L Court

TU‐EE‐A4‐05: Validation of CT‐Assisted Targeting (CAT) Software for Soft Tissue and Bony Target Localization

Purpose: To evaluate the performance of an automatic CT-to-CT image registration algorithm for both soft tissue and bony targets. Method and Materials: CT-Assisted Targeting (CAT) software was developed for on-line CT-guided radiotherapy using a CT-on-rails system. The algorithm was tested in two phantom studies and cross-compared with other radiotherapy imaging techniques available in the same room. A BAT phantom (North American Scientific, Chatsworth) was intentionally shifted and imaged each time by (1) an electronic portal imaging device, (2) ultrasound, and (3) the CT-on-rails. A Rando pelvic phantom with imbedded gold markers for target localization using the ACCULOC™ software (Northwest Medical Physics Equipment) was also used. To test the software in patient images, 15 prostate cancer patients receiving 3 CT scans per week over 8 weeks treatment were selected. The prostate was chosen as the soft tissue target and a bony structure in the pelvic region (excluding the femoral heads) was used as the bony alignment target. A total of 366 treatment-day CT images were registered using the CAT software and verified by a single observer. Results: The phantom studies demonstrated that the CAT software can achieve sub-millimeter accuracy in detecting the intended shifts and were generally agreed well with other established imaging modalities in this controlled phantom study. The CAT software also performed well in patients CT images. The failure rate, as defined by greater than 3 mm differences between the automatic detected positions and the final positions adjusted by the human observer, was only 2.1 % for soft tissue target registration and 1.6% for bony target registration in 366 CT images. The automatic registration takes less than 12 seconds. Conclusion: We have designed a highly robust, accurate, and fast computer algorithm for CT-to-CT image registration. The software provides a quick and reliable application for CT-guided radiotherapy.

SU‐E‐T‐850: Selection of Initial DVH Constraints for IMRT Planning Based on Target Dose Gradient Characteristics

Purpose: One major obstacle for IMRT planning is to set up proper DVH constraints for organs at risk (OARs). Inexperienced IMRT planners often set OAR constraints using generic clinical guidelines, which may not always produce the best organ sparing. The goal of this study is to incorporate dose gradient information to set up the most likely achievable OAR dose tolerance. Methods: We investigated the dose falloff characteristics from typical IMRT plans. We found that the fastest dose falloff from a prostate plan can be universally used to describe the best scenario of dose gradient near the PTV. We also found the multi‐field near‐field dose falloff is different from the far‐field dose falloff, which is usually described by the attenuation of a single beam. We used an open‐field un‐modulated dose calculation to simulate the far‐field dose falloff. Also, we calculated the Euclidean distance map to translate the dose fall‐off to regional dose distribution. This dose distribution solely based on the distance from PTV and dose gradient information was reloaded back into the original plan. The DVHs calculated from this dose‐gradient plan represents the best scenario of OAR sparing. Those DVH values for OARs were used as initial estimation of DVH objective function for IMRT planning. Results: We applied our approach to 2 head‐and‐neck patients, 2 lung patients, and 2 prostate patients. We found the 40% dose falloff from the target dose prescription best represents the dose distribution in the near‐field. IMRT plans re‐optimized based on OAR constraints set by the dose‐gradient method produced better OAR sparing than the clinical plans for bladder (in prostate plans), heart (in lung plans), and oral cavity (in head‐and‐neck plans). All other organs produced similar results. Conclusions: We have designed a novel approach to incorporate dose gradient information as dose constraints for IMRT optimization.

TH‐D‐M100J‐01: Deformable Registration of KV/MV Projection Images for Quantifying Patient Setup Offsets and Anatomical Deformations in Head and Neck IMRT

Purpose: To develop an intensity‐based deformable image registration solution for detecting and quantifying both rigid and non‐rigid variations typically seen in head‐neck IMRTtreatment, including patient position offsets, involuntary organ movements and anatomical deformations. Methods & Materials: A two‐step deformable image registration solution was developed by implementing a BSpline‐based non‐rigid representation of head‐neck anatomy and similarity metrics of mean square differences and normalized mutual information. kV and MV X‐ray projection images were registered in two successive steps. First, rigid‐body registrations were performed to determine patient position offsets in terms of translations and in‐plane rotations. The outputs were then used to initiate deformable registrations from which non‐rigid local displacements between reference and target images were extracted in the form of deformation vector fields. Validation studies were performed for patient CT simulation data, phantom images, and setup images of 12 head‐neck IMRT patients. The accuracy of the registrations were examined by comparing registration results with known variations in simulation, phantom images, a set of pre‐shift/post‐shift confirmation images, and with a feature‐based registration by subtracting coordinates of well‐identified anatomical points in patient setup images of different fractions. Results: For all three data sets with known changes, the mean (SD) error in rigid‐body registration was 0.3 mm (0.3 mm) for translations and 0.1° (0.1°) for in‐plane rotations. The error in deformable registration of image pairs with known changes was 0.5 mm (0.9 mm). For patient images with unknown non‐rigid local displacements, the agreements between deformable registration and the feature‐based registration were within 2.0 mm (96.5% of registered points). Conclusion: A 2‐step deformable image registration solution was developed and validated for registering kV/MV X‐ray projection images. The accuracy of the registration is adequate for detecting and quantifying both rigid and nonrigid variations typically seen during patient setup and target anatomy localization for head‐neck IMRT delivery.

TH‐E‐224C‐06: The Effect of Dental Restorations and Fixed Prosthodontics On Radiation Therapy Dose Distribution: A Monte Carlo Study

Purpose: To investigate the effect of dental restorations, fixed prosthodontics, and implants on dose distributions in head and neck radiation therapy using Monte Carlo simulations. Specifically, we seek to understand how to prevent localized mucositis caused by backscatterdose.Method and Materials: Simplified models of a range of dental restorations, fixed prosthodontics, and implants were constructed using a representative sample of materials and configurations. These models were irradiated with a simulated 6MV lateral beam. The resulting dose distributions were compared against dose distributions on models without dental work. Results: Exposed dental alloy (Au‐Pd) caused the most significant amount of backscatter, and corresponding hot spots in the dose distribution. Dental alloy which was surrounded by porcelain also caused backscatter hot spots, although lower compared to exposed metal. These backscatter effects do not appear in pencil beam dose calculations. This work showed that backscatter from dental work caused a dose enhancement of up to 40% at a distance of 1mm in the upstream direction for exposed metal surfaces. The dose enhancement from porcelain‐veneered materials was up to 20% 1mm from the surface. The smaller enhancement was attributed to absorbtion within the ceramic veneer. Isodose lines for the backscatter formed a contour roughly conforming to the shape of the dental work. Beyond 3mm from the surface of the prosthodontic device, the dose enhancement had completely decayed. Conclusion: The metal content of dental restorations and fixed prosthodontics create significant enhanced dose to adjacent soft tissue. This is a major cause of morbidity. Since we have shown that the enhancement decays at a distance of about 3mm in tissue‐equivalent material, we may reduce the likelihood and intensity of mucositis by displacing the soft tissue from the teeth with 3mm of tissue‐equivalent material.

SU‐FF‐T‐149: Conformal Vs. IMRT Concomitant Boosts for IMRT Based Head and Neck Treatment

Purpose: To evaluate conformal 3D‐CRT and IMRT techniques for the boost portion of a concomitant boost treatment schedule for IMRT based head and neck radiation. Method and Materials: Nine‐field IMRT plans were generated using Eclipse for 4 stage IV oropharynx patients, treating all target volumes initially to 57Gy. Two alternative plans were then generated to deliver a 15Gy boost to gross disease: a 3D conformal plan, using 3–4 fields, and 5‐field IMRT plan. Boost volumes ranged from 25–60cc. The IMRT and 3D‐CRT boost plans were evaluated as individual graphic plans and as a cumulative with the first course treatment for a total dose of 72Gy (IMRT/IMRT and IMRT/3D‐CRT combinations). The comparison assessed target coverage, dose to critical structures (parotids, cord and oral cavity), hot spots and number of monitor units (MU). Results: Evaluated as a cumulative plan the IMRT/IMRT technique met all the constraints for critical structures (mean dose to parotid 26Gy, cord max 46Gy) and the hot spots were between 104–106%. The IMRT/3D conformal technique also met the constraints for the critical structures with hot spots between 103–105%. Both cumulative plans achieved 98.6–100% coverage of boost volumes. Evaluated as individual plans both the IMRT and 3D conformal boost plans achieved the desired coverage while keeping the dose to critical structures at a minimum; hot spots were located within the confines of the boost volume. The number of MUs ranged from 250–296 for the 3D‐CRT plan in comparison to 360–562 for the IMRT. Average planning time was 1.0 and 2.5 hours for the IMRT and 3D‐CRT boost, respectively. Conclusion: Both boost techniques are dosimetrically equivalent. Treatment technique can therefore be chosen based on the available clinical recourses.

SU‐FF‐J‐45: A Fluence Deformation Based Technique for Portal Image‐Guided Adaptive Head‐And‐Neck IMRT

Purpose: Recent studies have shown significant inter-fraction patient anatomy changes over the courses of fractionated head-and-neck IMRT. Current IMRT planning uses a fixed 3D-margin around CTV to account for these changes and patient setup errors, which results in high-dose to the normal tissues in the margin and may limit the treatment. We are developing an online portal image-guided adaptive technique aimed at reducing the margin by adapting the photon fluence to these inter-fraction changes. Method and Materials: This technique uses portal images taken at each treatment gantry angle and compares them with the corresponding DRRs from the planning CT. First, a deformable registration is performed to determine a 2D transformation between the two images. This transformation is then applied to the originally optimized fluence to obtain a deformed fluence map that adapts to the detected changes. Finally, MLC sequences and deliverable fluences are re-calculated for adaptive dose delivery. Initial development used planning studies where rigid anatomy changes were simulated in the plan by shifting the isocenter, gantry angle and couch angle. Simulated DRRs were used as approximate representations of online portal images. Dose distributions and DVHs were calculated and compared to those from the originally optimized IMRT plan. Results: Preliminary results of applying this technique to head-and-neck patient data: 1) Deformed fluences calculated from transformations obtained by registering the portal images to the DRRs; 2) Comparisons of the resulting dose distribution of the adaptive technique to the one from the originally optimized plan. Conclusion: Preliminary results suggest that this fluence deformation based adaptive technique can geometrically account for simple rigid anatomic variations including 2D shifts-rotations. Evaluation of the full extent of dosimetric outcome from applying this technique and implementation of deformable registration algorithms for adapting to more complex anatomic changes, such as 3D deformation and volume change, are in progress.

SU‐FF‐T‐291: Monte Carlo Calculation of Rectal Dose When Using An Endorectal Balloon During Prostate Radiation Therapy

Purpose: Air‐filled intrarectal balloons can be used to localize and immobilize the prostate for radiation therapy. This project investigates how well the Eclipse treatment planning system (Varian Inc.) includes the effect of this potentially significant heterogeneity on doses to the rectum. Method and Materials: The BEAMnrc/DOSXYZnrc Monte Carlo(MC) codes were used to simulate a 4‐field conformal therapy treatment for patients who have been treated for prostate cancer under an IRB‐approved protocol which includes a 27Gy cone‐down using a rectal balloon. The rectal doses calculated using MC were compared with those from Eclipse using isodose curves, dose‐volume histograms, and wall‐volume histograms. Results: The MC results showed that, for a 27Gy prescription to the 95% isodose line, Eclipse over‐estimates the volume of the rectum receiving more than 26 Gy by 2–10cc and the volume of the rectum receiving between 12–15 Gy by 10–20cc. Conclusion: The differences in the rectal dose calculated by Eclipse and MC are consistent and can be predicted. They can be explained by the scattering behavior of the PA and lateral fields inside the balloon: For each field, the lack of electronic equilibrium reduces the high dose region while the increased electron range widens the low dose region beyond the penumbra. The combined effect of the four fields divides the DVH into two major regions, corresponding to the AP‐PA and lateral fields, and four subregions, corresponding to the scattering behavior of electrons in air.

MO-D-T-617-07: Measurements of Surface Dose for 6MV and 10 MV X-Ray Beams Using Micro-MOSFET and Comparisons to Monte Carlo Skin Dose Calculations

Purpose: Accurate measurement of skindose in radiation therapy is of considerable clinical importance, especially in treating head‐and‐neck and breast cancers.MOSFETdosimeters have been introduced as a more efficient and easier‐to‐use alternative to TLD and radio‐chromic film for skindosemeasurement. However, existing data with standard‐size MOSFET suggest large differences from TLD or film measurements. We investigated the applications of a micro‐MOSFET for skindosemeasurements and studied the correlation between the measured surface dose by micro‐MOSFET and the skindose expected from a Monte Carlo calculation. Method and Materials: 1). Measurements were conducted for normally incident 6MV and 10MV beams onto a flat solid water phantom. MOSFET data were compared with both measurements using a parallel plate ion chamber and a MCdose calculation for the build‐up region. 2). Measurements of surface dose were conducted for 6MV oblique beams incident onto the surface of a semi‐cylindrical solid water phantom. Results were compared to a MC calculated dose in a skin layer extending 2mm down from the surface. Results: For normal beam incidence, depth dosesmeasured by micro‐MOSFET agree within 3% with parallel‐plate ion chamber data and MC calculation; In the build‐up region, comparison of MOSFET data with the MC calculation suggests that the MOSFET has a water‐equivalence thickness of ∼0.5mm. For oblique beams incident on the curved phantom, the micro‐MOSFET measurements correlate well with the MC calculated skindose for a 6 MV beam, with up to ∼ 6% differences depending on the positions of the MOSFET on the surface. Results from a 10 MV beam will also be presented. Conclusion: Preliminary results indicate that the measured surface dose with a micro‐MOSFET on a curved surface under a 6MV oblique beam irradiations provide a good approximation (within ∼ 6%) of the skindose.