S. Bahri

SU‐GG‐J‐84: Evaluation of Comparing Daily Ultrasound Images with a Reference Ultrasound Image for Prostate Localization

Purpose: To evaluate if acquiring an ultrasoundimage at the time of CT simulation for comparison with daily ultrasoundimages improves daily localization of prostates. Method and Materials: Resonant Medicals® ultrasound localization system was installed and implemented in our clinic. The technique relies on acquiring a 3D ultrasoundimage at the time of CT simulation for daily comparison whereas other ultrasound localization techniques compare daily ultrasoundimages to the original CTimage.Treatment planning is done on the CT. DRRs are also constructed from the CT, and fiducials implanted in the prostate are outlined on the DRRs. Each day a 3D ultrasoundimage was acquired and compared to the ultrasound that was acquired at the time of CT simulation. Daily, if the ultrasoundimage was approved by the physician, the couch was shifted to align the current prostate location with its location at the time of simulation. After the ultrasound, ports were taken as often as prescribed by the physician. The fiducial locations as seen in the ports were compared to their locations on the DRRs. Any necessary shifts were made to align the fiducials. Following the treatments, an analysis was made of the ultrasound localization as compared to the fiducial localization. 22 patients had 7 or more days in which both ultrasound and ports of fiducials were acquired and are included in this analysis. Results: The measured average difference between the ultrasound localization and the localization based on ports of fiducials is 7.2 mm. This is comparable to what is reported in literature for other ultrasound localization techniques. Conclusion: Using a 3D ultrasoundimage acquired at the time of CT simulation does not improve ultrasound localization accuracy as compared to techniques that compare daily ultrasoundimages to the simulation CT for localization.

Treatment planning comparison between high dose rate and intensity-modulated radiation therapy for prostate cancer as a means of boost dose

Purpose The main objective of this study was to compare dosimetric characterisation of high-dose-rate brachytherapy (HDR-BT) with external beam intensity-modulated radiation therapy (EX-IMRT) as a means of delivering boost dose. Materials and methods Five HDR patients were selected for IMRT planning. Patients underwent ultrasound-guided catheter placement for HDR. Computed tomography (CT) images were obtained and imported into the Nucletron PLATO Brachytherapy system. The prostate, urethra, bladder and rectum were contoured on axial slices. The dose was calculated and optimised by graphical optimisation. The CT images of these structures were exported from the PLATO to Eclipse workstation for IMRT planning. For each patient, the dose–volume histogram (DVH) of HDR and IMRT plans were generated, drawn on the same scale and compared. Results The dose distribution in HDR plans was non-uniform and conformed peripherally inside the planned target volume (PTV). A small volume of the prostate received a very high dose from HDR.In IMRT plans, a uniform dose distribution was observed. The DVH curves for PTV dropped sharply and reached to a zero volume of the prostate at about 6·4 Gy. In HDR plans, the DVH curves for PTV showed a long tail up to a very high dose. About 10% of the prostate received about 13·3 Gy, which is 222% of the prescribed dose (6 Gy) in HDR plans. In contrast, the same volume in IMRT plans received Conclusions HDR brachytherapy may reduce normal tissue toxicities in prostate boost treatments, even though the dose homogeneity inside the PTV is far worse than in IMRT treatments. Another advantage of HDR over IMRT is that the organ motion is not a significant concern as in IMRT.

SU‐E‐T‐397: A Review of Rapid Arc Standard Deviations

PURPOSE he purpose of this study is to acquire baseline values for the standard deviations of MU and angle reported by a Varian 23ex linac following the delivery of a RapidArc field. METHODS During the delivery of RapidArc fields, the Varian linac records the gantry angle and cumulative MU every 50 ms. By comparing this data to the corresponding planned data, the system calculates the standard deviations of the MUs and the gantry angles for each field. These standard deviations are reported to the linac screen at the completion of each field and documented daily for each field along with the patient, plan, treatment site, and date. The data was then entered into Micorsoft Excel for analysis. This data was acquired for every RapidArc patient (88 total patients) over the course of the first year of RapidArc use. RESULTS The maximum MU standard deviation was 0.09 and the maximum gantry angle standard deviation was 0.30°. The averages are 0.05 MU and 0.19° respectively. No specific treatment site (prostate/prostate fossa, head and neck, brain, thorax, or other) differed significantly from these averages. CONCLUSION If the MU standard deviation is greater than 0.1 or the gantry angle standard deviation is greater than 0.3°, the RapidArc QA tests of the linac and the plan should be promptly repeated to identify any possible problems. These baseline values may be specific to the linac in question but provide a point of reference for other linacs as well.

SU‐EE‐A4‐02: Evaluation of Ultrasound Localization Versus MV Portal Images of Fiducial Markers in Prostates

Purpose: This pilot study evaluated prostate localization by comparing ultrasoundimages to orthogonal MV portal images of fiducial markers implanted into the prostate. Method and Materials: Each prostate patient had gold fiducial markers implanted into his prostate prior to simulation. The Restitu™ ultrasound system (Resonant Medical, Inc.) was used to acquire the ultrasoundimages. The first ultrasoundimage was acquired immediately prior to acquiring the CT simulation image. These images were fused using the CT isocenter, and the prostate reference volume was contoured for each patient. This contour included a portion of the inferior bladder wall near the trigone to assist with daily localization. Each day, therapists acquired an ultrasoundimage and overlaid the prostate reference volume contour onto the current image. Orthogonal MV portal images were then acquired. Displacements of imaged fiducials from their expected locations as observed on the DRRs were removed by shifting the couch if the displacements exceeded 5 mm. Following treatment, the location of each fiducial in all 3 directions was measured on the final portal images for each day and averaged to measure the final prostate location. Differences in where the ultrasoundimage and where the portal images of fiducials would locate the prostate were compared. Results: For 11 patients, the differences are less than 6.1 mm within the 95% confidence interval. For 1 patient, ultrasound imaging did not consistently reproduce the prostate location to within 10 mm as compared to fiducials via our technique. Sources of deviation include slight discrepancies in calibrating the ultrasound systems, slice spacing, different users, fusion discrepancies, image quality, and random uncertainties. Conclusion: For most patients, ultrasound and ports of fiducials provide comparable localization information for prostates. However, sources of disagreement still exist. Anatomical landmarks can be useful in most cases but can also be misleading if improperly used.

TU‐EE‐A1‐03: Comparison of High Dose Rate (HDR) Vs Intensity Modulated Radiation Therapy (IMRT) for Prostate Boost Treatment

Purpose: The objective of this study is to compare dosimetric characteristics of prostate treatments using HDR brachytherapy and IMRT technique. Method and Materials: Five HDR patients were selected for IMRT planning. Patients underwent ultrasound guided catheter placement for HDR. CTimages were obtained and imported into the Nucletron PLATO Brachytherapy system. The prostate, urethra, bladder and rectum were contoured on axial slices. The dose was calculated and optimized by graphical optimization. The CTimages of these structures were exported from the PLATO to Eclipse workstation for IMRT planning and comparison. For each patient, the DVH of HDR and IMRT plans were generated, drawn on the same scale and compared. Results: In IMRT plans the DVH curves for PTV dropped sharply and reached to zero volume of the prostate at about 6.4 Gy. In HDR plans the DVH curves for PTV showed a long tail up to a very high dose. About 10% of the PTV for prostate received greater than 12 Gy (200%) of the prescribed dose (6 Gy) in HDR plans. In contrast, the same volume in IMRT plans received less than 6 Gy (100%). Average prostate V90 and V100 dose was about 6.3 Gy and 4.12 Gy respectively for HDR, and 6.09 Gy and 5.74 Gy for IMRT plans, respectively. UrethraV90 dose for IMRT plans showed similar levels (93%), whereas in HDR the dose varied widely (60 to 100%). In all plans, the dose to the bladder and rectum was significantly lower in HDR than in IMRT plans. Conclusions: HDR brachytherapy may reduce normal tissue toxicities in prostate boost treatments, even though the dose homogeneity inside the PTV is far worse than in IMRTtreatments. Another advantage of HDR over IMRT is that the organ motion is not a significant concern as in IMRT.

SU-FF-J-96: The Application of Varian's Markermatch Software in a Retrospective Study of Inter-Fractional Prostate Motion

Purpose: MarkerMatch is an automated marker match software feature developed by Varian in on-board imaging. It may calculate inter-fractional prostate motion with internal fiducial markers identified on CT scans. Before each treatment, a pair of portal images was taken and fiducial markers are identified. Based on the portal image pair, MarkerMatch calculates the optimized couch displacement in 3D to maximally restore the marker positions to their reference positions. To evaluate MarkerMatchs clinical performance, we did a phantom test and a retrospective study on patients implanted with radio-opaque fiducial markers. Method and Materials: We used a phantom implanted with 4 cylindrical-shaped markers of 1mm in diameter and 3mm in length. MarkerMatch localizes the markers based on CT images. In order to test MarkerMatchs ability to handle CT images of different quality, we scanned the phantom with four CT spacing. The portal image pair taken before treatment is normally at AP/Lateral gantry angles, but sometimes it is difficult to identify markers from the lateral image. To test MarkerMatchs ability to handle non-orthogonal portal image pair, we took portal images at 7 different gantry angles. As a preliminary test for the use of Markermatch in clinic, we retrospectively analyzed five patients implanted with 2–3 gold markers based on 43 pairs of weekly setup portal images. Results: In our phantom test, MarkerMatch is able to measure overall marker displacements within 1mm in each direction, regardless of the spacing used in the CT scans. Using different gantry separation angles, the measured overall marker displacements agree with each other within 1mm. Retrospective analysis of five patients is also presented. Conclusion: Initial studies indicate that MarkerMatch is robust in detecting and analyzing patient motion in 3D and can provide valuable information of inter-fractional prostate motion in clinic. Conflict of Interest: Funded in part by Varian Research Grant.