Ion Chamber Collection Efficiencies for Proton Spot Scanning Calibration

Charge accumulation was measured under calibration conditions in the spread-out Bragg peak (SOBP) using the calibration bias as well as a range of voltages from 10V to 500V and a Farmer-style ion chamber. Collection efficiency was determined by extrapolating to infinite voltage. Similar measurements were taken in an identical dose distribution with a much shorter spot duration. The impact of each of the three models on calibration was then quantified using the TRS-398 protocol. The collection efficiency for the standard calibration was determined to agree well with the prediction of a continuous beam recombination correction. The standard calibration field was found to persistently agree with a continuous beam recombination correction for much lower operating biases. The collection efficiency result for the short spot duration field did not agree with either the continuous or pulsed-beam correction. Using the incorrect recombination model under the standard calibration conditions resulted in a 0.5% calibration difference. We have determined that our spot scanning system would be most appropriately calibrated using a recombination correction with continuous beam model. Physicists responsible for the calibration of such systems are advised to take measurements described here to correctly identify the applicable recombination model for their clinics.Comment: Submitted December 16, 2015 to Medical Physic

The Mayo Clinic Florida Microdosimetric Kinetic Model of Clonogenic Survival: Application to Various Repair-Competent Rodent and Human Cell Lines

The relative biological effectiveness (RBE) calculations used during the planning of ion therapy treatments are generally based on the microdosimetric kinetic model (MKM) and the local effect model (LEM). The Mayo Clinic Florida MKM (MCF MKM) was recently developed to overcome the limitations of previous MKMs in reproducing the biological data and to eliminate the need for ion-exposed in vitro data as input for the model calculations. Since we are considering to implement the MCF MKM in clinic, this article presents (a) an extensive benchmark of the MCF MKM predictions against corresponding in vitro clonogenic survival data for 4 rodent and 10 cell lines exposed to ions from 1H to 238U, and (b) a systematic comparison with published results of the latest version of the LEM (LEM IV). Additionally, we introduce a novel approach to derive an approximate value of the MCF MKM model parameters by knowing only the animal species and the mean number of chromosomes. The overall good agreement between MCF MKM predictions and in vitro data suggests the MCF MKM can be reliably used for the RBE calculations. In most cases, a reasonable agreement was found between the MCF MKM and the LEM IV

A Geant4 shielding design for the first US carbon multi-ion hybrid synchrotron facility

The Mayo Clinic Florida Integrated Oncology Building will be the home of the first spot-scanning only carbon/proton hybrid therapy system by Hitachi, Ltd. It will provide proton beams up to kinetic energies of 230 MeV and carbon beams up to 430 MeV n−1 for clinical deployment. To provide adequate radiation protection, the Geant4 (v10.6) Monte Carlo toolkit was utilized to quantify the ambient dose equivalent at a 10 mm depth (H*(10)) for photons and neutrons. To perform accurate calculations of the ambient dose equivalent, three-dimensional computer-aided design files of the entire planned facility were imported into Geant4, as well as certain particle system components such as the bending magnets, fast Faraday cup, and gantry. Particle fluence was scored using 60 cm diameter spheres, which were strategically placed throughout areas of interests. Analytical calculations were performed as first-pass design checks. Major shielding slabs were optimized using Geant4 simulations iteratively, with more than 20 alternative designs evaluated within Geant4. The 430 MeV n−1 carbon beams played the most significant role in concrete thickness Requirements. The primary wall thickness for the carbon fixed beam room is 4 meters. The presence of the proton gantry structure in the simulation caused the ambient dose equivalent to increase by around 67% at the maze entrance, but a decrease in the high energy beam transport corridor. All shielding primary and secondary goals for clinical operations were met per state regulation and national guidelines

Modified COMS plaques for 125I and 103Pd iris melanoma brachytherapy

Purpose: Novel plaques are used to treat iris melanoma at the Mayo Clinic Rochester. The plaques are a modification of the Collaborative Ocular Melanoma Study (COMS) 22 mm plaque design with a gold alloy backing, outer lip, and silicone polymer insert. An inner lip surrounds a 10 mm diameter cutout region at the plaque center. Plaques span 360°, 270°, and 180° arcs. This article describes dosimetry for these plaques and others used in the treatment of anterior eye melanomas. Methods and Materials: The EGSnrc user-code BrachyDose is used to perform Monte Carlo simulations. Plaques and seeds are fully modeled. Three-dimensional dose distributions for different plaque models, TG-43 calculations, and 125I (model 6711) and 103Pd (model 200) seeds are comp