D Jacob

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

Technical Note: Dosimetry of Leipzig and Valencia applicators without the plastic cap

PURPOSE High dose rate (HDR) brachytherapy for treatment of small skin lesions using the Leipzig and Valencia applicators is a widely used technique. These applicators are equipped with an attachable plastic cap to be placed during fraction delivery to ensure electronic equilibrium and to prevent secondary electrons from reaching the skin surface. The purpose of this study is to report on the dosimetric impact of the cap being absent during HDR fraction delivery, which has not been explored previously in the literature. METHODS geant4 Monte Carlo simulations (version 10.0) have been performed for the Leipzig and Valencia applicators with and without the plastic cap. In order to validate the Monte Carlo simulations, experimental measurements using radiochromic films have been done. RESULTS Dose absorbed within 1 mm of the skin surface increases by a factor of 1500% for the Leipzig applicators and of 180% for the Valencia applicators. Deeper than 1 mm, the overdosage flattens up to a 10% increase. CONCLUSIONS Differences of treating with or without the plastic cap are significant. Users must check always that the plastic cap is in place before any treatment in order to avoid overdosage of the skin. Prior to skin HDR fraction delivery, the timeout checklist should include verification of the cap placement.

SU‐F‐T‐63: Dosimetric Relevance of the Valencia and Leipzig HDR Applicators Plastic Cap

PURPOSE Utilization of HDR brachytherapy treatment of skin lesions using collimated applicators, such as the Valencia or Leipzig is increasing. These applicators are made of cup-shaped tungsten material in order to focalize the radiation into the lesion and to protect nearby tissues. These applicators have an attachable plastic cap that removes secondary electrons generated in the applicator and flattens the treatment surface. The purpose of this study is to examine the dosimetric impact of this cap, and the effect if the cap is not placed during the HDR fraction delivery. METHODS Monte Carlo simulations have been done using the code Geant4 for the Valencia and Leipzig applicators. Dose rate distributions have been obtained for the applicators with and without the plastic cap. An experimental study using EBT3 radiochromic film has been realized in order to verify the Monte Carlo results. RESULTS The Monte Carlo simulations show that absorbed dose in the first millimeter of skin can increase up to 180% for the Valencia applicator if the plastic cap is absent and up to 1500% for the Leipzig applicators. At deeper distances the increase of dose is smaller being about 10-15%. CONCLUSION Important differences have been found if the plastic cap of the applicators is absent in the treatment producing an overdosage in the skin. The user should have a checklist to remind him check always before HDR fraction delivery to insure the plastic cap is placed on the applicator. This work was supported in part by Generalitat Valenciana under Project PROMETEOII/2013/010, by the Spanish Government under Project No. FIS2013-42156, and by a research agreement with Elekta Brachytherapy, Veenendaal, The Netherlands.

SU‐D‐19A‐07: Dosimetric Comparison of HDR Plesiotherapy and Electron Beam Therapy for Superficial Lesions

PURPOSE Large superficial (skin, soft tissue sarcoma) lesions located on curved areas are hard to treat with electrons. The Freiburg Flap (Nucletron, Netherlands) is a flexible mesh style surface which can be easily shaped to fit curved surfaces for reproducible HDR fraction delivery. To understand the fundamental dosimetric differences, a dosimetric comparison was made between HDR plesiotherapy (Freiburg applicator for lesions over 4cm) and external electron beam radiotherapy over cases with varying target curvature (both stylized and clinical cases). METHODS Four stylized cases with variable complexity were created using artificial DICOM axial CT slices and RT structures (a square and three curved structures on a 4.5cm radius cylinder). They were planned using Oncentra v4.3 and exported to Pinnacle v9.6 for electrons planning. The HDR source dwell positions were optimized for the best coverage of the targets using graphical optimization. Electron treatment plans were created in Pinnacle using the same CT and RT structures of three HDR cases with surface lesions previously treated with the Freiburg flap. The En face electron plans used 6-12 MeV electrons and 0.5-1 cm bolus was added to increase surface dose. The electron plans were prescribed to an isodose line to conform to the target. RESULTS For all lesions, the average target dose coverage was similar (D90ave of 100% for HDR vs 101% for electrons). For lesions with high curvature, the HDR coverage was better (D90 102% vs D90 97% for electron). For all cases, adjacent structures high dose region was lower for HDR than electrons (D1cc 100% for HDR vs D1cc 111% for electrons). CONCLUSION HDR plesiotherapy offers excellent target conformity for superficial targets similar to electrons. However, for lesions with complex curved surfaces, HDR has the advantage to achieve better dose distributions using graphical optimization to spare adjacent normal tissue while maximizing target coverage.

SU‐E‐T‐236: Brachytherapy for Breast Cancer Patients with Implanted Pacemeker, Using Multi Lumen Partial Breast Applicator

PURPOSE Discuss optimised Accelerated Partial Breast Irradiation (APBI) using a Strut-Adjused Volume Implant (SAVI) applicator for patients with implanted pacemeker, without overdosing or relocating the pacemaker. METHODS A right breast cancer patient with an in situ pacemaker (Medtronic model ADDR01) on the right side underwent breast conservative surgery and was referred for Partial Breast Irradiation (PBI) using Ir-192 High Dose Rate Brachytherapy. 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 1cm margin without exceeding the 2Gy limit to the pacemaker and the leads. A seven catheter SAVI device was selected and implanted in the right 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. Optimization tools namely anatomy based Inverse Planning Simulated Annealing (IPSA), and graphical optimization were utilized to load and optimize the dwell time in the catheters and reduce the dose received by the pacemaker. Device interrogation was conducted before and after the treatment. All operational parameters of the pacemaker were found to be normal.No change in the baseline reference values were observed. RESULTS Using the SAVI device it was possible to limit the pacemaker/ leads dose to less than 2 Gy. Target V100, V95 and V90 were 90.1%, 95.5% and 98.5% respectively. V150 and V200 being 18.6cc and 10.6cc respectively.Calculated average point doses on pacemaker assembly for ten fractions was 140cGy. CONCLUSIONS By combining the optimization tools of todays Brachytherapy planning system and a multilumen SAVI applicator, HDR partial breast irradiation can be safely delivered for breast cancer patients with in situ pacemaker, with out the fear of interrupting pacemaker functionality.

Accelerated partial breast irradiation using a strut-based brachytherapy device: A multi-institutional initial report on acute and late toxicity with greater than 24-month follow-up.

149 Background: Accelerated partial breast irradiation (APBI) is commonly used in early-stage breast cancer. The SAVI Collaborative Research Group is a multi-institutional group created to study outcomes in patients who received APBI utilizing strut-based brachytherapy. This analysis reports the acute and late toxicities for patients with greater than 2-year follow-up (F/U) from this study. METHODS 904 APBI patients (ductal carcinoma in situ [n=267] or invasive breast cancer [n=637]), received HDR brachytherapy (34 Gy in 10 fractions) using the SAVI device (Cianna Medical). Patients with dosimetry and documented follow-up were evaluated within 6 weeks of treatment for early adverse events (AEs), and at 1 year, 2 years, and beyond for late AEs. Dosimetric parameters were evaluated with respect to toxicity and will be presented. RESULTS In 212 patients (median age 62.9 years, range 40-88) all with follow-up greater than 24 months, the median tumor size was 12mm. As of last follow-up (>24 months) cumulative rates of erythema and hyperpigmentation of grade 2 or higher were 1.4% and 0.5%. The incidence of grade 2 or higher telangiectasia, seroma and fat necrosis were 2.8%, 2.8%, and 0.5% respectively. CONCLUSIONS Adverse events for APBI with SAVI are low in incidence, low in grade and compare favorably to other HDR APBI methods.

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.