H Jiang

Accurate Monte Carlo simulations for nozzle design, commissioning and quality assurance for a proton radiation therapy facility

Monte Carlo dosimetry calculations are essential methods in radiation therapy. To take full advantage of this tool, the beam delivery system has to be simulated in detail and the initial beam parameters have to be known accurately. The modeling of the beam delivery system itself opens various areas where Monte Carlo calculations prove extremely helpful, such as for design and commissioning of a therapy facility as well as for quality assurance verification. The gantry treatment nozzles at the Northeast Proton Therapy Center (NPTC) at Massachusetts General Hospital (MGH) were modeled in detail using the GEANT4.5.2 Monte Carlo code. For this purpose, various novel solutions for simulating irregular shaped objects in the beam path, like contoured scatterers, patient apertures or patient compensators, were found. The four-dimensional, in time and space, simulation of moving parts, such as the modulator wheel, was implemented. Further, the appropriate physics models and cross sections for proton therapy applications were defined. We present comparisons between measured data and simulations. These show that by modeling the treatment nozzle with millimeter accuracy, it is possible to reproduce measured dose distributions with an accuracy in range and modulation width, in the case of a spread-out Bragg peak (SOBP), of better than 1 mm. The excellent agreement demonstrates that the simulations can even be used to generate beam data for commissioning treatment planning systems. The Monte Carlo nozzle model was used to study mechanical optimization in terms of scattered radiation and secondary radiation in the design of the nozzles. We present simulations on the neutron background. Further, the Monte Carlo calculations supported commissioning efforts in understanding the sensitivity of beam characteristics and how these influence the dose delivered. We present the sensitivity of dose distributions in water with respect to various beam parameters and geometrical misalignments. This allows the definition of tolerances for quality assurance and the design of quality assurance procedures.

Simulation of organ-specific patient effective dose due to secondary neutrons in proton radiation treatment.

Cancer patients undergoing radiation treatment are exposed to high doses to the target (tumour), intermediate doses to adjacent tissues and low doses from scattered radiation to all parts of the body. In the case of proton therapy, secondary neutrons generated in the accelerator head and inside the patient reach many areas in the patient body. Due to the improved efficacy of management of cancer patients, the number of long term survivors post-radiation treatment is increasing substantially. This results in concern about the risk of radiation-induced cancer appearing at late post-treatment times. This paper presents a case study to determine the effective dose from secondary neutrons in patients undergoing proton treatment. A whole-body patient model, VIP-Man, was employed as the patient model. The geometry dataset generated from studies made on VIP-Man was implemented into the GEANT4 Monte Carlo code. Two proton treatment plans for tumours in the lung and paranasal sinus were simulated. The organ doses and ICRP-60 radiation and tissue weighting factors were used to calculate the effective dose. Results show whole body effective doses for the two proton plans of 0.162 Sv and 0.0266 Sv, respectively, to which the major contributor is due to neutrons from the proton treatment nozzle. There is a substantial difference among organs depending on the treatment site.

Adaptation of GEANT4 to Monte Carlo dose calculations based on CT data

The GEANT4 Monte Carlo code provides many powerful functions for conducting particle transport simulations with great reliability and flexibility. However, as a general purpose Monte Carlo code, not all the functions were specifically designed and fully optimized for applications in radiation therapy. One of the primary issues is the computational efficiency, which is especially critical when patient CT data have to be imported into the simulation model. In this paper we summarize the relevant aspects of the GEANT4 tracking and geometry algorithms and introduce our work on using the code to conduct dose calculations based on CT data. The emphasis is focused on modifications of the GEANT4 source code to meet the requirements for fast dose calculations. The major features include a quick voxel search algorithm, fast volume optimization, and the dynamic assignment of material density. These features are ready to be used for tracking the primary types of particles employed in radiation therapy such as photons, electrons, and heavy charged particles. Recalculation of a proton therapy treatment plan generated by a commercial treatment planning program for a paranasal sinus case is presented as an example.

4D Monte Carlo simulation of proton beam scanning: modelling of variations in time and space to study the interplay between scanning pattern and time-dependent patient geometry

When dosimetric effects in time-dependent geometries are studied, usually either the results of individual three-dimensional (3D) calculations are combined or probability-based approaches are applied. These methods may become cumbersome and time-consuming if high time resolution is required or if the geometry is complex. Furthermore, it is difficult to study double-dynamic systems, e.g., to investigate the influence of time-dependent beam delivery (i.e., magnetically moving beam spots in proton beam scanning) on the dose deposition in a moving target. We recently introduced the technique of 4D Monte Carlo dose calculation to model continuously changing geometries. In intensity modulated proton therapy, dose is delivered by individual pristine Bragg curves. Dose spots are positioned in the patient by varying magnetic field and beam energy. If the movement of these dose spots occurs during significant respiratory motion, interplay effects can take place. Because of the inhomogeneity of individual subfields, the consequences of motion can be more severe than in conventional proton therapy. We demonstrate how the technique of 4D Monte Carlo can be used to study interplay effects in proton beam scanning. Time-dependent beam delivery to a changing patient geometry is simulated in a single 4D dose calculation. Interplay effects between respiratory motion and beam scanning speed are demonstrated.

Effects of Hounsfield number conversion on CT based proton Monte Carlo dose calculations.

The Monte Carlo method provides the most accurate dose calculations on a patient computed tomography (CT) geometry. The increase in accuracy is, at least in part, due to the fact that instead of treating human tissues as water of various densities as in analytical algorithms, the Monte Carlo method allows human tissues to be characterized by elemental composition and mass density, and hence allows the accurate consideration of all relevant electromagnetic and nuclear interactions. On the other hand, the algorithm to convert CT Hounsfield numbers to tissue materials for Monte Carlo dose calculation introduces uncertainties. There is not a simple one to one correspondence between Hounsfield numbers and tissue materials. To investigate the effects of Hounsfield number conversion for proton Monte Carlo dose calculations, clinical proton treatment plans were simulated using the Geant4 Monte Carlo code. Three Hounsfield number to material conversion methods were studied. The results were compared in forms of dose volume histograms of gross tumor volume and clinical target volume. The differences found are generally small but can be dosimetrically significant. Further, different methods may cause deviations in the predicted proton beam range in particular for deep proton fields. Typically, slight discrepancies in mass density assignments play only a minor role in the target region, whereas more significant effects are caused by different assignments in elemental compositions. In the presence of large tissue inhomogeneities, for head and neck treatments, treatment planning decisions could be affected by these differences because of deviations in the predicted tumor coverage. Outside the target area, differences in elemental composition and mass density assignments both may play a role. This can lead to pronounced effects for organs at risk, in particular in the spread-out Bragg peak penumbra or distal regions. In addition, the significance of the elemental composition effect (dose to water vs. dose to tissue) is tissue-type dependent and is also affected by nuclear reactions.

SU‐FF‐T‐333: Monte Carlo Simulations Using Whole‐Body Pediatric and Adult Phantoms as Virtual Patients to Assess Secondary Organ Doses in Proton Radiation Therapy

Introduction: Early cancer detection combined with new treatment technologies has resulted in higher numbers of long‐term cancer survivors. The risk of radiation‐induced secondary cancers to tissues away from the PTV is a growing concern in particular for pediatric patients. The focus of this project is to use whole‐body pediatric phantoms in Monte Carlodose calculations in order to determine the effective dose from secondary radiation in patients undergoing protontreatment.Methods: Age and gender specific pediatric phantoms have been implemented into the Geant4 Monte Carlo package for organdose calculations. A proton therapytreatment plan for a pediatric head and neck tumor case was chosen to address the significance of age dependent phantoms for radiation protection calculations. To mimic radiation therapytreatment, the setup of the phantom position was based on field parameters (based on a full treatment head model), including gantry angle, couch angle, and iso‐center position. We distinguish between secondary radiation from the treatment head and secondary radiation generated within the patient. Results: Results using an adult phantom as well as phantoms of a 4‐year old female and an 11‐year old male were analyzed. Organdoses and radiation and tissue weighting factors were used to calculate the effective dose. For protontreatments with double‐scattering system, range modulator and aperture, a significant number of secondary neutrons are generated in the treatment head. More important, differences between phantoms (age dependent) were found with respect to dose to specific organs and relative importance of neutrons generated in the patient versus neutrons from the treatment head. Conclusion: We present results of doses to various anatomic sites in the human body for whole‐body phantoms. The magnitude of secondary dose in organs/tissues depends on the distance from the PTV. For the first time, the significance of age‐dependent phantoms for secondary dose calculations was studied.

Comparison of pencil-beam and Monte Carlo calculated dose distributions for proton therapy of skull-base and para-spinal tumors

Purpose: Pencil beam algorithms rely on kernels to model proton range in density-scaled water equivalent material. Monte Carlo dose calculation methods are more accurate by design. This study addresses the issue of clinical significance of potential differences between a commercial pencil-beam algorithm and Monte Carlo dose calculation. Skull-base or paraspinal tumors are challenging for dose calculations due to interfaces between high and low density areas in the irradiated volume. In addition, in particular for para-spinal cases, there are often metallic implants that not only distort the CT image but also affect the accuracy of dose calculations.

Proton Monte Carlo in the clinic

Proton Monte Carlo dose calculation has been implemented clinically for use in parallel to a commercial treatment planning system at Massachusetts General Hospital, Boston, USA. Treatment heads were modeled in detail and a software link was created between the treatment machine control system and the Monte Carlo program to transfer information about the treatment head settings. To describe the patient anatomy, Hounsfield Units were converted into materials with explicit element composition and density. A link of the Monte Carlo program to the departmental patient database and the commercial planning system was established to import treatment information. This presentation gives a roadmap to clinical implementation of Monte Carlo dose calculation covering all relevant aspects.

SU‐E‐T‐370: Dosimetric Comparison Between Single‐Lumen and Two Types of Multi‐Lumen Catheters Used for Partial Breast Brachytherapy

Purpose: compare two optimization methods using brachyvision and compare the dosimetry between multimen‐lumen and single‐lumen catheters used for partial breast brachytherapy Methods: Single‐lumen catheter is the Mammosite device. Two multi‐lumen devices are used: contura 5‐lumen and Hologic 4‐lumen devices. The planning system is Brachyvision. PTV is constructed by expanding the balloon by 1cm, and shaping the volume to exclude the pectoralis muscle and leave a 0.5cm margin from the skin. PTV_EVAL is defined as PTV subtracts the balloon. We also define two critical‐organ ROIs: skin region and chest region, where skin region is the skin side to PTV excluding PTV and the chest region is the chest side to PTV excluding PTV. One optimization method is to achieve uniform 3.4 Gy/fx on the PTV surface, and the other method is an inverse optimization method to achieve at least 95% of PTV receive dose higher than 3.4 Gy/fx and less than 10% of PTV receive more than 5Gy/fx with upper bound to skin region and chest region. Patients are grouped into 4 categories: (a) Skin and chest wall are within 1cm from the balloon; (b) only skin (c) only chest wall is within 1cm from the balloon and (d) both skin and chest wall are within 1cm from the balloon. Results: For category (a) patients, both optimization methods give similar satisfactory Results: 98% and 99% of PTV_EVAL receive 3.4 Gy/fx for single‐lumen and multi‐lumen catheters, respectively; while the skindose are less than 3.4Gy/fx. For patients in categories (b) and (d), the inverse optimization method and multi‐lumen catheter gives significant lower skindose. Conclusions: single‐lumen, multi‐ lumen device provide similarly satisfactory result for patients with skin and chest at least 1cm from the balloon; for other patients, the inverse optimization method and multi‐lumen catheter device give significantly less skindose.

SU‐FF‐T‐21: The Optimal Number of Lumens for Multi‐Lumen Devices Used in Partial Breast Irradiation

Purpose: To compare the dosimetry of single‐lumen MammoSite device to multi‐lumen devices and to find the optimal number of lumens Methods and Materials: Four designs of multi‐lumen devices are studied: a central lumen with two, three, four, or six off‐set lumens which are 180°, 120°, 90° and 60° rotationally symmetric around the central lumen. All lumens are straight and parallel to the central lumen. Patients of three different clinical conditions are simulated: (1) skin and chest wall spacing from balloon are larger than 10 mm, (2) smaller than 5 mm and (3) between 5 mm and 10 mm. 10 mm extension from the balloon is defined as planning target volume (PTV). In clinical conditions (2) and (3), where the spacing of skin and/or the spacing of chest wall are less than 10 mm, PTV are shaped to avoid skin and chest wall. The region of PTV excluding balloon is defined as PTV_EVAL, which is used for DVH constraints and dosimetric comparison. Results: For clinic condition (1), where the PTV is symmetric, single‐lumen MammoSite and multi‐lumen devices provide similar dosimetric results, whereas in clinic condition (2) and (3), the PTV is asymmetric, the multi‐lumen devices provide good coverage to PTV_EVAL with V95 > 90%, and the multi‐lumen devices deliver significantly less incidental dose to skin and chest wall: three‐lumen device reduce the dose to skin/chest wall by 10%, while four‐ five‐ and six‐lumen device provide similar results, reducing the skin/chest wall dose to be less than 120% of the prescription dose, a 25% drop. Conclusion: Compared with single‐lumen MammoSite device, Multi‐lumen device have better dosimetric results in partial breast irradiation where the PTV spacing is less than 10 mm. Devices with more than four lumens provide similar results to four lumen device. Research sponsored by Hologic