Application of a directional palladium-103 brachytherapy device on a curved surface.

PURPOSE: The directional planar palladium-103 LDR device (CivaSheet TM ) may be used for intraoperative implantation at the interface between the tumor site and healthy tissue. Its dosimetric properties have been studied in the ideal case of application on a flat surface. The dosimetric impact of implanting this highly directional device on a curved surface that may be encountered in clinical treatments is analyzed. METHODS: CivaSheet is designed as an array of directional palladium-103 sources (CivaDots). From the postoperative computed tomography (CT) scans of three patients, the shape of each implanted CivaSheet was reconstructed. In order to obtain a realistic estimate of the distribution of curvatures, the mean radius of curvature at the location of each CivaDot was calculated. A Monte Carlo simulation (FLUKA) of a single CivaDot was designed, based upon published geometry and material specifications. Both the radial dose function analog and the two-dimensional anisotropy function analog for the CivaDot were validated in comparison with film measurements and benchmarked to published Monte Carlo data. A value for the dose-rate constant Λ = 0.587(19) cGy/h/U for a CivaDot source in water was calculated as well. Knowledge of the dose distribution in the vicinity of each source allowed the dose at any point around CivaSheets of different curvatures and orientations to be calculated. RESULTS: The local radius of curvature was found to be primarily between 2 and 8 cm in all three patient implants. On the unshielded side of an inward-facing curved CivaSheet implant of radius 2 cm, the calculated dose at 0.5 cm depth exceeded the prescribed dose by ∼20%, while on the shielded side the dose increased by a factor of two, thus compromising the shielding efficiency of the original design. On the unshielded side of an outward-facing curved implant, the dose at 0.5 cm depth decreased by ∼20%. CONCLUSIONS: When tumor bed curvature can be estimated from the preplanning CT scan, the results from this study provide quantitative guide for modifying the source strength to achieve the desired clinical results. In many intraoperative cases, however, accurate preplanning based on surface curvature may not be practical. In such situations, knowledge of the dosimetric impact of the surface curvature provides motivation for avoiding implantation geometries that can lead to either over/underdosing the target, or excess dose to healthy tissue.

3D inpatient dose reconstruction from the PET-CT imaging of 90Y microspheres for metastatic cancer to the liver: feasibility study.

PURPOSE: The introduction of radioembolization with microspheres represents a significant step forward in the treatment of patients with metastatic disease to the liver. This technique uses semiempirical formulae based on body surface area or liver and target volumes to calculate the required total activity for a given patient. However, this treatment modality lacks extremely important information, which is the three-dimensional (3D) dose delivered by microspheres to different organs after their administration. The absence of this information dramatically limits the clinical efficacy of this modality, specifically the predictive power of the treatment. Therefore, the aim of this study is to develop a 3D dose calculation technique that is based on the PET imaging of the infused microspheres. METHODS: The Fluka Monte Carlo code was used to calculate the voxel dose kernel for 90Y source with voxel size equal to that of the PET scan. The measured PET activity distribution was converted to total activity distribution for the subsequent convolution with the voxel dose kernel to obtain the 3D dose distribution. In addition, dose-volume histograms were generated to analyze the dose to the tumor and critical structures. RESULTS: The 3D inpatient dose distribution can be reconstructed from the PET data of a patient scanned after the infusion of microspheres. A total of seven patients have been analyzed so far using the proposed reconstruction method. Four patients underwent treatment with SIR-Spheres for liver metastases from colorectal cancer and three patients were treated with Therasphere for hepatocellular cancer. A total of 14 target tumors were contoured on post-treatment PET-CT scans for dosimetric evaluation. Mean prescription activity was 1.7 GBq (range: 0.58-3.8 GBq). The resulting mean maximum measured dose to targets was 167 Gy (range: 71-311 Gy). Mean minimum dose to 70% of target (D70) was 68 Gy (range: 25-155 Gy). Mean minimum dose to 90% of target (D90) was 53 Gy (range: 13-125 Gy). CONCLUSIONS: A three-dimensional inpatient dose reconstruction method has been developed that is based on the PET/CT data of a patient treated with 90Y microspheres. It allows for a complete description of the absorbed dose by the tumor and critical structures. It represents the first step in building predictive models for treatment outcomes for patients receiving this therapeutic modality as well as it allows for better analysis of patients' dose response and will ultimately improve future treatment administration.

SU-E-T-590: Procedure for Verification and Inter-Comparison of IMRT Beam Models.

PURPOSE: The implementation of an accurate beam model is an integral part of the commissioning of any planning system. This process is especially challenging in the case of IMRT beam models owing to the complexity of small field sizes and MLC leaf-end and tongue-and-groove effects. The question of how to judge the quality of an IMRT beam model in comparison with other versions of the same model is central to this work. METHODS: We make an important distinction between evaluation of the beam model and evaluation of the optimization routine that is a part of any IMRT planning system. The H-shaped target used in this work has several important features: it can only be covered by segments with small field size, for which all leaf design effects are important, and it has the overall dimensions of a common IMRT target. The procedure for inter-comparison of two IMRT beam models (old and new) involves the generation of two plans optimized with each beam model using identical IMRT prescriptions. Both plans are subsequently delivered on a solid water phantom with film located in two parallel planes with a small-volume ionization chamber inserted in the center. RESULTS: Four dose calculations are performed, such that each plan is calculated with either of the two beam models. The four dose distributions are subsequently compared with the two film measurements using gamma analysis. In addition, the absolute dose measured in the center of the dose distribution is compared with the calculated value. A score is assigned to each beam model based on the results. CONCLUSIONS: Using the procedure outlined in this presentation, different versions of an IMRT beam model can be compared and scored for quality. Adoption of a unified strategy for beam model inter-comparison can greatly facilitate the evaluation and commissioning of IMRT beam models.

SU-E-T-554: PTV to Skin Proximity for Head and Neck IMRT Treatment Planning.

PURPOSE: The goal of this work was to evaluate measured vs. calculated surface dose as a function of PTV-to-skin proximity and calculation matrix oxel size, determine effects on plan quality, and provide parameters and levels of uncertainty for clinical use. METHODS: A right-sided CTV with the lateral border 5mm from the surface was delineated on the CT data of a head and neck phantom. A 5mm PTV was generated except laterally where distances of 0-5mm were used. A 7-field IMRT plan was generated using the Eclipse TPS. Optimization was performed where 95% of the PTV receives the prescription dose using a matrix size of 2mm3 . Dose calculations were repeated for grid sizes of 1, 3 and 5mm3 . For each plan nine point dose values were obtained just inside the phantom surface, corresponding to a 2cm2 grid near the central target region. Nine ultra-thin TLDs were placed on the phantom surface corresponding to the grid. Measured and calculated dose values were compared. Conformality, homogeneity and target coverage were compared. RESULTS: Surface dose is over-estimated by the TPS by 21 and 8% for 5 and 3mm3 voxels, respectively and accurately predicted for 2mm3 voxels. A voxel size of 1mm3 results in underestimation of 13%. Conformality improves with increasing PTV to skin distance and a CI of unity results for grid sizes of 1-3mm3 between 4 and 4.5mm. Hot spot decreases as the PTV moves away from the surface and falls below 110% at 4mm. Underdosage worsens as the PTV approaches the skin. CONCLUSIONS: For decreasing PTV-to-skin distance with this TPS, isodose conformality decreases, 'hot spot' increases, and target coverage degrades. Surface dose is accurately predicted for a 2mm3 voxel size, while choosing a finer or coarser grid results in underestimation or overestimation, respectively. All of the above appear to hold for VMAT.

WE-G-BRB-01: Per-Frame Analysis of MatriXX Data.

PURPOSE: One of the most widely used IMRT QA devices (MatriXX) is an array of ionization chambers which are periodically read during plan delivery. Although the ionization chambers are not expected to exhibit strong angular dependence, the measured dose distribution is often found to significantly differ from the planned dose distribution. We identify the origin of all factors that affect the measurement accuracy of the MatriXX and develop a per-frame post-processing strategy that reduces their impact on the passing rate of IMRT and VMAT plans. METHODS: We developed software that reads the dose frame sequence recorded by the MatriXX and applies a number of correction factors to each frame. Angular correction factors are computed as ratios of measured dose at the isocenter of the phantom and planned dose at the same location for all clinically used photon energies. For every clinical case, the recorded movie file is read and the dose for every frame is corrected according to the angle of the beam. In addition, the background evolution is tracked in the 'beam-off' frames which are subsequently subtracted from the 'beam-on' frames according to a predictive model. Machine output correction is also implemented, which significantly improves the absolute dose measurements. The IMRT effective plane of measurement of the MatriXX was identified and found not to coincide with the isocentric plane. RESULTS: The clinical passing rates are significantly improved when the per-frame analysis software was introduced in our IMRT QA procedure. For a group of 800 patients with no corrections the average passing rate was 93.6%, while for the first 300 cases with per frame corrections the average passing rate was 97.3%. CONCLUSIONS: We identify all factors that impact the measurement accuracy of the MatriXX (angular effects, background evolution, machine output, plane of measurement) and propose a strategy for their elimination.

WE-G-BRB-09: CyberKnife Patient Specific QA Using a 4D Cylindrical Diode Array System.

PURPOSE: To examine and facilitate the feasibility of the ArcCheck cylindrical diode array system as a patient specific QA device for CyberKnife radiosurgery delivery. METHODS: There is an obvious necessity for CyberKnife robotic radiosurgery patient QA procedures for hypofractionated treatment of larger planned treatment volumes (PTV), e.g. prostate. This need will increase when the future CyberKnife MLC is introduced. The small unflattened CyberKnife fields, along with the variation of beam-to-detector spatial angles, pose a significant detection challenge for dosimetric systems. The feasibility of the ArcCheck (Sun Nuclear Inc.) cylindrical diode array system for patient-specific QA on the CyberKnife is demonstrated using a beam-to-diode specific angular correction that was developed and has been applied. For localization and tracking, four gold seed fiducial markers were embedded in the system's central plug. We used a Monte Carlo 1% uncertainty for the dose calculation. RESULTS: By disabling the Linac based corrections and applying the custom CyberKnife correction that we developed, the passing rate increased from 39.6% to 99.8% using a 3%3mm gamma criteria for a given lung case. An additional lung case passed 98.5%. In both cases, a 10% dose threshold was used. In addition, brain, trigeminal nerve and lung cases with synchrony tracking are being investigated. CONCLUSIONS: We demonstrated the ArcCheck feasibility for CyberKnife patient specific QA performance. The custom CK angular correction that we developed and applied showed a high passing rate for the lung cases. A verification of the polar angle response should be conducted, in addition to the azimuthal angle that was verified for Linacs. Any data that is being retrieved is additional data to the current chamber point measurement procedures.

Absolute dose reconstruction in proton therapy using PET imaging modality: feasibility study.

A simple analytical model is developed that allows efficient absolute dose reconstruction in patients undergoing radiation treatments using proton beams. The model is based on the solution of the inverse problem of dose recovery from the 3D information contained in the PET signal, obtained immediately after the treatment. The core of the proposed model lies in the analytical calculation of the introduced positron emitters' species matrix (PESM) or kernel, facilitated by previously developed theoretical calculations of the proton energy fluence distribution. Once the PESM is known, the absolute dose distribution in a patient can be found from the deconvolution of the 3D activity distribution obtained from the PET scanner with the calculated species matrix. As an example, we have used FLUKA Monte Carlo code to simulate the delivery of the radiation dose to a tissue phantom irradiated by a parallel-opposed beam arrangement and calculated the resultant total activity. Deconvolution of the calculated activity with the PESM leads to the reconstructed dose being within 2% of that delivered.

Shielding design for a laser-accelerated proton therapy system.

In this paper, we present the shielding analysis to determine the necessary neutron and photon shielding for a laser-accelerated proton therapy system. Laser-accelerated protons coming out of a solid high-density target have broad energy and angular spectra leading to dose distributions that cannot be directly used for therapeutic applications. A special particle selection and collimation device is needed to generate desired proton beams for energy- and intensity-modulated proton therapy. A great number of unwanted protons and even more electrons as a side-product of laser acceleration have to be stopped by collimation devices and shielding walls, posing a challenge in radiation shielding. Parameters of primary particles resulting from the laser-target interaction have been investigated by particle-in-cell simulations, which predicted energy spectra with 300 MeV maximum energy for protons and 270 MeV for electrons at a laser intensity of 2 x 10(21) W cm(-2). Monte Carlo simulations using FLUKA have been performed to design the collimators and shielding walls inside the treatment gantry, which consist of stainless steel, tungsten, polyethylene and lead. A composite primary collimator was designed to effectively reduce high-energy neutron production since their highly penetrating nature makes shielding very difficult. The necessary shielding for the treatment gantry was carefully studied to meet the criteria of head leakage <0.1% of therapeutic absorbed dose. A layer of polyethylene enclosing the whole particle selection and collimation device was used to shield neutrons and an outer layer of lead was used to reduce photon dose from neutron capture and electron bremsstrahlung. It is shown that the two-layer shielding design with 10-12 cm thick polyethylene and 4 cm thick lead can effectively absorb the unwanted particles to meet the shielding requirements.