L Simpson

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‐E‐T‐596: High Dose Brachytherapy Planning of a Left Breast Cancer Patient with in Situ Pacemaker:

Purpose: To investigate the benefits of multilumen partial breast Brachytherapy device, SAVI, in reducing dose to in situ pacemakers in patients with cancer of the left breast. Methods: A left breast cancer patient with an in situ pacemaker underwent breast conservative surgery and was referred for Partial Breast Irradiation (PBI) using Ir‐192 High Dose Rate Brachytherapy. The preliminary estimation of the pacemaker dose from a pre‐insertion CT study was about 8 % of the prescribed dose which exceeded the generally accepted dose of 2Gy. The challenge was to use a suitable applicator to treat the tumor bed and 1 cm margin without exceeding the 2Gy limit to the pacemaker and the leads. A seven catheter SAVI device was selected and implanted in the left breast in an optimal direction and a 3D treatment plan was generated following a post insertion CT scan, using the Oncentra Brachy treatment planning system. Several optimization tools available in the planning system namely inverse, graphical, and dose point optimization were utilized to selectively load the catheters and reduce the dose to pacemaker and leads. A set of calibrated TLD chips were used to determine the surface dose on the pacemaker, which was then compared to the calculated surface dose. The pacemaker parameters were monitored before and after the 10 fraction (bid 5days) HDR brachytherapy, by a vendor representative and were found to functioning properly. Results: Using the seven catheter SAVI device it was possible to limit the pacemaker/ leads dose to less than 2 Gy, with an overall satisfactory dose to the target volume. Conclusions: By combining the optimization tools of todays Brachytherapy planning system and a multilumen SAVI applicator, HDR partial breast irradiation can be successfully delivered for left breast cancer patients with in situ pacemaker, with out the concern of interrupting pacemaker functionality

SU-E-T-150: Filmless Quality Assurance for Linear Accelerator with Electronic Portal Image Device

Purpose: Our intention was to perform some machine quality assurance (QA) tests that require the use of films with electronic portal imaging device(EPID). A software tool was designed for these tasks which include Light vs Radiation Field, Isocenter Star Shot, Winston‐Lutz and MLC Picket Fence images. Methods: For Light vs Radiation Field, BB dots were placed on the tabletop to indicate the light field and double‐exposure technique was used to enhance the display of BB dots. For Table Isocenter Star Shot, a square field was used, and a steel ruler was placed on the table to show one of the major axis on the images. For Winston‐Lutz test, we placed a small steel ball at isocenter and irradiate it with a square field from various gantry angles. For MLC Picket Fence test, we used a multi‐segment MLC field to deliver six rectangular fields adjacent to each other. In addition to the composite image, the EPID also saved the image from each segment. Our software analyzed the images of all segments. Results: The ability to save multiple images without entering the treatment room greatly shortened the overall time spent. The test results of Light vs Radiation Field, Isocenter Star Shot and Winston‐Lutz from the EPID are similar to the film, except the image quality is much better on EPIDimages. A significant advantage of EPID is that individual image of every segment can be saved and analyzed; where as one cannot determine accurately the position errors on a composite image of film. This analysis of EPID based images facilitates easy calibration of MLC leaves Conclusions: Compared to film method, the EPIDimages are of higher quality and better consistency. The software we developed provides us with fast, convenient, and accurate analysis for this application.

SU‐E‐T‐251: Prototype QA Device for HDR Source Position Simulator

Purpose: Design and build prototype device to perform baseline and ongoing, monthly, tests of resistance‐tolerant, smooth, dummy HDR source, position‐measurements under controlled conditions. Methods: Seven helical loops of Nucletron‐compatible catheter were wound onto the surface of a 10″ long, polyethylene tube which was 1.875″ diameter. A Nucletron HDR transfer tube could be connected between a Source Position Simulator and the entrance, 1st top helical loop end (Figure 1). Results: The 2010 SPS was attached via the transfer tube to the SPS QA prototype. The SPS wire was advanced smoothly from the start position (1030mm) to the maximum extension at 1469mm (using the advance yellow edge of the SPS marker). There was minimal cumulative resistance braking of the wire source thru the last 50mm before that max point at 1469mm. Next, the older, heavily‐used, 2004 SPS was attached via the transfer tube to the SPS QA prototype device. This SPS wire would not advance to the limit smoothly. Actually it met some modest resistance at around 1152mm (identical to the conditions of the Medical Event). The SPS wire was more firmly pushed through that zone of resistance and advanced, with increasing resistance to about 1262mm before the decision was made not to push more firmly. Conclusion: It is possible, using cumulative resistance at increased path lengths in the helical catheter, to objectively measure a distance‐to‐stop. Any motion that is not pushably‐smooth and meets any resistance short of the max finish at 1469mm indicates the beginning of damage. That signal should suffice to warn that ‘it is time’ to refurbish or replace the SPS. Results show a proper range of cumulative resistance‐induced retardation of the pushing motion of the wire thru the full motion range in QA simulation. The clinical condition of use of an SPS is accurately emulated in this device.

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‐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.