Prostate IMRT Patients Research Articles

Evaluation of various common prostate IMRT techniques based on estimated tumor control and normal tissue complication probabilities in correlation with patients anatomical parameters derived from the CT scans

Abstract Background: The relationship between the prostate IMRT techniques and patients anatomical parameters has been rarely investigated. Objective: to evaluate various prostate IMRT techniques based on tumor control and normal tissue complication probability (TCP and NTCP) values and also the correlation of such techniques with patients anatomical parameters. Methods: Four IMRT techniques (9, 7 and 5 fields and also automatic) were planned on the CT scans of 63 prostate cancer patients. The sum of distances between the organs at risk (OARs) and target tissue and also their average joint volumes were measured and assumed as anatomical parameters. Selected dosimetric and radiobiological parameters (TCP and NTCP) values were compared among various techniques and the correlation with the above anatomical parameters were assessed using Pearsons’ correlation. Results: High correlations were found between the dosimetric/radiobiological parameters of OARs with the joint volumes and with the distances between the OARs and target tissue in all the techniques. The TCP and complication free tumor control probability (P+) values were decreased with increasing the joint volume and decreasing the distances between the OARs and target tissue (as poly-nominal functions). The NTCP values were increased with increasing the joint volumes and decreasing the distances (3-degree poly-nominal functions). For the low percent joint volumes (<20%) and high distances (>7 cm), The TCP, NTCP and P+ showed no statistical differences between various techniques (P-value>0.07). However, 9 and 7 fields techniques indicated better radiobiological results (P-value<0.05) in almost other ranges (>20% joint volumes and <7 cm distances). Conclusion: Based on our results, it would be possible to compare radiobiological effects of various common IMRT techniques and choose the best one regarding to patients anatomical parameters derived from the CT scans.

Incorporation of geometrical information on structure position to gamma analysis results for portal dosimetry based IMRT prostate verification

Introduction Gamma analysis became a standard tool to evaluate the agreement between planned and delivered fluence/dose distribution. The importance of points out of tolerance limit is always the same, because the analysis does not include any information on position of relevant structures for the current Beam’s eye view (BEV). We have suggested the method for incorporation this into the gamma analysis result. Purpose The aim of this work was to create the application in Matlab software that would use the corrective matrix to be applied to the result matrix of gamma analysis from portal dosimetry with EPID for each new prostate IMRT patient to warn about unacceptable results in the most critical areas. Materials and methods The results of gamma analysis for 30 prostate patients were analyzed. Patients were treated with 5 fields sliding window IMRT on Clinac DHX. Gamma map has been matched with BEV projection of PTV, rectum, and bladder for current patient. The sum matrix of BEV over all patients for each gantry angle was created to show the most frequent BEV localization of current structure. The weighting matrix was created to be applied to the gamma analysis results of prostate IMRT increasing the importance of unsatisfactory results at areas where projections of organs are most probable. Results and conclusion The weighting matrix is the result of optimization process which took into account frequent position of BEVs and frequent position of pixels out of tolerance within gamma analysis. Disclosure Authors have no potential conflict of interest to disclose.

SU‐F‐T‐229: A Novel Method for EPID‐Based In‐Vivo Exit Dose Verification for Intensity Modulated Radiotherapy

Purpose:Electronic portal imaging device (EPID) can be used to acquire a two‐dimensional exit dose distribution during treatment delivery, thus allowing the in‐vivo verification of the dose delivery through a comparison of measured portal images to predicted portal dose images (PDI). The aim of this study was to present a novel method to easily and accurately predict PDI, and to establish an EPID‐based in‐vivo dose verification method during IMRT treatments.Methods:We developed a model to determine the predicted portal dose at the same plane of the EPID detector location. The Varian EPID (aS1000) positions at 150cm source‐to‐detector‐distance (SDD), and can be used to acquire in‐vivo exit dose using Portal Dosimetry (PD) function. Our model was generated to make an equivalent water thickness represent the buildup plate of EPID. The exit dose at extend SDD plane with patient CT data in the beam can be calculated as the predicted PDI in the treatment planning system (TPS). After that, the PDI was converted to the fluence at SDD of 150cm using the inverse square law coded in MATLAB. Five head‐and‐neck and prostate IMRT patient plans contain 32 fields were investigated to evaluate the feasibility of this new method. The measured EPID image was compared with PDI using the gamma analysis.Results:The average results for cumulative dose comparison were 81.9% and 91.6% for 3%, 3mm and 4%, 4mm gamma criteria, respectively. Results indicate that the patient transit dosimetry predicted algorithm compares well with EPID measured PD doses for test situations.Conclusion:Our new method can be used as an easy and feasible tool for online EPID‐based in‐vivo dose delivery verification for IMRT treatments. It can be implemented for fast detecting those obvious treatment delivery errors for individual field and patient quality assurance.

Planned versus ‘delivered’ bladder dose reconstructed using solid and hollow organ models during prostate cancer IMRT

Background and purposeAll studies to date have evaluated the dosimetric effect of bladder deformation using an organ model that includes the dose to the urine. This research reconstructed bladder dose using both hollow and solid organ models, to determine if dose/volume differences exist. Materials and methods35 prostate IMRT patients were selected, who had received 78Gy in 39 fractions and full bladder instructions. Biomechanical modelling and finite element analysis were used to reconstruct bladder dose (solid and hollow organ model) using every third CBCT throughout the treatment course. ResultsReconstructed dose (ReconDose) was 11.3Gy greater than planned dose (planDose) with a hollow bladder model (p<0.001) and 12.3Gy greater with a solid bladder model (p<0.0001). Median reconstructed volumes within the 30Gy, 65Gy and 78Gy isodoses were 3–4 times larger with the solid organ model (p<0.0001). The difference between planning bladder volume and median treatment volume was associated with the difference between the planDose and reconDose below 78Gy (R2>0.61). ConclusionsSubstantial differences exist between planned and reconstructed bladder dose, associated with the differences in bladder filling between planning and treatment. Dose reconstructed using a solid bladder model over-reports the volume of bladder within key isodose levels and overestimates the differences between planned and reconstructed dose. Dose reconstruction with a hollow organ model is recommended if the goal is to associate that dose with toxicity.

SU‐E‐T‐652: GPU‐Based Automatic Treatment Planning Using Previously Delivered Treatment Plans as Prior Knowledge

Purpose: To demonstrate the feasibility of automatically and efficiently developing a clinically optimal treatment plan for a new patient using a GPU‐based automatic planning engine and a library of previously delivered treatment plans. Methods: An in‐house GPU‐based platform called SCORE was used for automatic treatment planning. A library of 65 prostate IMRT plans previously delivered at our institution has been assembled. Leave‐one‐out cross validation was performed whereby each of the 65 patients was selected as a ‘new’ patient and the other 64 patients were treated as reference patients. A treatment plan was automatically generated for the ‘new’ patient using the same beam setup and guided by the DVH of the reference plan. A set of candidates were generated by filtering the 64 plans using institutional criteria for treatment plan quality. A GUI was developed to allow clinicians to navigate through the candidate plans at high efficiency to select the best plan. Results: For each prostate IMRT patient, it took 40–60 minutes to generate 64 new plans without any manual intervention. Compared with the original clinical plan created through the regular planning process, each of the final plans chosen from the candidates had equal or better plan quality in terms of DVH curves and specific plan quality metrics (D95% and Dmax for PTV, V65 and V40 for rectum and bladder, Dmax for femoral heads). Conclusion: We proposed a GPU‐based automatic treatment planning procedure as a solution to the current manual treatment planning process requiring significant human effort, planner experience, and clinician. By using this procedure that leverages prior planning experience, a set of candidate plans can be generated without any manual intervention from which it is easy for the clinician to find a clinically optimal plan using an interactive plan selection GUI.

SU‐E‐J‐47: EPID Based Target Tracking During Intensity‐Modulated Radiation Therapy

Purpose: This work investigates the feasibility of using real‐time EPID images of the treatment fields acquired during IMRT for target localization and tracking. Methods: In this study, 35 patients treated with IMRT were retrospectively investigated. These patients were grouped as: prostate with lymph node (n=14), prostate without lymph node (n=17), and lung (n=4). For each patient, two to four fiducial markers were implanted inside the tumor. The DRR, which projects the patient anatomy and the fiducial marker at the EPID location, was reconstructed for each field. The MLC aperture of each control point was overlay on its corresponding DRR to evaluate the fractional time when the fiducial marker was seen on the EPID image. The probability of seeing at least one, two, three, or four fiducial markers during the treatment was recorded. Results: Our results show that for prostate IMRT patients without lymph nodes included in the target volume, the average probability of seeing at least one, two, three, and four fiducial markers during the treatment was 50% (35%–59%), 39% (23%– 51%), 24% (7%–38%), and 12% (4%–29%), respectively. For prostate IMRT patients with lymph nodes, the probability was 41% (24%–51%), 29% (12%– 42%), 15% (3%–24%), and 7% (4%–15%), respectively. For lung IMRT treatment, the average probability of seeing at least one fiducial marker was 34% (20%–52%). Conclusion: The continuous image acquisition from the EPID during IMRT treatment provides sufficient target movement information for real‐time target localization and intrafractional target motion correction for advanced radiotherapy treatments.

Poster - Thur Eve - 21: ROC analysis in patient specific quality assurance.

Many institutions rely on a patient specific measurement for IMRT/VMAT patient QA. In diagnostic imaging, radiologists use Receiver Operator Curves (ROC) to help quantify the value of a diagnostic imaging test. The purpose of this work is to investigate the value or ROC methodology for patient specific IMRT QA. Beam fluences for 34 prostate IMRT patients were analyzed using gamma analysis. For half of these, measurements were done using the planned beam fluences. For the rest, perturbations to the MLC leaf positions were introduced. Gamma analysis was then used to measure fluence differences. Assuming that the unperturbed fluencies were positive measurements, distributions of true positive and false negatives were calculated. For poorly performing beam delivery systems the choice of γ-DTA criterion has little effect on test sensitivity and specificity. The AUC is increased by about 10% for high performance beam delivery systems. For a 3%/3mm γ-DTA condition, ideal cut off values are reasonably independent of MLC performance. At a tighter γ-DTA condition of 2%/2mm, then the optimal sensitivity and specificity of the test is more dependent on MLC performance. For a pass-fail test such as the γ-DTA map is, it is important to choose an optimal cut off value to maximize the sensitivity and specificity of the test. ROC methodology allows users to follow a prescriptive method to obtain ideal cut-off values for gamma analysis, and to assess improvements in sensitivity and specificity for higher performing beam delivery system.

TU-E-BRA-04: Real-Time Automatic Fiducial Marker Detection in Low Contrast Cine-MV Images.

Intrafraction motion tracking using beam-line MV images have gained much attention because no additional imaging dose is introduced. Since MV images have much lower contrast than kV images, a robust marker detection algorithm is a pre-requisite. In this work, we develop a novel, fast, and robust method to detect implanted markers in low-contrast cine-MV patient images. Several marker detection methods have been proposed in the recent years. These methods are all based on template matching or its derivatives. Template matching needs to match object shape that changes significantly for different implantation and projection angle. While these methods require a large number of templates to cover the different situations, they are often forced to use a smaller number of templates to reduce the computation load because their methods all require exhaustive search in the ROI. We solve this problem by synergetic use of modern but well-tested computer vision and AI techniques - detect implanted markers utilizing discriminant analysis for initialization and mean-shift feature space analysis for sequential tracking. This novel approach avoids exhaustive search by exploiting the temporal correlation between consecutive frames and makes it possible to perform more sophisticated detection at the beginning to improve the accuracy, followed by ultrafast sequential tracking after the initialization. The method was evaluated using 1149 cine-MV images from 2 prostate IMRT patients and compared with manual marker detection results from 6 researchers. The average of the manual detection results is considered as the ground truth. The average RMS errors of the automatic tracking from the ground truth are 1.9 and 2.1 pixels for the 2 patients (0.26mm/pixel). The standard deviations of the results from the 6 researchers are 2.3 and 2.6 pixels. The proposed method can achieve similar marker detection accuracy to manual detection in low-contract cine-MV images.

SU‐E‐T‐332: A Retrospective Review of Target Motion for Prostate IMRT Patients Using Calypso

Purpose: This limited study determines the fraction of prostate IMRT patients who do not need setup adjustments, following initial setup, for their first 5 and last 5 fractions. For patients needing setup adjustments during their first or last 5 fractions, the average time to the first adjustment is established for each interrupted fraction. Methods: The Calypso System (Varian Medical Systems, Palo Alto, CA) tracks the position of three implanted fiducials in the prostate. The treatment is manually interrupted if excursions &gt;3mm occur over a time period &gt;5 seconds. Post treatment reports were examined from 18 patients to determine the number of setup changes needed during their first and last 5 fractions and the time of the first setup change for each interrupted fraction. Results: Three of the 18 patients in their first 5 fractions and 5 of the 18 patients in their last 5 fractions did not require setup changes. On average, 7.6 (+/− 7.4) setup changes were needed per patient for the first 5 fractions and 5.9 (+/− 8.5) setup changes for the last 5 fractions. Over the 18 patients, there were a total of 137 setup changes in the first 5 fractions and 106 in the last 5 fractions and only 1 patient with no setup changes. For those patients requiring setup changes, the first setup change occurred, on average, after 6.3 (+/− 4.4) minutes for the first 5 fractions and 5.4 (+/− 2.5) minutes for the last 5 fractions. Conclusions: The majority of prostate patients required adjustments multiple times over the first and last 5 fractions. Using the Calypso System to monitor target motion appears to be beneficial for the majority of prostate patients. Treatment techniques decreasing treatment time, such as VMAT, can be helpful in decreasing the number of interruptions, but still require motion management systems.

SU-E-J-139: Feasibility of Using EPID for Real-Time Target Localization during Treatment.

This study aims to investigate the feasibility of using the images of the treatment fields acquired by an electronic portal imaging device (EPID) for real-time target localization. Forty one patients treated with IMRT and RapidArc were recruited in this study including 37 prostate patients and 4 lung patients. These patients were grouped as: prostate IMRT with lymph node (n=14), prostate IMRT without lymph node (n=17), prostate RapidArc (n=6), and lung IMRT (n=4). For each patient, two to four fiducial markers were implanted inside the tumor. The DRR, which projects the patient anatomy and the fiducial marker at the EPID location, was reconstructed for each field. The MLC aperture of each control point was overlay on its corresponding DRR to evaluate the fractional time when the fiducial marker was seen on the EPID image. The probability of seeing at least one, two, three, and four fiducial markers during the treatment was recorded. For the prostate IMRT patients without lymph nodes included in the target volume, the average probability of seeing at least one, two, three, and four fiducial markers during the treatment was 50% (35%-59%), 39% (23%-51%), 24% (7%-38%), and 12% (4%-29%), respectively. For the prostate IMRT patients with lymph nodes, the probability was 41% (24%-51%), 29% (12%-42%), 15% (3%-24%), and 7% (4%-15%), respectively. For prostate RapidArc treatments using two arcs, the average probability of seeing at least one fiducial marker was 81% (58%-90%) for the full arc and 74% (53%-94%) for the partial arc. For the lung IMRT treatment, the average probability of seeing at least one fiducial marker was 34% (20%-52%). The continuous image acquisition from the EPID during the treatment provides sufficient target movement information for real-time target localization and intrafractional target motion correction for advanced radiotherapy treatments.

SU-E-J-147: Dosimetric Consequences of Intrafraction Prostate Motion: Comparison Between Phantom Measurements and Three Different Calculation Methods.

Evaluation of different calculation methods for dose modification due to intrafraction prostate motion using film measurements as ground truth. We acquired intrafraction motion data with the Calypso tumor tracking system by Varian Medical Systems Inc for 4 prostate IMRT patients treated with 35 fractions each. These motion data were transferred to a phantom platform which reproduces the observed motion and has a 20 cm diameter cylindrical solid water phantom mounted. For each patient all fractions were irradiated on one radiochromic MD-V2-55 film placed in the isocentric transversal slice of this phantom. These films serve as ground truth for three calculation Methods: 1) Recalculation of the plan with shifted target point for every segment with the segment's mean Calypso position. 2)+3) Convolution of the static dose distribution with a probability density function of the observed positions. For 2) only Calypso positions with activated beam on signal were used whereas for 3) all Calypso positions between the first and the last beam on signal for all fractions were employed. The comparisons between films and calculated dose distributions were made with the verification software VeriSoft 3.2 (PTW, Freiburg, Germany) where an 8×8 cm̂2 ROI around the isocenter was selected for gamma evaluation. The segment shifted plans reach 3%/3mm gamma values above 90% against the films for all four patients. For both convolution methods three values are above 90%, only for the patient with the largest intrafraction motion they decrease to 89%. Shifting of the target point for every segment is well suited to estimate the dosimetric consequences of intrafraction prostate motion. This may facilitate the evaluation of different margin sizes or dose prescribing recipes under different motion conditions. If such a lengthy calculation is not possible, a convolution with motion data can be used for acceptable results, too. Our work was partially supported by Siemens Healthcare and Varian Medical Systems Inc.

SU-E-T-136: Measure the Actual Radiation Dose Delivered for Prostate IMRT Treatment Using An Implantable MOSFET Dosimeter

Purpose: To report the initial clinical experience using an in‐vivo, implantable metal oxide semiconductors field effect transistors(MOSFET)dosimeters for the daily dose verification of prostate IMRTtreatment.Methods: The dose verification system (DVS) from Sicel Technologies was used to measure the actual dose delivered for prostate IMRT patients. Fifteen patients (10 prostate IMRT; 5 Sequential IMRT, pelvis then prostate) were implanted with two DVS dosimeters in the prostate gland: one on the right and the other on the left. The location of the DVS dosimeters were identified on the CTimages and the expected doses were calculated. The measured readings of the dosimeters were compared to the expected dose values. Daily orthogonal portal film or CBCT was used for patient setup.Results: The average difference between predicted and measured doses was 1.1% for all 15 patients (30 dosimeters) over the whole course of treatment. The range of the difference between the measured cumulative doses to the planned doses for the whole treatment course was from −5.3% to 5.1%. For three out of fifteen patients, a new plan was created as the measured doses disagreed with the expected value for the original plans. For those three patients, the difference between measured and the predicted doses was as large as 11.3%. The discrepancies were much smaller after replanning. The average dosimeter measurement for the nine prostate IMRTtreatment (exclude one prostate IMRT patient who had been re‐planned) over the course of treatment was decreasing signifying a possible decrease in the sensitivity of the dosimeters Conclusions: This study demonstrated DVS dosimeters could provide valuable information about actual dose delivered and actual dose fluctuations of the daily treatment. The DVS dosimeters could serve as a patient specific quality assurance and guidance for adaptive radiation therapy.

Comparison of online IGRT techniques for prostate IMRT treatment: Adaptive vs repositioning correction

This study compares three online image guidance techniques (IGRT) for prostate IMRT treatment: bony-anatomy matching, soft-tissue matching, and online replanning. Six prostate IMRT patients were studied. Five daily CBCT scans from the first week were acquired for each patient to provide representative "snapshots" of anatomical variations during the course of treatment. Initial IMRT plans were designed for each patient with seven coplanar 15 MV beams on a Eclipse treatment planning system. Two plans were created, one with a PTV margin of 10 mm and another with a 5 mm PTV margin. Based on these plans, the delivered dose distributions to each CBCT anatomy was evaluated to compare bony-anatomy matching, soft-tissue matching, and online replanning. Matching based on bony anatomy was evaluated using the 10 mm PTV margin ("bone10"). Soft-tissue matching was evaluated using both the 10 mm ("soft10") and 5 mm ("soft5") PTV margins. Online reoptimization was evaluated using the 5 mm PTV margin ("adapt"). The replanning process utilized the original dose distribution as the basis and linear goal programming techniques for reoptimization. The reoptimized plans were finished in less than 2 min for all cases. Using each IGRT technique, the delivered dose distribution was evaluated on all 30 CBCT scans (6 patients x 5 CBCT/patient). The mean minimum dose (in percentage of prescription dose) to the CTV over five treatment fractions were in the ranges of 99%-100% (SD = 0.1%-0.8%), 65%-98% (SD = 0.4%-19.5%), 87%-99% (SD = 0.7%-23.3%), and 95%-99% (SD = 0.4%-10.4%) for the adapt, bone10, soft5, and soft10 techniques, respectively. Compared to patient position correction techniques, the online reoptimization technique also showed improvement in OAR sparing when organ motion/deformations were large. For bladder, the adapt technique had the best (minimum) D90, D50, and D30 values for 24, 17, and 15 fractions out of 30 total fractions, while it also had the best D90, D50, and D30 values for the rectum for 25, 16, and 19 fractions, respectively. For cases where the adapt plans did not score the best for OAR sparing, the gains of the OAR sparing in the repositioning-based plans were accompanied by an underdosage in the target volume. To further evaluate the fast online replanning technique, a gold-standard plan ("new" plan) was generated for each CBCT anatomy on the Eclipse treatment planning system. The OAR sparing from the online replanning technique was compared to the new plan. The differences in D90, D50, and D30 of the OARs between the adapt and the new plans were less than 5% in 3 patients and were between 5% and 10% for the remaining three. In summary, all IGRT techniques could be sufficient to correct simple geometrical variations. However, when a high degree of deformation or differential organ position displacement occurs, the online reoptimization technique is feasible with less than 2 min optimization time and provides improvements in both CTV coverage and OAR sparing over the position correction techniques. For these cases, the reoptimization technique can be a highly valuable online IGRT tool to correct daily treatment uncertainties, especially when hypofractionation scheme is applied and daily correction, rather than averaging over many fractions, is required to match the original plan.

SU-GG-T-54: On-Line Adaptive IMRT for Prostate Cancer

Purpose: This study proposes a novel on‐line IGRT technique that can modify the original IMRT plan to match with the “anatomy‐of‐the‐day” within few minutes. The performance of this technique is compared with current on‐line IGRT strategies using bone/soft‐tissue based alignment. Method and Materials: Six prostate IMRT patients were studied. An initial IMRT plan was designed on a commercial treatment planning system, using five/seven co‐planar 15‐MV beams. The plan information was exported to an in‐house adaptive planning system. Five daily CBCTimages of each patient were analyzed. Daily GTVs (prostate and seminal vesicles), bladders and rectums were contoured on each CBCT. The original plan dose distribution was deformed to match the daily anatomy and to serve as prescription dose distribution for the daily re‐optimization of the original plan. The re‐optimization process utilized linear programming technique which can reach the solution within less than two minutes. The delivered dose distributions were compared among the ART versus alignment based IGRT techniques. Three bone and soft‐tissue based alignment techniques: Bone10, Soft5, and Soft10, were included. The number 5 and 10 indicate the PTV margins; with Adapt indicating ART technique with 5mm margin. Results: The average GTV D99 doses for all six patients are 99%–100%, 65%–98%, 87%–99%, and 95%–99% for Adapt, Bone10, Soft5, and Soft10 plans respectively. The Bone10 missed GTV coverage at D99 and D95 in 2 of the 6 patients, while the Soft5 technique also missed D95 coverage in 3 patients. The Soft10 and Adapt techniques show adequate GTV coverage for all. Compare to Soft5, the Adapt technique shows improvement on bladder/rectal sparing when local volume/shape variation occurs. Conclusion: This demonstrates the advantage of the online ART technique over patient alignment techniques in handling complex anatomical positioning and shape changes during prostate IMRT treatment. Conflict of Interest: Research sponsored by Varian Medical Systems.

SU-GG-T-53: Integrating An Online Adaptive IMRT Process to the Prostate IGRT

Purpose: To investigate the feasibility of a novel online IMRT re‐optimization process. This online process can be an integral part to the current prostate IGRT paradigm to augment the existing patient alignment techniques for cases with complex daily organ motion and deformation. Method and Materials: Initial IMRT planning is designed with simulation CT using commercial treatment planning system. The planning data is exported to the in‐house adaptive treatment planning platform (ART). Prior to daily treatment, a 3D imaging guidance is performed. If the position correction can not ensure the optimal dose coverage/sparing, ART is performed. Deformable registration of two sets of organs/structures is performed so the original planned dose distribution matches the CBCTanatomy. This matched dose is set as prescription dose distribution for re‐optimization. The IMRT plan is re‐optimized using a goal programming optimization model in less than 2 minutes and the adaptive plan is evaluated. Dynamic MLC sequencing of the fluence map is calculated by 2 independent algorithms for verification and QA. Results: Six prostate IMRT patients were tested. For each patient 5 daily CBCTs were selected. A total of 30 adaptive IMRT plans were performed; and the re‐optimization was less than 2 minutes for all cases. The soft‐tissue based IGRT technique works for 24 out of 30 cases. The failed cases were indeed due to complex geometrical change among the target volumes and the organs. The adaptive technique achieved optimal target coverage for all cases. To assess the quality of the re‐optimization algorithm, the OAR sparing in adaptive plans were compared with the new plans generated from the Eclipse system, and were found to be of comparable quality. Conclusion: The feasibility of integrating an online adaptive IMRT process to the prostate IGRT paradigm is presented, and its applicability is demonstrated with limited clinical cases.

SU-GG-T-377: Evaluation of Cone Beam CT in Prostate IMRT

Purpose: The purpose of this study was to evaluate the volume changes of target and critical organs using cone‐beam CT (CB‐CT) and to investigate their effects on the organ doses for prostate IMRT cases. Previously the feasibility of CB‐CT based treatment planning was demonstrated by researchers (L. Lee et al., 2006; S. Yoo et al., 2008, Int. J. Rad. Onc. Biol. Phys.) Method and Materials: Ten prostate IMRT patients had daily onboard imaging and weekly kV CB‐CT using a Trilogy system for treatment position verification. The latest CB‐CT data were imported into Eclipse treatment planning system and used for drawing CB‐CT based contours for prostate, rectum, and bladder. The volume of the organs in CB‐CT were measured and compared to the organ volumes from the initial CTimages.Organ contours were exported to the primary IMRT plan and the plan was rerun using CB‐CT based volumes with the same field parameters. The volume and mean dose changes were measured. Results: The average interval between the first CT and latest CB‐CT was 29 days. The mean volume changes of the prostate, rectum and bladder were −8.3%, 10.8%, and −5.6%, respectively between the first CT and latest CB‐CT for 10 prostate IMRT cases. The dose coverage ratios of CB‐CT based volumes to primary volumes were 99.9%, 111.7% and 100.6% for prostate, rectum and bladder, respectively. Conclusion: The prostate volume got decreased about 8% (n=10) after a month based on kV CB‐CT and the dose coverage remained the same. However, in the average approximately 10% increase in rectal volume and dose were observed. The greater mean volume was measured for bladder with CB‐CT, however, the mean dose was almost equal to the primary plan. Therefore, based on the observation, updated volumes with CB‐CT can be used for improving the rectal dose as needed.

SU‐DD‐A3‐06: The Impact of Probe Angle and Tissue Elastic Module in Ultrasound‐Guided Prostate IMRT

Purpose: An integrated ultrasound and CT‐Sim system can be used to assist the daily setup of prostate IMRT patient. The purpose of this study is to investigate the influence of probe angle and the tissue elastic module on the displacement of prostate during ultrasound localization using Finite Element Method (FEM). Method and Materials: An ultrasound localization system (Resonant Medical System, Montreal, Canada) integrated with a CT‐Sim was used to obtain a full set of 3‐D ultrasound (US)‐CT images. The patients' anatomical structures, such bone, bladder, and prostate, were contoured on the CT images by radiation oncologists. The ultrasound probe was positioned at 1cm inferior to bladder superior boundary. A 3D finite element model was generated for each of the patient. The corresponding displacement of prostate during ultrasound localization was calculated by FEM software (Ansys). Results: Under normal tissue elastic module (body (E=15kPa), bone (E=10GPa), bladder wall (E=300kPa), prostate (E=100kPa), and probe (E=3GPa)), when the angle of ultrasound probe increased from 10 to 60 degree to vertical plane with 2cm compression of ultrasound probe, the total displacement of prostate was 0.46∼0.6mm (0.43∼0.54mm inferior, 0.15∼0.25mm posterior). When the elastic module of bladder wall changed from 50kPa to 1MPa with the probe angle of 45 degree, the total displacement of prostate was increased from 0.26mm to 0.76mm (0.23 to 0.68mm inferior, 0.1 to 0.32mm posterior). There was almost no left and right displacement during compression. Conclusion: For the displacement of prostate, there was no significant dependent on the probe angle and limited dependent on the tissue elastic module. With proper controlled compression, the total prostate displacement can be limited within 2mm. This displacement can be corrected by FEM calculation.

SU‐EE‐A3‐03: Comparison of CBCT and Electromagnetic Transponders for Prostate Localization

Purpose: In prostate cancer radiotherapy, it is unknown how large the PTV margins must be to account for the isocenter correction tolerance and intrafraction motion. The risk of geographic miss can be minimized by the placement of fiducial markers in the prostate gland for daily pretreatment localization and adjustment of patient position if necessary. In this study, we assess the magnitude of interfraction and intrafraction isocenter displacement using implanted electromangnetic transponders, and validate the accuracy of interfraction localization using cone beam CT. Method and Materials: Fifteen supine prostate IMRT patients with three implanted transponders each were studied. Initial daily localization was based on three laser and skin marks. Daily localization error distribution was determined from offsets between the initial setup position and that determined by Calypso. Post setup with the Calypso system, isocenter localization was immediately independently verified by imaging the radio‐opaque transponders using an integrated cone beam CT imaging system. Both localization techniques produced lateral, longitudinal, and vertical target offsets from machine isocenter. Organ motion or patient movement during treatment was continuously monitored by the Calypso system at a 4‐mm threshold. Results: The mean interfraction displacement (± SD) in cm in the lateral, vertical, and longitudinal directions were −0.2 ± 0.6, 1.8 ± 1.3, and 0.3 ± 0.9, respectively. After any necessary isocenter corrections, the mean isocenter placement error relative to the cone beam CT (± SD) in cm in the lateral, vertical, and longitudinal directions were 0.0 ± 0.1, 0.1 ± 0.2, and 0.0 ± 0.1, respectively. Conclusion: Compared with use of skin marks, electromagnetic isocenter repositioning provides an increased degree of isocenter localization. Good agreement was observed between cone beam CT isocenter localization and electromagnetic repositioning. However, the electromagnetic technique, with real time continuous tracking, has the added advantage of threshold‐based intervention with no additional radiation dose.

Posters basic dosimetry

Introduction Early implementation of image-guided radiotherapy using cone-beam CT (CBCT) has allowed observations of 3D patient anatomy relative to the planning CT scan, but clinical decisions are ultimately required based on re-planning (i.e. re-calculation of delivered dose) using CBCT scans. However, incorporating treatment re-planning into an efficient image-guided workflow is problematic due to limitations of treatment planning systems (TPSs) and poor consistency of density calibration and image artefacts in CBCT scans. This study reports on the validation of image-guided workflow for treatment re-planning at our institution. Methods Image-guided workflow was incorporated into the Pinnacle TPS via automated scripts. This allowed all relevant treatment plan information to be stored and imported onto a treatment plan based on the CBCT scan. In addition, CBCT dose calculations were validated using the treatment plans for six prostate IMRT patients, each with at least five CBCT images acquired during treatment. Each CBCT was enhanced to remove artefacts and spurious variations in density. Dose distributions at each fraction were then calculated using the CBCT scan. CBCT images before and after enhancement were compared in terms of the density in different tissue regions. Dose distributions based on the CBCT images were compared to the planned distribution in terms of isocentre dose and 95% isodose position. Results The image-guided workflow is now an efficient process due to the automation of re-planning. The CT numbers in regions of fat and muscle tissue in the enhanced CBCT were both within 1% of the values in the planning CT, as opposed to 10-20% different for the original CBCT. The average difference in isocentre dose between planning CT and CBCT calculation was 0.1% for the enhanced CBCT and 3.4% for the original CBCT. Changes in 95% isodose position were less than 2mm in the majority of cases. Changes greater than this were explained by the presence or absence of rectal gas. Conclusions Image-guided workflow has been implemented into the treatment process and validated with prostate IMRT cases. Enhanced CBCT images greatly improve the accuracy of the CBCT calculated doses, with differences explainable by observed changes in patient anatomy.

SU-FF-T-357: Preliminary Results of In-Vivo Dosimetry in Prostate IMRT Patients Using DVS Implantable Dosimeters

Purpose: Report the initial clinical experience using an in-vivo, implantable MOSFET dosimeters for measuring dose in prostate IMRT patients. Method and Materials: Six patients were implanted with 2 DVS® (Dose Verification System) dosimeters. CT scans for treatment planning were performed 4–5 days after implantation. The predicted dose for each dosimeter was calculated and compared to the measured dose. Patients were positioned daily with a MV digital images and ultrasound. The daily dose delivered was recorded and compared to the expected dose. Weekly analysis was performed to confirm the accuracy of the dose delivered and identify dose variation trends (random or systematic). For cases when the dose reading deviated more than 10% from the expected, an immediate analysis of the patient setup and treatment plan was performed. Results: The majority of the patient cases had an average variance ⩽ 5%. However, there were significant daily dose variations identified by the DVS that could easily have gone unnoticed without the DVS. One patient had deviations of 12–20% on the first fraction due to misreadings in the patient positioning images and the ultrasound localization. In addition to helping identify dose variations, the DVS confirmed that the prostate boost margins were appropriate and that target coverage was adequate on a daily basis. Conclusions: Significant variations between the measured and predicted doses were found in 2 out of 6 (33%) patients and required immediate action. The cause for the variation was easily identified and corrected. The DVS acts as an adjunct to IMRT and IGRT since it provides a final confirmation of the dose received in the target measuring the net effect of all the variables that can affect accurate dose delivery. The DVS can be a valuable tool in the continuing radiation therapy goal for improving accuracy of treatment delivery.