B Reitz

A region growing method for tumor volume segmentation on PET images for rectal and anal cancer patients.

The application of automated segmentation methods for tumor delineation on 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) images presents an opportunity to reduce the interobserver variability in radiotherapy (RT) treatment planning. In this work, three segmentation methods were evaluated and compared for rectal and anal cancer patients: (i) Percentage of the maximum standardized uptake value (SUV% max), (ii) fixed SUV cutoff of 2.5 (SUV2.5), and (iii) mathematical technique based on a confidence connected region growing (CCRG) method. A phantom study was performed to determine the SUV% max threshold value and found to be 43%, SUV43% max. The CCRG method is an iterative scheme that relies on the use of statistics from a specified region in the tumor. The scheme is initialized by a subregion of pixels surrounding the maximum intensity pixel. The mean and standard deviation of this region are measured and the pixels connected to the region are included or not based on the criterion that they are greater than a value derived from the mean and standard deviation. The mean and standard deviation of this new region are then measured and the process repeats. FDG-PET-CT imaging studies for 18 patients who received RT were used to evaluate the segmentation methods. A PET avid (PETavid) region was manually segmented for each patient and the volume was then used to compare the calculated volumes along with the absolute mean difference and range for all methods. For the SUV43% max method, the volumes were always smaller than the PETavid volume by a mean of 56% and a range of 21%-79%. The volumes from the SUV2.5 method were either smaller or larger than the PETavid volume by a mean of 37% and a range of 2%-130%. The CCRG approach provided the best results with a mean difference of 9% and a range of 1%-27%. Results show that the CCRG technique can be used in the segmentation of tumor volumes on FDG-PET images, thus providing treatment planners with a clinically viable starting point for tumor delineation and minimizing the interobserver variability in radiotherapy planning.

IMRT planning and delivery incorporating daily dose from mega-voltage cone-beam computed tomography imaging.

The technology of online mega-voltage cone-beam (CB) computed tomography (MV-CBCT) imaging is currently used in many institutions to generate a 3D anatomical dataset of a patient in treatment position. It utilizes an accelerator therapy beam, delivered with 200 degrees gantry rotation, and captured by an electronic portal imager to account for organ motion and setup variations. Although the patient dose exposure from a single volumetric MV-CBCT imaging procedure is comparable to that from standard double-exposure orthogonal portal images, daily image localization procedures can result in a significant dose increase to healthy tissue. A technique to incorporate the daily dose, from a MV-CBCT imaging procedure, in the IMRT treatment planning optimization process was developed. A composite IMRT plan incorporating the total dose from the CB was optimized with the objective of ensuring uniform target coverage while sparing the surrounding normal tissue. One head and neck cancer patient and four prostate cancer patients were planned and treated using this technique. Dosimetric results from the prostate IMRT plans optimized with or without CB showed similar target coverage and comparable sparing of bladder and rectum volumes. Average mean doses were higher by 1.6 +/- 1.0 Gy for the bladder and comparable for the rectum (-0.3 +/- 1.4 Gy). In addition, an average mean dose increase of 1.9 +/- 0.8 Gy in the femoral heads and 1.7 +/- 0.6 Gy in irradiated tissue was observed. However, the V65 and V70 values for bladder and rectum were lower by 2.3 +/- 1.5% and 2.4 +/- 2.1% indicating better volume sparing at high doses with the optimized plans incorporating CB. For the head and neck case, identical target coverage was achieved, while a comparable sparing of the brain stem, optic chiasm, and optic nerves was observed. The technique of optimized planning incorporating doses from daily online MV-CBCT procedures provides an alternative method for imaging IMRT patients. It allows for daily treatment modifications where other volumetric tomographic imaging techniques may not be feasible and/or available and where accurate patient localization with a high degree of precision is required.

Dosimetric and technical aspects of intraoperative I-125 brachytherapy for stage I non-small cell lung cancer

Initial treatment outcome data from our institution for stage I non-small cell lung cancer (NSCLC) patients have shown that sublobar resection in combination with iodine-125 (I-125) brachytherapy is associated with recurrence rates of 2.0%, compared to 18.6% with sublobar resection alone. In this work, the technical and dosimetric aspects required to execute this procedure from the radiation oncology perspective as well as an analysis of the dose distributions of patients treated with this technique are presented. In this treatment technique, I-125 seeds in vicryl suture are embedded into vicryl mesh and surgically inserted providing a 2.0 cm margin on each side of the resection staple line. A nomogram is developed to determine the suture spacing in the vicryl mesh, as a function of seed activity in order to deliver 120 Gy at a distance of 0.5 cm above and below the seed array. Post-operative dosimetry consists of a CT-based planning and dose volume analysis. Dose distributions, dose volume histograms and mean dose data for lung are analysed in a group of patients. Dosimetric results show significant lung sparing with only a small volume of lung irradiated for all patients with mean lung dose values ranging from 1.5 Gy to 5.4 Gy. Lung brachytherapy with I-125 at the time of sublobar resection is a highly conformal option of dose delivery for stage I NSCLC patients with compromised physiologic reserve. Patient-related toxicity clinically measured by loss of pulmonary function and radiation-induced pneumonitis have not been linked to this procedure.

Monitoring tumor motion with on-line mega-voltage cone-beam computed tomography imaging in a cine mode

Accurate daily patient localization is becoming increasingly important in external-beam radiotherapy (RT). Mega-voltage cone-beam computed tomography (MV-CBCT) utilizing a therapy beam and an on-board electronic portal imager can be used to localize tumor volumes and verify the patients position prior to treatment. MV-CBCT produces a static volumetric image and therefore can only account for inter-fractional changes. In this work, the feasibility of using the MV-CBCT raw data as a fluoroscopic series of portal images to monitor tumor changes due to e.g. respiratory motion was investigated. A method was developed to read and convert the CB raw data into a cine. To improve the contrast-to-noise ratio on the MV-CB projection data, image post-processing with filtering techniques was investigated. Volumes of interest from the planning CT were projected onto the MV-cine. Because of the small exposure and the varying thickness of the patient depending on the projection angle, soft-tissue contrast was limited. Tumor visibility as a function of tumor size and projection angle was studied. The method was well suited in the upper chest, where motion of the tumor as well as of the diaphragm could be clearly seen. In the cases of patients with non-small cell lung cancer with medium or large tumor masses, we verified that the tumor mass was always located within the PTV despite respiratory motion. However for small tumors the method is less applicable, because the visibility of those targets becomes marginal. Evaluation of motion in non-superior-inferior directions might also be limited for small tumor masses. Viewing MV-CBCT data in a cine mode adds to the utility of MV-CBCT for verification of tumor motion and for deriving individualized treatment margins.

Investigation of simple IMRT delivery techniques for non-small cell lung cancer patients with respiratory motion using 4DCT.

Techniques for generating simplified IMRT treatment plans for treating non-small cell lung cancer (NSCLC) patients with respiratory motion were investigated. To estimate and account for respiratory motion, 4-dimensional computed tomography (4DCT) datasets from 5 patients were used to design 5-field 6-MV ungated step-and-shoot intensity modulated radiotherapy (IMRT) plans delivering a dose of 66 Gy to the planning target volume (PTV). For each patient, 2 plans were generated using the mean intensity and the maximum intensity of 10 CT datasets from different breathing phases. The plans also utilized different margins around the clinical target volume/internal target volume (CTV/ITV) to account for tumor motion. To reduce the treatment time and ensure accurate dose delivery to moving targets, the number of intensity levels was minimized while maintaining dose coverage to PTV and minimizing dose to organs at risk (OARs). Dose-volume histograms (DVHs), dosimetric metrics, and outcome probabilities were evaluated for all plans. Plans using the averaged CT image dataset were inferior, requiring larger margins around the PTV, with a maximum of 1.5 cm, to ensure coverage of the tumor, and therefore increased the dose to OARs located in proximity of the tumor. The plans based on superimposed CT image datasets achieved full coverage of the tumor, while allowing tight margins around the PTV and minimizing the dose to OARs. A small number of intensity-levels (3 to 5), resulting in IMRT plans with a total of 13 to 30 segments, were sufficient for homogeneous PTV coverage, without affecting the sparing of OARs. In conclusion, a technique involving treatment planning with the superimposed CT scans of all respiratory phases, and the application of IMRT with only a small number of segments was feasible despite significant tumor motion; however, greater patient numbers are needed to support the statistical significance of the results presented in this work.

TH‐D‐AUD B‐08: Initial Experience with the Monaco IMRT Treatment Planning System

Purpose: To evaluate the commercial CMS Monaco IMRTtreatment planning system which employs a Monte Carlo(MC) based dose calculation engine, biological motivated cost functions, multi‐criteria optimization, and an efficient sequencing algorithm. Method and Materials: For a head and neck, a liver, a prostate and a rectal cancer patient, step‐and‐shoot IMRT plans were designed using Monaco. The plans were compared to ones generated by the established CMS XiO treatment planning system. The plans were optimized to achieve the same clinical objectives concerning dose to the tumor and to the relevant organs‐at‐risk. However, whereas the XiO plans were formulated using DVH and minimum/maximum dose constraints, the Monaco plans utilized the biological cost functions offered by the system. DVHs, EUD, mean‐ and maximum‐doses were compared, as well as the number of beam segments and MUs. Finally the plans were delivered on a MapCheck device to verify the agreement between the MC calculated dose distributions and measurements to be less than 3% and 3 mm. Results: Plans optimized with Monaco achieved at least similar and in some cases superior dose distributions. The multi‐criteria optimization tools and the sensitivity analysis helped to reduce the time needed to optimize the plan. The Monaco plans resulted in fewer segments and lower number of MUs and therefore reduced delivery time. All calculated dose distributions passed the dose verification with the MapCheck device. Conclusion: The commercially available Monaco system produces clinical relevant plans, which are dosimetrically equivalent or superior to plans from the conventional XiO system, feature shorter delivery times, and can easily be verified with normal QA procedures using MapCheck.

SU-GG-J-14: A Novel Integrated Approach for Individualizing Beam Directions and Motion Management

Purpose: To develop an integrated technique to individualize treatment planning (TP) and management of inter‐ and intra‐fraction motion for NSCLC patients treated with 3DCRT or SBRT.Method and Materials: The technique involves first using MV‐CBCT in a cine mode prior to planning for optimal beam angle selection and verification of ITV and PTV 4DCT planning‐based margins. The angles are selected based on optimal geometrical tumor mass separation with respect to the surrounding OARs and optimal viewing of tumor motion in longitudinal (superior‐inferior/SI), vertical (anterior‐posterior/AP), and lateral (left‐right/LR) directions. Secondly, MV‐CBCT is used for daily tumor volume localization just prior to treatment. Thirdly, the EPID is deployed during treatment delivery to verify tumor motion and margins by capturing 7 frames per sec over 30 sec. Since the beam angles were selected to optimally view the target motion, the clinical benefits of using MV‐fluoroscopy (MV‐fluoro) to monitor tumor motion are maximized. To improve the contrast‐to‐noise ratio on the CB projection data and MV‐fluoro image frames, post‐processing with filtering techniques was used. Volumes of interest from the planning 4DCT were projected onto the MV‐cine and MV‐fluoro. Results: Data show optimal planning beam angles that ensured highly conformal dose distributions and viewing tumor motion in SI, AP, and LR directions derived from the cine data were feasible. The patient tumor volume was localized with MV‐CBCT, which represents an average static volumetric image of the patient over 60 sec. The MV‐fluoro data confirmed the tumor mass was located within the PTV during treatment despite respiratory motion. Conclusion: Individualizing margins using 4DCT, deriving optimal beam angles based on quasi 3D motion data from MV‐cine, localizing with CB, and verifying tumor motion and margins with MV‐fluoro is a clinically viable integrated technique, allowing for inter‐ and intra‐fraction motion management.

TH‐C‐351‐05: Quantitative Methods for Contouring On PET‐CT Images

Purpose: FDG‐PET imaging is routinely used to diagnose and stage cancer patients. It is also gaining wide acceptance as a tool to assist in tumor delineation in radiotherapy (RT) treatment planning. However, target volume definition is subject to inter‐observer variability. The objective of this study was to evaluate several existing auto‐contouring methods and develop a technique that would reduce inter‐observer variability. Method and Materials: Eighteen rectal and anal cancer patients who had undergone PET‐CT imaging and received RT were retrospectively reviewed. For each patient, a FDG‐PET avid (AVID) region was contoured by an experienced clinician without the use of the CT scan. The AVID volume was compared to volumes derived by the automated methods. Three automated methods were used: a fixed SUV cutoff of 2.5, previously suggested in the literature, a percentage of the maximum SUV (%SUVmax), and an in‐house derived mathematical technique. A 43% threshold was found for %SUVmax using a phantom study with cylinders of known volumes filled with varying concentrations of FDG. The mathematical approach generated 3D volumes using a Confidence Connected Region Growing (CCRG) technique that calculated the mean and standard deviation from pixel intensities contained in a 3D volume grown from a seed pixel. Results: The class solution of using a single value of SUV or a %SUVmax proved limited. These two methods depend on the correct threshold being applied and need to be different for each patient. The resulting volume differences ranged from 1%–129%. The CCRG based volumes were within 8% of the AVID volumes with a range of 1%–23%. Conclusion: Assuming that the same seed pixel is chosen, the CCRG method reduces inter‐observer contouring variability on FDG‐PET images and provides a viable clinical solution by always growing the same volume. Research partially supported by Siemens Medical Solutions.

WE-E-M100F-03: Improving Soft Tissue Contrast in Megavoltage Cone-Beam CT Images for Adaptive Radiotherapy

Purpose: To investigate image quality improvement in Megavoltage Cone‐Beam CT (MV‐CBCT) images using image filtering techniques for adaptive radiotherapy (ART) protocols. MV‐CBCT imaging is often used for daily patient localization. However, soft tissuecontrast in MV‐CBCT images is limited and accurate delineation of targets and organs‐at‐risk is sometimes challenging. Image post‐processing with advanced image filtering techniques can improve the quality without the need to increase the dose exposure for the imaging procedure. Method and Materials: MV‐CBCT images of two image‐quality phantoms and of patients with prostate cancer were post‐processed using noise‐reducing, edge‐preserving image filters. On the phantom images, the contrast‐to‐noise ratio and the spatial resolution before and after filtering were evaluated. The improvement in image quality for the prostate patients was qualitatively judged by physicians based on the ability to delineate the prostate, rectum, bladder and seminal‐vesicle volumes. The optimal combination of MV‐CBCT delivery protocols with different patient doses and filtering techniques was determined for online and offline ART protocols. Results: Using an edge‐preserving noise‐reducing curvature flow image filter, the quality of MV‐CBCT images was improved. The contrast‐to‐noise ratio on the phantoms was improved by up to 30%, while maintaining the spatial resolution. Although the raw cone‐beam image quality of the prostate patients was sufficient for patient treatment localization, the ability to contour anatomical structures was increased on the post‐processed images.Conclusion: Using advanced filtering tools for MV‐CBCT images can improve the image quality, especially soft‐tissue contrast. This allows delineation of organs not clearly visible on the raw images. The tools are potentially beneficial for deformable registration of MV‐CBCT images with planning CTs, for monitoring the delivered dose to targets and organs‐at‐risk, and for ART protocols. Finally using protocols with lower exposure, patient dose from daily imaging procedures can be reduced.

SU‐FF‐J‐46: Dosimetric Effects of Daily Localization for Prostate Cancer Patients Using MV‐CBCT

Purpose: To evaluate the effect of daily shifts observed with mega‐voltage cone beam CT (MV‐CBCT) localization on the IMRT dose distribution received by prostate cancer patients. Method and Materials: Eight patients who received a dose of 77.4 Gy to the PTV, which included the prostate and the seminal vesicles with a 1 cm margin, were selected for this retrospective study. Prior to each daily treatment fraction, the prostate was localized using MV‐CBCT, and the treatment couch position was corrected accordingly in the lateral (RL), longitudinal (SI) and vertical (AP) directions. The shifts for each of the 308 fractions were recorded, and the 308 corresponding dose distributions that the patients would have received if the shifts were not applied were calculated. Dose volume histograms (DVH) and mean dose for target and organs‐at‐risk were derived from these dose distributions, and compared to the treatment plan. Results: The average shifts for each patient were less than 6, 5 and 5 mm in the RL, SI and AP directions, respectively, with standard deviations ranging from 2 to 7 mm. The relative mean dose difference for the prostate was less that 1%, however effects as large as 15% and 20% were observed for the rectum and bladder, respectively. Rectum dose differences were correlated to AP shifts, while the bladder dose was affected by the SI shifts. Conclusion: For IMRT plans with a 1 cm margin, daily localization of the prostate is necessary to reduce the risk of bladder and rectum complication. The dose to these organs is very sensitive to systematic errors, while the effects of random errors cancel each other due to the essentially spherical shape of the dose distribution. Our results show that accurate patient positioning is an important step in any dose‐escalation and/or margin reduction strategy to further improve the therapeutic ratio.