Multiple Isocenters Research Articles

Single isocenter treatment planning techniques for stereotactic radiosurgery of multiple cranial metastases.

Whole brain radiation therapy use has decreased in favor of stereotactic radiosurgery (SRS) for the treatment of multiple brain metastases due to reduced neurotoxicity. Here we compare two single isocenter radiosurgery planning techniques, volumetric modulated arc therapy (VMAT) and dynamic conformal arcs (DCA) in terms of their dosimetric and delivery performance. Sixteen patients with 2- 18 brain metastases (total 103; median 4) previously treated with single fraction SRS were replanned for multiple lesion single isocenter treatments using VMAT and DCA using different treatment planning systems for each and three different plan geometries for DCA. Plans were evaluated using the Paddick conformity index, normal tissue V12Gy, the probability for symptomatic brain necrosis (S-NEC), maximum organ-at-risk (OAR) point doses, and total number of monitor units (MU). Conformity was not significantly different between VMAT and DCA plans. VMAT plans showed a trend towards higher MU with a median difference between 18% and 24% (p≤0.09). Median V12Gy differences were 7.0cm3-8.6cm3 favoring DCA plans (p<0.01). VMAT plans had median excess absolute and relative S-NEC risks compared to DCA plans of 8%-10% and 25%-31%, respectively (p<0.01). Moreover for VMAT compared to DCA, maximum OAR doses were significantly higher for the brainstem (1.9Gy; p<0.01), chiasm (0.5Gy; p≤0.02), and optic nerves (0.5Gy; p≤0.04). In most cases DCA plans were found to be dosimetrically superior to VMAT plans with reduced V12Gy and associated risk for S-NEC. Maximum doses to important OARs showed significant improvement, increasing the ability for subsequent salvage treatments involving radiation.

Quy trình đặt bệnh xạ trị trục não tủy với kỹ thuật điều biến thể tích theo cung tròn bằng máy Truebeam Bệnh viện Ung bướu Thành phố Hồ Chí Minh

The purpose of external beam radiation therapy is to precisely deliver a specified dose into target volumes, while minimizing radiation dose to at-risk organs (OARs). Radiation therapy requires precision throughout entire process from simulation to planning and administering radiation therapy. Cranial Spinal Irradiation (CSI) is a complex technique, due to the large target volumes and the limited field-size of the accelerator machine, requiring multiple isocenters and coupling fields. The irradiation field coupling must be highly accurate. Setup errors lead to field-overlaping resulting in OARs overdose and recurrence risk due to underdose to target volumes. New radiation techniques which help highly optimizing radiation dose to the target volumes and better protecting organs at risk, even require more accuracy in setting up process. Here, we introduce the patient setting up procedure using for VMAT- CSI process at the TrueBeam machine in Ho Chi Minh City Cancer Hospital.

Effect of setup error in the single-isocenter technique on stereotactic radiosurgery for multiple brain metastases.

In conventional stereotactic radiosurgery (SRS), treatment of multiple brain metastases using multiple isocenters is time‐consuming resulting in long dose delivery times for patients. A single‐isocenter technique has been developed which enables the simultaneous irradiation of multiple targets at one isocenter. This technique requires accurate positioning of the patient to ensure optimal dose coverage. We evaluated the effect of six degrees of freedom (6DoF) setup errors in patient setups on SRS dose distributions for multiple brain metastases using a single‐isocenter technique. We used simulated spherical gross tumor volumes (GTVs) with diameters ranging from 1.0 to 3.0 cm. The distance from the isocenter to the target's center was varied from 0 to 15 cm. We created dose distributions so that each target was entirely covered by 100% of the prescribed dose. The target's position vectors were rotated from 0°–2.0° and translated from 0–1.0 mm with respect to the three axes in space. The reduction in dose coverage for the targets for each setup error was calculated and compared with zero setup error. The calculated margins for the GTV necessary to satisfy the tolerance values for loss of GTV coverage of 3% to 10% were defined as coverage‐based margins. In addition, the maximum isocenter to target distance for different 6DoF setup errors was calculated to satisfy the tolerance values. The dose coverage reduction and coverage‐based margins increased as the target diameter decreased, and the distance and 6DoF setup error increased. An increase in setup error when a single‐isocenter technique is used may increase the risk of missing the tumor; this risk increases with increasing distance from the isocenter and decreasing tumor size.

VMAT Based TBI Prior to BMT in Malignant & Non-malignant Disorders – Single Institution Experience

The purpose of this paper is to report the feasibility of Total Body Irradiation (TBI) by multiple Iso-centric Volumetric Modulated Arc Therapy (VMAT) technique before Bone Marrow Transplant (BMT) in malignant & Non-malignant, hematological & non-hematological disorders. Total of 5 patients, 3 with ALL, 1each with Hurlor syndrome and Thalasemia Major were recruited for the TBI in our hospital from April 2018 to May 2019 before going for BMT. Immobilizations were done with thermoplastic masks and vaclocs for all patients in SBRT Multi-board. HFS&FFS two sets of CT scans done for 4 patients having more than 120 cm in heights with dosimetric reference marker in junction and only HFS scan done for a patient less than 120 cm in height. Dose prescribed to PTV excluding lung and kidneys was 12Gy in 6 fractions over 3 days (2 fractions per day) for ALL, single dose of 3 Gy for Hurler syndrome and 4 Gy in 2 fractions for Thalasemia Major. Treatment planning was done using VMAT technique in a treatment planning system. It consisted of average of 13 arcs for ALL and 8 arcs for other two patients with multiple isocenters and small static fields used to enhance the dose coverage. The best treatment plans were selected ensuring 90% PTV dose coverage and acceptable dose to OARs. Treatment executed with Linear accelerator equipped with OBI, CBCT used as Image Guided Radiation Therapy (IGRT) for each VMAT delivery. Quality assurance performed with mini-phantom and point dose measurements done with slab phantom and CC13 ion chamber followed by in vivo dosimetry performed with Optically Stimulated Luminescence Dosimeter nano chips. Contouring and planning time took 15-20 hours and quality assurance took 4-5 hours for each patient. PTV coverage of 90% achieved with PTV Dmean of 12.54 Gy for ALL patients and 3.02&4.32 respectively in other two cases. OAR average doses of lungs, kidneys and liver reduced to 9.8, 9.5 & 10.3 Gy respectively and lens restricted to 6 Gy for ALL patients and no OAR constraints for single and dual fraction low dose prescription patients. On couch treatment time on day one was 2.5 hours including setup and mounting of OSLD chips and irradiation for all cases. Further fractions time reduced to 1.5 hrs. 3 sets of CBCT performed for head first with average setup error of ±0.5 mm and two CBCT scans performed for feet first with average setup error of ±0.3 mm. OSLD dosimeter readings showed the results of within ±5% deviation. TBI by VMAT technique is feasible and provides satisfactory 3D dose distribution within PTV and reducing the dose to the critical OARs. VMAT provides a homogenous dose to the whole body, verified & confirmed by in vivo dosimetry. VMAT-TBI has advantage over extended SSD based conventional TBI methods in resource limited community as it can be used in regular radiotherapy department without requiring dedicated accessories.

Knowledge-Based VMAT Planning of Total Marrow Irradiation as a Part of Conditioning Regimen Before Allo-SCT in a Prospective Phase I/II Study

Total marrow irradiation (TMI) involves optimization of extremely large target volumes and requires extended time for treatment planning and delivery. Although volumetric modulated arc therapy (VMAT) achieves substantial reduction in the delivery time, the planning process still presents a challenge with multiple isocenters and multiple overlapping arcs to adequately cover sub-target volumes of head and neck, chest, and pelvis. We developed and applied a knowledge-based planning (KBP) model to set optimization objectives, and evaluated its performance for VMAT-TMI. We created 3 model libraries for head and neck, chest, and pelvis plans from 20 patients treated on a prospective study examining the utility of TMI. Plans utilized a 3-isocenter, 13-arc VMAT technique. Head and neck and pelvis plans were optimized utilizing the chest plans as the base dose to prevent hot spots around the field junctions. For the KBP model, they were re-optimized without the base-dose plan and were added to the corresponding library to minimize scatters in dose-volume histogram (DVH) plots of target volumes. The KBP models were validated on a cohort of 6 recent patients who were not used for the training. Optimization objectives for organs at risk (OARs) away from the junction regions were placed along the predicted DVHs. For OARs overlapping two sub plans, sums of predicted mean dose values from each sub plan were used as objectives. The KBP database generated achievable mean dose values along with the prediction ranges of DVHs for each OAR. Performance of KBP generated within 2∼3 iterative optimizations was comparable to clinical plans that used to take a few days of effort with indefinite number of iterations. KBP provided target coverages (95% volume covered by the prescribed dose of 9 Gy) and the homogeneity index (<1.4) in accordance with our institutional practice. Both clinical plans and KBP had mean dose values in the range of 104%-111% of the prescribed dose for planning target volumes, 23%-29% for lenses and oral cavity, and 42%-63% for brain, heart, lungs, bowels, liver, kidneys, eyes. Our study demonstrates that KBP can be implemented for Linac-based multiple-isocenter TMI plans by creating a model for each sub plan. VMAT-TMI powered by KBP can potentially support uniform dosimetric quality among different users and clinics, and can reduce treatment planning time and effort by providing a goal attainable within a few iterative optimizations.Tabled 1Abstract 2665; Table; Comparison of mean dose values to OAR (% of prescribed dose)bowelsbraineyesheartkidneysliverlenseslungsoral cavityclinical plans48±660±444±351±647±651±529±263±425±6KBP42±657±545±349±348±651±229±159±323±2 Open table in a new tab

The National Centre for Oncological Hadron therapy (CNAO): Present Status and Future Perspectives

Some elementary particles used for experiments of fundamental physics have properties useful to the treatments of patients affected by oncological pathologies. They are protons and carbon ions, collectively named hadrons, hence the term hadron therapy. Hadrons, in particular carbon ions, are more precise on the target than conventional X-rays and possess radiobiological characteristics suited to treat radioresistant or inoperable tumors. Italy is at the forefront of these techniques, and in Pavia a clinical facility called CNAO (Italian acronym that stands for National Centre for Oncological Hadron therapy) has treated so far more than 2800 patients with very good results. The CNAO was created by the Health Ministry and was realized by the CNAO Foundation in collaboration with the Italian Institute of Nuclear Physics (INFN), CERN, GSI and other institutions in Italy and abroad. The facility in Pavia delivers beams of hadrons in three treatment rooms with four fixed beam ports: three horizontal and one vertical. A new room, with an horizontal beamline and multiple isocenters, was completed and will be fully devoted to research applications. The CNAO has also launched a development programe to add a new single room for proton therapy with a gantry and a dedicated accelerator. Attention is also devoted to the most interesting aspects of research and development in the hadron therapy domain, like the creation of a new BNCT (Boron Neutron Capture Therapy) facility and the design of a novel gantry for carbon ions.

Prediction of conical collimator collision for stereotactic radiosurgery.

The purpose of this study is to predict the collision clearance distance of stereotactic cones with treatment setup devices in cone‐based stereotactic radiosurgery (SRS). The BrainLAB radiosurgery system with a Frameless Radiosurgery Positioning Array and dedicated couch top was targeted in this study. The positioning array and couch top were scanned with CT simulators, and their outer contours of were detected. The minimum clearance distance was estimated by calculating the Euclidian distances between the surface of the SRS cones and the nearest surface of the outer contours. The coordinate transformation of the outer contour was performed by incorporating the Beam's Eye View at a planned arc range and couch angle. From the minimum clearance distance, the collision‐free gantry ranges for each couch angle were sequentially determined. An in‐house software was developed to calculate the clearance distance between the cone surface and the outer contours, and thus determine the occurrence of a collision. The software was extensively tested for various combinations of couch and arc angles at multiple isocenter locations for two combinations of cone‐couch systems. A total of 50 arcs were used to validate the calculation accuracies of the software for each system. The calculated minimum distances and collision‐free angles from the software were verified by physical measurements. The calculated minimum distances were found to agree with the measurements to within 0.3 ± 0.9 mm. The collision‐free arc angles from the software also agreed with the measurements to within 1.1 ± 1.1° with a 5‐mm safety margin for 20 arcs. In conclusion, the in‐house software was able to calculate the minimum clearance distance with <1.0 mm accuracy and to determine the collision‐free arc range for the cone‐based BrainLab SRS system.

Residual positioning errors and uncertainties for pediatric craniospinal irradiation and the impact of image guidance

BackgroundOptimal alignment is of utmost importance when treating pediatric patients with craniospinal irradiation (CSI), especially with regards to field junctions and multiple isocenters and techniques applying high dose gradients. Here, we investigated the setup errors and uncertainties for pediatric CSI using different setup verification protocols.MethodsA total of 38 pediatric patients treated with CSI were identified for whom treatment records and setup images were available. The setup images were registered retrospectively to the reference image using an automated tool and matching on bony anatomy, subsequently, the impact of different correction protocols was simulated.ResultsFor an action-level (AL)-protocol and a non-action level (NAL)-protocol, the translational residual setup error can be as large as 24 mm for an individual patient during a single fraction, and the rotational error as large as 6.1°. With daily IGRT, the maximum setup errors were reduced to 1 mm translational and 5.4° rotational versus 1 mm translational and 2.4° rotational for 3- and 6- degrees of freedom (DoF) couch shifts, respectively. With a daily 6-DoF IGRT protocol for a wide field junction irradiation technique, the residual positioning uncertainty was below 1 mm and 1° for translational and rotational directions, respectively. The largest rotational uncertainty was found for the patients’ roll even though this was the least common type of rotational error, while the largest translational uncertainty was found in the patients’ anterior-posterior-axis.ConclusionsThese results allow for informed margin calculation and robust optimization of treatments. Daily IGRT is the superior choice for setup of pediatric patients treated with CSI, although centers that do not have this option could use the results presented here to improve their margins and uncertainty estimates for a more accurate treatment alignment.

High Quality Clinical Stereotactic Radiosurgery Planning and Delivery With Standard Resolution (5 mm) Multileaf Collimation and Multiple Isocenters

PurposeOur purpose was to demonstrate the use of novel planning techniques in producing high-quality stereotactic radiosurgery (SRS) plans using a standard 5 mm multileaf collimator (MLC) and multiple isocenters delivered clinically at a local institution. Methods and MaterialsNovel planning techniques consisted of offset isocenter, variable asymmetrical jaws, and Digital Imagine and Communications in Medicine (DICOM) edits to reduce leaf tip transmission, all with the aim of maximizing dose conformity. A local institution clinical cohort was planned (1-4 targets), and plan conformity metrics common to SRS were compared against conformity metrics from selected previous publications comparing Gamma Knife to linear accelerator SRS using high-definition MLC (2.5 mm). Additionally, local institution plan conformity metrics for 2 benchmark SRS planning cases (3 and 7 targets) were compared with metrics from other centers treating SRS clinically in England. Pretreatment quality assurance results, both point dose measurement and film analysis, are presented to demonstrate plan deliverability. ResultsClinical conformity metrics are shown to be comparable to previously published results using either Gamma Knife or linear accelerator with high-definition MLC. Metrics from benchmark planning cases are shown to be comparable and to have better prescription dose conformity than average nationally in England. Pretreatment quality assurance results demonstrate suitable plan deliverability. ConclusionsSRS planning using standard 5 mm MLC and multiple isocenters produces high-quality treatment plans for a limited number of targets with a high degree of dose conformity and dose fall off when employing novel planning techniques to compensate for MLC leaf size and multiple isocenters.

Patient motion tracking for non-isocentric and non-coplanar treatments via fixed frame-of-reference 3D camera.

PurposeAs C‐arm linac radiation therapy evolves toward faster, more efficient delivery, and more conformal dosimetry, treatments with increasingly complex couch motions are emerging. Monitoring the patient motion independently of the couch motion during non‐coplanar, non‐isocentric, or dynamic couch treatments is a key bottleneck to their clinical implementation. The goal of this study is to develop a prototype real‐time monitoring system for unconventional beam trajectories to ensure a safe and accurate treatment delivery.MethodsAn in‐house algorithm was developed for tracking using a couch‐mounted three‐dimensional (3D) depth camera. The accuracy of patient motion detection on the couch was tested on a 3D printed phantom created from the body surface contour exported from the treatment planning system. The technique was evaluated against a commercial optical surface monitoring system with known phantom displacements of 3, 5, and 7 mm in lateral, longitudinal, and vertical directions by placing a head phantom on a dynamic platform on the treatment couch. The stability of the monitoring system was evaluated during dynamic couch trajectories, at speeds between 10.6 and 65 cm/min.ResultsThe proposed monitoring system agreed with the ceiling mounted optical surface monitoring system in longitudinal, lateral, and vertical directions within 0.5 mm. The uncertainty caused by couch vibration increased with couch speed but remained sub‐millimeter for speeds up to 32 cm/min. For couch speeds of 10.6, 32.2, and 65 cm/min, the uncertainty ranges were 0.27– 0.73 mm, 0.15–0.87 mm, and 0.28–1.29 mm, respectively.ConclusionBy mounting a 3D camera in the same frame‐of‐reference as the patient and eliminating dead spots, this proof of concept demonstrates real‐time patient monitoring during couch motion. For treatments with non‐coplanar beams, multiple isocenters, or dynamic couch motion, this provides additional safety without additional radiation dose and avoids some of the complexity and limitations of room mounted systems.

Characterizing a deformable registration algorithm for surface-guided breast radiotherapy.

Surface-guided radiation therapy (SGRT) is a nonionizing imaging approach for patient setup guidance, intra-fraction monitoring, and automated breath-hold gating of radiation treatments. SGRT employs the premise that the external patient surface correlates to the internal anatomy, to infer the treatment isocenter position at time of treatment delivery. Deformations and posture variations are known to impact the correlation between external and internal anatomy. However, the degree, magnitude, and algorithm dependence of this impact are not intuitive and currently no methods exist to assess this relationship. The primary aim of this work was to develop a framework to investigate and understand how a commercial optical surface imaging system (C-RAD, Uppsala, Sweden), which uses a nonrigid registration algorithm, handles rotations and surface deformations. A workflow consisting of a female torso phantom and software-introduced transformations to the corresponding digital reference surface was developed. To benchmark and validate the approach, known rigid translations and rotations were first applied. Relevant breast radiotherapy deformations related to breast size, hunching/arching back, distended/deflated abdomen, and an irregular surface to mimic a cover sheet over the lower part of the torso were investigated. The difference between rigid and deformed surfaces was evaluated as a function of isocenter location. For all introduced rigid body transformations, C-RAD computed isocenter shifts were determined within 1mm and 1˚. Additional translational shifts to correct for rotations as a function of isocenter location were determined with the same accuracy. For yaw setup errors, the difference in shift corrections between a plan with an isocenter placed in the center of the breast (BrstIso) and one located 12cm superiorly (SCFIso) was 2.3mm/1˚ in lateral direction. Pitch setup errors resulted in a difference of 2.1mm/1˚ in vertical direction. For some of the deformation scenarios, much larger differences up to 16mm and 7˚ in the calculated shifts between BrstIso and SCFIso were observed that could lead to large unintended gaps or overlap between adjacent matched fields if uncorrected. The methodology developed lends itself well for quality assurance (QA) of SGRT systems. The deformable C-RAD algorithm determined accurate shifts for rigid transformations, and this was independent of isocenter location. For surface deformations, the position of the isocenter had considerable impact on the registration result. It is recommended to avoid off-axis isocenters during treatment planning to optimally utilize the capabilities of the deformable image registration algorithm, especially when multiple isocenters are used with fields that share a field edge.

Efficient quality assurance method with automated data acquisition of a single phantom setup to determine radiation and imaging isocenter congruence.

We developed a quality assurance (QA) method to determine the isocenter congruence of Optical Surface Monitoring System (OSMS, Varian, CA, USA), kilovoltage (kV), and megavoltage (MV) imaging, and the radiation isocenter using a single setup of the OSMS phantom for frameless Stereotactic Radiosurgery (SRS) treatment. After aligning the phantom to the OSMS isocenter, a cone‐beam computed tomography (CBCT) of the phantom was acquired and registered to a computed tomography (CT) scan of the phantom to determine the CBCT isocenter. Without moving the phantom, MV and kV images were simultaneously acquired at four gantry angles to localize MV and kV isocenters. Then, Winston‐Lutz (W‐L) test images of the central BB in the phantom were acquired to analyze the radiation isocenter. The gantry and couch were automatically controlled using the TrueBeam Developer Mode during MV, kV, and W‐L image acquisition. All the images were acquired weekly for 17 weeks to track the congruence of all the imaging modalities' isocenter in six‐dimensional (6D) translations and rotations, and the radiation isocenter in three‐dimensional (3D) translations. The shifts of isocenters of all imaging modalities and the radiation isocenter from the OSMS isocenter were within 0.2 mm and 0.2° on average over 17 weeks. The maximum discrepancy between OSMS and other imaging modalities or radiation isocenters was 0.8 mm and 0.3°. However, systematic shifts of radiation isocenter anteriorly and laterally relative to the OSMS isocenter were observed. The measured discrepancies were consistent from week‐to‐week except for two weeks when the isocenter discrepancies of 0.8 mm were noted due to drifts of the OSMS isocenter. Once recalibration was performed on OSMS, the discrepancy was reduced to 0.3 mm and 0.2°.By performing the proposed QA on a weekly basis, the isocenter congruencies of multiple imaging systems and radiation isocenter were validated for a linear accelerator.

Experimental Verification of Dosimetric Uncertainty Related to Rotational Error of Single Isocenter for Multiple Targets Technique

Treatment of multiple brain mets using single isocenter on Linac is a more efficient technique than using multiple isocenters on robotic radiosurgery. This study investigated the uncertainties in target coverage due to the rotational set up errors with film measurements. A commercial radiosurgery head phantom built with a stack of film inserts within the brain was CT scanned at 1mm spacing. Three spherical targets with the diameter of 1cm (2) and 2cm (1) were generated at a distance of 4 ∼ 6cm from the isocenter to simulate multiple mets in the brain. A volumetric modulated arc therapy (VMAT) plan was optimized with a 6X beam to have a minimum 92% coverage for each GTV. The head phantom was aligned on 6 degrees of freedom (6DoF) couch with CBCT. Three pieces of Galchromic films were cut exactly the size of film insert and placed at the locations across each target along the Sup/Inf direction (film plane being close to the CT coronal cut). Once the positioning was confirmed by CBCT, the VMAT plan was delivered first at the planned setup (0° rotational error). A rotational error of 1° and 2° in yaw was then introduced and verified by CBCT. The same VMAT plan was then delivered again to both setups with the 1° and 2° of rotational error in yaw, respectively, with a new set of Galchromic films in place. All films were analyzed to have 1D absolute dose profiles extracted along a line of interest for comparison with the calculation. Comparison of dose profiles between the measurement and the calculation shows a good agreement for the planned delivery, i.e., no rotational errors introduced. With the rotational error of 1° and 2°, the measured dose profiles are seen an obvious shift from the planned. The max shift at the prescription dose was 0.6mm and 1.2mm for the yaw error of 1° and 2°, respectively. The deviation for each target is consistent with the magnitude of rotational errors. Table 1 summarized the max measured deviations of the prescription isodose line in distance from the planned for each GTV at a rotational error of 0°, 1° and 2°, respectively. Experiment for the delivery of a VMAT plan with single isocenter has confirmed the accuracy of the setup using 6DoF and CBCT. Distant targets are susceptible to the deviation of dose coverage due to the rotational errors. A planning margin up to 2mm should be sufficient to ensure the GTV coverage should the rotational tolerance of 6DoF is maintained less than 2°.Abstract 3813; Table 1Rotational Error in YawMax deviations (mm) relative to the planGTV1GTV2GTV30°0.20.30.31°0.60.40.42°0.91.21.0 Open table in a new tab

Dosimetric Comparison of Multiple vs Single Isocenter Technique for Linear Accelerator-Based Stereotactic Radiosurgery: The Importance of the Six Degree Couch

Previous studies have evaluated the feasibility of a single isocenter technique (SIT) for linear accelerator-based stereotactic radiosurgery (SRS). However, recent data has demonstrated that not controlling for rotational error reduces target coverage. With the advent of the robotic six degree of freedom (6DoF) couch, we sought to evaluate its effect on patients with multiple lesions. Patients treated with SRS at our institution to ≥ 3 metastases were identified. Original treatment plans involved multiple isocenters (MP). The lesions were re-planned at same prescription dose and normalization using SIT with 2 coplanar arcs and 3 non-coplanar half arcs at approximately 45, 90, and 135 degrees (SP). SPs were evaluated to identify planning treatment volumes (PTVs) >5cm from isocenter. Lesions 5-10cm from isocenter had an additional 1mm of margin, and lesions > 10cm from isocenter had an additional 2mm of margin added. Plans were re-run with these margins to account for inability to correct rotational error (SP+1). Volume of brain receiving 12 Gy (V12) and mean brain dose (MBD) were evaluated. Dosimetry planning time (DPT) was estimated at 0.5 hours for normal structures, 2 hours per isocenter for MPs, and 5 hours for SPs. Patient time on table (PTT) was estimated at 20 minutes per isocenter for set-up and beam on time of 2 minutes per arc. Statistics were calculated using Related-Sample Wilcoxon Signed Rank test. A total of 10 patients with 73 brain metastases receiving SRS to a median of 6 lesions (range 3-16) were analyzed. All PTVs had max heterogeneity between 105% and 135%. MP treated 73 lesions with 63 isocenters. On average, MP had a 19.2% higher V12 than SP (range -7.9%-45.6%, p=0.017). However, MBD was higher with SP compared to MP, with MBD increased by 34.9% on average (range 0.25%-80.1%, p=0.005); the highest mean brain dose recorded was 4.6 Gy. For creation of SP+1, 30 lesions required 1mm of margin, while none required 2 mm of margin. V12 increased by 47.8% on average per patient (range 6.9%-97.6%, p=0.008) from SP to SP+1. MBD also increased from SP to SP+1 by an average of 19.2% (range 3.6-73.3, p=0.008), with max MBD of 4.7 Gy. DPT was 5.5 hours for SP, while median for MP was 12.5 hours (range 6.5-30.5, p=0.005). PTT was 30 minutes for SP, while median for MP was 144 minutes (range 72-352, p=0.005). SIT results in dosimetry comparable to multi-isocenter plans for patients being treated to multiple brain metastases as long as rotational error can be corrected for with the use of a 6DoF couch. While mean brain dose is slightly higher, this is unlikely to have clinical significance. The time savings to both DPT and PTT are significant using SIT. Increasing margin to account for rotational error (rather than correcting for it with 6DoF couch) leads to a nearly 50% increase in V12, which could result in higher rates of radiation necrosis.

RADI-15. CLUSTERING AND GROUPING OF BRAIN METS IN RADIOSURGERY TREATMENTS

INTRODUCTION: Radiosurgical treatment of numerous lesions in the brain with ‘single-isocenter’ radiosurgery on a linac often requires using multiple isocenters. With our TPS (Elements, Brainlab) multiple plans need to be generated for each set of lesions, and a sum plan calculated. We investigated how to distribute multiple lesions into two groups for two isocenters to achieve a good summed dose distribution. METHODS: The DICOM RS file is exported and the PTV data is extracted by a MATLAB program that calculates the convex hulls, estimated radii, and the centers of mass for each PTV. Two approaches were tried: (1) Lesions close to each other (closer than a certain limit) are put in different groups and (2) Create clusters by kMeans clustering, which allows close lesions but the groups are distant from each other. MATLAB programs were written for all approaches. Treatment plans were generated for three patients (20, 13, 15 lesions) using each method and compared with the actual treatment plan used to treat the patient based on the intuitive grouping of lesions by the planners. Dose maximums outside the lesions, and volumes in the normal tissue exceeding 75, 50 and 25% of the prescription dose were evaluated. RESULTS AND DISCUSSION: The coverage of all lesions for all plans were 95% of the prescription dose. The first approach allowed lowering the maximum dose between lesions, but with summing dose distributions this advantage disappeared. The maximum dose and the 75, 50 and 25% dose volumes were also all worse than in plans generated by experienced planners and higher normal brain doses are delivered if closely spaced lesions are separated into different isocenters for treatment. However, the clustering approach resulted in the same or better values of these same parameters, i.e. improved dose distributions over the dosimetrist’s intuitively chosen separation.

Efficacy and safety of gamma knife radiosurgery for posterior cranial fossa meningioma: a systematic review.

The management of posterior cranial fossa meningioma [PCFM] is challenging and many neurosurgeons advise gamma knife radiosurgery [GKRS] as a modality for its upfront or adjuvant treatment. Due to the varying radiosurgical response based on lesion location, tumor biology, and radiation dosage, we performed a pioneer attempt in doing a systematic review analyzing the treatment efficacy and safety profile of GKRS for PCFM based on current literature. A systematic review was performed according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses [PRISMA] guidelines. A thorough literature search was conducted on PubMed, Web of science, and Cochrane data base; articles were selected systematically based on PRISMA protocol, reviewed completely, and relevant data was summarized and discussed. A total of 18 publications pertaining to GKRS for PCFM were included with a pooled sample size of 2131 patients. The median pre-GKRS tumor volume ranged from 2.28 to 10.5cm [3]. Primary GKRS was administered in 61.1% of the pooled study cohorts, adjuvant treatment in 32.9%, and salvage therapy in 6.5% patients. Majority of the meningiomas were WHO grade 1 tumors (99.7%). The pooled mean marginal dose in the studies was 13.6Gy (range 12-15.2Gy) while the mean of maximum doses was 28.6Gy (range 25-35Gy). Most studies report an excellent radiosurgical outcome including the tumor control rate and the progression-free survival [PFS] of over 90%. The tumor control, PFS, and adverse radiation effect [ARE] rates in author's series were 92.3%, 91%, and 9.6%, respectively. The favorable radiosurgical outcome depends on multiple factors such as small tumor volume, absence of previous radiotherapy, tumor location, elderly patients, female gender, longer time from symptom onset, and decreasing maximal dose. GKRS as primary or adjuvant treatment modality needs to be considered as a promising management strategy for PCFM in selected patients in view of the growing evidence of high tumor control rate, improved neurological functions, and low incidence of ARE. The use of multiple isocenters, 3-D image planning, and limit GKRS treatment to tumors less than 3.5cm help to avoid complications and achieve the best results. The treatment decisions in PCFM cases must be tailored and should consider the factors such as radiological profile, symptom severity, performance level, and patient preference for a good outcome.

Total marrow irradiation for hematopoietic malignancies using volumetric modulated arc therapy: A review of treatment planning studies.

Total Marrow Irradiation (TMI) has been introduced in the management of hematopoietic malignancies with the aim of reducing toxicities induced by total body irradiation. TMI is one of the most challenging planning and delivery techniques of radiotherapy, as the whole skeleton should be irradiated, while sparing nearby organs at risk (OARs). Target volumes of 7–10 k cm3 and healthy tissue volumes of 50–90 k cm3 should be considered and inverse treatment planning is needed. This review focused on aspects of TMI delivery using volumetric modulated arc therapy (VMAT). In particular, multiple arcs from isocenters with different positions are required for VMAT-TMI as the cranial-caudal lengths of patients are much larger than the jaw aperture. Therefore, many field junctions between arcs with different isocenters should be managed. This review covered, in particular, feasibility studies for managing multiple isocenters, optimization of plan parameters, plan optimization of the lower extremities, robustness of field junctions and dosimetric plan verification of VMAT-TMI. This review demonstrated the possibility of VMAT in delivering TMI with multi-arcs and multi-isocenters. Care should be paid in the patient repositioning, with particular attention to the cranial-caudal direction.

Application of failure mode and effects analysis to optimization of linac quality controls protocol.

To apply Failure Mode and Effects Analysis (FMEA) to optimize linac quality control (QC) protocol in order to ensure patient safety and treatment quality, taking maximum advantage of the available resources. Each parameter tested by the QC was considered as a potential failure mode (FM). For each FM, likelihood of occurrence (O), severity of effect (S), and lack of detectability (D) were evaluated and corresponding Risk Priority Number (RPN) was calculated from the product of three indexes. The scores were assigned using two methods: (a) A survey submitted to the medical physicists; (b) A semi-quantitative analysis (SQA) performed through: simulation of FMs in the treatment planning system; studies reported in literature; results obtained by the QC data analysis. A weighted RPN for all FMs was calculated taking into account both the methods. For each linac, the tests were then sorted by their frequency and the RPN value. A high variability was found in the scores of the survey, although in many it was reduced in RPN values, highlighting the more relevant tests as on beam output and imaging system. Integrating these results with those obtained by SQA, the RPN-based ranking of tests has been provided considering the specific use of the accelerator: for example, more accurate tests on dose modulation and multileaf collimator speed were required in linacs where intensity-modulated treatment is performed, while, more specific tests on couch and jaw position indicators were necessary where treatments with multiple isocenters and/or junctions between adjacent fields were often delivered. Failure Mode and Effects Analysis is a useful tool to prioritize the linac QCs, taking into account the specific equipment and clinical practice. The integration of SQA and survey results reduces subjectivity of the FMEA scoring and allows to optimize linac QCs without "losing" the expertise and experience of medical physicists and clinical staff.

147. Setup accuracy in Lung Stereotactic Body Radiation Therapy (SBRT): Single lesion versus multiple lesions

Purpose To analyse setup accuracy for patients we use Octavius® (PTW Freiburg) 4D checks and post-treatment off-line images provided by Varian Aria (v.13.5). We focused attention on lung SBRT treatments with 6XFFF MV arcs delivered by Trilogy LINAC (Varian). An interesting case for quality assurance and SWOT (Strengths/Weaknesses/Opportunities/Threats) analysis is represented by multiple or bilateral diseases, that need more than one isocenter. The Aria system gives instructions to linac to manage plans with two isocenters as if dose distributions are disjointed. We would therefore like to avoid a dose increase between multiple isocenters (Threat). Methods Image Guided RT (IGRT) is particularly useful when high tumoricidal doses are prescribed with highly conformal treatment modalities, including SBRT [1] . We retrospectively evaluated 55 consecutive patients that underwent Cone Beam Computed Tomography (CBCT) before each SBRT session and receive high dose in few fractions. Pre-treatment checks are evaluated with γ(3%;3 mm)-index. Forced compression method using a plate to compress abdomen is used in the 96% of treatments, in others a thermoplastic orfit is customized. We collect vertical, lateral and longitudinal shifts of the isocenter to calculate the 3D vector, defined as E = vert 2 + long 2 + lat 2 session by session [2] . Results γ (3%;3 mm) is always 93%, instead the median E vector is 0,71 cm for a total of 44 treatments with single-isocenter and 0,36 cm for 27 multi-isocenter plans. An assessment of the dose changes of the recalculated plans on calibrated CBCT is underway preliminary results are shown in the table. Displayed dynamic range is optimal for all the ultrasound scanners (coefficient of determination R2 ⩾ 95%) and the maximum value is ABUS and Samsung score of 99%. Distance measures are coherent, there is no geometric distortion. The analysis of the Point Spread Function is underway with advanced mathematical methods. Conclusions Even if the dose from CBCT for patients with multi-isocenter plans increases proportionally to the number of isocenters, we can say that the quality of positioning seems to have really improved.

Could the Jawless Flattening-Filter-Free Double-Mlc Layer Machine Reduce Low-Dose Regions Compared to Conventional Linear Accelerator for Large-Field Pelvic and Abdominal IMRT/VMAT Cases?

For large-field static IMRT/VMAT treatment plans for pelvic & abdominal targets, multi-leaf collimator (MLC) leakage is a dominant factor that contributes to low-dose regions, and is related to maximum jaw opening and/or multiple isocenters, and field-size dependent MLC leakage. The goal of this study was to evaluate the impact of jawless flattening-filter-free (FFF) double-MLC machine recently introduced in the clinic on low-dose regions for large-field pelvic & abdominal targets and to compare its delivery efficiency with conventional linear accelerators using total monitor unit (MU) as a metric. Ten challenging clinical pelvic & abdominal cases were retrospectively selected and treatment plans were generated using commercial treatment planning software for the jaw-less FFF double-MLC layer machine, and these research plans were comparable to clinical plans in terms of target coverage and Organ-at-Risk (OAR) doses. Prescription dose was 45Gy in 25 fx in all but 3 replanned cases. The cases were all large-field IMRT/VMAT cases with the median target size of 1713 cm3 (max=2269 cm3, SD=505 cm3). To minimize any subjective bias, static IMRT or VMAT techniques with the same arcs or gantry angles, which were identical to the clinical plans, were used for dose calculation. After achieving comparable target coverage as well as OAR doses, low-dose regions (10%, 20%, & 30% of prescription dose) and total MU were compared between two different plans. Target coverages were found to be over 95% with the median values of 96.3% for clinical plans and 97.3% for this study, and doses for various OARs were mostly comparable between clinical plans and this study. The median total MU was 684(SD=227) for the clinical plans and 878(SD=287) for this study. The higher MU for this study was related to the limitation of maximum field size for the jawless double-MLC layer machine; two isocenters had to be used for three cases due to the limited maximum field size of 28cm. Since the separation of two isocenters for the jawless machine should be within 8cm apart, total number of arcs or fields for the jawless machine were twice as that of clinical plans for three out of 10 clinical cases. The median value of low-dose region (10% of prescription dose) was 48.4% (SD=11.8%) for clinical plans vs. 48.6% (SD=11.7%) for this study, and the ratios of low-dose regions (10%, 20%, & 30%) between clinical plans and this study were 1.03(SD=0.04), 1.01(SD=0.03), and 0.97(SD=0.07), respectively. The comparable low-dose regions for this study resulted from the double-MLC layer features, which shows the extremely low measured MLC leakage rate of 0.1% For large-field static IMRT/VMAT cases, low-dose regions (10%, 20%, & 30% of prescription dose) for the jawless FFF double-MLC layer machine were comparable to those of conventional machines.