P Zhang

Commissioning and Quality Assurance of RapidArc Radiotherapy Delivery System

PURPOSEnThe Varian RapidArc is a system for intensity-modulated radiotherapy (IMRT) treatment planning and delivery. RapidArc incorporates capabilities such as variable dose-rate, variable gantry speed, and accurate and fast dynamic multileaf collimators (DMLC), to optimize dose conformality, delivery efficiency, accuracy and reliability. We developed RapidArc system commissioning and quality assurance (QA) procedures.nnnMETHODS AND MATERIALSnTests have been designed that evaluate RapidArc performance in a stepwise manner. First, the accuracy of DMLC position during gantry rotation is examined. Second, the ability to vary and control the dose-rate and gantry speed is evaluated. Third, the combined use of variable DMLC speed and dose-rate is studied.nnnRESULTSnAdapting the picket fence test for RapidArc, we compared the patterns obtained with stationary gantry and in RapidArc mode, and showed that the effect of gantry rotation on leaf accuracy was minimal (< or =0.2 mm). We then combine different dose-rates (111-600 MU/min), gantry speeds (5.5-4.3 degrees /s), and gantry range (Deltatheta = 90-12.9 degrees ) to give the same dose to seven parts of a film. When normalized to a corresponding open field (to account for flatness and asymmetry), the dose of the seven portions show good agreement, with a mean deviation of 0.7%. In assessing DMLC speed (0.46, 0.92, 1.84, and 2.76 cm/s) during RapidArc, the analysis of designed radiation pattern indicates good agreement, with a mean deviation of 0.4%.nnnCONCLUSIONSnThe results of these tests provide strong evidence that DMLC movement, variable dose-rates and gantry speeds can be precisely controlled during RapidArc.

WE‐E‐BRB‐06: Optimization of Collimator Trajectory in Volumetric Modulated Arc Therapy: Development and Evaluation for Paraspinal SBRT

Purpose: To develop a collimator trajectory optimization paradigm for volumetric modulated arc therapy (VMAT) and evaluate this technique in paraspinal stereotactic body radiation therapy(SBRT).Method and Materials: We propose a novel VMAT paradigm, Coll‐VMAT, which integrates collimator rotation into the synchronization of gantry rotation, multileaf collimator(MLC) motion, and dose rate modulation. A principle component analysis (PCA) calculates the primary cord orientation using the cord contour projection on the beams eye view for each gantry angle, thereafter determining a collimator angle such that MLC travel is parallel to the PCA‐derived direction. In this way, cord can be constantly blocked while minimizing resultant blocking of the planning target volume (PTV), and more MLC pairs are available for dose modulation. Standard VMAT optimization follows the geometry‐based collimator trajectory calculation, to obtain the optimal MLC position and monitor units (MU) at each gantry angle. Evaluation compares Coll‐VMAT‐optimized plans to standard VMAT and IMRT in 5 paraspinal SBRT patients. Prescription dose to the PTV is 24Gy while constraining the cord maximum dose to 14Gy. Plan evaluation statistics include PTV V22.8Gy, PTV D95%, cord mean dose, and total beam‐on time. Results:Collimator rotation in the five Coll‐VMAT plans ranges from 26°–54°, with a median of 40°. Patient‐averaged PTV V22.8Gy (94.6% with Coll‐VMAT vs 92.1% VMAT and 93.3% IMRT) and D95% (22.5Gy vs 21.4Gy and 22Gy) are higher, while cord mean dose is lower (9.8Gy vs 10.0Gy and 11.7Gy). Total beam‐on time is comparable to standard VMAT and substantially lower than IMRT (5164MU vs 4868MU and 13,283MU). Conclusion: The dosimetric advantage of Coll‐VMAT is significant especially for small targets. Collimator trajectory optimization provides an additional degree of freedom in VMAT optimization that can improve dosimetric quality of paraspinal SBRT compared to standard VMAT and IMRT.Conflict of Interest: Research agreement with Varian Corporation.

SU‐D‐105‐04: Automated QA Analysis of VMAT Delivery Errors Using Trajectory Log Files

PURPOSEnTo investigate the sensitivity of trajectory log analysis for detecting small VMAT delivery errors with an automated program.nnnMETHODSnFor each treatment, Varian TrueBeam™ generates a set of trajectory log files that record accumulated MU, MLC positions, and gantry angle every 10 ms during delivery. An automated computer program analyzes discrepancies between planned and actual leaf positions. Any leaf that exceeds 0.5 mm discrepancy for >15% of the total beam-on time is flagged and an alert sent to a physicist for further investigation. To validate the method, 6 different induced-error VMAT plans were generated by modifying a lung VMAT plan with an intended error: gantry angle error of 0.5 and 1.0 deg, position error of one leaf of 0.5, 1.0, 1.5, and 2.0 mm. All plans were delivered and trajectory logs were collected. The dosimetric effect of the induced errors was evaluated by reconstructing dose distributions from the trajectory logs and comparing to the original error-free plan.nnnRESULTSnThe proposed method is sensitive and can detect gantry error down to 0.5 deg and MLC leaf error of 0.5 mm without any false positives (i.e. no error detected for other leaves). No dose difference was observed for gantry angle errors of 0.5-1.0 deg, while the maximum dose discrepancy from a MLC position error of 0.5 mm was 1% and increased to 3% for a leaf error of 2 mm, with a passing rate of 99.6% and 97.7% for gamma analysis (1%/0 mm), respectively.nnnCONCLUSIONnAnalysis of trajectory log files is highly sensitive and provides an efficient and accurate method of detecting treatment delivery errors down to sub-millimeters. The automated program can be run daily for treatment validation and can be used also as a pre-treatment QA tool. Research grant from Varian Medical Systems.

TU‐C‐213CD‐04: Tracking Implanted Fiducials Using Kilovoltage (kV) Projection Images: A Feasibility Study

Purpose: We have developed a method of tracking irregularly shaped implanted markers using KV projection images acquired in rotational mode and assess its potential for detecting intra‐fractional target motion. This is a feasibility study directed toward long‐range goals of acquiring such images during rotational treatment and using them for motion correction. Methods: KV projection images were acquired (Varian TrueBeam) during seven cone beam scans of two gastroesophageal and two pancreas cancer patients (IRB‐approved protocol). Each had at least one irregularly shaped radiopaque marker (Visicoil) implanted in or near the tumor. Specialized digitally reconstructed radiographs (DRRs) used for template based tracking were created from a breath‐hold planning CT at end expiration, in which the ray tracing was confined to a small volume of interest surrounding each marker. Sobel filter preprocessing of KV images served to enhance marker visibility and suppress background features. DRRs were matched with processed KV images both manually (ground truth) and automatically (normalized cross‐correlation with simplex minimization). Anthropomorphic phantom studies were also done to evaluate measurement uncertainty.Results: The mean (over patient scans) and standard deviation of the differences (Auto‐manual) were −0.04 ± 0.68 mm and 0.08 ± 0.89 mm in transverse and superior‐inferior (SI) directions respectively. The percentages of matches with difference exceeding 2 mm were 1.8% transverse and 5.0% SI. Intra‐observer consistency of manual registration was checked by repeating the manual registration for all 657 projections in one patient; the standard deviation of the difference was 0.4 mm. Phantom studies showed the measurement uncertainty of automatic registration to be approximately 0.15 mm. Conclusions: The proposed method can track arbitrary marker shapes using templates generated from a breath‐hold CT or alternatively, respiration‐correlated CT scan at one phase. Preliminary results indicate accuracy and robustness are adequate for clinical application but confirmation in larger numbers of patients is required. Research grant from Varian Medical Systems

TU-A-BRA-04: Choreographing Couch and Collimator in Volumetric Modulated Arc Therapy

Purpose: The Trilogy® MX (Varian Medical Systems) supports simultaneous motion of the couch, gantry and collimator during volumetric modulated arc therapy (VMAT). This study investigates the feasibility and potential advantages of treating CNStumors using VMAT with such capabilities. Method and Materials: To guide the selection of couch, gantry and collimator trajectories, a score function estimates the geometric overlap between tumor and organs at risk (OAR) for each couch/gantry combination. An initial set of beam trajectory candidates includes couch and gantry combinations, that correspond to minima found in a search of the overlap score map excluding zones of collision and long beam paths through the patient. This set is clustered into multiple continuous arcs subject to mechanical limitations. Following determination of couch/gantry trajectories, a principal component analysis finds the collimator angle at each beam orientation that minimizes residual tumor‐OAR overlap. A purpose‐developed VMAT optimization algorithm determines the optimal MLC position and MU for control points within each arc. A planning study of seven CNS patients compared trajectory VMAT with dynamic gantry, leaf, couch and collimatormotion (Tra‐VMAT), standard non‐coplanar VMAT (Std‐VMAT, no couch/collimator motion within subarcs), and non‐coplanar IMRT plans. Results: Tra‐VMAT yields improved tumor dose conformality, lowered maximum dose to optic nerve, brainstem and chiasm, by 6%, 1.8%, 0.9%, 1.3%, respectively, relative to Std‐VMAT. Tra‐VMAT also yields higher PTV minimum dose, reduced maximum dose to chiasm, optic nerve, and cochlea, and reduced cochlea mean dose, by 4.4%, 1.5%, 6.4%, 10.2%, 8.9%, respectively, relative to IMRT. Tra‐VMAT average beam‐on time is comparable to Std‐VMAT, but significantly (42%) less compared to IMRT.Conclusion: Optimized couch/collimator trajectories can be integrated with VMAT. Trajectory VMAT, with improved mechanical flexibility, results in better dosimetric properties and improved treatment efficiency in the treatment of CNStumors. Conflict of Interest: Sponsered by Varian Medical System

SU‐FF‐J‐81: A Feasibility Study for Real‐Time Tomosynthesis‐Guided Rapid Arc Therapy

Purpose: To investigate the feasibility of real‐time mis‐alignment correction in Rapid Arc treatment and design a corresponding tomosynthesis acquisition protocol. Method and Materials: A CTimage set of an anthropomorphic pelvic phantom was used in the study. Simulated projection images were produced to resemble a simultaneous kV fluoro in Rapid Arc treatment. A modified Feldkamp algorithm was used to reconstruct the tomosynthesisimages. Various combinations of imagingreconstruction parameters including scan angle, angular interval, and slice thickness (mm) were tested: 1) 60°, 6°, 2.4; 2) 60°, 6°, 0.8; 3) 60°, 3°, 2.4; and 4) 30°, 3°, 2.4. A predefined 5 mm displacement in all three orthogonal directions modeled patient motion during treatment. After each successive tomosynthesis acquisition, registrations were performed between current reconstructions and reference images. The phantom position was corrected accordingly by shifting the treatment couch. Residual errors and their root mean square (RMS) values were recorded for evaluation. Results: The residual errors (L‐R, A‐P and S‐I directions in mm) for the 4 schemes after the first tomosynthesis acquisition were (1.2, 2.5, −0.2), (−1.2, 1.1, 0.0), (1.1, 1.9, 0.0) and (−1.2, 3.1, 0.0), and the corresponding RMSs were 1.6, 0.9, 1.3 and 1.9 respectively. The RMSs after a full arc delivery were 0.7, 0.5, 0.5 and 0.7. All schemes tested accurately corrected displacement in the SI direction after first acquisition. Scheme 2 performed better than scheme 1 at the expense of more computation time. By doubling projection numbers in scheme 1, scheme 3 improved correction ability in the L‐R direction. With a smaller 30° scan angle, scheme 4 was acceptable and will be improved after several acquisitions. Conclusions:Tomosynthesis scans can be used for real‐time mis‐alignment correction in Arc therapy after 30° gantry rotation.

TH‐D‐AUD C‐02: Reliability Study of Ultrasound Tissue Characterization in Quantitative Measurement of Radiation‐Induced Breast Tissue Toxicity

Purpose: To evaluate the reliability of the ultrasoundtissue characterization (UTC) as a measure for breast‐tissue radiation toxicity. Method and Materials: We have recently reported that UTC midband‐fit could be used to assess radiation‐induced tissue toxicity. The reliability of this method is presented in this report. Twenty‐three breast patients previously treated with radiation were recruited. Both treated and untreated breasts were scanned by one radiation oncologist. The untreated breasts were used for reliability study, in which two ultrasound radio‐frequency (RF) images were acquired at the same position. A region‐of‐interest (ROI) was selected either manually by a physician or a computer program, in which a fixed ROI was used. Between‐scan repeatability and the correlation between physician and computer program were assessed using intraclass correlation coefficient (ICC). To evaluate the ability of UTC to scale tissue toxicity, the patients were divided into four groups according to patient self‐assessment of breast hardening and the medians of the midband‐fit differences between treated and untreated breast in each group were investigated. Results: The repeatability using single measurement is 0.860 and 0.804 for physician and computer program, respectively. When the average of two measurements is used, the repeatability for computer program is 0.891, which suggests computer program can contest with the physician at the expense of double measurements. The correlation between physician and computer program is very good (ICC = 0.897), which indicates a substantial agreement between the physician and the computer program. For both method, the UTC midband‐fit increases with higher tissue toxicity of the patients. Conclusion: Both physician and computer program can assess toxicity reliably. The physician may prefer the computer program for automatic evaluation of tissue toxicity. There is a clear concordance between the UTC evaluations and the patients self‐assessment, which proves the reliability of using UTC in quantitative measurement of radiation toxicity.

Review of Real-Time 3-Dimensional Image Guided Radiation Therapy on Standard-Equipped Cancer Radiation Therapy Systems: Are We at the Tipping Point for the Era of Real-Time Radiation Therapy?

PURPOSEnToxa0review real-time 3-dimensional (3D) image guided radiation therapy (IGRT) on standard-equipped cancer radiation therapy systems, focusing on clinically implemented solutions.nnnMETHODS AND MATERIALSnThree groups in 3 continents have clinically implemented novel real-time 3D IGRT solutions on standard-equipped linear accelerators. These technologies encompass kilovoltage, combined megavoltage-kilovoltage, and combined kilovoltage-optical imaging. The cancer sites treated span pelvic and abdominal tumors for which respiratory motion is present. For each method the 3D-measured motion during treatment is reported. After treatment, dose reconstruction was used to assess the treatment quality in the presence of motion with and without real-time 3D IGRT. The geometric accuracy was quantified through phantom experiments. A literature search was conducted to identify additional real-time 3D IGRT methods that could be clinically implemented in the near future.nnnRESULTSnThe real-time 3D IGRT methods were successfully clinically implemented and have been used to treat more than 200 patients. Systematic target position shifts were observed using all 3 methods. Dose reconstruction demonstrated that the delivered dose is closer to the planned dose with real-time 3D IGRT than without real-time 3D IGRT. In addition, compromised target dose coverage and variable normal tissue doses were found without real-time 3D IGRT. The geometric accuracy results with real-time 3D IGRT had a mean error of <0.5xa0mm and a standard deviation of <1.1xa0mm. Numerous additional articles exist that describe real-time 3D IGRT methods using standard-equipped radiation therapy systems that could also be clinically implemented.nnnCONCLUSIONSnMultiple clinical implementations of real-time 3D IGRT on standard-equipped cancer radiation therapy systems have been demonstrated. Many more approaches that could be implemented were identified. These solutions provide a pathway for the broader adoption of methods to make radiation therapy more accurate, impacting tumor and normal tissue dose, margins, and ultimately patient outcomes.

SU-F-J-124: Reduction in Dosimetric Impact of Motion Using VMAT Compared to IMRT in Hypofractionated Prostate Cancer Patients

PURPOSEnTo quantify and compare the dosimetric impact of motion management correction strategies during VMAT and IMRT for hypofractionated prostate treatment.nnnMETHODSnTwo arc VMAT and 9 field IMRT plans were generated for two prostate cancer patients undergoing hypofractionated radiotherapy (7.5Gy × 5 and 8Gy × 5). 212 motion traces were retrospectively extracted from treatment records of prostate cancer patients with implanted Calypso beacons. Dose to the CTV and normal tissues was reconstructed for each trace and plan taking into account the actual treatment delivery time. Following motion correction scenarios were simulated: (1) VMAT plan - (a) No correction, (b) correction between arcs, (c) correction every 20 degrees of gantry rotation and (2) IMRT plan - (a) No correction,(b) correction between fields. Two mm action threshold for position correction was assumed. The 5-95% confidence interval (CI) range was extracted from the family of DVHs for each correction scenario.nnnRESULTSnTreatment duration for 8Gy plan (VMAT vs IMRT) was 3 vs 12 mins and for 7.5Gy plan was 3 vs 9 mins. In the absence of correction, the VMAT 5--95% CI dose spread was, on average, less than the IMRT dose spread by 2% for CTVD95, 9% for rectalwall (RW) D1cc and 9% for bladderwall (BW) D53. Further, VMAT b/w arcs correction strategy reduced the spread about the planned value compared to IMRT b/w fields correction by: 1% for CTVD95, 2.6% for RW1cc and 2% for BWD53. VMAT 20 degree strategy led to greater reduction in dose spread compared to IMRT by: 2% for CTVD95, 4.5% for RW1cc and 6.7% for BWD53.nnnCONCLUSIONnIn the absence of a correction strategy, the limited motion during VMATs shorter delivery times translates into less motion-induced dosimetric degradation than IMRT. Performing limited periodic motion correction during VMAT can yield excellent conformity to planned values that is superior to IMRT. This work was partially supported by Varian Medical Systems.

SU-C-202-04: Adapting Biologically Optimized Dose Escalation Based On Mid-Treatment PET/CT for Non-Small-Cell Lung Cancer

PURPOSEnTo develop a biological modeling strategy which incorporates the response observed on the mid-treatment PET/CT into a dose escalation design for adaptive radiotherapy of non-small-cell lung cancer.nnnMETHODnFDG-PET/CT was acquired midway through standard fractionated treatment and registered to pre-treatment planning PET/CT to evaluate radiation response of lung cancer. Each mid-treatment PET voxel was assigned the median SUV inside a concentric 1cm-diameter sphere to account for registration and imaging uncertainties. For each voxel, the planned radiation dose, pre- and mid-treatment SUVs were used to parameterize the linear-quadratic model, which was then utilized to predict the SUV distribution after the full prescribed dose. Voxels with predicted post-treatment SUV≥2 were identified as the resistant target (response arm). An adaptive simultaneous integrated boost was designed to escalate dose to the resistant target as high as possible, while keeping prescription dose to the original target and lung toxicity intact. In contrast, an adaptive target volume was delineated based only on the intensity of mid-treatment PET/CT (intensity arm), and a similar adaptive boost plan was optimized. The dose escalation capability of the two approaches was compared.nnnRESULTnImages of three patients were used in this planning study. For one patient, SUV prediction indicated complete response and no necessary dose escalation. For the other two, resistant targets defined in the response arm were multifocal, and on average accounted for 25% of the pre-treatment target, compared to 67% in the intensity arm. The smaller response arm targets led to a 6Gy higher mean target dose in the adaptive escalation design.nnnCONCLUSIONnThis pilot study suggests that adaptive dose escalation to a biologically resistant target predicted from a pre- and mid-treatment PET/CT may be more effective than escalation based on the mid-treatment PET/CT alone. More plans and ultimately clinical protocols are needed to validate this approach. MSKCC has a research agreement with Varian Medical System.