Satoshi Yoshidome

Computerized estimation of patient setup errors in portal images based on localized pelvic templates for prostate cancer radiotherapy

We have developed a computerized method for estimating patient setup errors in portal images based on localized pelvic templates for prostate cancer radiotherapy. The patient setup errors were estimated based on a template-matching technique that compared the portal image and a localized pelvic template image with a clinical target volume produced from a digitally reconstructed radiography (DRR) image of each patient. We evaluated the proposed method by calculating the residual error between the patient setup error obtained by the proposed method and the gold standard setup error determined by consensus between two radiation oncologists. Eleven training cases with prostate cancer were used for development of the proposed method, and then we applied the method to 10 test cases as a validation test. As a result, the residual errors in the anterior–posterior, superior–inferior and left–right directions were smaller than 2 mm for the validation test. The mean residual error was 2.65 ± 1.21 mm in the Euclidean distance for training cases, and 3.10 ± 1.49 mm for the validation test. There was no statistically significant difference in the residual error between the test for training cases and the validation test (P = 0.438). The proposed method appears to be robust for detecting patient setup error in the treatment of prostate cancer radiotherapy.

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.

SU‐E‐J‐40: Automated Estimation of Lung Tumor Locations for Tumor‐Based Patient Setup Using MV‐CBCT Images in Stereotactic Body Radiotherapy

PURPOSE Patient setup procedure in stereotactic radiation therapy should be performed based on a tumor region, not bone structures. The goal of this study was to develop an automated method of estimation of lung tumor locations for tumor-based patient setup using megavoltage cone-beam computed tomography (MV-CBCT) images in stereotactic body radiotherapy (SBRT). METHODS Planning CT, treatment MV-CBCT images (4.125 MV-CBCT images/patient), and DICOM-RT structure sets for 8 patients were employed for this study. The patients had solitary lung tumors smaller than 25 mm (range of effective diameter: 23 - 8.8 mm and median: 17.7 mm) and received SBRT. In the proposed method, the lung tumor locations were estimated in MV-CBCT images by using tumor templates obtained from corresponding planning CT images. First, a MV-CBCT image was globally aligned with a planning CT image by finding the location with the maximum cross-correlation coefficient, and then a gross target volume (GTV) region in the structure set was placed in the planning CT and MV-CBCT images. Second, a tumor template was produced by cropping the dilated GTV region in the planning CT image. Finally, a tumor location was estimated within the dilated GTV region in the MV-CBCT image by using the tumor template matching and calculating the centroid of the dilated GTV region. Gold standards of tumor locations were determined by a radiation oncologist and two radiological technologists in the clinical practice. RESULTS A mean error between the gold standard and the tumor location estimated by the proposed method was 4.1 and standard deviation was 2.3 mm for 8 patients.. CONCLUSION The results suggest that the proposed method using the MV-CBCT may be one of useful tools for tumor-based patient setup in SBRT.

Feasibility Study of Automated Framework for Estimating Lung Tumor Locations for Target-Based Patient Positioning in Stereotactic Body Radiotherapy

Objective. To investigate the feasibility of an automated framework for estimating the lung tumor locations for tumor-based patient positioning with megavolt-cone-beam computed tomography (MV-CBCT) during stereotactic body radiotherapy (SBRT). Methods. A lung screening phantom and ten lung cancer cases with solid lung tumors, who were treated with SBRT, were employed to this study. The locations of tumors in MV-CBCT images were estimated using a tumor-template matching technique between a tumor template and the MV-CBCT. Tumor templates were produced by cropping the gross tumor volume (GTV) regions, which were enhanced by a Sobel filter or a blob structure enhancement (BSE) filter. Reference tumor locations (grand truth) were determined based on a consensus between a radiation oncologist and a medical physicist. Results. According to the results of the phantom study, the average Euclidean distances of the location errors in the original, Sobel-filtered, and BSE-filtered images were 2.0 ± 4.1 mm, 12.8 ± 9.4 mm, and 0.4 ± 0.5 mm, respectively. For clinical cases, these were 3.4 ± 7.1 mm, 7.2 ± 11.6 mm, and 1.6 ± 1.2 mm, respectively. Conclusion. The feasibility study suggests that our proposed framework based on the BSE filter may be a useful tool for tumor-based patient positioning in SBRT.

SU‐E‐J‐26: Automated Estimation Method of Patient Setup Errors Using Simulated Portal Images for Prostate Cancer Radiotherapy

PURPOSE We developed a novel automated estimation method for patient setup errors based on simulated and real portal images for prostate cancer radiotherapy. METHODS The estimation of patient setup errors in this study was based on a template matching technique with a cross-correlation coefficient and Sobel filter between the real portal image and localized pelvic template of reference image, which were DRR (digitally reconstructed radiography) images and simulated portal images. The simulated portal image was derived by projecting a CT image according to an inverse exponential power law of x-ray attenuation for a water-equivalent path length of each voxel of the CT image on each ray from a source to each pixel on the EPID (electric portal imaging device). A localized pelvic template of each patient in AP (anterior-posterior) or lateral view was automatically extracted from the DRR or simulated portal images by cropping a rectangular region, which was determined by using the mean pelvic template and four anatomical feature points. We applied the proposed method to three prostate cancer cases, and evaluated it using the residual error between the patient setup error obtained by proposed method and the gold standard setup error determined by two radiation oncologists. RESULTS The average residual errors of the patient setup error for the DRR and simulated portal images were 0.79 and 1.26 mm in the left-right (LR) direction, 3.17 and 2.05 mm in the superior-inferior (SI) direction, 1.69 and 5.82 mm in the anterior-posterior (AP) direction, 3.84 and 6.94 mm in Euclidean distance (ED), respectively. If we used the simulated portal image for LR and SI directions and the DRR image for AP direction, the Euclidean distance was 3.22 mm. CONCLUSIONS The proposed method has a potential to correctly estimate patient setup errors for prostate cancer radiotherapy.

SU‐GG‐J‐44: Estimation of Lateral Scatter Kernels in EPID and Water Equivalent Phantom for Dose Verification in Stereotactic Lung Radiotherapy

Purpose: Lateral scatter kernels (LSKs) of an electronic portal imaging device(EPID) and a water equivalent phantom are essential data for estimating the three‐dimensional dose distributions in lungcancer patients who receive stereotactic body radiotherapy. The LSKs are used for converting portal images to portal doseimages. The purpose of this study was to investigate a method of estimating LSKs of the EPID and water equivalent phantom, and then compare the LSKs between experimental measurements and the Monte Carlo(MC) method. Method and Materials: The experimental LSKs were derived from the differentiation of the signal (mean pixel value or absorbed dose) as a function of equivalent circular radius of irradiation area. Mean pixel values in a region of interest of the EPID and absorbed doses in the water equivalent phantom were measured for estimating the LSKs of the EPID and water equivalent phantom, respectively, by changing the irradiation area of 3×3 to 20×20 cm2. For evaluation of the experimental method, the theoretical LSKs were obtained based on the Monte Carlo method by simulating the same geometry of the experimental set‐up including the EPID and water equivalent phantom. Two x‐ray energies of 6 and 10 MV were employed respectively, at a medicallinear accelerator in conjunction with an EPID.Results: The experimental method overestimated the LSKs of the EPID and water equivalent phantom compared with those of the Monte Carlo simulation method. The full width half maximum values at 6 and 10 MV of the theoretical LSKs were 0.166 and 0.193 cm for the EPID, respectively, and 0.177 and 0.211 cm for the water equivalent phantom, respectively. Conclusion: We should continue to investigate the experimental and theoretical methods for estimation of LSKs of the EPID and water equivalent phantom for dose verification in stereotactic lungradiotherapy.

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.