Evidence for the appearance of atmospheric tau neutrinos in super-Kamiokande

Super-Kamiokande atmospheric neutrino data were fit with an unbinned maximum likelihood method to search for the appearance of tau leptons resulting from the interactions of oscillation-generated tau neutrinos in the detector. Relative to the expectation of unity, the tau normalization is found to be 1.42 ± 0.35(stat)(-0.12)(+0.14)(syst) excluding the no-tau-appearance hypothesis, for which the normalization would be zero, at the 3.8σ level. We estimate that 180.1 ± 44.3(stat)(-15.2)(+17.8) (syst) tau leptons were produced in the 22.5 kton fiducial volume of the detector by tau neutrinos during the 2806 day running period. In future analyses, this large sample of selected tau events will allow the study of charged current tau neutrino interaction physics with oscillation produced tau neutrinos.

SU-E-T-500: Pencil-Beam versus Monte Carlo Based Dose Calculation for Proton Therapy Patients with Complex Geometries. Clinical Use of the TOPAS Monte Carlo System

PURPOSE

To investigate the necessity of the verification of dose distributions using Monte Carlo (MC) simulations for proton therapy of head and neck patients and other complex patient geometries.

METHODS

TOPAS, a TOol for PArticle Simulations that makes MC simulations easy-to-use for research and clinical use and is layered on top of Geant4, has been used to simulate the treatments of head and neck patients at the Massachusetts General Hospital (MGH). The resulting dose distributions have been compared to pencil beam calculations based on the XiO treatment planning system. Dose difference distributions were used to highlight areas where the two algorithms did not agree. Dose volume histograms are utilized to investigate the overall agreement of the planned doses in target structures.

RESULTS

21 head and neck patients, both nasopharynx and spinal cord, were investigated. The field complexity ranges from a single field up to 13 fields. For all patients, the dose in the clinical target volume agrees well. Nevertheless, differences in critical structures around the targets have been observed mostly due to range differences between the two algorithms.

CONCLUSIONS

Pencil beam algorithms provide an accurate description of dose in the target volume. However, we conclude that the differences between MC simulations and pencil beam algorithms in regions outside the target for complex geometries, such as present in head and neck patients, support the necessity of routine use of MC simulations for treatment verifications before treatment. TOPAS is aiming to make such routine simulations available to all researchers and clinics. An automated interface utilizing TOPAS to enable such simulations has been developed at MGH and should become routinely used in the near future for patients with complex geometries. NIH/NCI R01 CA140735.

SU-E-T-475: Nano-Dosimetric Track Structure Scoring including Biological Modeling with TOPAS-NBio

PURPOSE

To develop a nano-dosimetric Monte Carlo simulation package, TOPAS-nBio, based on the TOPAS (TOol for PArticle Simulations) framework that is being developed in a collaboration between the Massachusetts General Hospital (MGH), the SLAC National Accelerator Laboratory and the University of California, San Francisco. The goal is to incorporate biological processes on a sub-cell level that will provide the basis for a wide range of research in the field of radiobiology, such as bystander effects, biological dose calculations and effects of nano-particles on radiation therapy.

METHODS

The TOPAS framework has been utilized to extend the functionality of this tool for particle transport to include nano- dosimetry. The physics lists of TOPAS have been extended to include efforts by the Geant4-DNA group to model physics on nanometer scales, including chemical processes of the first millisecond after irradiation. TOPAS-nBio uses the functionality of TOPAS to score energy depositions on nanometer scales. A simulation of the setup of a cell culture irradiation experiment has been used as to test the feasibility of the project.

RESULTS

Track structures for an irradiation of a cell culture experiment were successfully obtained. Delta-electron distributions have been produced and single track delta electrons and their energy depositions were observed.

CONCLUSIONS

This study is a first step in the development of TOPAS-nBio, a tool that aims at bringing nanometer scale radiation physics and biology together and make Monte Carlo simulations accessible for all radiobiology researchers. The results presented here show a first proof of concept for the development of TOPAS-nBio.

MO-F-BRB-03: A Method to Assess the Need for Clinical Monte Carlo Dose Calculations for Small Proton Therapy Fields

PURPOSE

Due to multiple Coulomb scattering in complex geometries, small field dosimetry in proton therapy is challenging. Our goal was to define an indicator for the accuracy of dose delivery based on analytical dose calculations in treatment planning systems for small (e.g. radiosurgery) proton therapy fields.

METHODS

Seven patients whose treatment involved one or more small fields (below ∼3.6cm in diameter) were selected. We developed a fast methodology to quantify the inhomogeneity of the tissue traversed by a single beam using a heterogeneity index (HI). The implementation was based on the dose calculation approach taken by our pencil beam algorithm. Plans created with the treatment planning system were verified against Monte Carlo dose calculations on a field- by-field basis. DVHs were analyzed and differences in the dose to the GTV were assessed. The correlation between the HI-values and the discrepancies between planning system and Monte Carlo in terms of absolute dose to the target was studied.

RESULTS

Our treatment planning system overestimates the dose within the GTV for very small fields by up to ∼8%, even if proper output factor normalization is done in water. The differences are strongly correlated to HI (Spearman's ρ=0.8, rho<0.0001). More complex heterogeneities within the beam path caused larger errors by the analytical algorithm. With the established correlation a threshold for the HI can be set by choosing a tolerance level.

CONCLUSIONS

The HI as defined in this study appears to be a good indicator of the accuracy of proton field delivery in terms of GTV prescription dose when small fields are being delivered. Each HI-value was obtained in less than 2 minutes allowing implementation of the HI algorithm in clinical routine. For HI- values exceeding a certain threshold, either a change in beam incidence or a Monte Carlo dose calculation should be considered.

SU-E-T-470: Comparison of Proton Treatment Planning and Monte Carlo Calculation Using TOPAS for Liver Cancer

PURPOSE

To compare Monte Carlo (MC) calculated and planned dose distributions (pencil beam algorithm) for patients with liver cancer treated with proton radiation therapy.

METHODS

Six patients with unresectable Hepatocellular carcinoma were chosen from the institutional protocol list. We applied the newly developed TOPAS (Tool for Particle Simulation) Monte Carlo (MC) tool and an in-house (mcauto) program, which connects the planning system with the MC. Two beams, typically right lateral (RL) and anterior-posterior (AP), were simulated for each patient with a total prescribed dose of 58 Gy. The calculated absolute dose was determined by separately simulating an SOBP dose in a water phantom for normalization to the prescription dose. The difference between MC and planned dose were calculated and Dose Volume Histograms (DVHs) for the critical organs with non-negligible dose (whole liver, heart, small and large bowel and chest wall) were analyzed.

RESULTS

The resulting dose distributions were in quite good agreement. The main discrepancy in all cases was observed in the lateral penumbrae. These discrepancies can mainly result from the range compensator gradient and tissue composition. The Dose Volume Histograms (DVHs) also presented good agreement between doses for the CTV as well as all the OARs. The difference in D95 ranged from 0.7-1.5 Gy that is translated to 1.3-2.5% of the prescribed dose.

CONCLUSIONS

TOPAS Monte Carlo tool presented an efficient and accurate method for dose calculation in liver and to validate clinical treatment planning. Discrepancies with doses calculated using the pencil beam algorithm were seen but were generally quite small.