J Xiong

Choreographing Couch and Collimator in Volumetric Modulated Arc Therapy

PURPOSE To design and optimize trajectory-based, noncoplanar subarcs for volumetric modulated arc therapy (VMAT) deliverable on both Varian TrueBEAM system and traditional accelerators; and to investigate their potential advantages for treating central nervous system (CNS) tumors. METHODS AND MATERIALS To guide the computerized selection of beam trajectories consisting of simultaneous couch, gantry, and collimator motion, a score function was implemented to estimate the geometric overlap between targets and organs at risk for each couch/gantry angle combination. An initial set of beam orientations is obtained as a function of couch and gantry angle, according to a minimum search of the score function excluding zones of collision. This set is grouped into multiple continuous and extended subarcs subject to mechanical limitations using a hierarchical clustering algorithm. After determination of couch/gantry trajectories, a principal component analysis finds the collimator angle at each beam orientation that minimizes residual target-organ at risk overlaps. An in-house VMAT optimization algorithm determines the optimal multileaf collimator position and monitor units for control points within each subarc. A retrospective study of 10 CNS patients compares the proposed method of VMAT trajectory with dynamic gantry, leaves, couch, and collimator motion (Tra-VMAT); a standard noncoplanar VMAT with no couch/collimator motion within subarcs (Std-VMAT); and noncoplanar intensity-modulated radiotherapy (IMRT) plans that were clinically used. RESULTS Tra-VMAT provided improved target dose conformality and lowered maximum dose to brainstem, optic nerves, and chiasm by 7.7%, 1.1%, 2.3%, and 1.7%, respectively, compared with Std-VMAT. Tra-VMAT provided higher planning target volume minimum dose and reduced maximum dose to chiasm, optic nerves, and cochlea by 6.2%, 1.3%, 6.3%, and 8.4%, respectively, and reduced cochlea mean dose by 8.7%, compared with IMRT. Tra-VMAT averaged beam-on time was comparable to Std-VMAT but significantly (45%) less than IMRT. CONCLUSION Optimized couch, gantry, and collimator trajectories may be integrated into VMAT with improved mechanical flexibility and may provide better dosimetric properties and improved efficiency in the treatment of CNS tumors.

Automatic tracking of arbitrarily shaped implanted markers in kilovoltage projection images: A feasibility study

PURPOSE Certain types of commonly used fiducial markers take on irregular shapes upon implantation in soft tissue. This poses a challenge for methods that assume a predefined shape of markers when automatically tracking such markers in kilovoltage (kV) radiographs. The authors have developed a method of automatically tracking regularly and irregularly shaped markers using kV projection images and assessed its potential for detecting intrafractional target motion during rotational treatment. METHODS Template-based matching used a normalized cross-correlation with simplex minimization. Templates were created from computed tomography (CT) images for phantom studies and from end-expiration breath-hold planning CT for patient studies. The kV images were processed using a Sobel filter to enhance marker visibility. To correct for changes in intermarker relative positions between simulation and treatment that can introduce errors in automatic matching, marker offsets in three dimensions were manually determined from an approximately orthogonal pair of kV images. Two studies in anthropomorphic phantom were carried out, one using a gold cylindrical marker representing regular shape, another using a Visicoil marker representing irregular shape. Automatic matching of templates to cone beam CT (CBCT) projection images was performed to known marker positions in phantom. In patient data, automatic matching was compared to manual matching as an approximate ground truth. Positional discrepancy between automatic and manual matching of less than 2 mm was assumed as the criterion for successful tracking. Tracking success rates were examined in kV projection images from 22 CBCT scans of four pancreas, six gastroesophageal junction, and one lung cancer patients. Each patient had at least one irregularly shaped radiopaque marker implanted in or near the tumor. In addition, automatic tracking was tested in intrafraction kV images of three lung cancer patients with irregularly shaped markers during 11 volumetric modulated arc treatments. Purpose-built software developed at our institution was used to create marker templates and track the markers embedded in kV images. RESULTS Phantom studies showed mean ± standard deviation measurement uncertainty of automatic registration to be 0.14 ± 0.07 mm and 0.17 ± 0.08 mm for Visicoil and gold cylindrical markers, respectively. The mean success rate of automatic tracking with CBCT projections (11 frames per second, fps) of pancreas, gastroesophageal junction, and lung cancer patients was 100%, 99.1% (range 98%-100%), and 100%, respectively. With intrafraction images (approx. 0.2 fps) of lung cancer patients, the success rate was 98.2% (range 97%-100%), and 94.3% (range 93%-97%) using templates from 1.25 mm and 2.5 mm slice spacing CT scans, respectively. Correction of intermarker relative position was found to improve the success rate in two out of eight patients analyzed. CONCLUSIONS The proposed method can track arbitrary marker shapes in kV images using templates generated from a breath-hold CT acquired at simulation. The studies indicate its feasibility for tracking tumor motion during rotational treatment. Investigation of the causes of misregistration suggests that its rate of incidence can be reduced with higher frequency of image acquisition, templates made from smaller CT slice spacing, and correction of changes in intermarker relative positions when they occur.

Simultaneous MV-kV imaging for intrafractional motion management during volumetric-modulated arc therapy delivery*

The purpose of this study was to evaluate the accuracy and clinical feasibility of a motion monitoring method employing simultaneously acquired MV and kV images during volumetric‐modulated arc therapy (VMAT). Short‐arc digital tomosynthesis (SA‐DTS) is used to improve the quality of the MV images that are then combined with orthogonally acquired kV images to assess 3D motion. An anthropomorphic phantom with implanted gold seeds was used to assess accuracy of the method under static, typical prostatic, and respiratory motion scenarios. Automatic registration of kV images and single MV frames or MV SA‐DTS reconstructed with arc lengths from 2° to 7° with the appropriate reference fiducial template images was performed using special purpose‐built software. Clinical feasibility was evaluated by retrospectively analyzing images acquired over four or five sessions for each of three patients undergoing hypofractionated prostate radiotherapy. The standard deviation of the registration error in phantom using MV SA‐DTS was similar to single MV images for the static and prostate motion scenarios (σ=0.25 mm). Under respiratory motion conditions, the standard deviation of the registration error increased to 0.7 mm and 1.7 mm for single MV and MV SA‐DTS, respectively. Registration failures were observed with the respiratory scenario only and were due to motion‐induced fiducial blurring. For the three patients studied, the mean and standard deviation of the difference between automatic registration using 4° MV SA‐DTS and manual registration using single MV images results was 0.07±0.52 mm. The MV SA‐DTS results in patients were, on average, superior to single‐frame MV by nearly 1 mm — significantly more than what was observed in phantom. The best MV SA‐DTS results were observed with arc lengths of 3° to 4°. Registration failures in patients using MV SA‐DTS were primarily due to blockage of the gold seeds by the MLC. The failure rate varied from 2% to 16%. Combined MV SA‐DTS and kV imaging is feasible for intratreatment motion monitoring during VMAT of anatomic sites where limited motion is expected, and improves registration accuracy compared to single MV/kV frames. To create a clinically robust technique, further improvements to ensure visualization of fiducials at the desired control points without degradation of the treatment plan are needed. PACS number(s): 87.55.km, 87.55.N‐

Learning the relationship between patient geometry and beam intensity in breast intensity-modulated radiotherapy

Intensity modulated radiotherapy (IMRT) has become an effective tool for cancer treatment with radiation. However, even expert radiation planners still need to spend a substantial amount of time adjusting IMRT optimization parameters in order to get a clinically acceptable plan. We demonstrate that the relationship between patient geometry and radiation intensity distributions can be automatically inferred using a variety of machine learning techniques in the case of two-field breast IMRT. Our experiments show that given a small number of human-expert-generated clinically acceptable plans, the machine learning predictions produce equally acceptable plans in a matter of seconds. The machine learning approach has the potential for greater benefits in sites where the IMRT planning process is more challenging or tedious

Fast radioactive seed localization in intraoperative cone beam CT for low-dose-rate prostate brachytherapy

A fast knowledge-based radioactive seed localization method for brachytherapy was developed to automatically localize radioactive seeds in an intraoperative volumetric cone beam CT (CBCT) so that corrections, if needed, can be made during prostate implant surgery. A transrectal ultrasound (TRUS) scan is acquired for intraoperative treatment planning. Planned seed positions are transferred to intraoperative CBCT following TRUS-to-CBCT registration using a reference CBCT scan of the TRUS probe as a template, in which the probe and its external fiducial markers are pre-segmented and their positions in TRUS are known. The transferred planned seeds and probe serve as an atlas to reduce the search space in CBCT. Candidate seed voxels are identified based on image intensity. Regions are grown from candidate voxels and overlay regions are merged. Region volume and intensity variance is checked against known seed volume and intensity profile. Regions meeting the above criteria are flagged as detected seeds; otherwise they are flagged as likely seeds and sorted by a score that is based on volume, intensity profile and distance to the closest planned seed. A graphical interface allows users to review and accept or reject likely seeds. Likely seeds with approximately twice the seed volume are automatically split. Five clinical cases are tested. Without any manual correction in seed detection, the method performed the localization in 5 seconds (excluding registration time) for a CBCT scan with 512×512×192 voxels. The average precision rate per case is 99% and the recall rate is 96% for a total of 416 seeds. All false negative seeds are found with 15 in likely seeds and 1 included in a detected seed. With the new method, updating of calculations of dose distribution during the procedure is possible and thus facilitating evaluation and improvement of treatment quality.

Adaptation, Commissioning, and Evaluation of a 3D Treatment Planning System for High-Resolution Small-Animal Irradiation

Although spatially precise systems are now available for small-animal irradiations, there are currently limited software tools available for treatment planning for such irradiations. We report on the adaptation, commissioning, and evaluation of a 3-dimensional treatment planning system for use with a small-animal irradiation system. The 225-kV X-ray beam of the X-RAD 225Cx microirradiator (Precision X-Ray) was commissioned using both ion-chamber and radiochromic film for 10 different collimators ranging in field size from 1 mm in diameter to 40 × 40 mm2. A clinical 3-dimensional treatment planning system (Metropolis) developed at our institution was adapted to small-animal irradiation by making it compatible with the dimensions of mice and rats, modeling the microirradiator beam orientations and collimators, and incorporating the measured beam data for dose calculation. Dose calculations in Metropolis were verified by comparison with measurements in phantoms. Treatment plans for irradiation of a tumor-bearing mouse were generated with both the Metropolis and the vendor-supplied software. The calculated beam-on times and the plan evaluation tools were compared. The dose rate at the central axis ranges from 74 to 365 cGy/min depending on the collimator size. Doses calculated with Metropolis agreed with phantom measurements within 3% for all collimators. The beam-on times calculated by Metropolis and the vendor-supplied software agreed within 1% at the isocenter. The modified 3-dimensional treatment planning system provides better visualization of the relationship between the X-ray beams and the small-animal anatomy as well as more complete dosimetric information on target tissues and organs at risk. It thereby enhances the potential of image-guided microirradiator systems for evaluation of dose–response relationships and for preclinical experimentation generally.

SU‐D‐144‐01: Implementation of a Clinical Treatment Planning System for Use with a Small Animal Irradiation System

PURPOSE Current commercially available small animal irradiators provide limited software tools for treatment planning. We report on the implementation of an in-house treatment planning system, Metropolis, for micro-irradiator and compare its capabilities with those of the vendor-supplied system, TPS(XRAD). METHODS The 225 kV beam of the micro-irradiator was commissioned using both ion chamber and radiographic film (EBT3). Dose calculation in Metropolis was verified by comparing with measured dose in phantom. Starting from a 3D CT image of a tumor-bearing mouse, the same treatment plans were designed in the two systems. In TPS(XRAD), a treatment point was selected on a CT slice, two orthogonal beams were defined based on tumor size on the CT slice, and isocenter depth was computed from a body contour determined by image thresholding. In Metropolis, structures (outer, tumor, and lung) were contoured on CT images, beams were defined based on tumor volume coverage on beams-eye-view (BEV), 3D dose calculation was performed, and plan was evaluated based on dose distribution and dose-volume histogram (DVH). RESULTS Measured dose in phantom agreed with Metropolis calculation within 3% for all collimators. The beam-on-times calculated by the two systems agreed within 1% at isocenter. Whereas TPS(XRAD) provides only beam-on-time for each beam, Metropolis capabilities include various image segmentation tools, multimodality image registration, verification of target coverage and normal tissue sparing using BEV, 3D isodose distribution overlaid on CT images, and DVH. DVH inspection of the two-beam treatment plan reveals a tumor dose variation (D95 : 388 and D05 : 401 cGy) and dose to lung (Dmean: 84 cGy) for a prescribed isocenter dose of 400 cGy. CONCLUSION By implementing a clinical TPS for a small animal irradiator system, both efficient planning and precise plan evaluation become possible, allowing the full potential of advanced micro-irradiator radiation treatment planning to be conducted for pre-clinical experimentation.

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

PURPOSE To quantify and compare the dosimetric impact of motion management correction strategies during VMAT and IMRT for hypofractionated prostate treatment. METHODS Two 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. RESULTS Treatment 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. CONCLUSION In 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.