Yoshihiko Onizuka

A simple table lookup method for PET/CT partial volume correction using a point-spread function in diagnosing lymph node metastasis

ObjectiveWe evaluated the partial volume effect in PET/CT images and developed a simple correction method to address this problem.MethodsSix spheres and the background in the phantom were filled with F-18 and we thus obtained 4 different sphere-to-background (SB) ratios. Thirty-nine cervical lymph nodes in 7 patients with papillary thyroid carcinoma (15 malignant and 24 benign) were also examined as a preliminary clinical study. First, we developed recovery coefficient (RC) curves normalized to the maximum counts of the 37-mm sphere. Next, we developed a correction table to determine the true SB ratio using three parameters, including the maximum counts of both the sphere and background and the lesion diameter, by modifying the approximation formula of the RC curves including the point-spread function correction. The full width at half maximum in this formula is estimated with the function of the SB ratio.ResultsIn the phantom study, a size-dependent underestimation of the radioactivity was observed. The degree of decline of RC was influenced by the SB ratio. In preliminary clinical examination, the difference in the SUVmax between malignant and benign LNs thus became more prominent after the correction. The PV correction slightly improved the diagnostic accuracy from 95 to 100%.ConclusionsWe developed a simple table lookup correction method for the partial volume effect of PET/CT. This new method is considered to be clinically useful for the diagnosis of cervical LN metastasis. Further examination with a greater number of subjects is required to corroborate its clinical usefulness.

Microdosimetric evaluation of the 400 MeV/nucleon carbon beam at HIMAC.

Microdosimetric single event spectra were determined as a function of depth in an acrylic phantom for the carbon beam at HIMAC using a tissue equivalent proportional counter (TEPC) coupled to a scintillation counter system. The fragments produced by the carbon beam were identified by the ΔE-time of flight distribution obtained from two scintillation counters which were positioned at the up- and down-stream of the TEPC. Lineal energy distribution for the carbon beam and its five fragments, namely, proton, helium, lithium, beryllium, and boron ions, were measured in the lineal-energy range of 5-1000keV∕μm at five phantom depths between 0 and 230mm. The dose distribution for the carbon beam and its fragments were obtained separately. The relative biological effectiveness (RBE) of the carbon beam in the phantom was calculated using a response function. The maximum RBE for the carbon beam was found to be about 5 near the Bragg peak. It was observed to rapidly decrease for Bragg peaks occurring at deeper positions in the phantom. The dose from the beam fragments accounted for about 30% to the total dose, however, its contribution to the RBE was less than 17%.

Microdosimetric study for secondary neutrons in phantom produced by a 290 MeV/nucleon carbon beam.

Absorbed doses from main charged-particle beams and charged-particle fragments have been measured with high accuracy for particle therapy, but there are few reports for doses from neutron components produced as fragments. This study describes the measurements on neutron doses produced by carbon beams; microdosimetric distributions of secondary neutrons produced by 290MeV∕nucleon carbon beams have been measured by using a tissue equivalent proportional counter at the Heavy Ion Medical Accelerator in Chiba, Japan at the National Institute of Radiological Sciences. The microdosimetric distributions of the secondary neutron were measured on the distal and lateral faces of a body-simulated acrylic phantom (300mmheight×300mmwidth×253mmthickness). To confirm the dose measurements, the neutron energy spectra produced by incident carbon beams in the acrylic phantom were simulated by the particle and heavy ion transport code system. The absorbed doses obtained by multiplying the simulated neutron energy spectra with the kerma factor calculated by MCNPX agree with the corresponding experimental data fairly well. Downstream of the Bragg peak, the ratio of the neutron dose to the carbon dose at the Bragg peak was found to be a maximum of 1.4×10-4 and the ratio of neutron dose was a maximum of 3.0×10-7 at a lateral face of the acrylic phantom. The ratios of neutrons to charged particle fragments were 11% to 89% in the absorbed doses at the lateral and the distal faces of the acrylic phantom. We can conclude that the treatment dose will not induce serious secondary neutron effects at distances greater than 90mm from the Bragg peak in carbon particle therapy.

Computerized method for measurement of displacement vectors of target positions on EPID cine images in stereotactic radiotherapy

The purpose of this study was to develop a computerized method for measurement of displacement vectors of target position on electronic portal imaging device (EPID) cine images in a treatment without implanted markers. Our proposed method was based on a template matching technique with cross-correlation coefficient between a reference portal (RP) image and each consecutive portal (CP) image acquired by the EPID. EPID images with 512×384 pixels (pixel size:0.56 mm) were acquired in a cine mode at a sampling rate of 0.5 frame/sec by using an energy of 4, 6, or 10MV on linear accelerators. The displacement vector of the target on each cine image was determined from the position in which took the maximum cross-correlation value between the RP image and each CP image. We applied our method to EPID cine images of a lung phantom with a tumor model simulating respiratory motion, and 5 cases with a non-small cell lung cancer and one case of metastasis. For validation of our proposed method, displacement vectors of a target position calculated by our method were compared with those determined manually by two radiation oncologists. As a result, for lung phantom images, target displacements by our method correlated well with those by the oncologists (r=0.972 - 0.994). Correlation values for 4 cases ranged from 0.854 to 0.991, but the values for the other two cases were 0.609 and 0.644. This preliminary result suggested that our method may be useful for monitoring of displacement vectors of target positions without implanted markers in stereotactic radiotherapy.

Microdosimetric evaluation of the neutron field for BNCT at Kyoto University reactor by using the PHITS code

In this study, microdosimetric energy distributions of secondary charged particles from the (10)B(n,α)(7)Li reaction in boron-neutron capture therapy (BNCT) field were calculated using the Particle and Heavy Ion Transport code System (PHITS). The PHITS simulation was performed to reproduce the geometrical set-up of an experiment that measured the microdosimetric energy distributions at the Kyoto University Reactor where two types of tissue-equivalent proportional counters were used, one with A-150 wall alone and another with a 50-ppm-boron-loaded A-150 wall. It was found that the PHITS code is a useful tool for the simulation of the energy deposited in tissue in BNCT based on the comparisons with experimental results.

Microdosimetric investigation of the spectra from YAYOI by use of the Monte Carlo code PHITS

The purpose of this study was to obtain the neutron energy spectrum on the surface of the moderator of the Tokyo University reactor YAYOI and to investigate the origins of peaks observed in the neutron energy spectrum by use of the Monte Carlo Code PHITS for evaluating biological studies. The moderator system was modeled with the use of details from an article that reported a calculation result and a measurement result for a neutron spectrum on the surface of the moderator of the reactor. Our calculation results with PHITS were compared to those obtained with the discrete ordinate code ANISN described in the article. In addition, the changes in the neutron spectrum at the boundaries of materials in the moderator system were examined with PHITS. Also, microdosimetric energy distributions of secondary charged particles from neutron recoil or reaction were calculated by use of PHITS and compared with a microdosimetric experiment. Our calculations of the neutron energy spectrum with PHITS showed good agreement with the results of ANISN in terms of the energy and structure of the peaks. However, the microdosimetric dose distribution spectrum with PHITS showed a remarkable discrepancy with the experimental one. The experimental spectrum could not be explained by PHITS when we used neutron beams of two mono-energies.

SU‐GG‐J‐58: Stereotactic Body Radiotherapy: Computer‐Assisted Verification of a Lung Tumor Region Using EPID without Implanted Markers

Purpose: Our purpose of this study was to develop a computer‐assisted verification method for lungtumor regions using a Gaussian image enhancement based on second derivatives of Gaussian functions in cine images on an EPID without implanted markers during SBRT.Method and Materials: The localization for a lungtumor was based on a template matching technique between a “tumor template” image obtained from a first EPID cine image and the subsequent image. The irradiation field region was cropped from an original EPID cine image by analyzing the histogram of this image. The “tumor template” image was segmented from the first EPID cine image, i.e., reference portal image, by using a Gaussian image enhancement based on second derivatives of Gaussian functions and a region growing technique. The tumor region was determined within the irradiation field as the position where the tumor template image took the maximum cross‐correlation value within each subsequent cine image. For performance evaluation of the proposed method, we applied the proposed method to EPIDimages acquired from twelve cases (age: 51–83 years old, mean: 72) with a non‐small cell lungcancer, and calculated the following two values: (1) the location error, i.e., the Euclidean distance from “tumor” point to the candidate point and (2) the overlap measure between the target candidate regions obtained by the manual method and our automated segmentation method. Results: The average location deviation between tumor center points obtained by the proposed method and the manual method was 1.80 ± 0.73 mm. The average overlap measure was 66.0 ± 10.0% for 12 cases. Conclusion: The results of this study suggest that the proposed method based on the tumor template matching technique might be feasible for localization of a lungtumor without implanted markers in SBRT.

SU‐GG‐T‐257: Modeling of Beam Profiles Based On Three Gaussian Functions in Lung Stereotactic Body Radiotherapy for Acceptance Test of Radiotherapy Planning System

Purpose: The purpose of this study was to estimate beam profiles in lung stereotactic body radiotherapy for acceptance test of radiotherapy planning system (RTP) system. The beam profiles measured by an ionization chamber were approximated by using three Gaussian functions, and compared with profile data calculated by two RTP systems. Method and Materials: X‐ray linear accelerator with 4,6,10MV (Varian 21EX) was used to deliver symmetric beam profile for a field size of 5×5cm2. A lung phantom consisted of a lung equivalent material (thickness : 170 mm) sandwiched by two Solid Waters, whose thickness were 30 mm and 50 mm for anterior and posterior sides, respectively. Measured beam profiles were approximated by manually determining three amplitudes and standard deviations of three Gaussian functions corresponding to three point spread functions of an x‐ray focus, x‐ray or electron scatter, and a detector,and by integrating the composed function. Finally, we evaluated our method by comparing the approximated beam profiles with those calculated by two algorithms in two RTP systems, i.e., Convolution/superposition (CS) (Philips Pinnacle) and analytical anisotropic algorithm (AAA) (Varian Eclipse).Results: Difference between the measured and approximated beam profiles were 4% at 20–80% doses, and 1.5% difference at the other doses. The fringe values (distance between the 50 and 90% levels) of beam profiles approximated by Gaussian functions and calculated by CS and AAA algorithms were 4.7, 6.1, 6.4 mm for 4MV x‐ray, 5.7, 6.9, 7.1 mm for 6MV x‐ray, and 6.8, 7.7, 8.2 mm for 10MV x‐ray, respectively. Conclusion: It was suggested the beam profile model based on the three Gaussian functions may be useful for acceptance test of a RTP system.

SU‐GG‐J‐41: Automated Estimation of a Tumor Region and Its Displacement On EPID Cine Images Without Implanted Markers in Lung Stereotactic Body Radiotherapy

Purpose: The purpose of this study was to develop a method for automated estimation of a lungtumor region and its displacement on an electronic portal imaging device(EPID) during lung stereotactic body radiotherapy(SBRT) without implanted markers. Method and Materials: Our method for automated estimation of the tumor region and its displacement was based on a template matching technique with cross‐correlation coefficient between a target template image and each consecutive portal (CP) image, which was acquired in cine mode with the EPID in each treatment. Each target region was segmented in the first EPID cine image, which was referred to as the reference portal (RP) image, based on a multiple‐gray level thresholding technique and a region growing technique, and then a target template image was extracted as “a tumor template”. The displacement vector of a target was determined from the position in which the target template image took the maximum cross‐correlation value within the CP image.EPIDimages with 512×384 pixels (pixel size: 0.56 mm) were acquired in a cine mode at a sampling rate of 0.5 frame/sec by using x‐ray energies of 4, 6, or 10 MV on linear accelerators. We applied our proposed method to EPID cine images of 12 cases (ages: 51–83, mean: 73) with a non‐small cell lungcancer.Results: For 12 cases, the target displacements obtained by our method agreed with those determined by the manual method by a mean correlation value of 0.839. Each tumor region segmented by our proposed method was overlapped by 60% on average with that determined by the manual method. Conclusion: This preliminary result suggested that our proposed method may be useful for estimating of displacements of target positions without implanted markers in lungSBRT.

Microdosimetric evaluation of the 400MeV∕nucleon carbon beam at HIMAC: Microdosimetric evaluation of HIMAC carbon beam

Microdosimetric single event spectra were determined as a function of depth in an acrylic phantom for the carbon beam at HIMAC using a tissue equivalent proportional counter (TEPC) coupled to a scintillation counter system. The fragments produced by the carbon beam were identified by the ΔE-time of flight distribution obtained from two scintillation counters which were positioned at the up- and down-stream of the TEPC. Lineal energy distribution for the carbon beam and its five fragments, namely, proton, helium, lithium, beryllium, and boron ions, were measured in the lineal-energy range of 5-1000keV∕μm at five phantom depths between 0 and 230mm. The dose distribution for the carbon beam and its fragments were obtained separately. The relative biological effectiveness (RBE) of the carbon beam in the phantom was calculated using a response function. The maximum RBE for the carbon beam was found to be about 5 near the Bragg peak. It was observed to rapidly decrease for Bragg peaks occurring at deeper positions in the phantom. The dose from the beam fragments accounted for about 30% to the total dose, however, its contribution to the RBE was less than 17%.