Cindy Hancox

Magnetic trapping of rare-earth atoms at millikelvin temperatures

The ability to create quantum degenerate gases has led to the realization of Bose–Einstein condensation of molecules, atom–atom entanglement and the accurate measurement of the Casimir force in atom–surface interactions. With a few exceptions, the achievement of quantum degeneracy relies on evaporative cooling of magnetically trapped atoms to ultracold temperatures. Magnetic traps confine atoms whose electronic magnetic moments are aligned anti-parallel to the magnetic field. This alignment must be preserved during the collisional thermalization of the atomic cloud. Quantum degeneracy has been reached in spherically symmetric, S-state atoms (atoms with zero internal orbital angular momentum). However, collisional relaxation of the atomic magnetic moments of non-S-state atoms (non-spherical atoms with non-zero internal orbital angular momentum) is thought to proceed rapidly. Here we demonstrate magnetic trapping of non-S-state rare-earth atoms, observing a suppression of the interaction anisotropy in collisions. The atoms behave effectively like S-state atoms because their unpaired electrons are shielded by two outer filled electronic shells that are spherically symmetric. Our results are promising for the creation of quantum degenerate gases with non-S-state atoms, and may facilitate the search for time variation of fundamental constants and the development of a quantum computer with highly magnetic atoms.

Comparison of out-of-field photon doses in 6 MV IMRT and neutron doses in proton therapy for adult and pediatric patients.

The purpose of this study was to assess lateral out-of-field doses in 6 MV IMRT (intensity modulated radiation therapy) and compare them with secondary neutron equivalent dose contributions in proton therapy. We simulated out-of-field photon doses to various organs as a function of distance, patients age, gender and treatment volumes based on 3, 6, 9 cm field diameters in the head and neck and spine region. The out-of-field photon doses to organs near the field edge were found to be in the range of 2, 5 and 10 mSv Gy(-1) for 3 cm, 6 cm and 9 cm diameter IMRT fields, respectively, within 5 cm of the field edge. Statistical uncertainties calculated in organ doses vary from 0.2% to 40% depending on the organ location and the organ volume. Next, a comparison was made with previously calculated neutron equivalent doses from proton therapy using identical field arrangements. For example, out-of-field doses for IMRT to lung and uterus (organs close to the 3 cm diameter spinal field) were computed to be 0.63 and 0.62 mSv Gy(-1), respectively. These numbers are found to be a factor of 2 smaller than the corresponding out-of-field doses for proton therapy, which were estimated to be 1.6 and 1.7 mSv Gy(-1) (RBE), respectively. However, as the distance to the field edge increases beyond approximately 25 cm the neutron equivalent dose from proton therapy was found to be a factor of 2-3 smaller than the out-of-field photon dose from IMRT. We have also analyzed the neutron equivalent doses from an ideal scanned proton therapy (assuming not significant amount of absorbers in the treatment head). Out-of-field doses were found to be an order of magnitude smaller compared to out-of-field doses in IMRT or passive scattered proton therapy. In conclusion, there seem to be three geometrical areas when comparing the out-of-target dose from IMRT and (passive scattered) proton treatments. Close to the target (in-field, not analyzed here) protons offer a distinct advantage due to the lower integral dose. Out-of-field, but within approximately 25 cm from the field edge, the scattered photon dose in IMRT turned out to be roughly a factor of 2 lower than the neutron equivalent dose from proton therapy for the fields considered in this study. At larger distances to the field (beyond approximately 25 cm), protons offer an advantage, resulting in doses that are roughly a factor of 2-3 lower.

Calculated organ doses from selected prostate treatment plans using Monte Carlo simulations and an anatomically realistic computational phantom

There is growing concern about radiation-induced second cancers associated with radiation treatments. Particular attention has been focused on the risk to patients treated with intensity-modulated radiation therapy (IMRT) due primarily to increased monitor units. To address this concern we have combined a detailed medical linear accelerator model of the Varian Clinac 2100 C with anatomically realistic computational phantoms to calculate organ doses from selected treatment plans. This paper describes the application to calculate organ-averaged equivalent doses using a computational phantom for three different treatments of prostate cancer: a 4-field box treatment, the same box treatment plus a 6-field 3D-CRT boost treatment and a 7-field IMRT treatment. The equivalent doses per MU to those organs that have shown a predilection for second cancers were compared between the different treatment techniques. In addition, the dependence of photon and neutron equivalent doses on gantry angle and energy was investigated. The results indicate that the box treatment plus 6-field boost delivered the highest intermediate- and low-level photon doses per treatment MU to the patient primarily due to the elevated patient scatter contribution as a result of an increase in integral dose delivered by this treatment. In most organs the contribution of neutron dose to the total equivalent dose for the 3D-CRT treatments was less than the contribution of photon dose, except for the lung, esophagus, thyroid and brain. The total equivalent dose per MU to each organ was calculated by summing the photon and neutron dose contributions. For all organs non-adjacent to the primary beam, the equivalent doses per MU from the IMRT treatment were less than the doses from the 3D-CRT treatments. This is due to the increase in the integral dose and the added neutron dose to these organs from the 18 MV treatments. However, depending on the application technique and optimization used, the required MU values for IMRT treatments can be two to three times greater than 3D CRT. Therefore, the total equivalent dose in most organs would be higher from the IMRT treatment compared to the box treatment and comparable to the organ doses from the box treatment plus the 6-field boost. This is the first time when organ dose data for an adult male patient of the ICRP reference anatomy have been calculated and documented. The tools presented in this paper can be used to estimate the second cancer risk to patients undergoing radiation treatment.

Lineshape asymmetry for joint coherent population trapping and three-photon N resonances

We show that a characteristic two-photon lineshape asymmetry arises in coherent population trapping (CPT) and three-photon (N) resonances, because both resonances are simultaneously induced by modulation sidebands in the interrogating laser light. The N resonance is a three-photon resonance in which a two-photon Raman excitation is combined with a resonant optical pumping field. This joint CPT and N resonance can be the dominant source of lineshape distortion, with direct relevance for the operation of miniaturized atomic frequency standards. We present the results of both an experimental study and theoretical treatment of the asymmetry of the joint CPT and N resonance under conditions typical to the operation of an N resonance clock.

SU‐FF‐T‐439: Dynamic Monte Carlo Dose Calculations for IMRT in Geant4

Purpose: The effectiveness of IMRT to deliver highly conformal doses can be compromised in thoracic patients due to the presence of breathing motion. In order to study motion effects we have implemented a dynamic MLC model in a Geant4 based Monte Carlo system that allows movement of the leaves during the course of a simulation in direct analog to dynamic treatment delivery. We use Geant4 to investigate the effect of breathing motion in three lung patients. Method and Materials: We created a model of the Varian 2100C/D x/y jaws and MLC in Geant4 based on machine drawings. We validated our model by comparing open‐field penumbra and closed‐leaf transmission with measurements taken in‐house and found in the literature. We compared field‐by‐field photon fluence for a seven‐field IMRT treatment plan using dynamic dose delivery in Geant4 with that from an established Monte Carlo package. Step‐and‐shoot dose delivery was used, with each field utilizing between 23 and 39 distinct leaf configurations. Motion effects were investigated for three lung patients. For each patient, 4DCT data were binned into six breathing phases and then registered onto the exhale phase using deformable registration. Dynamic dose delivery was simulated for each breathing phase for comparison. Results: Closed‐leaf transmission of 1.5% using our Geant4 MLC model is in good agreement with values published in the literature and exhibits the characteristic picket‐fence profile expected from the tongue‐and‐groove geometry. Field‐by‐field comparison of photon fluence projected onto the isocenter plane shows 2%/2 mm agreement between Geant4 and the MLC model developed by Siebers et al (Phys. Med. Biol. 47, 3225 (2002)). Conclusion: We have implemented 4D Monte Carlo in Geant4 for IMRT using an in‐house model of the Varian x/y jaws and MLC. We have validated our model and simulated IMRT treatments in three lung patients based on 4DCT.

SU‐FF‐T‐409: Simulation of the Risk of Developing a Second Cancer Due to the Scattered Radiation for Different Treatment Modalities at Different Sites

Purpose: Pediatric patients undergoing radiation therapy receive higher doses at organs and tissues lateral to the primary field edge and stand a higher risk of developing secondary cancers during the span of their longer post treatment lives. There is a concern in particular for modern treatment techniques, like IMRT or proton therapy. Consequently, we did extensive simulations on organ specific equivalent doses.Method and Materials: Age and gender specific whole‐body phantoms (an adult, a 9 month old male, a 4‐year old female, an 8‐year old female, an 11‐year old male, and a 14‐year old male) have been implement in Geant4 in order to determine the doses from the secondary radiation in patients undergoing proton and IMRTtreatments. We have used several fields planned for different treatment sites (head & neck, abdominal) to determine the secondary organdoses. Lifetime attributable risk (LAR) is also determined based on two risk models, namely, excess relative risk and excess absolute risk. Results: For passive scatteredproton therapy,neutron equivalent doses to various organs are dominated by the treatment head contribution for small fields and by the patient contribution for large fields. Passive scatteredproton therapy and scanned beam proton therapy were simulated separately. The maximum doseorgans received in the spinal field was 8.5mSv/Gy. Simulated LAR for passive scatteredproton therapy for an 8‐year old female and 11‐year old male patients for the lung are found to be 1.05% and 0.38%, respectively. Conclusions: We find the scattered photon and neutrondoses to various organs and tissues decrease with phantom age and it depends on the field size. Breast in female and lung/bronchi in male patients show the largest risk but also the baseline values for these organs are the highest. For most organ the risks are below the baseline risk.

SU‐FF‐T‐430: Comparison of Dose Volume Histograms From Selected Proton Therapy Treatments Between Geant4.5.0 and Geant4.9.0 Monte Carlo Simulations with and Without Dynamic Densities

Introduction: In order to reduce computation time for protonMonte Carlo dose calculations, our group has introduced a method to improve the voxel navigation algorithm in Geant4.5.0 and to dynamically assign material density in a given voxel element during particle transport. In the meantime, we have developed a new fast voxel navigation technique tailored to Geant4.9.0. Furthermore, Geant4.9.0 shows differences in the physics setup and tracking algorithm compared to the Geant4.5.0. Thus, the aim of this study was to assess whether defining all material densities corresponding to a given HU from the CT scan, i.e. without applying dynamic densities, would significantly impact the runtime for the new Geant4 version. In this study we compare dose‐volume histograms as well as runtime from selected proton therapy treatments between Geant4 Monte Carlo calculations using Geant4.5.0 with the dynamic density method and Geant4.9.0 without the dynamic density method. Methods and Materials: The two plans considered in this study were a spinal cord astrocytoma treatment and a prostate treatment. The differences between the Monte Carlo simulations with and without dynamic densities for each treatment were evaluated by plotting the dose‐volume histograms of the PTV and several other organs‐at‐risk considered in the treatment plan. In addition, the dose contours for each treatment and the contour differences were evaluated. Results: There are small discrepancies between dose calculations with and without dynamic densities, particularly in the GTV of both treatments. These discrepancies may suggest problems with the dynamic density method or problems with the CT conversion process used for generating the patient geometry. Conclusion: A comparison of dose calculations using Geant4.5.0 (including improved voxel navigation and dynamic density algorithm) and using Geant4.9.0 (including our new voxel navigation algorithm and no dynamic density algorithm) yields slight but not clinically significant differences with comparable run time.

SU‐GG‐T‐357: Geant4 Benchmarking for IMRT Dose Calculations

Purpose: The purpose of this work is to test the accuracy of the Geant4 release 9.0 for use in IMRT patient dose calculations. This is of importance since major changes were made in the multiple scattering engine starting with Geant4 release 8.0, but the package has not since been extensively tested for low energy photons relevant for simulating patient treatments. We compare calculated dose distributions in homogeneous and heterogeneous multilayer phantoms with experiments and with results from a well‐benchmarked Monte Carlo toolkit, DPM (Dose Planning Method). Method and Materials: We investigated the ability of Geant4 to reproduce experimentally measured lateral and depth dose profiles in a homogeneous water phantom irradiated with 10×10 cm2, 30×30 cm2 and 40×40 cm2 open fields. The results were compared with both DPM and experimental data. Next, Geant4 was used to calculate dose distributions in heterogeneous phantoms consisting of slabs of water, bone and lung, paying particular attention to the handling of material interfaces. The results were compared with DPM. Results: We find that Geant4 and DPM give nearly identical results for each of the heterogeneous phantoms, agreeing to better than 2% in the depth dose through each material and the lateral dose profiles across all material boundaries. The agreement between Geant4, DPM and experimental data in the homogeneous water phantom is also quite good, with all three agreeing to within 2% in depth dose (neglecting the buildup region), and in lateral dose profiles taken at 5 and 10 cm depths (neglecting the beam penumbra). Conclusion: Homogeneous and heterogeneous phantom studies show that Geant4 yields accurate dose distributions for clinically relevant photon fields.

SU‐GG‐T‐340: A Comparison of Geant4 and DPM for 4D Monte Carlo of Lung IMRT

Purpose: Respiratory motion during IMRT for lungtumors presents a particular challenge. Geant4 is an attractive tool for investigating motion effects since it can accommodate time‐dependent geometries. Leaf positions and patient geometry may be updated during the course of the simulation and interplay effects directly studied. Here we investigate the applicability of Geant4 for patient dose calculations in the lung, comparing field‐by‐field results for an IMRT patient with results obtained using the code DPM. Method and Materials: Geant4, being a general purpose code, has many adjustable parameters available to the user and their default values may not be appropriate for patient dose calculations. Additionally, while it can import CT data and accurately model the complex patient geometry, it is not optimized for fast patient dose calculations. In particular, it does not efficiently handle voxel‐to‐voxel variations in material density. We modified the Geant4 (release 9.0) source code to optimize the handling of density variations in the voxelized patient to greatly speed up simulation time. Similar modifications had been made to an earlier release (release 5.0) for proton therapy but this is the first such optimization for IMRT. As part of our project on using Geant4 for 4D dose calculation in the lung we compared the results from Geant4 using different user parameters with the results obtained using DPM for a five‐field IMRT treatment plan for a lung patient. Results: The efficiency of Geant4, version 9.0, was greatly improved by modifying the voxel tracking algorithm. We found that DPM and Geant4 agreed to within a few percent after adjusting just two user parameters, the simulation stepsize limit and the secondary range cutoff (1 mm and 0.1 mm, respectively). Conclusion: Geant4 provides a natural platform for 4D studies and has been shown to be sufficiently accurate for patient dose calculations in lung.