H Zhao

SU‐E‐U‐08: Presentation of a New Intrafractional Prostate Monitoring Method with Ultrasound Image Guidance During Radiotherapy Treatment

Purpose: Clinical presentation of a new intrafractional ultrasound image guidance (USIG) prostate monitoring/tracking method. Methods: Clarity ultrasound system has recently released a new feature to monitor/track intrafractional prostate motion using a trans‐perineal image acquisition position. After initial localization and image guidance correction, the ultrasound probe remains fixed in the sagittal‐plane, trans‐perineal imaging position via a couch‐mounted arm‐support system. The software then enters a live monitoring mode, where the scanning ultrasound probe continuously acquires fanned sagittal images for tracking of during‐treatment prostate position. Compared to other tracking technologies (e.g. RF implanted beacon tracking, or real‐time fluoro monitoring), the ultrasound system has the unique advantages of live 3D image display, without invasive procedure or imaging radiation dose. We present our initial experience using this monitoring feature for 8 patients and 218 treatment sessions. Results: The monitoring software functioned as expected during treatment with consistent reporting of prostate deviation from isocenter location, and notification of instances where positional error exceeded our tolerance of 3 mm for 5 second. Comparisons of prostate tracking data streams obtained from Clarity USIG with data streams obtained in our clinic from Calypso RF tracking of a similar patient population show similarity of recorded motion information for both methods. Average during‐treatment prostate motion for both tracking methods was on the order of 1–2 mm in 3 cardinal directions, with roughly 5% of instances where motion exceeded 3 mm, 5 second tolerance level. When Monitoring reported that tolerance was exceeded we confirmed this by performing redundant, static imaging Alignment, with confirmation that target position had, indeed, changed as reported. Conclusion: The newly released trans‐perineal ultrasound Monitoring approach was confirmed to function well clinically, and to report accurate intrafractional monitoring data. Acquired motion streams were consistent with our Calypso monitoring experience obtained on a similar patient population in our own clinic.

SU-E-T-527: Comparison of CT-On-Rails and a 3D Surface Imaging System for Image Guided Partial Breast Radiation Therapy

Purpose: We evaluate the feasibility of using a 3D surfaceimaging system (AlignRT) for image guidance for partial breast radiation treatments through comparison with the CT‐on‐rails image guidance currently clinically employed at our institution. Methods/Materials: Four patients receiving external beam partial breast radiation therapy with multiple image guidance datasets were analyzed retrospectively. A total of 51 fractions were studied, each with a CT‐on‐rails dataset and an AlignRT body surface contour snapshot. For each fraction, the patient was aligned to treatment isocenter with room lasers and scanned with both CT‐on‐rails and AlignRT. Image guidance shifts were calculated for both modalities by comparison to the simulation CT and the differences between the two sets of shifts were analyzed. Results: Two patients showed significant change in surgical cavity volume and shape between simulation and fraction 1. For these patients it was found that the discrepancy between CT‐on‐rails based shifts and AlignRT shifts was significant (up to 11.1 mm) indicating that use of AlignRT for image guidance of these cases would be inadvisable. The remaining two patients did not show noticeable change in surgical cavity and the discrepancy between the two sets of shifts for these patients was minimal, thus indicating that AlignRT is potentially valuable for these cases. Conclusion: AlignRT appears to be a reasonable and potentially valuable image guidance approach for partial breast radiation therapy for patients with small changes in surgical cavity between CT simulation and treatment. However, there may be potential for alignment inaccuracies when using AlignRT for patients with dramatic changes in surgical cavity.

SU-F-J-55: Feasibility of Supraclavicular Field Treatment by Investigating Variation of Junction Position Between Breast Tangential and Supraclavicular Fields for Deep Inspiration Breath Hold (DIBH) Left Breast Radiation.

PURPOSE To explore the feasibility of supraclavicular field treatment by investigating the variation of junction position between tangential and supraclavicular fields during left breast radiation using DIBH technique. METHODS Six patients with left breast cancer treated using DIBH technique were included in this study. AlignRT system was used to track patients breast surface. During daily treatment, when the patients DIBH reached preset AlignRT tolerance of ±3mm for all principle directions (vertical, longitudinal, and lateral), the remaining longitudinal offset was recorded. The average with standard-deviation and the range of daily longitudinal offset for the entire treatment course were calculated for all six patients (93 fractions totally). The ranges of average ± 1σ and 2σ were calculated, and they represent longitudinal field edge error with the confidence level of 68% and 95%. Based on these longitudinal errors, dose at junction between breast tangential and supraclavicular fields with variable gap/overlap sizes was calculated as a percentage of prescription (on a representative patient treatment plan). RESULTS The average of longitudinal offset for all patients is 0.16±1.32mm, and the range of longitudinal offset is -2.6 to 2.6mm. The range of longitudinal field edge error at 68% confidence level is -1.48 to 1.16mm, and at 95% confidence level is -2.80 to 2.48mm. With a 5mm and 1mm gap, the junction dose could be as low as 37.5% and 84.9% of prescription dose; with a 5mm and 1mm overlap, the junction dose could be as high as 169.3% and 117.6%. CONCLUSION We observed longitudinal field edge error at 95% confidence level is about ±2.5mm, and the junction dose could reach 70% hot/cold between different DIBH. However, over the entire course of treatment, the average junction variation for all patients is within 0.2mm. The results from our study shows it is potentially feasible to treat supraclavicular field with breast tangents.

PO-0943: Clinical comparison of 2D transabdominal and 3D transperineal ultrasound image guidance methods for prostate RT

Purpose/Objective: Our clinic is a long-term user of a 1st generation trans-abdominal (TA) ultrasound image guidance (USIG) system (BAT, Best Nomos Inc) for prostate cancer treatments. We are also an early adopter and development partner for a new, second generation 3D USIG system (Clarity, Elekta Inc), which allows for trans-perineal (TP) localization and intra-fractional tracking of the prostate. This new system has been evaluated at our institution, by direct comparison with the previously established TA method for prostate alignment. Materials and Methods: Patients were positioned according to routine clinical protocol and aligned to skin marks using treatment room lasers. TP USIG was performed and TP shifts from tattoo were performed and recorded prior to performing TA USIG for verification purposes only. The observed differences of TA USIG from TP shifts were recorded. A total of 569 fractions delivered to 30 prostate cancer patients were thus analyzed for agreement between the two USIG systems. For each patient, a graph and tables showing shift of skin marks to TP USIG and agreement between the USIG systems of all applicable fractions were generated. Results: The mean TP-based initial shift from tattoo was 1.78, -0.27, and -2.36 mm in left-right (LR), anteriorposterior (AP), and superior-inferior (SI) directions, respectively. The average difference (AD) between the two USIG systems was -0.06, -0.05, and -0.02 mm in LR, AP and SI directions respectively. The respective standard deviations of the AD were 0.19, 0.45 and 0.38 mm. Image 1 shows a sample of patients, with the dot representing the mean agreement between the USIG systems for a patient. The error bars represent the patient specific standard deviation. Image 1: Agreement of prostate localization for two US IG systems.

SU-E-T-166: Characterization of Efficiency and Plan Quality for the FFF Beams On Various Anatomical Sites

Purpose: To characterize efficiency and plan quality for the FFF beams for various sites. Methods: 5 brain, Head and Neck, prostate, lung and liver cancer patients IMRT plans (25 total) were generated on either Prowess 4.6 or Eclipse 13.5 using the same dose constraints for each treatment site. Step and shoot with static gantry IMRT was used for treatment delivery. PTV coverage, critical structure doses, MUs, number of segments and beam on times were compared. Results: The average PTV size was 29.0, 34.9, 89.2, 257.6, 289.2 cm3 for liver, lung, prostate, head and neck and brain respectively.All plans were normalized such that 95% of the PTV volume would receive at least 95% of prescribed dose. All doses to the critical structures for both the FFF and flat beam met the targeted dose constraints.For plans with field sizes 10 cm, required 10% – 20% more segments and MUs for the FFF beam to achieve the same plan quality as the flat beam. Despite this, for fraction sizes less than 2.5 Gy the FFF beam is still approximately 13.9% more efficient in terms of delivery time. Conclusion: For the various treatment sites studied here, plans generated with the FFF beam were dosimetrically similar to those generated with a flattened beam. Despite the greater number of MUs and segments required to achieve the same plan quality as the flat beam for some plans, the FFF beam is still more efficient compared to the flat beam.

SU-E-T-38: A Head to Head Comparison of Two Commercial Phantoms Used for SRS QA

Multiple commercial phantoms are now available for performing end-to-end QA testing for stereotactic procedures. This project aims at directly comparing one of the newest phantoms on the market against a more established one by performing similar tests to determine whether results are similar and they can be used interchangeably. Both phantoms were used to evaluate the coincidence of radiation and laser isocenters of a linear accelerator. End-to-end dosimetric tests were also performed using both an ion chamber and film. As part of the testing, both phantoms were also evaluated in terms of their efficiency of setup as well as the time required to switch inserts for different tests. Results showed that the laser/radiation isocenter coincidence as determined from each phantom was highly correlated. Ion chamber results were within 0.5% of the expected values. Gamma (2%, 2mm) pass rates of corresponding films were within 2% between phantoms. These results show that both phantoms are capable of producing equivalent results for the QA tests evaluated here.

SU‐E‐T‐562: Scanned Percent Depth Dose Curve Discrepancy for Photon Beams with Physical Wedge in Place (Varian IX) Using Different Sensitive Volume Ion Chambers

PURPOSE To investigate and report the discrepancy of scanned percent depth dose (PDD) for photon beams with physical wedge in place when using ion chambers with different sensitive volumes. METHODS/MATERIALS PDD curves of open fields and physical wedged fields (15, 30, 45, and 60 degree wedge) were scanned for photon beams (6MV and 10MV, Varian iX) with field size of 5×5 and 10×10 cm using three common scanning chambers with different sensitive volumes - PTW30013 (0.6cm3), PTW23323 (0.1cm3) and Exradin A16 (0.007cm3). The scanning system software used was OmniPro version 6.2, and the scanning water tank was the Scanditronix Wellhoffer RFA 300. The PDD curves from the three chambers were compared. RESULTS Scanned PDD curves of the same energy beams for open fields were almost identical between three chambers, but the wedged fields showed non-trivial differences. The largest differences were observed between chamber PTW30013 and Exradin A16. The differences increased as physical wedge angle increased. The differences also increased with depth, and were more pronounced for 6MV beam. Similar patterns were shown for both 5×5 and 10×10 cm field sizes. For open fields, all PDD values agreed with each other within 1% at 10cm depth and within 1.62% at 20 cm depth. For wedged fields, the difference of PDD values between PTW30013 and A16 reached 4.09% at 10cm depth, and 5.97% at 20 cm depth for 6MV with 60 degree physical wedge. CONCLUSION We observed a significant difference in scanned PDD curves of photon beams with physical wedge in place obtained when using different sensitive volume ion chambers. The PDD curves scanned with the smallest sensitive volume ion chamber showed significant difference from larger chamber results, beyond 10cm depth. We believe this to be caused by varying response to beam hardening by the wedges.

SU-E-T-508: End to End Testing of a Prototype Eclipse Module for Planning Modulated Arc Therapy On the Siemens Platform

PURPOSE The latest clinical implementation of the Siemens Artiste linac allows for delivery of modulated arcs (mARC) using full-field flattening filter free (FFF) photon beams. The maximum doserate of 2000 MU/min is well suited for high dose treatments such as SBRT. We tested and report on the performance of a prototype Eclipse TPS module supporting mARC capability on the Artiste platform. METHOD our spine SBRT patients originally treated with 12/13 field static-gantry IMRT (SGIMRT) were chosen for this study. These plans were designed to satisfy RTOG0631 guidelines with a prescription of 16Gy in a single fraction. The cases were re-planned as mARC plans in the prototype Eclipse module using the 7MV FFF beam and required to satisfy RTOG0631 requirements. All plans were transferred from Eclipse, delivered on a Siemens Artiste linac and dose-validated using the Delta4 system. RESULTS All treatment plans were straightforwardly developed, in timely fashion, without challenge or inefficiency using the prototype module. Due to the limited number of segments in a single arc, mARC plans required 2-3 full arcs to yield plan quality comparable to SGIMRT plans containing over 250 total segments. The average (3%/3mm) gamma pass-rate for all arcs was 98.5±1.1%, thus demonstrating both excellent dose prediction by the AAA dose algorithm and excellent delivery fidelity. Mean delivery times for the mARC plans(10.5±1.7min) were 50-70% lower than the SGIMRT plans(26±2min), with both delivered at 2000 MU/min. CONCLUSION A prototype Eclipse module capable of planning for Burst Mode modulated arc delivery on the Artiste platform has been tested and found to perform efficiently and accurately for treatment plan development and delivered-dose prediction. Further investigation of more treatment sites is being carried out and data will be presented.

TH-C-19A-08: PDD Discrepancies at Opposite Biases From Very Small Volume Ion Chambers When Using Water Scanners

PURPOSE As more so-called micro ion chambers become commercially available, medical physicists may be inclined to use them during the linear accelerator commissioning process, in an attempt to better characterize the beam in steep dose gradient areas. The purpose of this work is to inform the medical physics community of a non-trivial, anomalous behavior observed when very small chambers are used in certain beam scanning configurations. METHODS A total of six ion chambers (0.007cc to 0.6cc) were used to scan PDDs from a 10×10cm2 field at both +300V and -300V biases. PDDs were scanned using three different water tank scanning systems to determine whether different scanners exhibit the same abnormality. Finally, PDDs were sampled using an external electrometer to bypass the internal electrometer of the scanner to determine the potential contributions of the scanner electronics to the abnormality observed. RESULTS We observed a reproducible, significant difference (over-response with depth) in PDDs acquired when using very small ion chambers with certain bias and watertank combinations, on the order of 3-5% at a depth of 25 cm in water. This difference was not observed when the PDDs were sampled using the ion chambers in conjunction with an external electrometer. This suggests a contribution of interference produced by the controller box and scanning system, which becomes significant for the very small signals collected by very small ion chambers, especially at depth, as the signal level is reduced even further. CONCLUSION Based on the results observed here, if currently available very small ion chambers are used with specific bias and scanning water-tank combinations, erroneous PDD data may be collected. If this data is used as input to the Treatment Planning System, systematic errors on the order of 3%-5% may be introduced into the treatment planning process.

SU‐E‐U‐09: Direct Clinical Comparison of 2D Versus 3D Transabdominal Ultrasound Image Guidance Methods for Prostate Radiotherapy Treatment

Purpose: Direct clinical comparison of 2D versus 3D transabdominal ultrasound image guidance (USIG) methods for prostate radiotherapy Methods: Image guidance shifts were compared on the same subject, same treatment session between Nomos BAT and Elekta Clarity ultrasound methods for 20 patients and 662 prostate treatments. Both systems use unique approaches to USIG. BAT system acquires two static 2D ultrasound images (axial and sagittal) and compares them to contours derived from simulation CT (i.e. intermodality comparison). Clarity system acquires a 3D ultrasound data set at CT simulation and compares to treatment‐day 3D ultrasound image set for deriving image guidance shifts (i.e. intramodality comparison). In this study, transabdominal Clarity image guidance was performed and the shifts were implemented first. Then transabdominal ultrasound images were acquired approximately 1minute later with BAT to derive image guidance shifts. These BAT‐reported shifts were recorded as the difference between BAT and Clarity USIG. The same experienced operator performed both BAT and Clarity paired alignments. Results: The average differences between the two systems were less than 1.0 mm in all three principle directions. The average standard deviation of these differences in lateral, vertical, and longitudinal direction were 1.2, 1.7, and 1.4 mm, respectively. Average Max difference (across all patients) was 1.9 mm; absolute Max across all image sessions was 10.2 mm. Differences between the two USIG approaches can be partially explained by prostate motion as a function of the time elapsed between imaging sessions, and 10.2 mm Max is not inconsistent with prostate motion we have observed during a similar time frame when using Calypso RF tracking. Results were further stratified into two groups: intact prostate and post‐prostatectomy, and no systematic differences were observed between these two groups. Conclusion: Good agreement was observed in this head‐to‐head comparison study between BAT and Clarity USIG methods for prostate localization.