Cranial Radiosurgery Research Articles

709: Exploring Varian RapidPlan models for prostate, cranial radiosurgery, (SRS), and lung SBRT

709: Exploring Varian RapidPlan models for prostate, cranial radiosurgery, (SRS), and lung SBRT

Evaluation of plan quality and treatment efficiency in cranial stereotactic radiosurgery treatment plans with a variable source-to-axis distance.

Radiotherapy deliveries with dynamic couch motions that shorten the source-to-axis distance (SAD) on a C-arm linac have the potential to increase treatment efficiency through the increase of the effective dose rate. In this investigation, we convert clinically deliverable volumetric modulated arc therapy (VMAT) and dynamic conformal arc (DCA) plans for cranial radiosurgery into virtual isocenter plans through implementation of couch trajectories that maintain the target at a shortened but variable SAD throughout treatment. A randomly sampled population of patients treated with cranial radiosurgery from within the last three years were separated into groups with one, two, and three lesions. All plans had a single isocenter (regardless of number of targets), and a single prescription dose. Patient treatment plans were converted from their original delivery at a standard isocenter to a dynamic virtual isocenter in MATLAB. The virtual isocenter plan featured a variable isocenter position based upon the closest achievable source-to-target distance (referred to herein as a virtual source-to-axis distance [vSAD]) which avoided collision zones on a TrueBeam STx platform. Apertures were magnified according to the vSAD and monitor units at a given control point were scaled based on the inverse square law. Doses were calculated for the plans with a virtual isocenter in the Eclipse (v13.6.23) treatment planning system (TPS) and were compared with the clinical plans. Plan metrics (MU, Paddick conformity index, gradient index, and the volume receiving 12 Gy or more), normal brain dose-volume differences, as well as maximum doses received by organs at risk (OARs) were assessed. The values were compared between standard and virtual isocenter plans with Wilcoxon Sign Ranked tests to determine significance. A subset of the plans were mapped to the MAX-HD anthropomorphic phantom which contained an insert housing EBT3 GafChromic film and a PTW 31010 microion chamber for dose verification on a linac. Delivering plans at a virtual isocenter resulted in an average reduction of 20.9% (p = 3×10-6 ) and 20.6% (p = 3.0×10-6 ) of MUs across all VMAT and all DCA plans, respectively. There was no significant change in OAR max doses received by plans delivered at a virtual isocenter. The low dose wash (1.0-2.0 Gy or 5-11% of the prescription dose) was increased (by approximately 20 cc) for plans with three lesions. This was equivalent to a 2.7%-3.8% volumetric increase in normal tissue receiving the respective dose level when comparing the plan with a virtual isocenter to a plan with a standard isocenter. Gamma pass rates with a 5%/1mm analysis criteria were 96.40% ± 2.90% and 95.07% ± 3.10% for deliveries at standard and virtual isocenter, respectively. Absolute point dose agreements were within -0.36% ± 3.45% and -0.55% ± 3.39% for deliveries at a standard and virtual isocenter, respectively. Potential time savings per arc were found to have linear relationship with the monitor units delivered per arc (savings of 0.009 s/MU with an r2 = 0.866 when fit to plans with a single lesion). Converting clinical plans at standard isocenter to a virtual isocenter design did not show any losses to plan quality while simultaneously improving treatment efficiency through MU reductions.

HSR22-135: Telemedicine for Cranial Radiosurgery Patients in a Rural US Population: Patterns and Predictors of Patient Utilization

HSR22-135: Telemedicine for Cranial Radiosurgery Patients in a Rural US Population: Patterns and Predictors of Patient Utilization

Telemedicine for Cranial Radiosurgery Patients in a Rural U.S. Population: Patterns and Predictors of Patient Utilization

Introduction: Telemedicine retains potential for increasing access to specialty providers in underserved and rural communities. COVID-19 accelerated adoption of telehealth beyond rural populations, serving as a primary modality of patient-provider encounters for many nonemergent diagnoses. Methods: From 2020 to 2021, telemedicine was incorporated in management of stereotactic radiosurgery patients. Retrospective data on diagnoses, demographics, distance to primary clinic, and encounter type were captured and statistically analyzed using descriptive measures and Cox proportional regression modeling. Graphical representation of service areas was created using geo-mapping software. Results: Patients (n = 208) completed 331 telemedicine encounters over 12 months. Metastases and meningiomas comprised 60% of diagnoses. Median age was 62 years with median household income and residential population of $44,752 and 7,634 people. The one-way mean and median travel distances were 74.6 and 66.3 miles. The total potential road mileage for all patients was 44,596 miles. A total of 118 (57%) patients completed video visits during the first encounter, whereas 90 (43%) opted for telephone encounters. At 12 months, 138 patients (66%) utilized video visits and 70 (34%) used telephone visits. Predictors of video visit use were video-enabled visit during the first encounter (hazard ratio [HR] 2.806, p < 0.001), total potential distance traveled (HR 1.681, p < 0.05), and the need for more than one visit per year (HR 2.903, p < 0.001). Discussion: Telemedicine can be effective in radiosurgery practice with predictors of video-enabled use being pre-existing patient comfort levels with videoconferencing, total annual travel distance, and number of visits per year. Age, rural population status, and household income did not impact telemedicine use in our patient cohort.

Impact of MRI Sequence Resolution for Target Volume Definition in Stereotactic Radiosurgery

MRI is a standard image modality in cranial radiosurgery (SRS) for defining the target volume. There is disparity with different MRI sequences however due to differences in acquisition resolution, which can directly impact accuracy of target segmentation. Here we acquired MRI using different sequences on phantoms to evaluate the effect on volume delineation in the context of cranial SRS.Four different T1-weight MR pulse sequences were included: (1) SE5mm: axial and coronal Spin Echo (SE) 2D acquisition with 0.4 × 0.4 mm2 in-plane resolution and 5 mm cross-plane; (1) SE3mm: axial and coronal SE with 0.5 × 0.5 mm2 in-plane and 3 mm cross-plane; (3) TFE: gradient echo 3D acquisition with 0.92 × 0.92 mm2 in axial plane and 1.25 mm cross-plane; (4) BRAVO: gradient echo 3D with 0.4 × 0.4 mm2 in axial plane and 0.5 mm cross-plane. Four phantoms with different shape and volume (0.54 to 25 cm3) were imaged, resulting in 16 sets of MRIs. Four radiation oncologists provided contours on individual sets of MRIs. All observer contours were compared with ground truth, which was defined on CT image according to the absolute dimensions of the phantom structures. Dice coefficients (DSC), Hausdorff distance (MaxHD, MeanHD, σHD) as well as ratio between reconstructed and true volume (Ratiovol) were evaluated between observer and ground truth contours. A two-sided signed-rank test was performed to determine whether differences were statistically significant.The comparison to ground truth as a function of MR sequence is presented in the Table. All indices improved as the MR resolution increased from SE5mm to BRAVO. All differences, with the exception of Ratiovol between SE3mm and TFE, were statistically significant (P < 0.01). Inter-observer variation which must be considered was likely due to: (1) differences in user window/level preferences and related impact on visualization of target boundaries; (2) discrepancies in visualization at the superior/inferior aspect of the target. The first factor leads to systematic over- or under-estimation of the target volume. The second factor affects only SE5mm and SE3mm and leads to over- or under-estimation at superior/inferior aspect of the target. Both factors have ramification in clinical SRS treatment planning.Significant improvement in target definition was observed as the MR image resolution improved. Results imply that the highest resolution 3D MR sequences should be used to minimize potential errors in target definition, and multi-slice 2D sequence should be avoided.

Comparison of VMAT complexity-reduction strategies for single-target cranial radiosurgery with the Eclipse treatment planning system.

Complexity in MLC‐based radiosurgery treatment delivery can be characterized by the efficiency of monitor unit (MU) utilization and the average MLC leaf separation distance for a treatment plan. A reduction in plan complexity may be desirable if plan quality is not impacted. In this study, a number of strategies are explored to determine how plan quality is affected by efforts to reduce plan complexity. Ten radiosurgery cases of varying complexity are retrospectively planned using six optimization strategies: an unconstrained volumetric modulated arc therapy (VMAT) technique, a MU‐constrained VMAT technique, three techniques using various strengths of the aperture shape controller (ASC), and a hybrid technique consisting of a final‐stage VMAT optimization applied to a dynamic conformal arc leaf sequence (ODCA). The plans are compared in terms of MU efficiency, MLC leaf‐separation, conformity index (CI), gradient index (GI), and QA measurement results. The five VMAT techniques exhibited only minor differences in CI and GI values, though the ASC and MU‐constrained techniques did require 6–20% fewer MU and had mean field apertures 5–19% larger. On average, the ODCA technique had CI values 3.5% lower and GI values 1.0–2.5% higher than the VMAT techniques, but also had a mean field aperture 24–47% larger and required 16–32% fewer MU. The QA measurement results showed a 0.61% variation in mean per‐field 2%/1 mm gamma passing rates across all techniques (range 96.81%–97.42%), with no observed correlation between passing rate and technique. For simple targets, the ODCA technique achieved CI results that were equivalent to the unconstrained VMAT technique with an average 30% reduction in required MU, an average 50% increase in mean leaf separation distance, and brain V12Gy values within 0.38 cc of the VMAT technique for targets up to approximately 2 cm diameter. For MLC‐based single‐target radiosurgery, plan complexity can often be significantly reduced without an equivalent reduction in plan quality.

Radiotherapy for Melanoma: More than DNA Damage.

Despite its reputation as a radioresistant tumour, there is evidence to support a role for radiotherapy in patients with melanoma and we summarise current clinical practice. Melanoma is a highly immunogenic tumour and in this era of immunotherapy, there is renewed interest in the potential of irradiation, not only as an adjuvant and palliative treatment, but also as an immune stimulant. It has long been known that radiation causes not only DNA strand breaks, apoptosis, and necrosis, but also immunogenic modulation and cell death through the induction of dendritic cells, cell adhesion molecules, death receptors, and tumour-associated antigens, effectively transforming the tumour into an individualised vaccine. This immune response can be enhanced by the application of clinical hyperthermia as evidenced by randomised trial data in patients with melanoma. The large fraction sizes used in cranial radiosurgery and stereotactic body radiotherapy are more immunogenic than conventional fractionation, which provides additional radiobiological justification for these techniques in this disease entity. Given the immune priming effect of radiotherapy, there is a strong but complex biological rationale and an increasing body of evidence for synergy in combination with immune checkpoint inhibitors, which are now first-line therapy in patients with recurrent or metastatic melanoma. There is great potential to increase local control and abscopal effects by combining radiotherapy with both immunotherapy and hyperthermia, and a combination of all three modalities is suggested as the next important trial in this refractory disease.

EP-2181 Use of treatment log-files for QA of cranial radiosurgery adaptive plans

EP-2181 Use of treatment log-files for QA of cranial radiosurgery adaptive plans

EP-2071 Redefining the classical Winston-Lutz test for cranial radiosurgery in terms of dose

EP-2071 Redefining the classical Winston-Lutz test for cranial radiosurgery in terms of dose

Intrafraction Verification Using ExacTrac 6D Imaging for Frameless Cranial Radiosurgery

Background: Radiosurgery has been an integral component in the management of intracranial tumors. It involves the administration of a high dose to the tumor. An essential aspect of this kind of treatment delivery is accurate localization of the tumor target that is precisely reproducible during treatment. To achieve this, the methods of positioning and immobilization of the patient play a critical role. Conventionally, the patient has been immobilized with invasive head frame and the location of internal anatomy deduced by external coordinates. This method has some drawbacks. - It works on the assumption that the external coordinates represent the isocentre location correctly. - It depends on the spatial relationships between the frame and the skull. Slippage and deformity of the frame, although rare, can cause an error in positioning. - From the patient's perspective, the placement of the frame is a source of discomfort. It also puts them at risk for bleeding and infection. Frameless SRS has come into use in recent times. This is a noninvasive method. However, giving up the use of an invasive head frame leads to increased intrafraction motion. To overcome this, we require constant verification of the patient's positioning at every step of the treatment. Aim: The objective of this study is to compare the precision and accuracy between Brainlab ExacTrac imaging and CBCT and evaluate the intrafraction motion of patient undergoing intracranial SRT. Methods: For this study we included 10 patients with intracranial tumors who are eligible to undergo SRS. A frameless stereotactic head frame mask was prepared to immobilize the head during SRS. During treatment the patients were immobilized with the same frame and the 6D positional accuracy was checked for every single field before the treatment execution by taking verification image using 6D ExacTrac and Onboard Imagers. The 6D ExacTrac tolerance were set to rotation &lt; 1.00 and translational shift &lt; 0.7 mm. We make sure of intrafarction movement by taking frequent ExacTrac images during the treatment of every 30 s. After the treatment the registration between the planning CT image and CBCT image taken before treatment were cross verified 6 dimensionally using offline review. The difference in shift between ExacTrac and CBCT are evaluated by means of RMS. Results: The RMS of the difference in the translational dimensions between the ExacTrac System and CBCT was &lt; 1.0 for online matching and &lt; 1.10 for offline matching. Furthermore, the RMS of the difference in the rotational dimensions between the ExacTrac System and CBCT were &lt; 0.81 for online matching and &lt; 0.96 for offline matching. Conclusion: It is concluded that the discrepancies in residual set-up error between the ExacTrac 6D x-ray and the CBCT were minor. ExacTrac 6D imaging provides additional advantage of patient intrafraction movement control by means of imaging during treatment.

Patient-specific quality control for stereotactic cranial radiosurgery

Patient-specific quality control (PSQC) has been carried out for 25 cranial radiosurgery plans delivered using a TrueBeam STx (Varian, Palo Alto, CA) equipped with an ExacTrac (BrainLab, Munich, Germany) imaging system. The StereoTactic End-to-End Verification (STEEV) phantom (CIRS Inc., Norfolk, VA) was used to measure both point doses using a microDiamond detector (PTW, Freiburg, Germany) and 2D dose planes using gafchromic type EBT-XD film (Ashland, NJ, USA). The Octavius 4D and 1000SRS array (PTW, Freiburg, Germany) was used to measure 3D dose distributions. Point doses were recorded using the planned couch positions and then with the couch angle reset to zero degrees to remove the influence of directional dependence of the microDiamond. The mean difference between measured and calculated point doses was −3.2 ± 1.1% using planned couch positions and −1.8 ± 0.9% with the influence of couch angle removed. The mean difference in the point dose measured using the Octavius was −1.1 ± 0.7% and the mean gamma pass rate for 2%/1 mm criteria (10% threshold) was 99.4 ± 0.7%. The mean pass rate using 5%/1 mm global gamma criteria for the film analysis was 97.1 ± 3.4% with only 1 result below 90%. The mean pass rate using 3%/2 mm local gamma (10% threshold) was 82.3 ± 12.5% with 5 results less than 70% however all these consisted of plans with average equivalent square field sizes of

Dosimetric effects of incorrect jaw settings in cranial radiosurgery

A small circular field used in stereotactic radiosurgery is obtained using a cone accessory attached to a LINAC. Such beam modification requires that the LINAC jaws be set to a specific size, an adjustment which, in many cases, must be explicitly made by the LINAC operator. Given that treatment planning systems are commissioned with correct jaw settings, it is difficult to quantify the leakage dose to the patient when the jaw setting is larger than recommended. However, there are documented cases indicating that the resulting overexposure can lead to long-term disability and death. This study quantifies the dose differences between correct and incorrect jaw settings by simulating a realistic treatment plan using a Varian LINAC with a BrainLab circular cone accessory. Using Monte Carlo methods, the details of the LINAC head and cone accessory were simulated, with calculated doses benchmarked against measurements. Calculations were based on a realistic treatment plan for right side trigeminal neuralgia delivering 90 Gy using a 6 MV (4 mm cone) beam with 7 arcs. The calculated doses to target and normal tissue with jaws set correctly (5 × 5 cm2) and incorrectly (settings ranging from 10 × 10 cm2 to 20 × 20 cm2) were then compared. Incorrect jaw settings resulted in an increase in the target dose of less than 6%. This implies that pre-treatment QA based only on point dose measurements at the isocenter will not detect jaw setting errors. However, extremely high doses (>25 Gy) occurred 5 cm away from the isocenter. For example, dose to 50% of the volume, D50, in the eyes increased from 15 cGy with the correct jaw setting to 5318 cGy with an incorrect jaw setting of 20 × 20 cm2. D50 in the brain increased from 27 cGy with the correct jaw setting to 4335 cGy for a setting of 20 × 20 cm2. The leakage radiation is shown to originate beyond the outer edge of the beam limiting cone (outer radius = 5 cm). Leakage dose can reach 15%, 40%, and 60% of target dose when incorrect jaw settings of 10 × 10 cm2, 15 × 15 cm2 and 20 × 20 cm2 are used in the beam delivery.

Characterization and Dosimetric Effects of Intra-fraction Motion During Cranial Radiosurgery

The purpose of this study is to retrospectively review intra-fraction patient motion during image-guided cranial radiosurgery and evaluate the effects of the intra-fraction motion on the delivered dose for the subset of patients with motion greater than one millimeter. Patient alignment data were collected for all consecutive single and five fraction radiosurgery treatments at our institution during an approximately one-year period (n=1016 radiographs). These data, comprised of three-dimensional coordinates were recorded at initial alignment, at intervals during the treatment, and at treatment completion. The differences between the initial alignment and the subsequent radiographs represent patient motion. In cases with motion greater than one millimeter in any orthogonal direction, an automated script was used to simulate delivered treatment, based on the prescribed treatment plan and a shift in patient position based on the measured patient motion. The delivered treatment plans were then analyzed based on their conformity (ratio of volume at prescription dose over total planned target volume), heterogeneity (max dose over prescription dose), and gradient (volume at 50% prescription dose over volume at prescription dose) indices. 73 patients received 270 1-5 fraction radiosurgery treatments. 3D magnitude error between initial alignment and subsequent films was 0.4 mm ± 0.3 mm. 2.5% of shifts were greater than 1 mm, and 0.2% greater than 2 mm. There were 8 treatment fractions with errors greater than 1 mm in at least one direction, some with multiple radiographs greater than 1 mm. Among these 8 treatments, the median motion magnitude was 1.27 ± 0.31 mm. For seven out of the eight cases, intra-fraction motion changed conformality index by less than 5%. In one case 75% of the delivered treatment was altered by intra-fraction motion with a 3D magnitude value of 1.86 mm. This motion resulted in a change in the planned conformity, heterogeneity, and gradient indices from 3.13, 1.18, and 5.83, respectively, to 0.24, 1.04, and 71.01. On clinical follow-up, this patient had a partial response. In the other 7 cases, there was one case of radiation necrosis with the balance achieving partial or complete responses. Clinically relevant intra-fraction motion during image-guided cranial radiosurgery was evaluated for all consecutive patients during an approximately one-year period. 2.5% of intrafraction shifts were greater than 1 mm, and 0.2% greater than 2 mm. Shifts greater than 1 mm in a given direction were analyzed, and found to have limited impact on the dose distribution and clinical followup.

Frameless Volumetric Intracranial Stereotactic Radiosurgery with Non-coplanar Arcs: Clinical Experience, Accuracy, and Dosimetric Evaluation

In the last decades technological advances are increasing with the development of new treatment techniques and ways to deliver cranial radiosurgery(SRS), that is based into precision and accuracy, Volumetric Arc Therapy (VMAT) have been used in brain radio surgery in terms of non coplanar rotational arc beams with the aid of multi-leaf(MLC) to provide beam collimation. Using the traditional Leksell Stereotactic System, the time to do a treatment in average was around 6 hours; however the workflow using a frameless system decreases the mean treatment time in less than an hour, turning out to be a resources optimization for its realization, all this with the help of all accessories, quality assurance that should be made before deliver a proper treatment. The goal of this study is to show our clinical experience doing frameless SRS, with VMAT technique in the treatment of Arteriovenous Malformations(AVMs), Brain Metastases, Meningiomas, among others. The issues discussed in this study with regard to not only the clinical features, but also beam characteristics, and dosimetric features. 20 patients have been treated since May up to September 2016 with different diagnostics, doses given are related to each pathology treated, each prescription isodose was higher than 96%. Frameless system used was trUpoint ARCH™ SRS/SRT System, the Immobilization is accomplished utilizing multiple immobilization components - custom head support, bite tray, thermoplastic mask, and nasion cushion - allowing a truly custom hold from CIVCO, Patients were simulated in Computed tomography (CT) simulation General Electric(GE) Optima model, slides acquisition 1.25 mm; Magnetic Resonance was also done in a Siemens de 1.5T with 1 mm slices in contrast enhanced T1 MPR, T2 Flair, T2 Ciss, Diffusion, Perfusion, DTI Tractography, Angiography was done in GE Discovery; for image fusion for PTV and OAR contouring; calculation were done in Monaco® planning system version 5.10.02 with Monte Carlo algorithms; treatment delivery were make in a LINAC Elekta Infinity™ with Agility™ head with 160 interdigitating leaves with 0.5 cm width at isocenter; positioning verification XVI versión 4.5.1 b141. Treatment Delivery time 22 +/- 1.30 minutes for VMAT. Mean Monitor Units (MU) were 3795; For each patient IGRT is made with 3 XVI (Cone Beam CT), Pre and Post Treatment, what makes the procedure safe, confortable and accurate. With regard to CIRTOG a 0.9825 was obtained in average and QA was made using the OCTAVIUS® 1000 SRS with a Gamma Index 3mm/3% over 96%. Data found in this study with regard to the 20 patients treated; suggest that Frameless arch design provides a secure hold with no surgical attachment to the patient, displacements are less than 1 mm of patient movement permitted during intrafraction immobilization testing; also is safe, confortable and easy to use even allows us to rapid response for anxious patients with its release mechanism. Less treatment times and also excellent clinical outcomes.

Adaptation and validation of a commercial head phantom for cranial radiosurgery dosimetry end-to-end audit.

To adapt and validate an anthropomorphic head phantom for use in a cranial radiosurgery audit. Two bespoke inserts were produced for the phantom: one for providing the target and organ at risk for delineation and the other for performing dose measurements. The inserts were tested to assess their positional accuracy. A basic treatment plan dose verification with an ionization chamber was performed to establish a baseline accuracy for the phantom and beam model. The phantom and inserts were then used to perform dose verification measurements of a radiosurgery plan. The dose was measured with alanine pellets, EBT extended dose film and a plastic scintillation detector (PSD). Both inserts showed reproducible positioning (±0.5 mm) and good positional agreement between them (±0.6 mm). The basic treatment plan measurements showed agreement to the treatment planning system (TPS) within 0.5%. Repeated film measurements showed consistent gamma passing rates with good agreement to the TPS. For 2%-2 mm global gamma, the mean passing rate was 96.7% and the variation in passing rates did not exceed 2.1%. The alanine pellets and PSD showed good agreement with the TPS (-0.1% and 0.3% dose difference in the target) and good agreement with each other (within 1%). The adaptations to the phantom showed acceptable accuracies. The presence of alanine and PSD do not affect film measurements significantly, enabling simultaneous measurements by all three detectors. Advances in knowledge: A novel method for thorough end-to-end test of radiosurgery, with capability to incorporate all steps of the clinical pathway in a time-efficient and reproducible manner, suitable for a national audit.

Investigating the spatial accuracy of CBCT-guided cranial radiosurgery: A phantom end-to-end test study

PurposeTo evaluate the spatial accuracy of a frameless cone-beam computed tomography (CBCT)-guided cranial radiosurgery (SRS) using an end-to-end (E2E) phantom test methodology. Methods and materialsFive clinical SRS plans were mapped to an acrylic phantom containing a radiochromic film. The resulting phantom-based plans (E2E plans) were delivered four times. The phantom was setup on the treatment table with intentional misalignments, and CBCT-imaging was used to align it prior to E2E plan delivery. Comparisons (global gamma analysis) of the planned and delivered dose to the film were performed using a commercial triple-channel film dosimetry software. The necessary distance-to-agreement to achieve a 95% (DTA95) gamma passing rate for a fixed 3% dose difference provided an estimate of the spatial accuracy of CBCT-guided SRS. Systematic (∑) and random (σ) error components, as well as 95% confidence levels were derived for the DTA95 metric. ResultsThe overall systematic spatial accuracy averaged over all tests was 1.4mm (SD: 0.2mm), with a corresponding 95% confidence level of 1.8mm. The systematic (Σ) and random (σ) spatial components of the accuracy derived from the E2E tests were 0.2mm and 0.8mm, respectively. ConclusionsThe E2E methodology used in this study allowed an estimation of the spatial accuracy of our CBCT-guided SRS procedure. Subsequently, a PTV margin of 2.0mm is currently used in our department.

The spatial accuracy of two frameless, linear accelerator-based systems for single-isocenter, multitarget cranial radiosurgery.

Single‐isocenter, multitarget cranial stereotactic radiosurgery (SRS) is more efficient than using an isocenter for each target, but spatial positioning uncertainties can be magnified at locations away from the isocenter. This study reports on the spatial accuracy of two frameless, linac‐based SRS systems for multitarget, single‐isocenter SRS as a function of distance from the isocenter. One system uses the ExacTrac platform for image guidance and the other localizes with cone beam computed tomography (CBCT). For each platform, a phantom with 12 target BBs distributed up to 13.8 cm from the isocenter was aligned starting from five different initial offsets and then imaged with the treatment beam at seven different gantry and couch angles. The distribution of the resulting positioning errors demonstrated the value of adding a 1‐mm PTV margin for targets up to about 7–8 cm from the isocenter. For distances 10 cm or more, the CBCT‐based alignment remained within 1.1 mm while the ExacTrac‐based alignment differed by up to 2.2 mm.

EP-1656: Feasibility of an “off-target isocenter” technique for cranial intensity-modulated radiosurgery

EP-1656: Feasibility of an “off-target isocenter” technique for cranial intensity-modulated radiosurgery

EP-1544: Dose conformation evaluation of volumetric modulated arc therapy for cranial radiosurgery

EP-1544: Dose conformation evaluation of volumetric modulated arc therapy for cranial radiosurgery

Analysis of translational errors in frame-based and frameless cranial radiosurgery using an anthropomorphic phantom.

ObjectiveTo evaluate three-dimensional translational setup errors and residual errors in image-guided radiosurgery, comparing frameless and frame-based techniques, using an anthropomorphic phantom.Materials and MethodsWe initially used specific phantoms for the calibration and quality control of the image-guided system. For the hidden target test, we used an Alderson Radiation Therapy (ART)-210 anthropomorphic head phantom, into which we inserted four 5mm metal balls to simulate target treatment volumes. Computed tomography images were the taken with the head phantom properly positioned for frameless and frame-based radiosurgery.ResultsFor the frameless technique, the mean error magnitude was 0.22 ± 0.04 mm for setup errors and 0.14 ± 0.02 mm for residual errors, the combined uncertainty being 0.28 mm and 0.16 mm, respectively. For the frame-based technique, the mean error magnitude was 0.73 ± 0.14 mm for setup errors and 0.31 ± 0.04 mm for residual errors, the combined uncertainty being 1.15 mm and 0.63 mm, respectively.ConclusionThe mean values, standard deviations, and combined uncertainties showed no evidence of a significant differences between the two techniques when the head phantom ART-210 was used.