A Sarkar

Anatomy-Based Inverse Planning Simulated Annealing Optimization in High-Dose-Rate Prostate Brachytherapy: Significant Dosimetric Advantage Over Other Optimization Techniques

PURPOSE To perform an independent validation of an anatomy-based inverse planning simulated annealing (IPSA) algorithm in obtaining superior target coverage and reducing the dose to the organs at risk. METHOD AND MATERIALS In a recent prostate high-dose-rate brachytherapy protocol study by the Radiation Therapy Oncology Group (0321), our institution treated 20 patients between June 1, 2005 and November 30, 2006. These patients had received a high-dose-rate boost dose of 19 Gy to the prostate, in addition to an external beam radiotherapy dose of 45 Gy with intensity-modulated radiotherapy. Three-dimensional dosimetry was obtained for the following optimization schemes in the Plato Brachytherapy Planning System, version 14.3.2, using the same dose constraints for all the patients treated during this period: anatomy-based IPSA optimization, geometric optimization, and dose point optimization. Dose-volume histograms were generated for the planning target volume and organs at risk for each optimization method, from which the volume receiving at least 75% of the dose (V(75%)) for the rectum and bladder, volume receiving at least 125% of the dose (V(125%)) for the urethra, and total volume receiving the reference dose (V(100%)) and volume receiving 150% of the dose (V(150%)) for the planning target volume were determined. The dose homogeneity index and conformal index for the planning target volume for each optimization technique were compared. RESULTS Despite suboptimal needle position in some implants, the IPSA algorithm was able to comply with the tight Radiation Therapy Oncology Group dose constraints for 90% of the patients in this study. In contrast, the compliance was only 30% for dose point optimization and only 5% for geometric optimization. CONCLUSIONS Anatomy-based IPSA optimization proved to be the superior technique and also the fastest for reducing the dose to the organs at risk without compromising the target coverage.

A quality assurance method with submillimeter accuracy for stereotactic linear accelerators

The Stereotactic Alignment for Linear Accelerator (S. A. Linac) system is developed to conveniently improve the alignment accuracy of a conventional linac equipped with stereotactic cones. From the Winston‐Lutz test, the SAlinac system performs three‐dimensional (3D) reconstruction of the quality assurance (QA) ball coordinates with respect to the radiation isocenter, and combines this information with digital images of the laser target to determine the absolute position of the room lasers. A handheld device provides near‐real‐time repositioning advice to enable the user to align the QA ball and room lasers to within 0.25 mm of the centroid of the radiation isocenter. The results of 37 Winston‐Lutz tests over 68 days showed that the median 3D QA ball alignment error was 0.09 mm, and 97% of the time the 3D error was ≤0.25 mm. All 3D isocentric errors in the study were 0.3 mm or less. The median x and y laser alignment coordinate error was 0.09 mm, and 94% of the time the x and y laser error was ≤0.25 mm. A phantom test showed that the system can make submillimeter end‐to‐end accuracy achievable, making a conventional linac a “Submillimeter Knife”. PACS numbers: 87.53.Ly, 87.55.Qr

SU-FF-J-77: Image Quality Assessment for An Investigational Megavoltage Cone-Beam CT Device

Purpose: Megavoltage Cone‐Beam CT (MVCBCT) is an essential image guided radiation therapy(IGRT) device to acquire patients daily treatmentCT for accurate localization of treatment targets. The objective of this research was to assess its image qualities. Method and Materials: The image quality of MVCBCT was assessed by four indicators: noise, contrast, spatial resolution, and CT intensity stability. A CT electron density phantom and a Siemens calibration phantom were used. The images were acquired under various MU settings. The Siemens Syngo image processing software was used to sample and analysis the data. Results: The noise factor was used and found that the more MU to acquire the images produced less noise. 6 MU is the cut‐off value for noise factor of less than 5%. For contrast of the outer ring of the CT phantom, the electron density range of 0.976 were visible on all MUs. For the inner ring, we only see 1.052 on MU 1.043 for MU > 15. For the CT intensity stability, if the CT number differences has to be criteria of 0.25. Conclusion: The images from MVCBCT device were assessed for quality indicators; we conclude that the MU of 6 or above would have satisfactory results. For the future application of dose calculation on the MVCBCT images, the CT intensity stability is important, and we found that for 6 MU and above would have stable CT numbers.

SU-F-BRA-07: Dosimetric Verification of the Valencia Skin Applicator Using Gafchromic EBT3 Film

Purpose: The Valencia applicators have recently been introduced for HDR treatment of small and shallow superficial skin lesions (< 20 mm diameter and 3-mm depth). Per AAPM TG 56, any HDR applicator internal dimensions must be verified prior to clinical use. However radiographic and tomographic imaging to validate the Valencia applicators is impractical due to the Tungsten alloy housing and flattening filter. In this study, we propose to use EBT3 film to indirectly confirm the physical integrity of the Valencia applicators. Methods: Treatment plans were created using the Oncentra MasterPlan TPS v4.5 for the H2 (20-mm dia.) and H3 (30-mm dia.) Valencia Applicators. A virtual CT phantom (2-mm slice thickness) was created with one source position in water. The published effective depth method was used for each applicator to delivery 500 cGy to a 3-mm depth using the TG-43 formalism. Film measurements (n=3) at 3-mm depth and vertical plane in solid water were performed for each applicator to verify the prescribed dose calculated by the TPS. Percent depth dose curves and off-axis profiles (phantom surface and 3-mm depth) were measured and compared to published data. Films were analyzed using an in-house written program and RIT113 v6 software. Film calibration was performed per TG-55 protocol using the Ir-192 source with NIST-traceable calibration. Results: The prescription absolute dose difference was 1% for the Valencia H2 applicator and 4% for the Valencia H3 applicator. The measured percent depth dose curves and off-axis dose profiles measured for the H2 and H2 Valencia applicators are in excellent agreement with the Granero et al. Monte Carlo results1. Conclusion: Gafchromic EBT3 film can be used to indirectly verify the internal components of special HDR skin applicators constructed from high Z materials.1Granero et al. “Design and evaluation of a HDR skin applicator with flattening filter”, Med. Phys. 35(2), 495–503, 2008

SU‐FF‐T‐351: Using EPID Images to Verify the Dose Delivered From 3D Conformal Or Step‐N‐Shoot IMRT Fields

Purpose: The electronic portal imaging device(EPID) has particular advantages as a dosimeter such as high resolution, large detection area, real‐time acquisition capability and linear dose response. The doses from 3D or IMRT fields can be verified by the images acquired on EPID. This study demonstrates a calibration and calculation method to verify the doses from 3D conformal or step‐and‐shoot IMRT fields. Method and Materials: To use EPID for dosimetric purpose, a series of calibrationsimages would need to be acquired. The uniformity of the dose response, dose linearity, and field size dependence were measured and the results were used to correct the subsequent clinical measurements. A few 3D conformal and step‐and‐shoot IMRT fields were delivered directly to EPID. The pixel values were calculated to project the delivered doses and the images were processed to determine the MLC positions. The ion chamber measurements were also performed to be the benchmark of delivered dose. Results: The dose accuracy of this method was investigated by comparing the ion chamber to EPID results. The MLC positions from EPIDimages were compared to the treatment plans. Conclusion: There are some basic corrections needed when the EPID is used for dose measurements. By applying the proposed method, the MLC positions and delivered doses from 3D conformal or step‐and‐shoot IMRT fields can be verified with EPIDimages.

SU‐GG‐J‐24: Additional Skin Entrance Dose Delivered with Radiographic Image Guidance System in Cyber Knife Robotic Treatment Delivery

Purpose: To independently report the additional radiationdose from the image guidance and tracking part of the treatment delivery of Cyber Knife system. Method and Materials: The entrance dose on the skin and dose at 1cm depth in a specially designed phantom using calibrated diagnostic dosemeasurement equipment was measured. Using a Barracuda MPD probe placed perpendicularly to the beam measureddose from one exposure from one of the X‐ray source and was converted to pair of images to account for both X‐ray sources. The doses were measured for varying potential (kV) and beam current (mA) representing common clinical situations. Dose estimation from exposures was determined on the basis of the average number of treatment nodes and the number of exposures per treatment in setup, treatment and realignment for non‐lung and lung targets. A rough estimate of the number of exposures for a course of 5 treatments for a non lung target is 150–280 while for a lung target or a moving target is 515–925. Results: Using a fixed imaging technique of 250mA, exposure time of 100 ms and kV varying from 100–125, we obtained a range of 0.51–0.68 mGy for an image pair on skin and 0.38–0.51 mGy at 1cm depth. Using the estimated number of image pairs, this translates to an additional therapy skindose of approximately 11–15cGy for non lung targets and 37–49 cGy for lung target, where as corresponding dose at 1 cm depth is 8–11 cGy for non lung and 27–37cGy for lung targets respectively. The doses were computed for varying beam currents and exposure times. Conclusion: It is recommended that the dose from imaging devices in CyberKnife radiation treatments should be closely monitored and be accounted in the total delivered dose such that a unified approach is maintained for clinical trial outcome analysis.

SU‐GG‐T‐434: Submillimeter XKnife End‐To‐End Alignment Accuracy Using SAlinac and the Lucy Phantom

Purpose: To achieve submillimeter end‐to‐end alignment accuracy on a conventional linac with stereotactic cones using the Stereotactic Alignment for Linac (S. A. linac) system. Method and Materials: Previous conference publications have already shown that the SAlinac system can enable the user to align the Winston‐Lutz radiopaque ball and the room lasers to within 0.2 mm of the isocenter. In this study, the Lucy phantom from Standard Imaging has been used to show that submillimeter end‐to‐end alignment accuracy can also be achieved with the SAlinac system. In the 20 years since the seminal paper by Winston and Lutz, we have not seen any other system that can achieve submillimeter end‐to‐end alignment accuracy on a conventional gantry mounted linear accelerator. The SAlinac system captures live images of the laser targets with digital cameras and combines this information with analysis of the Winston‐Lutz test to provide near‐real time alignment advice. As the user repositions the Winston‐Lutz ball, the room lasers, or the head frame, the SAlinac system continuously captures and processes live digital images to provide updated repositioning advice. Results: To prepare for this study, two years ago the SAlinac system was used to correct a 0.5 mm gantry skew on the Siemens Mevatron MXE2 at Christiana Hospital, and four months ago the ceiling and wall lasers were remounted to minimize laser divergence. In this study we CT scanned the Lucy phantom, generated a treatment plan, and delivered the plan to expose the Gafchromic film inside the phantom. The final end‐to‐end results were x=−0.73, y=0.27, z=−0.41 mm, for a total 3D error of 0.87 mm. Conclusion: Submillimeter end‐to‐end alignment accuracy has been achieved — it is now the Submillimeter Knife (SKnife).

SU‐FF‐J‐11: Defining Picture Archiving and Communication System — Radiation Therapy Extension (PACS‐RT) for Progressive Needs for IGRT, 4D CT/PET, TPS and the RT Workflow Management

Purpose:IGRT has become an essential modality to accurately setup patients for radiation treatment. 4D/CT/PET systems generating multiple phases of respirationimage‐sets have greatly helped clinicians contour integrated target volumes. However, IGRT creates 30–40‐times more image data. PACS become important in modern Radiotherapy for massive storage needs. In this study, we defined a new system by expanding PACS into RT extension to cover additional non‐DICOM, RT‐specified data and intra‐departmental communications. Method and Materials: PACS‐RT was designed with two essential core architectures: data storage (archiving) and quality tracking (communications). The RT data stored includes DICOM/RT and non‐DICOM‐objects: TPS, RV it safeguards patient information and tracks quality in each step we perform. In a high‐volume cancer center with advanced treatments, this type of PACS‐RT system is proving essential. Conflict of Interest: N/A.

SU‐FF‐T‐81: Anatomy‐Based Inverse Planning Simulated Annealing (IPSA) Optimization in HDR Prostate Brachytherapy

Purpose: Independent validation of IPSA algorithm in obtaining superior target coverage and reduced dose to organs at risk, by comparing with the results of other optimization techniques in HDR brachytherapy.Method and Materials: In a recent prostate HDR brachytherapy protocol study (RTOG‐0321) we used an anatomy based inverse optimization technique known as Inverse Planning Simulated Annealing (IPSA), incorporated as a beta version in the Plato TPS Version 14.2.3 (Nucletron Corp., Veenendaal, The Netherlands) in an effort to satisfy the dose constraints set by the protocol for Prostate, Bladder, Rectum and Urethra. Between the period of June 2005 and November 2006, 20 patients received HDR boost dose of 19 Gy in two fractions to the prostate in addition to an external beam dose of 45 Gy with IMRT technique. 3D Brachytherapytreatment plans were generated using Plato Version 14.2.3. Dosimetry was obtained for the following optimization schemes in Plato, for the same dose constraints for all the patients treated during this time period: Anatomy based IPSA optimization, Geometric optimization on volume and Dose Point optimization on volume. Dose volume histograms were generated for PTV and organs at risk for each optimization method, from which V75 volumes for Rectum, Bladder, and V125 volume for Urethra were determined. Also dose homogeneity index as well as conformal index for the PTV was calculated for each optimization technique. Results: Despite suboptimal needle placement in some implants, using the same dose constraints on all patients, IPSA algorithm was able to comply with the tight RTOG dose constraints for 90% of the patients in this study, whereas it was only 30 % for DPO and only 5% for GO. Conclusion:Anatomy based IPSA optimization proved to be the overall best and fast technique for reducing the dose to organs at risk without compromising target coverage.