Y Song

Concise Matrix Analysis of Point-Based Prostate Targeting for Intensity Modulated Radiation Therapy

Intensity Modulated Radiation Therapy (IMRT) has recently emerged as an effective clinical treatment tool to treat various types of cancers by limiting the external beam dose to the surrounding normal tissue. However, the process of limiting external radiation dose to the tissue surrounding the tumor volume is not a trivial task. Several parameters including tumor volume and inhomogeneity, position and shape of the tumor volume, and the geometrical distribution of the radiation beams directly affect the determination of the external radiation dose. In addition, a major variable in effective delivery of the radiation dose is “set-up error” caused by the changes in patient position. Any changes in the position of the patient affect the geometrical location of the tumor volume and, therefore, need to be accommodated in the delivery of radiation beams during the treatment. This work presents a complete matrix representation required to calculate the three-dimensional rigid body homogeneous transformation matrices corresponding to external beam radiotherapy setup error and subsequent corrections in treatment beam parameters. A new concise orthogonal rotation solution is presented for use with clinical noisy data. Monte Carlo simulations prove the new matrix results are consistently better than the standard inverse solution. The required corrections in beam table, gantry, and collimator angles as function of the planned beam gantry angle are derived. For transformations that include a rotation on the sagittal plane, the required offsets to beam parameters are complex functions of the planned gantry angle but are clearly documented graphically for clinical use. A case study is presented with an error analysis that supports the use of the presented method in a clinical environment. Clinical implementation and evaluation of the presented method with patient data is also included in the paper.

The development of a novel radiation treatment modality – volumetric modulated arc therapy

Recent theoretical studies and clinical investigations have indicated that volumetric modulated arc therapy (VMAT) can produce equal or better treatment plans than intensity modulated radiation therapy (IMRT), while achieving a significant reduction in treatment time. Built upon the concept of aperture-based multi-level beam source sampling optimization, VMAT has overcome many engineering constraints and become a clinically viable radiation treatment modality. At this point in time, however, there are only two commercial VMAT treatment planning systems (TPS) on the market, which severely limit the dissemination of this novel technology. To address this issue, we recently have successfully developed our own version of VMAT TPS. In this paper, we present our preliminary test results.

A clinical procedure for case-specific analytical validation of mono-modality image fusion in image guided radiotherapy.

Image guided radiotherapy can be performed by fusing the daily treatment and reference planning computed tomography scans. Decreased errors in patient setup can lead to smaller target margins that significantly improve treatment efficacy and outcome. The purpose of this work is to present a clinical procedure to analytically compute the accuracy of the registration. Accepted techniques such as normalized mutual information intensity based three-dimensional image registration can be validated using a large automated point sample. Without a user independent metric it is not possible to determine effect of the fusion error on the calculated correction in patient setup

SU-F-T-226: QA Management for a Large Institution with Multiple Campuses for FMEA

PURPOSE To redesign our radiation therapy QA program with the goal to improve quality, efficiency, and consistency among a growing number of campuses at a large institution. METHODS A QA committee was established with at least one physicist representing each of our six campuses (22 linacs). Weekly meetings were scheduled to advise on and update current procedures, to review end-to-end and other test results, and to prepare composite reports for internal and external audits. QA procedures for treatment and imaging equipment were derived from TG Reports 142 and 66, practice guidelines, and feedback from ACR evaluations. The committee focused on reaching a consensus on a single QA program among all campuses using the same type of equipment and reference data. Since the recommendations for tolerances referenced to baseline data were subject to interpretation in some instances, the committee reviewed the characteristics of all machines and quantified any variations before choosing between treatment planning system (i.e. treatment planning system commissioning data that is representative for all machines) or machine-specific values (i.e. commissioning data of the individual machines) as baseline data. RESULTS The configured QA program will be followed strictly by all campuses. Inventory of available equipment has been compiled, and additional equipment acquisitions for the QA program are made as needed. Dosimetric characteristics are evaluated for all machines using the same methods to ensure consistency of beam data where possible. In most cases, baseline data refer to treatment planning system commissioning data but machine-specific values are used as reference where it is deemed appropriate. CONCLUSION With a uniform QA scheme, variations in QA procedures are kept to a minimum. With a centralized database, data collection and analysis are simplified. This program will facilitate uniformity in patient treatments and analysis of large amounts of QA data campus-wide, which will ultimately facilitate FMEA.

SU-G-TeP2-01: Can EPID Based Measurement Replace Traditional Daily Output QA On Megavoltage Linac?

PURPOSE To investigate the long term stability and viability of using EPID-based daily output QA via in-house and vendor driven protocol, to replace conventional QA tools and improve QA efficiency. METHODS Two Varian TrueBeam machines (TB1&TB2) equipped with electronic portal imaging devices (EPID) were employed in this study. Both machines were calibrated per TG-51 and used clinically since Oct 2014. Daily output measurement for 6/15 MV beams were obtained using SunNuclear DailyQA3 device as part of morning QA. In addition, in-house protocol was implemented for EPID output measurement (10×10 cm fields, 100 MU, 100cm SID, output defined over an ROI of 2×2 cm around central axis). Moreover, the Varian Machine Performance Check (MPC) was used on both machines to measure machine output. The EPID and DailyQA3 based measurements of the relative machine output were compared and cross-correlated with monthly machine output as measured by an A12 Exradin 0.65cc Ion Chamber (IC) serving as ground truth. The results were correlated using Pearson test. RESULTS The correlations among DailyQA3, in-house EPID and Varian MPC output measurements, with the IC for 6/15 MV were similar for TB1 (0.83-0.95) and TB2 (0.55-0.67). The machine output for the 6/15MV beams on both machines showed a similar trend, namely an increase over time as indicated by all measurements, requiring a machine recalibration after 6 months. This drift is due to a known issue with pressurized monitor chamber which tends to leak over time. MPC failed occasionally but passed when repeated. CONCLUSION The results indicate that the use of EPID for daily output measurements has the potential to become a viable and efficient tool for daily routine LINAC QA, thus eliminating weather (T,P) and human setup variability and increasing efficiency of the QA process.

SU-F-T-562: Validation of EPID-Based Dosimetry for FSRS Commissioning

PURPOSE The prevailing approach to frameless SRS (fSRS) small field dosimetry is Gafchromic film. Though providing continuous information, its intrinsic uncertainties in fabrication, response, scan, and calibration often make film dosimetry subject to different interpretations. In this study, we explored the feasibility of using EPID portal dosimetry as a viable alternative to film for small field dosimetry. METHODS Plans prescribed a dose of 21 Gy were created on a flat solid water phantom with Eclipse V11 and iPlan for small static square fields (1.0 to 3.0 cm). In addition, two clinical test plans were computed by employing iPlan on a CIRS Kesler head phantom for target dimensions of 1.2cm and 2.0cm. Corresponding portal dosimetry plans were computed using the Eclipse TPS and delivered on a Varian TrueBeam machine. EBT-XD film dosimetry was performed as a reference. The isocenter doses were measured using EPID, OSLD, stereotactic diode, and CC01 ion chamber. RESULTS EPID doses at the center of the square field were higher than Eclipse TPS predicted portal doses, with the mean difference being 2.42±0.65%. Doses measured by EBT-XD film, OSLD, stereotactic diode, and CC01 ion chamber revealed smaller differences (except OSLDs), with mean differences being 0.36±3.11%, 4.12±4.13%, 1.7±2.76%, 1.45±2.37% for Eclipse and -1.36±0.85%, 2.38±4.2%, -0.03±0.50%, -0.27±0.78% for iPlan. The profiles measured by EPID and EBT-XD film resembled TPS (Eclipse and iPlan) predicted ones within 3.0%. For the two clinical test plans, the EPID mean doses at the center of field were 2.66±0.68% and 2.33±0.32% higher than TPS predicted doses. CONCLUSION We found that results obtained with EPID portal dosimetry were slightly higher (∼2%) than those obtained with EBT-XD film, diode, and CC01 ion chamber with the exception of OSLDs, but well within IROC tolerance (5.0%). Therefore, EPID has the potential to become a viable real-time alternative method to film dosimetry.

SU-F-J-27: Segmentation of Prostate CBCT Images with Implanted Calypso Transponders Using Double Haar Wavelet Transform

PURPOSE Segmentation of prostate CBCT images is an essential step towards real-time adaptive radiotherapy. It is challenging For Calypso patients, as more artifacts are generated by the beacon transponders. We herein propose a novel wavelet-based segmentation algorithm for rectum, bladder, and prostate of CBCT images with implanted Calypso transponders. METHODS Five hypofractionated prostate patients with daily CBCT were studied. Each patient had 3 Calypso transponder beacons implanted, and the patients were setup and treated with Calypso tracking system. Two sets of CBCT images from each patient were studied. The structures (i.e. rectum, bladder, and prostate) were contoured by a trained expert, and these served as ground truth. For a given CBCT, the moving window-based Double Haar transformation is applied first to obtain the wavelet coefficients. Based on a user defined point in the object of interest, a cluster algorithm based adaptive thresholding is applied to the low frequency components of the wavelet coefficients, and a Lee filter theory based adaptive thresholding is applied to the high frequency components. For the next step, the wavelet reconstruction is applied to the thresholded wavelet coefficients. A binary/segmented image of the object of interest is therefore obtained. DICE, sensitivity, inclusiveness and ΔV were used to evaluate the segmentation result. RESULTS Considering all patients, the bladder has the DICE, sensitivity, inclusiveness, and ΔV ranges of [0.81-0.95], [0.76-0.99], [0.83-0.94], [0.02-0.21]. For prostate, the ranges are [0.77-0.93], [0.84-0.97], [0.68-0.92], [0.1-0.46]. For rectum, the ranges are [0.72-0.93], [0.57-0.99], [0.73-0.98], [0.03-0.42]. CONCLUSION The proposed algorithm appeared effective segmenting prostate CBCT images with the present of the Calypso artifacts. However, it is not robust in two scenarios: 1) rectum with significant amount of gas; 2) prostate with very low contrast. Model based algorithm might improve the segmentation in these two scenarios.

SU-E-T-52: Beam Data Comparison for 20 Linear Accelerators in One Network

PURPOSE To compare photon beam data for the 20 Varian linear accelerators (TrueBeam, iX, and EX models) in use at five centers in the same network with the intent to model with one set of beam data in Eclipsec. METHODS Varian linear accelerators, TrueBeam (3), 21 EX, iX, and Trilogy (14), and 6 EX (3), installed between 1999 and 2014 have their 6 MV and 15 MV x-ray beams reevaluated. Full commissioning, including output factors (St), percent depth doses (PDD), and off-axis profiles, was recently performed for a TrueBeam with a cc04 ion chamber in an IBA Blue phantom. Similarly, a subset of beam data for each of the other accelerators was measured recently as follows: for 3×3, 10×10, and 30×30 cm2 field sizes, flatness and penumbra (80-20%) were measured at dmax and 10 cm depths, PDD were measured at 10 and 20 cm depths, and St were measured at 5 cm depth. Measurement results for all machines were compared. RESULTS For 15 high-energy (6 and 15 MV) and 3 low-energy machines (6MV only): 1) PDD agreed within 1.4% at 10 and 20 cm depths; 2) penumbra agreed within 1.0 mm at dmax and 10 cm depths; 3) flatness was within 1.3% at dmax and 10 cm depths; and 4) with exception of the three low energy machines, output factors were within 1.1% and 0.5% for 3×3 and 30×30 cm2 , respectively. Measurement uncertainty, not quantified here, accounts for some of these differences. CONCLUSION Measured beam data from 15 high-energy Varian linacs are consistent enough that they can be classified using one beam data set in Eclipse. Two additional high-energy machines are removed from this group until their data are further confirmed. Three low-energy machines will be in a separate class based upon differences in output factors (St).

SU‐C‐108‐03: Investigation of MLC Leaf Tolerance and Dose Accuracy in VMAT Delivery

PURPOSE VMAT employs a multi-parameter modulation methodology. It tends to perform less reliably than the one-dimensional IMRT. Therefore, setting a suitable leaf positioning tolerance on the machine is crucial. In this study, we investigated the relationship between the leaf tolerance and the dose delivery accuracy for VMAT SBRT lung plans. The goal was to determine the optimal leaf tolerance for daily VMAT SBRT treatments. METHODS The study was performed on a VMAT-enabled Trilogy LINAC. To minimize the systematic bias in leaf and gantry speeds, all the VMAT SBRT plans had a prescription dose of 1800 cGy × 3. Prior to each plan delivery, the leaf positioning tolerance was reset to a new value in Machine Hardware Configuration in Treatment Administration. Upon completion of the plan delivery, the Dynalog Files and, more importantly, the Treatment Log File, i.e., the Dlog File, were saved and exported for post-processing. For our initial investigation, we tested leaf tolerance values of 1, 2, 3, 4, 5, and 6 mm, respectively. RESULTS We found that when the leaf tolerance was set to a tighter value, such as 1, 2, and 3 mm, a good delivery accuracy was preserved without any MLC interlock occurrences. However, when the leaf tolerance was increased to be greater than a threshold value, such as 3 mm in this case, the delivery accuracy deteriorated almost linearly beyond that point. We also found that except for the leaf tolerance = 1 mm, the gantry angle accuracy virtually remained unchanged as the leaf tolerance was increased, indicating a decoupling between the leaf tolerance and the gantry angle uncertainty. CONCLUSION We recommend that for VMAT SBRT lung plans with a prescription dose of 1800 cGy × 3, the optimal leaf tolerance should be set to 3 mm.

MO-F-108-06: A Novel Analytic Approach to Assessing Rotation-Induced Dosimetric Errors of Stereotactic Radiosurgery Cases

PURPOSE To assess 3D rotational setup error effect on dose distribution, we derive a new set of beams such that the beams-eye-views of this new set of beams with respect to the planning CT are the same as those of the original plan for the patient, but with setup rotation errors incorporated. METHODS To assess the rotation effects on the dose distribution of stereotactic radiosurgery (SRS) cases, a methodology was developed and two sets of table, gantry, and collimator angles were obtained in analytical forms. Those new angles emulate the rotation effects of setup errors. Eight SRS cases were computed with a series of different combinations of patient rotation errors, ranging from (-5°, -5°, -5°) to (5°, 5°, 5°) (roll, pitch and yaw) with an increment of 1°. For each set of rotational errors, its corresponding equivalent beams were computed using our analytical solution and used for dose calculation. RESULTS The above new beams were implemented in a treatment planning system (TPS). Based on all the plans that accounted for different rotations, we have found that rotations have an insignificant effect on the minimum, maximum, mean doses, and V80% of the planning target volume when the rotations were relatively small. This was particularly true for the small and near-spherical targets. They, however, did change V95% significantly when the rotations approached 5°. Our theory has been validated with SRS cases and proven to be practical and viable. CONCLUSIONS We have derived analytical solutions to a new set of table, gantry and collimator angles for a given beam configuration as a function of patient rotation errors. Compared to the traditional method of rotational effect assessment by importing rotated CT images into TPS, our equivalent beam approach is simple and accurate. It is a promising technique for real time adaptive treatment planning.