Radiosurgical Treatment Planning Research Articles

Ga68] DOTATATE PET/MRI-Guided Radiosurgical Treatment Planning and Response Assessment in Meningiomas.

Our purpose was to determine the utility of [68Ga]-DOTATATE PET/MRI in meningioma response assessment following radiosurgery. Patients with meningioma prospectively underwent postoperative DOTATATE PET/MRI. Co-registered PET and gadolinium-enhanced T1-weighted MRI were employed for radiosurgery planning. Follow-up DOTATATE PET/MRI was performed at 6-12 months post radiosurgery. Maximum absolute standardized uptake value (SUV) and SUV ratio (SUVRSSS) referencing superior sagittal sinus (SSS) blood pool were obtained. Size change was determined by Response Assessment in Neuro-Oncology (RANO) criteria. Association of SUVRSSS change magnitude and PFS was evaluated using Cox regression. 27 patients with 64 tumors (26% WHO-1, 41% WHO-2, 26% WHO-3, 7% WHO-unknown) were prospectively followed post stereotactic radiosurgery (SRS) or stereotactic body radiotherapy (SBRT) (mean dose: 30 Gy, modal dose 35 Gy, mean of 5 fractions). Post-irradiation SUV and SUVRSSS decreased by 37.4% and 44.4%, respectively (p < 0.0001). Size product decreased by 8.9%, thus failing to reach the 25% significance threshold as determined by RANO guidelines. Mean follow-up time was 26 months (range: 6-44). Overall mean PFS was 83% and 100%/100%/54% in WHO-1/-2/-3 subcohorts, respectively, at 34 months. At maximum follow-up (42-44 months), PFS was 100%/83%/54% in WHO-1/-2/-3 subcohorts, respectively. Cox regression analyses revealed a hazard ratio of 0.48 for 10-unit reduction in SUVRSSS in the SRS cohort. DOTATATE PET SUV and SUVRSSS demonstrated marked, significant decrease post radiosurgery. Lesion size decrease was statistically significant, however it was not clinically significant by RANO criteria. DOTATATE PET/MR thus represents a promising imaging biomarker for response assessment in meningiomas treated with radiosurgery.

Dosimetric comparison between RapidArc and HyperArc in hippocampal-sparing whole-brain radiotherapy with a simultaneous integrated boost

The HyperArc technique is known for generating high-quality radiosurgical treatment plans for intracranial lesions or hippocampal-sparing whole-brain radiotherapy (WBRT). However, there is no reported feasibility of using the HyperArc technique in hippocampal-sparing WBRT with a simultaneous integrated boost (SIB). This study aimed to compare dosimetric parameters of 2 commercially-available volumetric-modulated arc radiotherapy techniques, HyperArc and RapidArc, when using hippocampal-sparing WBRT with a SIB to treat brain metastases.Treatment plans using HyperArc and RapidArc techniques were generated retrospectively for 19 previously treated patients (1 to 3 brain metastases). The planning target volumes for the whole brain (excluding the hippocampal avoidance region; PTVWB) and metastases (PTVmet) were prescribed 25 and 45 Gy, respectively, in 10 fractions. Each plan included homogeneous and inhomogeneous delivery to the PTVmet. Dosimetric parameters for the target (conformity index [CI], homogeneity index [HI], target coverage [D95%]), and nontarget organs at risk were compared for the HyperArc and RapidArc plans.For homogeneous delivery, dosimetric parameters, including mean CI, HI, and target coverage in PTVWB and PTVmet, were superior for HyperArc than RapidArc plans (all p < 0.01). The PTVWB and PTVmet target coverage for HyperArc plans was significantly greater than for RapidArc plans (96.17% vs 93.38%, p < 0.01; 94.02% vs 92.21%, p < 0.01, respectively). HyperArc plans had significantly lower mean hippocampal Dmax and Dmin values than RapidArc plans (Dmax: 15.53 Gy vs, 16.71 Gy, p < 0.01; Dmin: 8.33 Gy vs 8.93 Gy, p < 0.01, respectively). Similarly, inhomogeneous delivery of hyperArc produced a superior target and lower hippocampal dosimetric parameters than RapidArc, except for the HI of PTVmet (all p < 0.01).HyperArc generated superior conformity and target coverage with lower hippocampal doses than RapidArc. HyperArc could be an attractive technique for hippocampal-sparing WBRT with an SIB.

P17.10.B VOLUMETRIC DIFFERENCES IN TARGET DEFINITON OF RECURRENT HIGH-GRADE GLIOMAS USING CONTRAST ENHANCED MRI VS.[18F] FDOPA PET IN RADIOSURGICAL PLANNING

Abstract BACKGROUND Radiosurgical treatment of high-grade gliomas usually bases target definition on contrast-enhanced T1-weighted MRI images. Despite advances in several MRI techniques, a number of limitations exist, primarily due to the disruption of the blood-brain barrier, making the target delineation less accurate, particularly when treating recurrences. Several studies support the use of amino acid PET/MRI in distinguishing progression from post-operative/post-radiation inflammation. However, no established guidelines define the biological target volume when using amino acid PET/MRI in radiosurgical treatment planning. We aimed to explore the volumetric differences in target volumes obtained with standard contrast-enhanced MRI versus [18F]FDOPA PET in patients with recurrent high-grade gliomas (rHGG). MATERIAL AND METHODS Ten patients obtained [18F]FDOPA PET/MRI within 1 week before radiosurgical treatment. We co-registered these images with the 3D MPRAGE-MRI typically used for stereotactic target definition and performed a follow-up PET/MRI scan 4-6 weeks after treatment. We reviewed and compared the method of tumor delineation, target volumes, and stereotactic accuracy in both modalities. RESULTS Radiosurgical treatment delivers high radiation doses to defined targets. The target volume and the proximity to eloquent structures limit the dose, thus depending on high-accuracy imaging. The invasive nature of malignant brain tumors makes anatomical delineation insufficient, particularly when treating recurrences. In such cases, amino acid PET supplies additional information. However, the spatial resolution afforded by [18F]FDOPA uptake alone is insufficient for stereotactic definition, which to a large degree is based on contrast-enhanced T1-weighted MRI images in order to achieve the sub-millimeter resolution needed for radiosurgical planning. Hybrid PET/MRI allows the combination of metabolic information of [18F]FDOPA-PET with the anatomical precision of stereotactic MRI. In the recurrent setting, [18F]FDOPA-PET supplies the necessary information for targeting the biologically active tumour, differentiating it from radiation necrosis, thus limiting the target volumes. This enables us to treat the recurrent tumours more efficiently. CONCLUSION [18F]FDOPA-PET is a useful supplement in radiosurgical planning and treatment of rHGG. It is helpful in the delineation of the biologically active tumor, thus limiting the target volume significantly. This enables higher radiation doses and potentially more efficient treatment. Further results will be presented.

Stratified Assessment of a Commercial Deep Learning Algorithm for Automated Detection and Contouring of Metastatic Brain Tumors in Stereotactic Radiosurgery

<h3>Purpose/Objective(s)</h3> Artificial intelligence-based tools can be leveraged to improve detection and segmentation of brain metastases for stereotactic radiosurgery (SRS). This study explores a deep learning algorithm with recent FDA clearance to assist in brain tumor contouring. In this study, we aimed to assess the performance of this tool by various demographic and clinical characteristics among patients with brain metastases treated with SRS. <h3>Materials/Methods</h3> We randomly selected 100 patients with brain metastases who underwent initial SRS on a frameless robotic radiosurgery system from 2017 to 2020 at a single institution. Cases with resection cavities were excluded from the analysis. Computed tomography (CT) and axial T1-weighted post-contrast magnetic resonance (MR) image data were extracted for each patient and uploaded to VBrain. A brain metastasis was considered "detected" when the VBrain "predicted" contours overlapped with the corresponding physician contours ("ground-truth" contours). We evaluated performance of VBrain against ground-truth contours using the following metrics: lesion-wise Dice similarity coefficient (DSC), lesion-wise average Hausdorff distance (AVD), false positive count (FP), and lesion-wise sensitivity (%). Wilcoxon rank-sum tests were performed to assess the relationships between patient characteristics including sex, race, primary histology, age, and size and number of brain metastases, and performance metrics such as DSC, AVD, FP, and sensitivity. <h3>Results</h3> We analyzed 100 patients with 422 intact brain metastases treated with SRS. Our cohort consisted of patients with a median number of 2 brain metastases (range: 1 to 47), median age of 79 (range: 19 to 91), and 50% male and 50% female patients. The primary site breakdown was 56% lung, 10% melanoma, 9% breast, 8% gynecological, 5% gastrointestinal, 2% prostate, and 5% other, while the race breakdown was 60% White, 18% Asian, 3% Black/African American, 2% Native Hawaiian or other Pacific Islander, and 17% other/unknown/not reported. The median tumor size was 0.118 c.c. (range: 0.003-28.838 c.c.). We found mean lesion-wise DSC to be 0.714, mean lesion-wise AVD to be 8.44% of lesion size (0.796 mm), mean FP count to be 0.17 tumors per case, and lesion-wise sensitivity to be 83.65% for all lesions. Moreover, mean sensitivity was found to be 98%, 91.93%, and 79.87% for lesions with diameter equal to and greater than 10 mm, 5 mm, and 3 mm, respectively. No other significant differences in performance metrics were observed across demographic or clinical characteristic groups. <h3>Conclusion</h3> A commercial deep learning algorithm showed promising results in segmenting brain metastases, with 98% sensitivity for metastases with diameters of 1 cm or higher. Performance declined for smaller lesions. As advances in imaging and radiosurgery enable the detection and treatment of smaller brain metastases, future work is ongoing to develop and improve these tools to assist in radiosurgical treatment planning.

Impact of 68Ga-DOTATOC PET/MRI on robotic radiosurgery treatment planning in meningioma patients: first experiences in a single institution.

OBJECTIVEFor stereotactic radiosurgery (SRS) planning, precise contouring of tumor boundaries and organs at risk is of utmost importance. Correct interpretation of standard neuroimaging (i.e., CT and MRI) can be challenging after previous surgeries or in cases of skull base lesions with complex shapes. The aim of this study was to evaluate the impact of 68Ga-DOTATOC PET/MRI on treatment planning for image-guided SRS by CyberKnife.METHODSThe authors retrospectively identified 11 meningioma treatments in 10 patients who received a 68Ga-DOTATOC PET/MRI prior to SRS. The planning target volume (PTV) used for the patients' treatment was defined as the reference standard. This was contoured by a treating radiosurgeon (RS0) using fused planning CT and PET/MRI data sets. The same tumors were then contoured by another experienced radiosurgeon (RS1) and by a less-experienced radiosurgeon (RS2), both blinded to PET data sets. A comparison of target volumes with focus on volume-based metrics and distance to critical structures was performed. RS1 and RS2 also filled in a questionnaire analyzing the confidence level and the subjective need for the implementation of PET data sets for contouring.RESULTSAnalysis showed a subjective personal preference for PET/MRI in all cases for both radiosurgeons, particularly in proximity to critical structures. The analysis of the planning volumes per physician showed significantly smaller RS2-PTV in comparison to RS1-PTV and to RS0-PTV, whereas the median volumes were comparable between RS1-PTV and RS2-PTV (median: RS0: 4.3 cm3 [IQR 3.4-6.5 cm3] and RS1: 4.5 cm3 [IQR 2.7-6 cm3] vs RS2: 2.6 cm3 [IQR 2-5 cm3]; p = 0.003). This was also reflected in the best spatial congruency between the 2 experienced physicians (RS0 and RS1). The percentage of the left-out volume contoured by RS1 and RS2 compared to RS0 with PET/MRI demonstrated a relevant left-out-volume portion in both cases with greater extent for the less-experienced radiosurgeon (RS2) (RS1: 19.1% [IQR 8.5%-22%] vs RS2: 40.2% [IQR 34.2%-53%]). No significant differences were detected regarding investigated critical structures.CONCLUSIONSThis study demonstrated a relevant impact of PET/MRI on target volume delineation of meningiomas. The extent was highly dependent on the experience of the treating physician. This preliminary study supports the relevance of 68Ga-DOTATOC PET/MRI as a tool for radiosurgical treatment planning of meningiomas.

A novel index for assessing treatment plan quality in stereotactic radiosurgery.

OBJECTIVEStereotactic radiosurgery (SRS) is characterized by high levels of conformity and steep dose gradients from the periphery of the target to surrounding tissue. Clinical studies have backed up the importance of these factors through evidence of symptomatic complications. Available data suggest that there are threshold doses above which the risk of symptomatic radionecrosis increases with the volume irradiated. Therefore, radiosurgical treatment plans should be optimized by minimizing dose to the surrounding tissue while maximizing dose to the target volume. Several metrics have been proposed to quantify radiosurgical plan quality, but all present certain weaknesses. To overcome limitations of the currently used metrics, a novel metric is proposed, the efficiency index (η50%), which is based on the principle of calculating integral doses: η50% = integral doseTV/integral dosePIV50%.METHODSThe value of η50% can be easily calculated by dividing the integral dose (mean dose × volume) to the target volume (TV) by the integral dose to the volume of 50% of the prescription isodose (PIV50%). Alternatively, differential dose-volume histograms (DVHs) of the TV and PIV50% can be used. The resulting η50% value is effectively the proportion of energy within the PIV50% that falls into the target. This value has theoretical limits of 0 and 1, with 1 being perfect. The index combines conformity, gradient, and mean dose to the target into a single value. The value of η50% was retrospectively calculated for 100 clinical SRS plans.RESULTSThe value of η50% for the 100 clinical SRS plans ranged from 37.7% to 58.0% with a mean value of 49.0%. This study also showed that the same principles used for the calculation of η50% can be adapted to produce an index suitable for multiple-target plans (Gη12Gy). Furthermore, the authors present another adaptation of the index that may play a role in plan optimization by calculating and minimizing the proportion of energy delivered to surrounding organs at risk (OARη50%).CONCLUSIONSThe proposed efficiency index is a novel approach in quantifying plan quality by combining conformity, gradient, and mean dose into a single value. It quantifies the ratio of the dose "doing good" versus the dose "doing harm," and its adaptations can be used for multiple-target plan optimization and OAR sparing.

RETRACTED ARTICLE: Application of Monte Carlo calculation method based on special graph in medical imaging

With the rapid development of medical imaging technology and information technology, digital medical image acquisition equipment is constantly updated, especially the rise of proton medical technology, which has higher requirements on human diseased cell imaging technology. In this paper, based on the image data acquired by the large aperture CT, the Monte Carlo calculation and analysis of edge-blurred electronic density images have carried out to achieve accurate diagnosis and provide a new method for efficient pathological treatment. This article made a preliminary study and analysis on the application of Monte Carlo method in radio surgical treatment planning. Monte Carlo method, which is known as random sampling or statistical test methods, can truly simulate the actual physical process, problem solving and actual in good compliance. It can be very successful results. Combined with the image processing method, this paper carries out the detection methods of image pre-processing, image segmentation, feature extraction, classification and recognition to the lesion area of human body to realize the effective recognition of the special lesion image area. Experimental results show that the edge of lesion area has obtained by image segmentation, contour extraction and Monte-Carlo calculation. Particle-space information has taken as input. Based on Monte Carlo method, the space-absorbed dose in phantom is analysed distributed. By using the computerized liver cancer diagnosis technology, the doctor’s working pressure and labour intensity are reduced, the doctor’s working efficiency is improved, the accuracy of the diagnosis is improved, the hospital saves many expenses and the patient’s burden is reduced. Therefore, the research of computer-aided diagnosis has important significance and application value.

Brainstem Dose Constraints in Nonisometric Radiosurgical Treatment Planning of Trigeminal Neuralgia: A Single-Institution Experience.

CyberKnife stereotactic radiosurgery (SRS) for trigeminal neuralgia (TGN) administers nonisometric, conformational high-dose radiation to the trigeminal nerve with risk of subsequent hypoesthesia. We performed a retrospective, single-institution review of 66 patients with TGN treated with CyberKnife SRS to compare outcomes from 2 distinct treatment periods: standard dosing (n= 38) and reduced dosing (n= 28). Standard and reduced dosing permitted a maximum brainstem dose of 45 Gy and 25 Gy, respectively, each with a prescription dose of 60 Gy. Primary and secondary outcomes were Barrow Neurologic Institute pain and numbness scores. Maximum brainstem dose, prepontine nerve length, and treatment history were recorded for their predictive contributions by logistic regression. After matching, patients in the standard dosing and reduced dosing groups were followed for a median of 25 months and 19.5 months, respectively. Mean trigeminal nerve length was 8.55 mm in the standard dosing group and 9.46 mm in the reduced dosing group. Baseline rates of poorly controlled pain were 97% and 88%, respectively, which improved to 23.4% and 8.3%, respectively (P < 0.001 for both). The baseline rates of bothersome numbness were null in both groups, and increased to 25% in the standard group (P= 0.006) and to 21% in the reduced group (P= 0.07). Regression analyses suggested that reduced brainstem exposure (P= 0.01), as well as a longer trigeminal nerve (P= 0.01), were predictive of durable pain control. These outcomes demonstrate that a lower maximum brainstem dose can provide excellent pain control without affecting facial numbness. Longer nerves may achieve better long-term outcomes and help optimize individual plans.

A hybrid algorithm for packing identical spheres into a container

Packing identical spheres occur in several commercial and industrial contexts, like automated radio-surgical treatment planning and in materials science for studying the dynamic behavior of granular material systems. In this paper, a hybrid algorithm is proposed for approximately solving the identical sphere packing problem. Given a set of identical spheres and a large container (open or spherical container), the goal of the problem is to find a smallest container that contains all spheres without overlapping between spheres and between spheres and the container. The proposed algorithm combines both particle swarm optimization and an efficient continuous local optimization procedure. The swarm optimization generates a series of diversified populations of particles whereas the continuous optimization serves either to repair the infeasibility of solutions or improve their qualities. The performance of the proposed algorithm is evaluated on a set of standard benchmark instances and its obtained results are compared to those reached by the more recent methods available in the literature. The experimental part shows that the proposed approach remains competitive, where it is able to reach 47 new upper bounds out of the 93 tested instances.

Analysis of Dose Distribution in the Heart for Radiosurgical Ablation of Atrial Fibrillation.

In a treatment planning study, radiosurgical treatment plans designed to produce lesions on the left atrium were created using two different methodologies. In one, structures in the heart (mitral valve and coronary arteries) were designated as critical structures while this was not done in the second plan. The treatment plans that were created were compared with standards for heart dose used when treating spine tumors. Although the dosage for the whole heart greatly exceeded the dose standards, when only the dose to the ventricles was considered, the plan where the mitral valve was spared was very close to the dose standards. The ventricles received a substantially higher dose in the plan where the mitral valve was not a critical structure. Although neither treatment plan was delivered, this study demonstrated the feasibility of treating the heart while minimizing dose to the ventricles.

Integration of navigated brain stimulation data into radiosurgical planning: potential benefits and dangers.

Radiosurgical treatment of brain lesions near motor or language eloquent areas requires careful planning to achieve the optimal balance between effective dose prescription and preservation of function. Navigated brain stimulation (NBS) is the only non-invasive modality that allows the identification of functionally essential areas by electrical stimulation or inhibition of cortical neurons analogous to the gold-standard of intraoperative electrical mapping. To evaluate the feasibility of NBS data integration into the radiosurgical environment, and to analyze the influence of NBS data on the radiosurgical treatment planning for lesions near or within motor or language eloquent areas of the brain. Eleven consecutive patients with brain lesions in presumed motor or language eloquent locations eligible for radiosurgical treatment were mapped with NBS. The radiosurgical team prospectively analyzed the data transfer and classified the influence of the functional NBS information on the radiosurgical treatment planning using a standardized questionnaire. The semi-automatized data transfer to the radiosurgical planning workstation was flawless in all cases. The NBS data influenced the radiosurgical treatment planning procedure as follows: improved risk-benefit balancing in all cases, target contouring in 0 %, dose plan modification in 81.9 %, reduction of radiation dosage in 72.7 % and treatment indication in 63.7 % of the cases. NBS data integration into radiosurgical treatment planning is feasible. By mapping the spatial relationship between the lesion and functionally essential areas, NBS has the potential to improve radiosurgical planning safety for eloquently located lesions.

Feasibility of integrating a multi-camera optical tracking system in intra-operative electron radiation therapy scenarios

Intra-operative electron radiation therapy (IOERT) combines surgery and ionizing radiation applied directly to an exposed unresected tumour mass or to a post-resection tumour bed. The radiation is collimated and conducted by a specific applicator docked to the linear accelerator. The dose distribution in tissues to be irradiated and in organs at risk can be planned through a pre-operative computed tomography (CT) study. However, surgical retraction of structures and resection of a tumour affecting normal tissues significantly modify the patient's geometry. Therefore, the treatment parameters (applicator dimension, pose (position and orientation), bevel angle, and beam energy) may require the original IOERT treatment plan to be modified depending on the actual surgical scenario. We propose the use of a multi-camera optical tracking system to reliably record the actual pose of the IOERT applicator in relation to the patient's anatomy in an environment prone to occlusion problems. This information can be integrated in the radio-surgical treatment planning system in order to generate a real-time accurate description of the IOERT scenario. We assessed the accuracy of the applicator pose by performing a phantom-based study that resembled three real clinical IOERT scenarios. The error obtained (2 mm) was below the acceptance threshold for external radiotherapy practice, thus encouraging future implementation of this approach in real clinical IOERT scenarios.

A method for scoring treatment time efficiency of Gamma Knife radiosurgical treatment plans for brain metastases

In Gamma Knife radiosurgery, an efficient plan is one that achieves dosimetric quality while minimizing treatment time. Although minimization of treatment time to improve throughput and benefit patient comfort is a common and important goal of radiosurgery planning, to date no studies have attempted to specifically quantify efficiency. The aim of this study was to define simple index to score efficiency, and by quantifying time savings achieved by replanning those cases identified as least efficient, so demonstrate the efficacy of the index. To quantify efficiency, it is necessary to determine treatment times expected for specified lesions. However, because of numerous case-specifics, efficiency cannot be quantified in terms of treatment times alone. This study defines a new quantity, the attenuation-corrected normalized treatment time-dose rate product, nTRP(corr), to account for differing dose rates, prescriptions, and attenuation. A plan efficiency index (PEI) is then defined for lesions of similar volume and shape in terms of expected and planned nTRP(corr). nTRP(corr) was retrospectively calculated for metastatic lesions of comparable shape. A curve fitted to data describing how nTRP(corr) typically varied with volume for these lesions was then used to determine expected nTRP(corr). For each lesion, PEI was calculated as the ratio of expected-to-planned nTRP(corr). Plans with the lowest PEI were replanned, with the aim of maintaining dosimetric quality while minimizing treatment time. Dosimetric quality was defined in terms of coverage, conformity, and gradient index. Statistical significance of differences between original and replans was quantified via paired t-tests. The mean(standard deviation) PEI of all reviewed lesions was 1.08(0.28). The 14 least efficient plans across the range of investigated volumes (45-19 800 mm(3)) had a mean PEI of 0.64, versus 1.18 when replanned (p < 0.0001). This corresponded to a mean(range) time saving of 42%(19%-62%), [29(8-52) min at date of treatment] with no statistically significant (p > 0.05) change in dosimetric quality. The PEI is a viable metric for identifying those plans that benefit from a more efficient planning strategy.

Clinical outcomes of brain metastases treated with Gamma Knife radiosurgery with 3.0 T versus 1.5 T MRI‐based treatment planning: Have we finally optimised detection of occult brain metastases?

The goal of this study was to determine if clinically relevant endpoints were changed by improved MRI resolution during radiosurgical treatment planning. Between 2003 and 2008, 200 consecutive patients with brain metastases treated with Gamma Knife radiosurgery (GKRS) using either 1.5 T or 3.0 T MRI for radiosurgical treatment planning were retrospectively analysed. The number of previously undetected metastases at time of radiosurgery, distant brain failures, time delay to whole brain radiotherapy (WBRT), overall survival and likelihood of neurological death were determined. Additional metastases were detected in 31.3% and 24.5% of patients at time of radiosurgery with 3.0 T and 1.5 T MRI, respectively (P = 0.27). Patients with multiple metastases at diagnostic scan were more likely to have additional metastases detected by 3.0 T MRI (P < 0.1). Median time to distant brain failure was 4.87 months and 5.43 months for the 3.0 T and 1.5 T cohorts, respectively (P = 0.44). Median time to WBRT was 5.8 months and 5.3 months for the 3.0 T and 1.5 T cohorts, respectively (P = 0.87). Median survival was 6.4 months for the 3.0 T cohort, and 6.1 months for the 1.5 T cohort (P = 0.71). Likelihood of neurological death was 25.3% and 16.7% for the 3.0 and 1.5 T populations, respectively (P = 0.26). The 3.0 T MRI-based treatment planning for GKRS did not appear to affect the likelihood of distant brain failure, the need for WBRT or the likelihood of neurological death in this series.

SU-E-T-618: Error Compensated Sparse Optimization for Fast Radiosurgery Treatment Planning.

Radiosurgical treatment planning requires a good approximation of the dose distribution which is typically computed on a high resolution grid. However, the resulting optimization problem is large, and leads to substantial runtime. We study a sparse grid approach, for which we estimate and compensate for the expected deviations from the bounds. We buildup an estimate of the hotspot error distribution by measuring the maximum dose deviation within a voxel for a large number of randomly generated beam configurations. This results in a conservative estimation of overdosage as a function of upper bound reduction for different grid sizes. We adjust the bounds for voxels inside the target volume (PTV) according to our estimation thus maintaining the likelihood of dose deviations within acceptable limits. The approach was applied to a prostate case, where the volumes of interest are large and close to each other. Our planning objective is a prescribed dose of 36.25 Gy to the 87% isodose. We employed constrained optimization to optimize the lower PTV bound on 2, 4, and 8mm isotropic grids. Results were computed on 1mm grid. The initial coverage was 93.7%, 92%, and 91%, and the volume exceeding the upper bound was 0.74%, 1.71%, and 9% for grid sizes of 2, 4, and 8mm, respectively. Changing the upper bound by 0.5% and 2.5% for the 4 and 8 mm grids resulted in only 0.75% and 2.2% of the volume exceeding the bound. The coverage did not change. Mean optimization times were 141.1, 22.6 and 3.4 minutes using the 2, 4 or 8mm grid, respectively. Experiments show that planning on a sparse grid can achieve comparable results with those of a high resolution grid, as long as the bounds are carefully balanced. This leads to substantially lower optimization times which facilitates interactive planning. This work was supported by the Graduate School for Computing in Medicine and Life Sciences funded by Germany’s Excellence Initiative [DFG GSC 235/1].

“Donut’s Shape” Radiosurgical Treatment Planning for Large Cystic Metastatic Brain Tumors

Radiosurgical management of large cystic metastatic brain tumors represents a significant challenge. Nevertheless, modified dose planning has shown beneficial results in such cases. "Donut's shape" radiosurgical treatment planning is based on the chain-like application of multiple, small-sized isocenters for selective coverage of the contrast-enhancing tumor capsule and minimal irradiation of the central cystic area. Such an approach was used for the management of large cystic intracranial metastases, which were not accompanied by a significant mass effect and did not require immediate volume reduction. Treatment was done using Leksell Gamma Knife model C with automatic positioning system. The majority of treated lesions showed significant shrinkage after radiosurgery and no major complications were met. Large cystic metastatic brain tumors may be successfully treated with gamma knife radiosurgery alone using the proposed "donut's shape" dose planning with coverage of the contrast-enhancing tumor capsule by multiple small-sized isocenters.

Integration of Three-Dimensional Rotational Angiography in Radiosurgical Treatment Planning of Cerebral Arteriovenous Malformations

Accuracy in delineating the target volume is a major issue for successful stereotactic radiosurgery for arteriovenous malformations. The aim of the present study was to describe a method to integrate three-dimensional (3D) rotational angiography (3DRA) into CyberKnife treatment planning and to investigate its potential advantages compared with computed tomography angiography (CTA) and magnetic resonance angiography. A total of 20 patients with a diagnosis of cerebral arteriovenous malformation were included in the present study. All patients underwent multislice computed tomography and 3D-volumetric CTA, 3DRA, and 3D magnetic resonance angiography. The contouring of the target and critical volumes was done separately using CTA and thereafter directly using 3DRA. The composite, conjoint, and disjoint volumes were measured. The use of CTA or 3DRA resulted in significant differences in the target and critical volumes. The target volume averaged 3.49 ± 3.01 mL measured using CTA and 3.26 ± 2.93 mL measured using 3DRA, for a difference of 8% (p < .05). The conjoint and disjoint volume analysis showed an 88% volume overlap. The qualitative evaluation showed that the excess volume obtained using CTA was mostly tissue surrounding the nidus and venous structures. The mean contoured venous volume was 0.67 mL measured using CTA and 0.88 mL (range, 0.1-2.7) measured using 3DRA (p < .05). 3DRA is a volumetric angiographic study that can be integrated into computer-based treatment planning. Although whether 3DRA provides superior accuracy has not yet been proved, its high spatial resolution is attractive and offers a superior 3D view. This allows a better 3D understanding of the target volume and distribution of the radiation doses within the volume. Additional technical efforts to improve the temporal resolution and the development of software tools aimed at improving the performance of 3D contouring are warranted.

Radiosurgical treatment planning of AVM following embolization with Onyx: possible dosage error in treatment planning can be averted

Treatment of arteriovenous malformations (AVM) of the brain is challenging due to the size and location of the nidus-proper and its proximity to the cerebrovascular circulation. Recent advances in catheter techniques and new embolization materials such as Onyx (a liquid agent that is less adhesive and slowly polymerizing) have increased the probability of achieving obliteration. When planning radiosurgical cases following such embolization, however, one must be cognizant of the distortions introduced by this novel substance on imaging studies. A sample of Onyx was irradiated to define the attenuation per mm thickness. The difference in attenuation compared to water was determined. Dose calculations were performed using 3 methods of inhomogeneity corrections. Homogeneous calculations were compared to "standard" heterogeneity corrections and to "modified" heterogeneity corrections by assigning individual electron densities to the normal brain and the Onyx. The difference between the attenuation of water in comparison to the Onyx was approximately 3% for beam energy of 6 MV. Best calculation results were achieved when using the modified inhomogeneity corrections which were based on the actual attenuation of the Onyx. The use of Onyx caused significant image artifact on MR and especially CT. As such, a correction must be manually introduced into the planning system to account for this potential error. Otherwise, dose calculation may be unreliable and could have dire consequences for patients receiving high doses of radiotherapy.

Radiosurgical Treatment Planning of AVM following Embolization with Onyx: A Cautionary Note

Treatment of arterio-venous malformations (AVM) of the brain represents a challenge due to the size and location of the nidus proper and its proximity to the cerebrovascular circulation. Recent advances in catheter techniques and new embolization materials, in particular Onyx (a novel liquid agent that is less adhesive and slowly polymerizing), have increased the probability of achieving total and near-total obliteration. In planning radiosurgical cases following such embolizations, one must be cognizant of the distortions introduced by Onyx on imaging studies (CT/MR). Between 12/06 and 12/08 we treated 13 AVM patients with SRS after Onyx embolization. One ml of Onyx stored in a circular plastic membrane (15mm diameter and 2cm high) was irradiated to define the attenuation of the onyx per mm thickness. We calculated the difference in the attenuation compared to water. Then, the Onyx and the projected volume of the brain were contoured in the planning system and a density correction was applied for the heterogeneity of the virtual organ that was drawn (i.e., onyx and brain). A pinpoint chamber was used for relative measurements (0.015cm3) due to the small amounts of onyx collected. The brain tissue around the Onyx had to be contoured due to the high intensity projection of the Onyx on CT images, which caused the CT to apply an incorrect brain density adjacent to the Onyx, thereby directly impacting the radiation calculation. We compared the monitor unit per arc of treatment between the corrected density value and the standard heterogeneity correction of the planning system. The absorption of the onyx with thickness of about 5mm was up to 2.3% higher compared to 5mm of water. The difference in the dose calculation due to the absorption differences, from a 30 degrees radiation arc treatment through the Onyx was higher by up to 30%. In contrast, the dose calculation for the same arc that does not pass through the Onyx caused a reduction of up to 26% due to the distortion of the images and assigned density values similar to air around the Onyx. The use of Onyx brought about significant image artefacts on MR and especially on CT. Larger Onyx-embolized AVMs showed significant imaging artefacts especially on CT, which rendered this imaging modality useless for radiation treatment planning purposes. These findings show the importance of the correction that needs to be applied manually in the planning system. For patient with large areas and volumes of onyx, this result underscores the importance of the correction that must be applied. A dose calculation may be completely wrong, particularly for AVM- radiosurgery patients receiving high doses in single fractions with no margins.

Radiosurgical Treatment Planning for Intracranial AVM Based on Images Generated by Principal Component Analysis-A Simulation Study

One of the most important factors in stereotactic radiosurgery (SRS) for intracranial arteriovenous malformation (AVM) is to determine accurate target delineation of the nidus. However, since intracranial AVMs are complicated in structure, it is often difficult to clearly determine the target delineation. To investigate the usefulness of principal component analysis (PCA) on intra-arterial contrast enhanced dynamic CT (IADCT) images as a tool for delineating accurate target volumes for stereotactic radiosurgery of AVMs. IADCT and intravenous contrast-enhanced CT (IVCT) were used to examine 4 randomly selected cases of AVM. PCA images were generated from the IADCT data. The first component images were considered feeding artery predominant, the second component images were considered draining vein predominant, and the third component images were considered background. Target delineations were first carried out from IVCT, and then again while referring to the first and second components of the PCA images. Dose calculation simulations for radiosurgical treatment plans with IVCT and PCA images were performed. Dose volume histograms of the vein areas as well as the target volumes were compared. In all cases, the calculated target volumes based on IVCT images were larger than those based on PCA images, and the irradiation doses for the vein areas were reduced. In this study, we simulated radiosurgical treatment planning for intracranial AVM based on PCA images. By using PCA images, the irradiation doses for the vein areas were substantially reduced.