were one ⁶⁰ Co beam and ten photon beams with nominal energy between 4 MV and 25 MV. Then, the change in k_Q for ionization chambers listed in the Addendum to AAPM's TG-51 protocol was calculated by changing the specification of the simple-model of ionization chamber. The simple-models were made with only cylindrical component modules. In this simulation, the radiation sources of ⁶⁰ Co beam and 24 MV photon beam were used.

RESULTS

The significant changes (p < 0.05) were observed for s_{w, air} , s_{g, air} , the wall correction factor P_wall , and the waterproofing sleeve correction factor P_sleeve . The decrease in s_{w, air} varied from -0.57% for a ⁶⁰ Co beam to -0.36% for the highest beam quality. The decrease in s_{g, air} varied from -0.72% to -1.12% in the same range. The changes in P_wall and P_sleeve were up to 0.41% and 0.14% and those maximum changes were observed for the ⁶⁰ Co beam. All changes in the central electrode correction factor P_cel , the stem correction factor P_stem , and the replacement correction factor P_repl were from -0.02% to 0.12%. Those changes were statistically insignificant (p = 0.07 or more) and were independent of photon energy. The change in k_Q was mainly characterized by the change in s_{w, air} , P_wall , and P_sleeve . The relationship between the change in k_Q and the beam quality index was linear approximately. The changes in k_Q of the simple-models were agreed with those of the precise-models within 0.08%.

CONCLUSION

The effects of ICRU-90 recommendations on k_Q for the ionization chambers listed in the Addendum to AAPM's TG-51 protocol were from -0.15% to 0.30%. To remove the known systematic effect on the clinical reference dosimetry, the k_Q based on ICRU-37 should be updated to the k_Q based on ICRU-90.

A simple technique to improve calculated skin dose accuracy in a commercial treatment planning system

Radiation dermatitis during radiotherapy is correlated with skin dose and is a common clinical problem for head and neck and thoracic cancer patients. Therefore, accurate prediction of skin dose during treatment planning is clinically important. The objective of this study is to evaluate the accuracy of skin dose calculated by a commercial treatment planning system (TPS). We evaluated the accuracy of skin dose calculations by the anisotropic analytical algorithm (AAA) implemented in Varian Eclipse (V.11) system. Skin dose is calculated as mean dose to a contoured structure of 0.5 cm thickness from the surface. The EGSnrc Monte Carlo (MC) simulations are utilized for the evaluation. The 6, 10 and 15 MV photon beams investigated are from a Varian TrueBeam linear accelerator. The accuracy of the MC dose calculations was validated by phantom measurements with optically stimulated luminescence detectors. The calculation accuracy of patient skin doses is studied by using CT based radiotherapy treatment plans including 3D conformal, static gantry IMRT, and VMAT treatment techniques. Results show the Varian Eclipse system underestimates skin doses by up to 14% of prescription dose for the patients studied when external body contour starts at the patient's skin. The external body contour is used in a treatment planning system to calculate dose distributions. The calculation accuracy of skin dose with Eclipse can be considerably improved to within 4% of target dose by extending the external body contour by 1 to 2 cm from the patient's skin. Dose delivered to deeper target volumes or organs at risk are not affected. Although Eclipse treatment planning system has its limitations in predicting patient skin dose, this study shows the calculation accuracy can be considerably improved to an acceptable level by extending the external body contour without affecting the dose calculation accuracy to the treatment target and internal organs at risk. This is achieved by moving the calculation entry point away from the skin.

Monte Carlo evaluation of the potential benefits of flattening filter free beams from the Oncor® clinical linear accelerator

OBJECTIVES

To evaluate the potential privileges of flattening filter-free (FFF) photon beams from Oncor® linac for 6 MV and 18 MV energies.

METHODS

A Monte Carlo (MC) model of Oncor® linac was built using BEAMnrc MCCode and verified by the measured data using 6 MV and 18 MV energies. A comprehensive set of data was also characterized for MC model of Oncor® machine running with and without flattening filter (FF) for 6 MV and 18 MV beams in six field sizes. The investigated characteristics included mean energy, energy spectrum, photon spatial fluence, superficial dose, percent depth dose (PDD), dose output, and out-of-field dose with two indexes of lateral dose profile and isodose curve at three depths.

RESULTS

Using FFF enhanced the energy uniformity 3.4±0.11% (6 MV) and 2.05±0.09% (18 MV) times and improved dose output by factor of 2.91 (6 MV) and 4.2 (18 MV) on the central axis, respectively. Using FFF also reduced the PDD dependencies by 9.1% (6 MV) and 5.57% (18 MV). In addition, using FFF had a lower out-of-field dose due to the reduced head scatter and softer spectra.

CONCLUSIONS

The findings in this study suggested that using FFF, Oncor® machine could achieve better treatment results with lower dose toxicity and a shorter beam-on time.

Characteristics of 2.5 MV beam and imaging dose to patients

PURPOSE

This work provides the beam characteristics and evaluates the imaging dose to patients for a 2.5 MV portal imaging beam.

METHOD AND MATERIALS

The Monte Carlo technique has been used to simulate the 2.5 MV imaging beam. Beam characteristics have been analyzed including the energy spectra and the fluence distributions as a function of position away from the beam central axis. The accuracy of a simulated beam was validated through comparisons between the Monte Carlo calculated and measured dose distributions in a water phantom. The simulated 2.5 MV beam was also used to obtain the absorbed-dose beam quality conversion factor, k_Q, for absorbed dose calibration. The simulated beams were then used to evaluate the imaging dose to patients compared with that from a conventional therapeutic 6 MV beam.

RESULTS

The mean energies of photons and electrons in the 2.5 MV beam are 0.48 MeV and 0.37 MeV respectively. The photon fluence decreases at 20 cm away from the central axis by only up to 30% for this flattening-filter free beam. The values of %dd curves at depth = 10 cm are 53% and 63% for 10 × 10 cm² and 40 × 40 cm² fields respectively. Portal imaging doses (D50 of the DVHs) to the eyes, heart and bladder from representative pairs of 2.5 MV (or 6 MV) setup images are 1.8 cGy (3.5 cGy), 1.1 cGy (2.5 cGy) and 1.0 cGy (2.4 cGy) for head, thorax and pelvis image acquisitions respectively.

CONCLUSION

We provide dosimetric data, as well as estimates of organ imaging doses, for this 2.5 MV beam. When clinical default imaging protocols are used, the imaging dose from the 2.5 MV beam is about 50% of that from a 6 MV beam. The information can be used to select image procedures and to estimate organ dose from imaging procedures.

Monte Carlo Calculations Supporting Patient Plan Verification in Proton Therapy

Patient’s treatment plan verification covers substantial amount of the quality assurance (QA) resources; this is especially true for Intensity-Modulated Proton Therapy (IMPT). The use of Monte Carlo (MC) simulations in supporting QA has been widely discussed, and several methods have been proposed. In this paper, we studied an alternative approach from the one being currently applied clinically at Centro Nazionale di Adroterapia Oncologica (CNAO). We reanalyzed the previously published data (Molinelli et al. (1)), where 9 patient plans were investigated in which the warning QA threshold of 3% mean dose deviation was crossed. The possibility that these differences between measurement and calculated dose were related to dose modeling (Treatment Planning Systems (TPS) vs. MC), limitations on dose delivery system, or detectors mispositioning was originally explored, but other factors, such as the geometric description of the detectors, were not ruled out. For the purpose of this work, we compared ionization chambers’ measurements with different MC simulation results. It was also studied that some physical effects were introduced by this new approach, for example, inter-detector interference and the delta ray thresholds. The simulations accounting for a detailed geometry typically are superior (statistical difference – p-value around 0.01) to most of the MC simulations used at CNAO (only inferior to the shift approach used). No real improvement was observed in reducing the current delta ray threshold used (100 keV), and no significant interference between ion chambers in the phantom were detected (p-value 0.81). In conclusion, it was observed that the detailed geometrical description improves the agreement between measurement and MC calculations in some cases. But in other cases, position uncertainty represents the dominant uncertainty. The inter-chamber disturbance was not detected for the therapeutic protons energies, and the results from the current delta threshold are acceptable for MC simulations in IMPT.

Medical physics aspects of the synchrotron radiation therapies: Microbeam radiation therapy (MRT) and synchrotron stereotactic radiotherapy (SSRT)

Stereotactic Synchrotron Radiotherapy (SSRT) and Microbeam Radiation Therapy (MRT) are both novel approaches to treat brain tumor and potentially other tumors using synchrotron radiation. Although the techniques differ by their principles, SSRT and MRT share certain common aspects with the possibility of combining their advantages in the future. For MRT, the technique uses highly collimated, quasi-parallel arrays of X-ray microbeams between 50 and 600 keV. Important features of highly brilliant Synchrotron sources are a very small beam divergence and an extremely high dose rate. The minimal beam divergence allows the insertion of so called Multi Slit Collimators (MSC) to produce spatially fractionated beams of typically ∼25-75 micron-wide microplanar beams separated by wider (100-400 microns center-to-center(ctc)) spaces with a very sharp penumbra. Peak entrance doses of several hundreds of Gy are extremely well tolerated by normal tissues and at the same time provide a higher therapeutic index for various tumor models in rodents. The hypothesis of a selective radio-vulnerability of the tumor vasculature versus normal blood vessels by MRT was recently more solidified. SSRT (Synchrotron Stereotactic Radiotherapy) is based on a local drug uptake of high-Z elements in tumors followed by stereotactic irradiation with 80 keV photons to enhance the dose deposition only within the tumor. With SSRT already in its clinical trial stage at the ESRF, most medical physics problems are already solved and the implemented solutions are briefly described, while the medical physics aspects in MRT will be discussed in more detail in this paper.

Analysis of JSI TRIGA MARK II reactor physical parameters calculated with TRIPOLI and MCNP

New computational model of the JSI TRIGA Mark II research reactor was built for TRIPOLI computer code and compared with existing MCNP code model. The same modelling assumptions were used in order to check the differences of the mathematical models of both Monte Carlo codes. Differences between the TRIPOLI and MCNP predictions of keff were up to 100pcm. Further validation was performed with analyses of the normalized reaction rates and computations of kinetic parameters for various core configurations.

Validation of absolute axial neutron flux distribution calculations with MCNP with 197Au(n,γ)198Au reaction rate distribution measurements at the JSI TRIGA Mark II reactor

The calculation of axial neutron flux distributions with the MCNP code at the JSI TRIGA Mark II reactor has been validated with experimental measurements of the (197)Au(n,γ)(198)Au reaction rate. The calculated absolute reaction rate values, scaled according to the reactor power and corrected for the flux redistribution effect, are in good agreement with the experimental results. The effect of different cross-section libraries on the calculations has been investigated and shown to be minor.

Monte Carlo-based dose calculation engine for minibeam radiation therapy

Minibeam radiation therapy (MBRT) is an innovative radiotherapy approach based on the well-established tissue sparing effect of arrays of quasi-parallel micrometre-sized beams. In order to guide the preclinical trials in progress at the European Synchrotron Radiation Facility (ESRF), a Monte Carlo-based dose calculation engine has been developed and successfully benchmarked with experimental data in anthropomorphic phantoms. Additionally, a realistic example of treatment plan is presented. Despite the micron scale of the voxels used to tally dose distributions in MBRT, the combination of several efficiency optimisation methods allowed to achieve acceptable computation times for clinical settings (approximately 2 h). The calculation engine can be easily adapted with little or no programming effort to other synchrotron sources or for dose calculations in presence of contrast agents.

Consideration of the radiation dose delivered away from the treatment field to patients in radiotherapy

Radiation delivery to cancer patients for radiotherapy is invariably accompanied by unwanted radiation to other parts of the patient's body. Traditionally, considerable effort has been made to calculate and measure the radiation dose to the target as well as to nearby critical structures. Only recently has attention been focused also on the relatively low doses that exist far from the primary radiation beams. In several clinical scenarios, such doses have been associated with cardiac toxicity as well as an increased risk of secondary cancer induction. Out-of-field dose is a result of leakage and scatter and generally difficult to predict accurately. The present review aims to present existing data, from measurements and calculations, and discuss its implications for radiotherapy.