D Bernard

Evaluation of Four Volume-Based Image Registration Algorithms

We evaluated 4 volume-based automatic image registration algorithms from 2 commercially available treatment planning systems (Philips Syntegra and BrainScan). The algorithms based on cross correlation (CC), local correlation (LC), normalized mutual information (NMI), and BrainScan mutual information (BSMI) were evaluated with: (1) the synthetic computed tomography (CT) images, (2) the CT and magnetic resonance (MR) phantom images, and (3) the CT and MR head image pairs from 12 patients with brain tumors. For the synthetic images, the registration results were compared with known transformation parameters, and all algorithms achieved accuracy of submillimeter in translation and subdegree in rotation. For the phantom images, the registration results were compared with those provided by frame and marker-based manual registration. For the patient images, the results were compared with anatomical landmark-based manual registration to qualitatively determine how the results were close to a clinically acceptable registration. NMI and LC outperformed CC and BSMI, with the sense of being closer to a clinically acceptable result. As for the robustness, NMI and BSMI outperformed CC and LC. A guideline of image registration in our institution was given, and final visual assessment is necessary to guarantee reasonable results.

A feasibility study of the Dynamic Phantom scanner for quality assurance of beam profiles at various gantry angles

The effect of gantry rotation on beam profiles of photon and electron beams is an important issue in quality assurance for radiotherapy. To address variations in the profiles of photon and electron beams at different gantry angles, a Dynamic Phantom scanner composed of a 20×12×6 cm3 scanning Lucite block was designed as a cross‐beam‐profile scanner. To our knowledge, differences between scanned profiles acquired at different gantry angles with a small size Lucite block and those acquired a full‐size (60×60×50 cm3) water phantom have not been previously investigated. We therefore performed a feasibility study for a first prototype Dynamic Phantom scanner without a gantry attachment mount. Radiation beams from a Varian LINAC 21EX and 2100C were used. Photon beams (6 MV and 18 MV) were shaped by either collimator jaws or a Varian 120 Multileaf (MLC) collimator, and electron beams (6 MeV, 12 MeV, and 20 MeV) were shaped by a treatment cone. To investigate the effect on profiles by using a Lucite block, a quantitative comparison of scanned profiles with the Dynamic Phantom and a full‐size water phantom was first performed at a 0° gantry angle for both photon and electron beams. For photon beam profiles defined by jaws at 1.0 cm and 5.0 cm depths of Lucite (i.e., at 1.1 cm and 5.7 cm depth of water), a good agreement (less than 1% variation) inside the field edge was observed between profiles scanned with the Dynamic Phantom and with a water phantom. The use of Lucite in the Dynamic Phantom resulted in reduced penumbra width (about 0.5 mm out of 5 mm to 8 mm) and reduced (1% to 2%) scatter dose beyond the field edges for both 6 MV and 18 MV beams, compared with the water phantom scanner. For profiles of the MLC‐shaped 6 MV photon beam, a similar agreement was observed. For profiles of electron beams scanned at 2.9 cm depth of Lucite (i.e., at 3.3 cm depth of water), larger disagreements in profiles (3% to 4%) and penumbra width (3 mm to 4 mm out of 12 mm) were observed. Additional profiles with the gantry at 90° and 270° were performed for both MLC‐ and jaw‐shaped photon beams and electron beams to evaluate the effect of gantry rotation. General good agreement is seen (less than 1 % variation) at all field sizes for collimator‐shaped 6 MV and 18 MV photon beams. Similar variations observed for MLC‐shaped photon beams indicate that the uncertainty in MLC position is similar to that for the collimator jaws. We conclude that the Dynamic Phantom scanner is a useful device for the routine quality assurance on beam profiles of photon beams and for constancy check on electron beams at various gantry angles. Caution should be taken when using this device to acquire basic electron dosimetry data. PACS number: 87.53.‐j

TH‐CD‐207A‐08: Simulated Real‐Time Image Guidance for Lung SBRT Patients Using Scatter Imaging

PURPOSE To develop a comprehensive Monte Carlo-based model for the acquisition of scatter images of patient anatomy in real-time, during lung SBRT treatment. METHODS During SBRT treatment, images of patient anatomy can be acquired from scattered radiation. To rigorously examine the utility of scatter images for image guidance, a model is developed using MCNP code to simulate scatter images of phantoms and lung cancer patients. The model is validated by comparing experimental and simulated images of phantoms of different complexity. The differentiation between tissue types is investigated by imaging objects of known compositions (water, lung, and bone equivalent). A lung tumor phantom, simulating materials and geometry encountered during lung SBRT treatments, is used to investigate image noise properties for various quantities of delivered radiation (monitor units(MU)). Patient scatter images are simulated using the validated simulation model. 4DCT patient data is converted to an MCNP input geometry accounting for different tissue composition and densities. Lung tumor phantom images acquired with decreasing imaging time (decreasing MU) are used to model the expected noise amplitude in patient scatter images, producing realistic simulated patient scatter images with varying temporal resolution. RESULTS Image intensity in simulated and experimental scatter images of tissue equivalent objects (water, lung, bone) match within the uncertainty (∼3%). Lung tumor phantom images agree as well. Specifically, tumor-to-lung contrast matches within the uncertainty. The addition of random noise approximating quantum noise in experimental images to simulated patient images shows that scatter images of lung tumors can provide images in as fast as 0.5 seconds with CNR∼2.7. CONCLUSIONS A scatter imaging simulation model is developed and validated using experimental phantom scatter images. Following validation, lung cancer patient scatter images are simulated. These simulated patient images demonstrate the clinical utility of scatter imaging for real-time tumor tracking during lung SBRT.

SU‐F‐T‐45: Dosimetric Effects of Saline Filled Balloons During IORT Using Xoft Electronic Brachytherapy

PURPOSE The portability of Xoft Axxent Electronic Brachytherapy (EBx) System has made it a viable option for intraoperative radiation therapy (IORT) treatment of early-stage breast cancer. The low energy (50kVp) of the X-ray source makes the shielding easy, but also means its dose distribution is sensitive to the mediums composition. Current treatment planning systems (TPS) typically assume homogenous water for brachytherapy dose calculations, including the pre-calculated atlas plans for the Xoft IORT cases. However, Xoft recommends using saline to fill the balloon applicator. This study investigates the dosimetric difference due to the increased effective atomic number (Zeff) from water (7.42) to saline (7.56). METHODS The diameter of the balloon applicators ranges from 3-6cm, with 4cm being most frequently used. For the 4-cm and 6-cm diameter applicators, MCNP Monte Carlo program was used to calculate the dose at the surface (Ds) of the middle section of the balloon and 1 cm away (D1cm) for water- and saline-filled balloons: one plan with a single dwell at the center and another with multiple dwells as in the atlas plans. The single dwell plan is a simple estimation of the dosimetry, while the atlas plan is representative of the actual dose distribution. RESULTS The single-dwell plan showed a 5.1% and 6.1% decrease in Ds for the 4- and 6-cm applicators, respectively, due to the saline. The atlas plan showed similar RESULTS: 4.8% and 6.4% decrease, respectively. The decrease in D1cm is 4.3%-5.2% and 3.3%-5.3s% in the single-dwell and atlas plans, respectively, for the 4- and 6-cm applicator. CONCLUSION The dosimetric effect introduced by saline is on the order of 5%. This effect should be taken into account during both treatment planning and patient outcome studies.

TH-AB-202-06: BEST IN PHYSICS (JOINT IMAGING-THERAPY): A Real-Time Tumor Tracking Using Novel Scatter Imaging Modality During Lung SBRT

PURPOSE Compared to traditional radiotherapy techniques, stereotactic body radiation therapy (SBRT) provides more favorable outcomes during the treatment of certain lung tumors. Despite advancements in image guidance, accurate target localization still remains a challenge. In this work, we expand our knowledge of a novel scatter imaging modality in order to develop a real-time tumor localization method using scattered photons from the patient during treatment. METHODS Images of the QUASAR™ Respiratory Motion Phantom were taken by irradiating it on a Varian TrueBeam accelerator. The scattered radiation was detected using a flat panel-based pinhole camera detection system. Two motion settings were investigated: static and dynamic. In the former, the lung tumor was manually shifted between imaging. In the latter, the lung tumor was set to move at a certain frequency and amplitude while the images were acquired continuously for one minute. The accuracy of tumor localization and the irradiation time required to distinguish the lung tumor were studied. RESULTS The comparison of measured and expected location of the lung tumor during static motion was shown to be under standard deviation (STD) of 0.064 with a mean STD of 0.031cm. The dynamic motion was taken at a rate of 1400 MU/min for one minute and the measured location of the lung tumor was then compared with the QUASAR phantoms sinusoidal motion pattern and the agreement found to be at an average STD of 0.275cm. The location of the lung tumor was investigated using aggregate images consisting of 1 or 2 frames/image and the change was below STD of 0.30cm. The lung tumor also appeared to be blurrier in images consisting of two frames. CONCLUSION Based on our preliminary results real-time image guidance using the scatter imaging modality to localize and track tumors during lung SBRT has the potential to become clinical reality.

SU-E-J-176: Results of Images Acquired with Backscattered MV Radiation Using a Pinhole Collimator

PURPOSE To ascertain the feasibility of acquiring real time images of small lung tumors from scattered photons while undergoing radiation treatment. There are several methodologies currently used to track tumor location such as MV-cine acquisition and kV fluoroscopy. However, MVcine offers no information parallel to the beam axis while kV fluoroscopy offers little potential for soft tissue discernability while also increasing the patient dose. This study investigates the feasibility of observing an actual simulated tumor while exploring techniques that may improve image quality. METHODS A prototype imager consisting of a gamma camera pinhole collimator and a computed radiography (CR) plate were used in conjunction with a Varian TrueBeam linac. One study consisted of a 2.5 cm diameter solid water cylinder representing a solid tumor imbedded within a lung equivalent material slab. The cylinder with the lung slab was sandwiched between 1 cm lung equivalent slabs and these were sandwiched between 2 slabs of solid water. The top water slab was 1 cm thick. The other imaging study consisted of three different density plugs, 0.46, 1.09, and 1.82 g/cm3 placed on the accelerator couch. The gantry was orientated 70° relative to the CR plate. The slabs and plugs were irradiated with 2000 MU and 500 MU respectively using the 6FFF mode. RESULTS The solid water plug was visually discernible in the slab phantom. The ratio of the signal coming from the higher density plugs (placed on the treatment couch) to that between the plugs increased from 1.02 to about 3.0 after subtracting the background image acquired with no plugs present. CONCLUSION Preliminary results indicate that a lung tumor could be visualized with scattered radiation during treatment. Improvements in discerning an object can be enhanced by filtering out the head leakage and background scattered radiation not emanating from the imaged object.

SU-E-T-197: Helical Cranial-Spinal Treatments with a Linear Accelerator

PURPOSE Craniospinal irradiation (CSI) of systemic disease requires a high level of beam intensity modulation to reduce dose to bone marrow and other critical structures. Current helical delivery machines can take 30 minutes or more of beam-on time to complete these treatments. This pilot study aims to test the feasibility of performing helical treatments with a conventional linear accelerator using longitudinal couch travel during multiple gantry revolutions. METHODS The VMAT optimization package of the Eclipse 10.0 treatment planning system was used to optimize pseudo-helical CSI plans of 5 clinical patient scans. Each gantry revolution was divided into three 120° arcs with each isocenter shifted longitudinally. Treatments requiring more than the maximum 10 arcs used multiple plans with each plan after the first being optimized including the dose of the others (Figure 1). The beam pitch was varied between 0.2 and 0.9 (couch speed 5- 20cm/revolution and field width of 22cm) and dose-volume histograms of critical organs were compared to tomotherapy plans. RESULTS Viable pseudo-helical plans were achieved using Eclipse. Decreasing the pitch from 0.9 to 0.2 lowered the maximum lens dose by 40%, the mean bone marrow dose by 2.1% and the maximum esophagus dose by 17.5%. (Figure 2). Linac-based helical plans showed dose results comparable to tomotherapy delivery for both target coverage and critical organ sparing, with the D50 of bone marrow and esophagus respectively 12% and 31% lower in the helical linear accelerator plan (Figure 3). Total mean beam-on time for the linear accelerator plan was 8.3 minutes, 54% faster than the tomotherapy average for the same plans. CONCLUSIONS This pilot study has demonstrated the feasibility of planning pseudo-helical treatments for CSI targets using a conventional linac and dynamic couch movement, and supports the ongoing development of true helical optimization and delivery.

SU‐E‐T‐708: The Role of Small Bowel in Cervical Cancer Brachytherapy

Purpose: Small bowel (SB) is the organ at risk (OAR) that has the highest rate of radiation induced late toxicity in brachytherapy of cervical cancer. Historically, however, its dose has not been systematically reported as other OARs even during image guided brachytherapy (IGBT). This study aims to evaluate the effect of including SB in BT plan optimization and to propose a possible reporting schema. Methods: Thirteen patients were included in this retrospective study. All patients were treated with external beam radiotherapy of 45Gy in 25 fractions followed by high dose rate (HDR)‐BT of 28Gy in 4 fractions/2 implants using tandem/ring applicator. MRI and CTs were obtained to define the GTV and HR‐CTV, and OARs. Treatment plans were generated for each patient without considering small bowel dose constraint, and revised for those with D2cc of SB >5Gy. Results: Six of 13 cases required plan revision due to high D2cc for SB. By sparing the SB with an average reduction of 19% in D2cc, only 2 of the 6 replanned cases resulted in less optimal target coverage, i.e. D90 of HR‐CTV less than 77Gy. However, one of them already had difficulty in achieving the coverage in the first plan due to other OAR dose restriction. On the other hand, SB mobility was noticed in the CT images, which could help in SB tolerance. Conclusion: Although SB is typically mobile compared to other OARs, it is strongly recommended that it be used as an OAR for plan optimization. Prospective reporting of D2cc index will help to better define its potential association with late complications in the future.

SU‐E‐T‐625: A Method to Estimate Lung Doses under a Block during Total Body Irradiation (TBI) Treatment

Purpose: Total Body Irradiation(TBI) treatment with photon beams has been accepted as an important radiotherapytreatment for a variety of malignant diseases. Usually lung blocks are used to minimize the lungdose and prevent radiation‐induced pneumonitis. The purpose of this paper is to estimate the mid‐plane lungdoses under block during TBI treatments. Methods: The accuracy of the dose calculations under a block from a commercial planning system was first studied. The central axis depth doses under a block were measured using a parallel plate chamber in homogeneous solid water phantom at 100SSD for both 6MV and 18MV beams. A 7.5cm thick, 15×15cm2 cerrobend block positioned on the central axis was used to attenuate the beam with a setting of 30×30cm2. The measurements were compared with calculations generated with the Pinnacle treatment planning system. In addition, measurements were performed in a heterogeneous phantom positioned in a TBI geometry. The phantom consisted of two 4cm solid water slabs sandwiched 16cm foam slabs simulating lung tissues. A parallel plate chamber, garfchromic films and optically stimulated luminescent(OSL) dosimeters were used to measure depth doses for both 6MV and 18MV beams. These data were used to model the mid‐plane lungdoses under the blocks. Results: The depth dose curves calculated by Pinnacle showed significant deviations from measurements. An estimate of the mid‐plane doses was calculated by taking the average of measured doses at depth of 2cm and 24cm for 6MV, 4cm and 24cm for 18MV. Our estimated value for the mid‐plane doses within ±7% of measurement. Conclusions: We have observed that the Pinnacle cannot accurately model the dose in the build‐up region under a central block. A method to estimate the mid‐plane lungdose under a block was proposed. More measurements and Monte Carlo simulations are proceeding to investigate the robustness of this method.

SU‐E‐T‐536: SRS Diode and Diamond Detector Signal Response Correction Factors in Small Diameter Stereotactic Radiosurgery Fields

Purpose: To evaluate the magnitude of correction factors that need to be applied to the response of a SRS diode and a diamond detector used to measure the relative output factor of the 5.0 mm diameter BrainLab stereotactic cone in conjunction with a Varian Trilogy accelerator. Method and Materials: Required accuracy and precision in dose delivery during SRS can be achieved only when the geometric and dosimetric characteristics of the small radiation fields are completely understood. We have previously reported a 3% difference between the relative output factor (ROF) for the 5.0mm diameter cone measured with uncompensated Scanditronix/IBA Si‐ diode and ROF calculated by Monte Carlo simulations using MCSIM code. However, the reported values for ROF did not account for differences in response of Si under conditions lacking lateral electronic equilibrium. A Si‐ response correction factor has been calculated using MCSIM code and was applied to the measured value for ROF. In addition PTW 60003 diamond detector was used to verify previous measurements. Corrections for dose rate dependence and volume averaging have been made using MC simulations. Results: ROF obtained for SRS diode (0.683±0.011) and diamond detector (0.685±0.008) are in excellent agreement (within 0.2%) with each other and within 1–1.3% of previously reported MC calculated value of ROF (0.692±0.011). Conclusions: We have performed an experimental and theoretical investigation of ROF for radiation field created by BrainLab 5.0 mm SRS cone in conjunction with a Trilogy accelerator equipped with SRS mode. Methods of obtaining ROF using Scanditronix/IBA Si‐diode and PTW Diamond 60003 detector have been outlined. Monte Carlo simulations were used to correct for various effects contributing to potential detector response errors. We conclude that corrections for detector signal response up to ±4% were needed for both SRS diode and diamond detectors.