Yida Hu

Pre-clinic study of uniformity of light blanket for intraoperative photodynamic therapy

A large-size blanket composed of the parallel catheters and silica core side glowing fiber is designed to substitute the hand-held point source in the photodynamic therapy treatment (PDT) of the malignant pleural or intraperitoneal diseases. It produces a reasonably uniform field for effective light coverage and is flexible to conform to anatomic structures in intraoperative PDT. The size of the blanket is 30cm×20cm. The light blanket composed of several PVC layers and a series of parallel catheters attached on both sides of the intralipid layer of 0.2% concentration. On one side of the intralipid layer, the parallel fiber catheters were attached using thermal sealing technique. On the other side, the parallel detect catheters were attached along the perpendicular direction. 0.1mm aluminum foil was used to construct the reflection layer to enhance the efficiency of light delivery. The long single side-glowing fiber goes through the fiber catheters according to the specific fiber pattern design. Compared with the prototype of the first generation, the new design is more cost-efficient and more applicable for clinical applications. The light distribution of the blanket was characterized by scanning experiments which were performed in flatness and on the curved surface of tissue body phantom. The fluence rate generated by the blanket can meet requirements for the light delivery in pleural or intraperitoneal (IP) PDT. Taking the advantage of large coverage and flexible conformity, it has great value to increase the reliability and consistency of PDT.

A light blanket for intraoperative photodynamic therapy.

A novel light source - light blanket composed of a series of parallel cylindrical diffusing fibers (CDF) is designed to substitute the hand-held point source in the PDT treatment of the malignant pleural or intraperitoneal diseases. It achieves more uniform light delivery and less operation time in operating room. The preliminary experiment was performed for a 9cmx9cm light blanket composed of 8 9-cm CDFs. The linear diffusers were placed in parallel fingerlike pockets. The blanket is filled with 0.2 % intralipid scattering medium to improve the uniformity of light distribution. 0.3-mm aluminum foil is used to shield and reflect the light transmission. The full width of the profile of light distribution at half maximum along the perpendicular direction is 7.9cm and 8.1cm with no intralipid and with intralipid. The peak value of the light fluence rate profiles per input power is 11.7mW/cm2/W and 8.6mW/cm2/W respectively. The distribution of light field is scanned using the isotropic detector and the motorized platform. The average fluence rate per input power is 8.6 mW/cm2/W and the standard deviation is 1.6 mW/cm2/W for the scan in air, 7.4 mW/cm2/W and 1.1 mW/cm2/W for the scan with the intralipid layer. The average fluence rate per input power and the standard deviation are 20.0 mW/cm2/W and 2.6 mW/cm2/W respectively in the tissue mimic phantom test. The light blanket design produces a reasonably uniform field for effective light coverage and is flexible to confirm to anatomic structures in intraoperative PDT. It also has great potential value for superficial PDT treatment in clinical application.

Backscatter correction factor for megavoltage photon beam

PURPOSE For routine clinical dosimetry of photon beams, it is often necessary to know the minimum thickness of backscatter phantom material to ensure that full backscatter condition exists. METHODS In case of insufficient backscatter thickness, one can determine the backscatter correction factor, BCF(s,d,t), defined as the ratio of absorbed dose measured on the central-axis of a phantom with backscatter thickness of t to that with full backscatter for square field size s and forward depth d. Measurements were performed in SAD geometry for 6 and 15 MV photon beams using a 0.125 cc thimble chamber for field sizes between 10 × 10 and 30 × 30 cm at depths between d(max) (1.5 cm for 6 MV and 3 cm for 15 MV) and 20 cm. RESULTS A convolution method was used to calculate BCF using Monte-Carlo simulated point-spread kernels generated for clinical photon beams for energies between Co-60 and 24 MV. The convolution calculation agrees with the experimental measurements to within 0.8% with the same physical trend. The value of BCF deviates more from 1 for lower energies and larger field sizes. According to our convolution calculation, the minimum BCF occurs at forward depth d(max) and 40 × 40 cm field size, 0.970 for 6 MV and 0.983 for 15 MV. CONCLUSIONS The authors concluded that backscatter thickness is 6.0 cm for 6 MV and 4.0 cm for 15 MV for field size up to 10 × 10 cm when BCF = 0.998. If 4 cm backscatter thickness is used, BCF is 0.997 and 0.983 for field size of 10 × 10 and 40 × 40 cm for 6 MV, and is 0.998 and 0.990 for 10 × 10 and 40 × 40 cm for 15 MV, respectively.

A heterogeneous optimization algorithm for reacted singlet oxygen for interstitial PDT

Singlet oxygen (1O2) is the major cytotoxic agent for type II photodynamic therapy (PDT). The production of 1O2 involves the complex reactions among light, oxygen molecule, and photosensitizer. From universal macroscopic kinetic equations which describe the photochemical processes of PDT, the reacted 1O2 concentration, [1O2]rx, with cell target can be expressed in a form related to time integration of the product of 1O2 quantum yield and the PDT dose rate. The object of this study is to develop optimization procedures that account for the optical heterogeneity of the patient prostate, the tissue photosensitizer concentrations, and tissue oxygenation, thereby enable delivery of uniform reacted singlet oxygen to the gland. We use the heterogeneous optical properties measured for a patient prostate to calculate a light fluence kernel. Several methods are used to optimize the positions and intensities of CDFs. The Cimmino feasibility algorithm, which is fast, linear, and always converges reliably, is applied as a search tool to optimize the weights of the light sources at each step of the iterative selection. Maximum and minimum dose limits chosen for sample points in the prostate constrain the solution for the intensities of the linear light sources. The study shows that optimization of individual light source positions and intensities is feasible for the heterogeneous prostate during PDT. To study how different photosensitizer distributions as well as tissue oxygenation in the prostate affect optimization, comparisons of light fluence rate were made with measured distribution of photosensitizer in prostate under different tissue oxygenation conditions.

Reconstruction of hemodynamics and sensitizer distributions during interstitial PDT using spectroscopy with linear light sources.

Light dosimetry for photodynamic therapy requires a knowledge of the optical absorption spectrum of the tissue being treated Here, we present a theoretical and experimental analysis of the capabilities of a system using interstitial linear light sources ranging in length from 2 to 5 cm to illuminate the tissue interstitially, and isotropic point-like detectors to measure the resulting diffusely transmitted light. The sources and detectors are translated in transparent plastic catheters under the control of a motorized positioning system designed for interstitial measurements in the prostate. The light source is a quartz-tungsten-halogen (QTH), and the spectrally resolved detection is accomplished using a CCD-based grating spectrometer. The data are analyzed using an approximation to the radiative transport equation, assuming homogeneous scattering and heterogeneous absorption spectra Absorption spectra are reconstructed independently for individual source-detector channel pairs. Sequential reconstruction can then be used to create a 3-dimensional reconstruction. The results of simulated data, measurements made in multi-component phantoms, and synthetic data reconstructed from in vivo measurements made with point sources demonstrate the feasibility of this method.

The design of a robotic multichannel platform for photodynamic therapy

A compact robotic platform is designed for simultaneous multichannel motion control for light delivery and dosimetry during interstitial photodynamic therapy (PDT). Movements of light sources and isotropic detectors are controlled by individual motors along different catheters for interstitial PDT. The robotic multichannel platform adds feedback control of positioning for up to 16 channels compared to the existing dual-motor system, which did not have positioning encoders. A 16-channel servo motion controller and micro DC motors, each with high resolution optical encoder, are adopted to control the motions of up to 16 channels independently. Each channel has a resolution of 0.1mm and a speed of 5cm/s. The robotic platform can perform light delivery and dosimetry independently, allowing arbitrary positioning of light sources and detectors in each catheter. Up to 16 compact translational channels can be combined according to different operational scheme with real-time optimal motion planning. The characteristic of high speed and coordinating motion will make it possible to use short linear sources (e.g., 1- cm) to deliver uniform PDT treatment to a bulk tumor within reasonable time by source stepping optimization of multiple sources simultaneously. Advanced robotic control algorithm handles the various unexpected circumstance in clinical procedure, e.g., positiontorque/ current control will be applied to prevent excessive force in the case of resistance in the fiber or motorized mechanism. The robotic platform is fully compatible with operation room (OR) environment and improves the light delivery and dosimetry in PDT. It can be adopted for diffusing optical tomography (DOT), spectroscopic DOT and fluorescent spectroscopy.

A heterogeneous algorithm for PDT dose optimization for prostate

The object of this study is to develop optimization procedures that account for both the optical heterogeneity as well as photosensitizer (PS) drug distribution of the patient prostate and thereby enable delivery of uniform photodynamic dose to that gland. We use the heterogeneous optical properties measured for a patient prostate to calculate a light fluence kernel (table). PS distribution is then multiplied with the light fluence kernel to form the PDT dose kernel. The Cimmino feasibility algorithm, which is fast, linear, and always converges reliably, is applied as a search tool to choose the weights of the light sources to optimize PDT dose. Maximum and minimum PDT dose limits chosen for sample points in the prostate constrain the solution for the source strengths of the cylindrical diffuser fibers (CDF). We tested the Cimmino optimization procedures using the light fluence kernel generated for heterogeneous optical properties, and compared the optimized treatment plans with those obtained using homogeneous optical properties. To study how different photosensitizer distributions in the prostate affect optimization, comparisons of light fluence rate and PDT dose distributions were made with three distributions of photosensitizer: uniform, linear spatial distribution, and the measured PS distribution. The study shows that optimization of individual light source positions and intensities are feasible for the heterogeneous prostate during PDT.

SU‐FF‐T‐88: Image Registration for Real‐Time Dose Calculation of Adaptive Radiation Therapy

Purpose: To enhance the speed and accuracy of certain treatment plan in radiation therapy, a real‐time dose calculation strategy is performed based on Fast Fourier Transformation (FFT) and CTimage registration. For this purpose, different registration methods are achieved by using Insight Segmentation and Registration Toolkit (ITK) in MATLAB. Method and materials: Our group developed an efficient method for kernel calculation for pencil beam convolution‐superposition based on FFT. In this paper, different image registration methods are developed in MATLAB by using ITK, such as rigid 3D, similarity 3D, Demons deformation and BSpline based Free Form. As ITK is written and used in a C++ template, smart pointer and generic structure, an open source wrapper MATITK is modified to facilitate the use of its image processing capabilities while working in the high‐level environment of MATLAB. The image registration methods are demonstrated on head and neck (H&N) CTimage sets of 6 patients. The results of evaluation and visualization for the registration are presented. Results and discussion: Transformation similarity and computation complexity as evaluation criteria are compared between rigid and deformable registration methods. Judging from the mutual information histogram and mean square metric, the deformable registration will get more improvement on similarity than the rigid registration. Checkerboard composites of the reference and registered images are displayed. The computational time of deformable registration are 10∼20 times longer with the whole set of images because of the pre‐processing, complicated transformation matrix and more parameters. A similarity3D registration method is adopted for the image registration of H&N radiation therapy to trade off between registration effect and the time consumption. Conclusion: The image registration can greatly contribute to the automatic contour adjustment and dose deformation for adaptive radiation therapy. A GUI program will be designed for the customized deformation region to speed up deformable registration.

SU‐FF‐T‐82: Rapid Dose Calculation for IMRT Fields in Heterogeneous Medium

Purpose: To develop a fast algorithm to calculate patient specific 3D dose distribution for megavoltage photon beams. Method and Materials: In our PC‐based convolution algorithm, the ratio of scatter‐to‐primary dose,Ds/Dp , for a 2‐D non‐uniform field is calculated as a 3D convolution: SPR(x,y,d) = ∫ x′ ∫ y′ ∫ z′ IM(x′,y′) IM(x,y) . aw((z−z′)+d 0 ) (w⋅r+z−z′+d 0 ) 3 . A(z′)A(z−z′) A(z) dx′dy′dz′ , with r = (x − x′) 2 + (y − y′) 2 and IM(x,y) = ∑ i fMU i p i (x′,y′) is the 2D intensity profile within the IMRT field, pi is 1 inside field and 0 outside field collimation for segment field i of the IMRT field. A(z) is the attenuation function. This convolution is calculated using a Fast Fourier Transform. The parameters a, w, and d0 characterize the phantom scatter properties and can be determined from measured PDD and Sp . In a CT phantom, fast ray‐tracing is used to calculate radiological depth for the attenuation function. Results: SPR calculated using MC simulation for 6 and 15 MV Mohan Spectrum is compared with the FFT convolution algorithm. They agree very well for depths beyond electron contamination. Agreement is also compared between FFT convolution and MC simulation in a heterogeneous phantom. They agree to within 1%, relative to the primary dose.Conclusion: This algorithm is ideally suitable for calculating patient‐specific MU for quality assurance of patient specific IMRT fields for photon beams. The major attraction is the fast speed, which allows potential real‐time applications.