Showing papers by "Timothy C. Zhu published in 2021"

Report of AAPM Task Group 219 on independent calculation-based dose/MU verification for IMRT.

Abstract: Independent verification of the dose per monitor unit (MU) to deliver the prescribed dose to a patient has been a mainstay of radiation oncology quality assurance. We discuss the role of secondary dose/MU calculation programs as part of a comprehensive Quality Assurance (QA) program. This report provides guidelines on calculation-based dose/MU verification for intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) provided by various modalities. We provide a review of various algorithms for "independent/second check" of monitor unit calculations for IMRT/VMAT. The report makes recommendations on the clinical implementation of secondary dose/MU calculation programs; on commissioning and acceptance of various commercially available secondary dose/MU calculation programs; on benchmark QA and periodic quality assurance; and on clinically reasonable action levels for agreement of secondary dose/MU calculation programs.

A Comparison of Two Probes to Determine Rectum Optical Properties

Abstract: Tissue optical properties are crucial for determining the light dose delivered to the tumor. Two probes are compared: the two-catheter probe is based on transmittance measurement between one point source and one isotropic detector inside parallel catheters spaced at 0.5 cm along a 1-inch diameter transparent cylinder; and a 1-inch trans-rectal diffuse optical tomography (DOT) probe designed for prostate measurements, using a multiple fiber-array with source-detector separations of 1.4-10 mm. The two-catheter probe uses an empirical model for primary and scatter light fluence rates in the cylindrical cavity condition for anal PDT to determine optical properties along the source catheter using dual motors to move the source and detector along the catheters. The DOT probe uses finite element method (FEM) to obtain distribution of optical properties in 3D. Validations for the two probes were performed in liquid and solid phantoms. For each method, validation was performed in tissue-mimicking liquid phantoms for a range of known optical properties (μa between 0.05 and 0.9 cm-1 and μs' between 5.5 and 16.5 cm-1). To cross-check the two methods, solid phantoms were created of known optical properties at the University of Pennsylvania and sent for measurement to Princess Margaret Cancer Centre (PMH) to mimic realistic patient simulating conditions. Measurements were taken and optical properties were then recovered without knowing the expected values to cross-validate each probe. The results show modest agreement between the measured μa and μs'values, but high degree of agreement between the measured μeff performed independently using the two methods.

Estimation of fluorescence probing depth dependence on the distance between source and detector using Monte Carlo modeling.

Abstract: Photosensitizer fluorescence emission during photodynamic therapy (PDT) can be used to estimate for in vivo photosensitizer concentration. We built a surface contact probe with 405nm excitation light source to obtain Photofrin fluorescence signal during clinical PDT. The probe was equipped with multiple detector fibers that were located at distances between 0.14 to 0.87 cm laterally from the excitation source fiber. In this study, we investigated the probing depth of fluorescence in biological tissue with different source-detector separation using our contact probe setup. We used Monte Carlo method to simulate the 405nm excitation light and 630nm fluorescence probing depth at various source and detector (SD) separations. The results provided insight to the most probable depth of origin of detected fluorescence at each SD separation and help to understand the in vivo depth distribution of clinically measured Photofrin concentration.

Determination of the distribution of drug concentration and tissue optical properties for ALA-mediated anal photodynamic therapy.

Abstract: PDT efficacy depends on the availability and dynamic interactions of photosensitizer, light, and oxygen. Tissue optical properties influence the delivered light dose and impact PDT outcome. In-vivo measurements of tissue optical properties and photosensitizer concentration enable determination of explicit and implicit dose factors affecting PDT and helps to understand the underlying biophysical mechanism of PDT. In this study, we measure tissue optical properties (absorption μa (λ) and scattering μs' (λ) coefficients) and PpIX concentration in tissue simulating liquid phantoms with a geometry that resembles anal canal. In-vivo light fluence rate and photosensitizer fluorescence of 405nm excitation light source were acquired using a dual-motor continuous wave transmittance spectroscopy system. We characterized the tissue optical properties correction factor of fluorescence signal using a series of tissue simulating phantoms with known PpIX concentrations and with absorption coefficient between 0.1 - 0.9 cm-1 and reduced scattering coefficient between 5 - 40 cm-1. The results demonstrated that our spectroscopy system could determine the distribution of tissue optical properties and PPIX concentration during anal PDT.

Monte Carlo (MC) study of dose distribution and Cherenkov imaging in total skin electron therapy (TSET) with TOPAS.

Abstract: Malignant tissues can be effectively treated by Total Skin Electron Therapy (TSET) over the entire body surface using 6 MeV electron beams. During the radiation treatment, Cherenkov photons are emitted from the patient's skin, and can potentially be used for in-vivo imaging of the radiation dose distribution. A Monte Carlo (MC) simulation toolkit TOPAS is used to study the generation and propagation of Cherenkov photons that are generated from the interaction of electron radiation with human tissues, and to understand the relationship between the dose distributions and the Cherenkov photon distributions. Validation of MC simulations with experiments are performed at 100 SSD and 500 SSD, and simulations of a patient phantom in realistic clinical treatment setups have been done. These simulations with TOPAS show that the emitted Cherenkov distributions at phantom surfaces closely follow their corresponding dose distributions.

Cherenkov imaging for Total Skin Electron Therapy - an evaluation of dose uniformity

Abstract: Total Skin Electron Therapy (TSET) utilizes high-energy electrons to treat cancers on the entire body surface. The otherwise invisible radiation beam can be observed via the optical Cherenkov photons emitted from interaction between the high-energy electron beam and tissue. Cherenkov emission can be used to evaluate the dose uniformity on the surface of the patient in real-time using a time-gated intensified camera system. Each patient was monitored during TSET by in-vivo detectors (IVD) as well as Scintillators. Patients undergoing TSET in various conditions (whole body and half body) were imaged and analyzed. A rigorous methodology for converting Cherenkov intensity to surface dose as products of correction factors, including camera vignette correction factor, incident radiation correction factor, and tissue optical properties correction factor. A comprehensive study has been carried out by inspecting various positions on the patients such as vertex, chest, perineum, shins, and foot relative to the umbilicus point (the prescription point).