Stereotactic Radiosurgery System Research Articles

Continuous versus step-by-step scanning mode of a novel 3D scanner for CyberKnife measurements

The purpose of the study is to investigate the continuous versus step-by-step scanning mode of a commercial circular 3D scanner for commissioning measurements of a robotic stereotactic radiosurgery system. The 3D scanner was used for profile measurements in step-by-step and continuous modes with the intent of comparing the two scanning modes for consistency. The profile measurements of in-plane, cross-plane, 15 degree, and 105 degree were performed for both fixed cones and Iris collimators at depth of maximum dose and at 10cm depth. For CyberKnife field size, penumbra, flatness and symmetry analysis, it was observed that the measurements with continuous mode, which can be up to 6 times faster than step-by-step mode, are comparable and produce scans nearly identical to step-by-step mode. When compared with centered step-by-step mode data, a fully processed continuous mode data gives rise to maximum of 0.50% and 0.60% symmetry and flatness difference respectfully for all the fixed cones and Iris collimators studied.

Clinical experience with two frameless stereotactic radiosurgery (fSRS) systems using optical surface imaging for motion monitoring.

The purpose of this study was to compare two clinical immobilization systems for intracranial frameless stereotactic radiosurgery (fSRS) under the same clinical procedure using cone‐beam computed tomography (CBCT) for setup and video‐based optical surface imaging (OSI) for initial head alignment and intrafractional motion monitoring. A previously established fSRS procedure was applied using two intracranial immobilization systems: PinPoint system (head mold and mouthpiece) and Freedom system (head mold and open face mask). The CBCT was used for patient setup with four degrees of freedom (4DOF), while OSI was used for 6DOF alignment prior to CBCT, post‐CBCT setup verification at all treatment couch angles (zero and nonzero), and intrafractional motion monitoring. Quantitative comparison of the two systems includes residual head rotation, head restriction capacity, and patient setup time in 25 patients (29 lesions) using PinPoint and 8 patients (29 fractions) using Freedom. The maximum possible motion was assessed in nine volunteers with deliberate, forced movement in Freedom system. A consensus‐based comparison of patient comfort level and clinical ease of use is reported. Using OSI‐guided corrections, the maximum residual rotations in all directions were 1.1°±0.5° for PinPoint and 0.6°±0.3° for Freedom. The time spent performing rotation corrections was 5.0±4.1 min by moving the patient with PinPoint and 2.7±1.0 min by adjusting Freedom couch extension. After CBCT, the OSI–CBCT discrepancy due to different anatomic landmarks for alignment was 2.4±1.3 mm using PinPoint and 1.5±0.7 mm using Freedom. Similar results were obtained for setup verification at couch angles (<1.5 mm) and for motion restriction: 0.4±0.3 mm/0.2°±0.2° in PinPoint and 0.6±0.3 mm/0.3°±0.1° in Freedom. The maximum range of forced head motion was 2.2±1.0 mm using Freedom. Both intracranial fSRS immobilization systems can restrict head motion within 1.5 mm during treatment as monitored by OSI. Setting a motion threshold for beam‐hold ensures that head motion is constrained within the treatment margin during beam‐on periods. The capability of 6D setup is useful to improve treatment accuracy. Patient comfort and clinical workflow should play a substantial role in system selection, and Freedom system outperforms PinPoint system in these two aspects.PACS number: 87.53.Ly, 87.55.D‐, 87.57.Q‐, 87.6s.L‐, 87.85.gi

Characteristics of a novel treatment system for linear accelerator-based stereotactic radiosurgery.

The purpose of this study is to characterize the dosimetric properties and accuracy of a novel treatment platform (Edge radiosurgery system) for localizing and treating patients with frameless, image‐guided stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Initial measurements of various components of the system, such as a comprehensive assessment of the dosimetric properties of the flattening filter‐free (FFF) beams for both high definition (HD120) MLC and conical cone‐based treatment, positioning accuracy and beam attenuation of a six degree of freedom (6DoF) couch, treatment head leakage test, and integrated end‐to‐end accuracy tests, have been performed. The end‐to‐end test of the system was performed by CT imaging a phantom and registering hidden targets on the treatment couch to determine the localization accuracy of the optical surface monitoring system (OSMS), cone‐beam CT (CBCT), and MV imaging systems, as well as the radiation isocenter targeting accuracy. The deviations between the percent depth‐dose curves acquired on the new linac‐based system (Edge), and the previously published machine with FFF beams (TrueBeam) beyond Dmax were within 1.0% for both energies. The maximum deviation of output factors between the Edge and TrueBeam was 1.6%. The optimized dosimetric leaf gap values, which were fitted using Eclipse dose calculations and measurements based on representative spine radiosurgery plans, were 0.700 mm and 1.000 mm, respectively. For the conical cones, 6X FFF has sharper penumbra ranging from 1.2−1.8 mm (80%‐20%) and 1.9−3.8 mm (90%‐10%) relative to 10X FFF, which has 1.2−2.2 mm and 2.3−5.1 mm, respectively. The relative attenuation measurements of the couch for PA, PA (rails‐in), oblique, oblique (rails‐out), oblique (rails‐in) were: −2.0%, −2.5%, −15.6%, −2.5%, −5.0% for 6X FFF and −1.4%, −1.5%, −12.2%, −2.5%, −5.0% for 10X FFF, respectively, with a slight decrease in attenuation versus field size. The systematic deviation between the OSMS and CBCT was −0.4±0.2 mm, 0.1±0.3 mm, and 0.0±0.1 mm in the vertical, longitudinal, and lateral directions. The mean values and standard deviations of the average deviation and maximum deviation of the daily Winston‐Lutz tests over three months are 0.20±0.03 mm and 0.66±0.18 mm, respectively. Initial testing of this novel system demonstrates the technology to be highly accurate and suitable for frameless, linac‐based SRS and SBRT treatment.PACS number: 87.56.J‐

SU‐E‐J‐13: Six Degree of Freedom Image Fusion Accuracy for Cranial Target Localization On the Varian Edge Stereotactic Radiosurgery System: Comparison Between 2D/3D and KV CBCT Image Registration

Purpose:To determine the 6 degree of freedom systematic deviations between 2D/3D and CBCT image registration with various imaging setups and fusion algorithms on the Varian Edge Linac.Methods:An anthropomorphic head phantom with radio opaque targets embedded was scanned with CT slice thicknesses of 0.8, 1, 2, and 3mm. The 6 DOF systematic errors were assessed by comparing 2D/3D (kV/MV with CT) with 3D/3D (CBCT with CT) image registrations with different offset positions, similarity measures, image filters, and CBCT slice thicknesses (1 and 2 mm). The 2D/3D registration accuracy of 51 fractions for 26 cranial SRS patients was also evaluated by analyzing 2D/3D pre‐treatment verification taken after 3D/3D image registrations.Results:The systematic deviations of 2D/3D image registration using kV‐ kV, MV‐kV and MV‐MV image pairs were within ±0.3mm and ±0.3° for translations and rotations with 95% confidence interval (CI) for a reference CT with 0.8 mm slice thickness. No significant difference (P&gt;0.05) on target localization was observed between 0.8mm, 1mm, and 2mm CT slice thicknesses with CBCT slice thicknesses of 1mm and 2mm. With 3mm CT slice thickness, both 2D/3D and 3D/3D registrations performed less accurately in longitudinal direction than thinner CT slice thickness (0.60±0.12mm and 0.63±0.07mm off, respectively). Using content filter and using similarity measure of pattern intensity instead of mutual information, improved the 2D/3D registration accuracy significantly (P=0.02 and P=0.01, respectively). For the patient study, means and standard deviations of residual errors were 0.09±0.32mm, −0.22±0.51mm and −0.07±0.32mm in VRT, LNG and LAT directions, respectively, and 0.12°±0.46°, −0.12°±0.39° and 0.06°±0.28° in RTN, PITCH, and ROLL directions, respectively. 95% CI of translational and rotational deviations were comparable to those in phantom study.Conclusion:2D/3D image registration provided on the Varian Edge radiosurgery, 6 DOF‐based system provides accurate target positioning for frameless image‐guided cranial stereotactic radiosurgery.

Stereotactic body radiotherapy (SBRT) reirradiation for pelvic recurrence from colorectal cancer

When surgery is not adequate or feasible, stereotactic body radiotherapy (SBRT) reirradiation has been used for recurrent cancers. We report the outcomes of a series of patients with pelvic recurrences from colorectal cancer reirradiated with SBRT. The Cyberknife(TM) Robotic Stereotactic Radiosurgery system with fiducial based real time tracking was used. Patients were followed with imaging of the pelvis. Four women and 14 men with 22 lesions were included. The mean dose was 25 Gy in median of five fractions. The mean prescription isodose was 77%, with a median maximum dose of 32.87 Gy. There were two local failures, with a crude local control rate of 89%. The median overall survival was 43 months. One patient had small bowel perforation and required surgery (Grade IV), two patients had symptomatic neuropathy (1 Grade III) and one patient developed hydronephrosis from ureteric fibrosis requiring a stent (Grade III). Local recurrence in the pelvis after modern combined modality treatment for colorectal cancer is rare. However it presents a therapeutic dilemma when it occurs; often symptomatic and eventually life threatening. SBRT can be a useful non-surgical modality to control pelvic recurrences after prior radiation for colorectal cancer.

Stereotactic radiosurgery for brain metastases from malignant melanoma.

Background:Surgical resection and stereotactic radiosurgery (SRS) are well-established treatment methods for patients with brain metastases, yet their respective roles in the management of brain metastases remain incompletely defined.Methods:To report on the role of SRS in the treatment of patients with brain metastases from malignant melanoma, a retrospective analysis of 381 intracranial melanoma metastases in 103 consecutive patients who underwent SRS between 2005 and 2011 at Beth Israel Deaconess Medical Center was conducted. The Cyberknife® SRS system was used to treat all patients. Clinical, technical, and radiographic data were recorded at presentation and on follow-up.Results:The patient cohort consisted of 40 female (39%) and 63 male (61%) patients with a median age of 57 years. The median overall survival from the time of radiosurgery for the entire patient cohort was 7.6 months. The local control rate at 1-year was 72% for the patients who received surgery followed by SRS and 55% for the entire patient population. Surgery followed by SRS was associated with significantly improved overall survival compared with SRS alone or whole-brain radiation therapy followed by salvage SRS (P < 0.0057).Conclusions:Both surgery plus SRS and SRS provide comparable local control. Despite the difference in lesion size in the subgroups who received surgery plus SRS and radiosurgery alone, similar outcomes were achieved in both groups, suggesting that surgical treatment of larger lesions can yield results that are not significantly different from small lesions treated by SRS alone.

CyberKnife radiosurgery: Precision without incision

CyberKnife stereotactic radiosurgery system is an innovative, effective, frameless, non-invasive substitute for conventional surgical treatment of cancer. It works on the principle of stereotaxy. It is used for the treatment of both cancerous and non-cancerous tumors, intracranial lesions, tumors of lung, spine, prostate, and kidney, recurrent cases of oral squamous cell carcinoma, arteriovenous malformation, and trigeminal neuralgia. It has an advantage over other systems like Gamma knife radiosurgery and linear accelerator (LINAC)-based systems, as it is frameless, has submillimeter accuracy, does not affect the normal cells adjacent to the lesion, and tracks the lesion in synchronization with the patient's respiratory rate. The future of CyberKnife encompasses possibilities such as incremental improvements in accuracy and better shaping of the field of radiation and would certainly allow extension of radiosurgery as an effective substitute for chemotherapy. This paper aims to review and highlight the immense potential that CyberKnife holds in the field of dentistry in treating disorders of the head and neck region, thereby ensuring enhanced longevity for the patients.

Cardiac Radiosurgery for Malignant Ventricular Tachycardia

Ventricular tachycardia is a frequent cause of mortality after myocardial infarction. Current treatment includes the implantation of cardioverter defibrillators and adjunctive therapies, such as catheter ablation or cardiac surgery. In patients where standard treatment fails, preclinical data showed that radiosurgery ablation of the ectopic substrate might be a viable option. Authors present a case report of cardiac radiosurgery in a patient with malignant ventricular tachycardia. The stereotactic radiosurgery system, CyberKnife, was used; the applied dose was 25 Gy in one fraction. Within the follow-up period of 120 days, no signs of toxicity were noted and no episode of malignant arrhythmia has been detected. This case report demonstrates that stereotactic radiosurgery of recurrent ventricular tachycardia after inefficient catheter ablation might be a viable option for patients unsuitable for cardiosurgical intervention. Further research on this topic is highly warranted.

A new strategy of CyberKnife treatment system based radiosurgery followed by early use of adjuvant bevacizumab treatment for brain metastasis with extensive cerebral edema.

Bevacizumab blocks the effects of vascular endothelial growth factor in leakage-prone capillaries and has been suggested as a new treatment for cerebral radiation edema and necrosis. CyberKnife is a new, frameless stereotactic radiosurgery system. This work investigated the safety and efficacy of CyberKnife followed by early bevacizumab treatment for brain metastasis with extensive cerebral edema. The eligibility criteria of the patients selected for radiosurgery followed by early use of adjuvant bevacizumab treatment were: (1) brain tumors from metastasis with one solitary brain lesion and symptomatic extensive cerebral edema; (2) >18 years of age; (3) the patient refused surgery due to the physical conditions and the risk of surgery; (4) no contraindications for bevacizumab. (5) bevacizumab was applied for a minimum of 2 injections and a maximum of 6 injections with a 2-week interval between treatments, beginning within 2 weeks of the CyberKnife therapy; (6) Karnofsky performance status (KPS) ≥30. Tumor size and edema were monitored by magnetic resonance imaging (MRI). Dexamethasone dosage, KPS, adverse event occurrence and associated clinical outcomes were also recorded. Eight patients were accrued for this new treatment. Radiation dose ranged from 20 to 33 Gy in one to five sessions, prescribed to the 61-71 % isodose line. Bevacizumab therapy was administered 3-10 days after completion of CyberKnife treatment for a minimum of two cycles (5 mg/kg, at 2-week intervals). MRI revealed average reductions of 55.8 % (post-gadolinium) and 63.4 % (T2/FLAIR). Seven patients showed significant clinical neurological improvements. Dexamethasone was reduced in all patients, with five successfully discontinuing dexamethasone treatment 4 weeks after bevacizumab initiation. Hypertension, a bevacizumab-related adverse event, occurred in one patient. After 3-8 months, all patients studied were alive and primary brain metastases were under control, 2 developed new brain metastases and underwent salvage CyberKnife treatment. Recurrent edema and emerging radiation necrosis were not observed. CyberKnife radiosurgery followed by early use of bevacizumab is promising and appears safe for treatment of brain metastases with extensive cerebral edema.

Use of an Image-Guided Robotic Radiosurgery System for the Treatment of Canine Nonlymphomatous Nasal Tumors

An image-guided robotic stereotactic radiosurgery (SRS) system can be used to deliver curative-intent radiation in either single fraction or hypofractionated doses. Medical records for 19 dogs with nonlymphomatous nasal tumors treated with hypofractionated image-guided robotic stereotactic body radiotherapy (SBRT), either with or without adjunctive treatment, were retrospectively analyzed for survival and prognostic factors. Median survival time (MST) was evaluated using Kaplan-Meier survival curves. Age, breed, tumor type, stage, tumor size, prescribed radiation dose, and heterogeneity index were analyzed for prognostic significance. Dogs were treated with three consecutive-day, 8-12 gray (Gy) fractions of image-guided robotic SBRT. Overall MST was 399 days. No significant prognostic factors were identified. Acute side effects were rare and mild. Late side effects included one dog with an oronasal fistula and six dogs with seizures. In three of six dogs, seizures were a presenting complaint prior to SBRT. The cause of seizures in the remaining three dogs could not be definitively determined due to lack of follow-up computed tomography (CT) imaging. The seizures could have been related to either progression of disease or late radiation effect. Results indicate that image-guided robotic SBRT, either with or without adjunctive therapy, for canine nonlymphomatous nasal tumors provides comparable survival times (STs) to daily fractionated megavoltage radiation with fewer required fractions and fewer acute side effects.

Quality Assurance Results for a Commercial Radiosurgery System: A Communication.

The purpose of this communication is to inform the radiosurgery community of quality assurance (QA) results requiring attention in a commercial FDA-approved linac-based cone stereo-tactic radiosurgery (SRS) system. Standard published QA guidelines as per the American Association of Physics in Medicine (AAPM) were followed during the SRS system's commissioning process including end-to-end testing, cone concentricity testing, image transfer verification, and documentation. Several software and hardware deficiencies that were deemed risky were uncovered during the process and QA processes were put in place to mitigate these risks during clinical practice. In particular, the present work focuses on daily cone concentricity testing and commissioning-related findings associated with the software. Cone concentricity/alignment is measured daily using both optical light field inspection, as well as quantitative radiation field tests with the electronic portal imager. In 10 out of 36 clini-cal treatments, adjustments to the cone position had to be made to align the cone with the collimator axis to less than 0.5 mm and on two occasions the pre-adjustment measured offset was 1.0 mm. Software-related errors discovered during commissioning included incorrect transfer of the isocentre in DICOM coordinates, improper handling of non-axial image sets, and complex handling of beam data, especially for multi-target treatments. QA processes were established to mitigate the occurrence of the software errors. With proper QA processes, the reported SRS system complies with tolerances set out in established guidelines. Discussions with the vendor are ongoing to address some of the hardware issues related to cone alignment.

Treating Adrenal Tumors in 26 Patients with CyberKnife: A Mono-Institutional Experience

BackgroundCyberKnife (CK) is a novel stereotactic radiosurgery system for treating tumors in any part of the body. It is a non-invasive or minimally invasive tumor treatment modality that can deliver high doses of spatially precise radiation and minimize exposure to neighboring healthy tissues or vital organs. The purpose of this study was to investigate the safety and efficacy of CK in the treatment of adrenal tumors. Methods and ResultsWe performed a retrospective analysis of 26 patients with adrenal tumors who had been treated with CK in the radiotherapy center of our hospital between March 2009 and March 2012. Eight patients had primary adrenal tumors and 18 patients had metastatic adrenal tumors. In addition to CK, 4 patients received chemotherapy and 2 patients received immunotherapy. The average tumor volume was 72.1 cm3 and the prescribed radiation dosage ranged from 30 to 50 Gy and was fractionated 3 to 5 times with a 58% to 80% isodose line. Abdominal CT was performed between 1 to 3 months after the CK treatment to evaluate the short-term efficacy with follow-up examinations once every 3 months. Three patients had complete remission, 12 patients had partial remission, 5 patients had stable disease, and 6 patients had progressive illness. The effective rate of pain relief was 93.8% and the disease control rate was 77% with a median overall survival of 17 months and a median progression-free survival of 14 months. Treatment Related toxicity was well-tolerated, but preventative measure need to be taken for radiation enteritis.Conclusions CK is safe and effective for treating adrenal tumors with few adverse reactions. Nonetheless, its long-term effects requires further follow-up.

SU‐E‐J‐22: Implementation and Validation of a 2D‐3D Rigid Registration Algorithm for Proton Gantry and Stereotactic Radiosurgery Systems

Purpose: To implement and validate the accuracy of an intensity‐based 2D to 3D rigid registration algorithm for proton treatment systems. Methods: A 2D‐3D rigid registration algorithm (REG23) was previously validated for linac machines. We adapted REG23 for use in both gantry‐based and Stereotactic Alignment in Radiosurgery (STAR) proton treatment systems. REG23 registration was run on NVIDIA Quadra600 GPU card using NCC metrics and AMOEBA optimizer and validated using dual orthogonal kV images acquired during cranial target treatments: 66 fractions to 7 patients in gantry system and 135 fractions to 9 patients in STAR. Rectangle ROIs covering the whole region superior to the base‐of‐skull were used for REG23 registration and the accuracy was evaluated using clinical utilized fiducial‐based 2D‐3D ray back‐projection rigid registration as the baseline. Results: For the gantry system, the differences between REG23 and baseline were ‐ 0.01±0.63 mm, 0.43±0.61 mm, 0.12±0.60 mm in the left‐right, superior‐inferior, and anterior‐posterior directions; 0.15±0.44°, 0.11±0.23°, and 0.19±0.37° in pitch, roll, and yaw, respectively. The vector difference was 1.04±0.48 mm. For STAR system, the REG23 results were − 0.34±0.45 mm, −0.10±0.36 mm, −0.38±0.75 mm, −0.14±0.54°, 0.04±0.18°, and 0.03±0.35° different than the baseline in the left‐right, superior‐inferior, and anterior‐posterior directions, pitch, roll, and yaw. The vector difference was 0.93±0.53 mm. The time for REG23 automatic registration was 29.7±9.5 seconds. Conclusion: We demonstrated that the intensity based 2D‐3D rigid registration algorithm REG23 provided sub‐millimeter accuracy and better than 0.5° for both gantry‐based and STAR proton treatment systems for cranial patients. This accuracy is within patient setup tolerance for current fractionated proton treatment systems. Given its accuracy and efficiency, we believe REG23 has great potential for clinical utilization in proton radiation therapy by replacing the implanted fiducial or anatomic feature based patient setup method.The project was supported by the Federal Share of program income earned by Massachusetts General Hospital on C06 CA059267, Proton Therapy Research and Treatment Center.

SU-E-T-116: Dosimetry On Beam Modeling at the Small Field for Stereotactic Radiosurgery

Purpose: Beam modeling performed based on the results of the present study, measuring the amount of radiation from a variety of small field of radiation measurement equipment beam modeling is based on the results of clinical practice, such as stereotactic radiosurgery and radiation therapy planning system applied to the treatment plan of each radiation dose distribution is to compare and analyze. Methods: A RapidArc medical linear accelerator was used with norminal 6 and 15 MV photon beam energies in this study. The small beams were generated that field sizes were 1×1, 2×2, 3×3 cm2 and reference field size was 10×10 cm2 to compare small field. We were generated the beam modeling and performed the automatic modeling with adjusted the mean and standard deviation of energy with the spectrum modeling base on the generated the PDD of the beam model. And, we were the lateral profile modeling by using adjusted each horn factor, effect source size, jaw transmission factor, MLC transmission factor, softening factor. Results: In this study, the edge detector measured a narrower penumbra width than the other detectors for small field sizes. For the 20–80% penumbra, the penumbra width measured with the ion chamber was approximately two times wider than the penumbra measured with the edge detector in all cases. The CC13 measured a penumbra width that was 25% wider for all field sizes. We found that the PDD measured with film had some signal fluctuation at the buildup region(6 MV: 1.5 cm, 15 MV: 2.9 cm) for the smallest field size when compared to the standard ionization chamber. Conclusion: The edge detector showed a greater suitability for small field dosimetry than the other detectors tested. We compared the detectors and evaluated the beam modeling results that as they have relatively large volumes, significant discrepancies could occur during the small field dosimetry.

SU‐E‐J‐55: Influence of Patient Weight and Couch Rotation On the Accuracy of a Surface Tracking System for Stereotactic Radiosurgery

Purpose: To evaluate the influence of patient weight and couch angle on the treatment accuracy of brain SRS guided by the AlignRT system integrated with TrueBeam couch system. Method and Materials: A Winston Lutz phantom with a facemask on top was used to simulate a brain SRS treatment with surface monitoring. The phantom contained small fiducial markers that guided the initial setup using kV and CBCT imaging. A CT scan of the phantom was taken and used to form a plan that included 15 sets of different combinations of couch and gantry angles. Once the phantom was set up, a reference image for surface was acquired and motion monitoring was initiated. The gantry and couch were then moved to their respective positions and an EPID image was acquired. A motion report was generated on the AlignRT system. EPID image registration was performed using the OBI system with DRRimage and the shifts were recorded. The method was repeated for evaluation of patient weight with five different weights in the range of 20 and 120 kg. Results: Examination of the AlignRT data suggests that the accuracy of TrueBeam couch system is slightly compromised for the longitudinal direction (&lt; 0.9 mm) after applying couch rotation during the tracking. The accuracy did not seem to be affected by varying patient weight if couch sagging is compensated for initial setup. All of the deviations of the AlignRT record showed that the accuracy was within 1 mm to the deviations from image registration of isocenter. Conclusions: Our data suggests that TrueBeam couch system maintains geometric accuracy of up to 120 kg of weight, at various gantry and couch angle positions, suggesting that the system is reliable for use with Brain SRS treatments. Addition of AlignRT for intrafraction monitoring helps to ensure that the treatment remains accurate.

Determination and verification of a 2D pencil-beam kernel for a radiosurgery system with cones

The quality and correctness of dosimetric data of small fields in stereotactic radiosurgery (SRS) depends significantly on the election of the detector employed in the measurements. This work provides an independent method of verification of these data through the determination of a polyenergetic 2-dimensional pencil-beam kernel for a BrainLAB SRS system with cones, employing the deconvolution/convolution of a reference experimental off-axis ratio (OAR) profile (cone diameter c0 = 35mm). The kernel in real space kc0(r,z0) is convolved with the ideal fluence Φ for the cones 7.5 to 35mm in diameter to obtain the OAR profiles, and the total scatter factors, St, which are compared with experimental values of the same quantities. The experimental OARs and St factors are measured in water with a PTW 60003 diamond detector. Additionally, the reference OAR is corrected for beam divergence and spectral fluence fluctuations defining a function of boundary correction factors (BF). The BF and Φ functions are transformed to the conjugate space with the zeroth-order Hankel transform, appropriated to the radial symmetry of the cones. Therefore, the kernel in real space kc0(r,z0) is the inverse Hankel transform of the ratio of the Hankel transforms of BF and Φ. Finally, an uncertainty analysis according to the Guide to the Expression of Uncertainty in Measurement is carried out for 3 different values of kc0(r,z0). Calculated and measured OARs agree within the dose/distance-to-agreement criteria of 2%/0.12mm; while, St factors agree within 2%. This procedure supplies an independent method to validate the dosimetric data necessary to feed treatment planning systems for SRS with cones.

Reliability of contour-based volume calculation for radiosurgery

Determining accurate target volume is critical for both prescribing and evaluating stereotactic radiosurgery (SRS) treatments. The aim of this study was to determine the reliability of contour-based volume calculations made by current major SRS platforms. Spheres ranging in diameter from 6.4 to 38.2 mm were scanned and then delineated on imaging studies. Contour data sets were subsequently exported to 6 SRS treatment-planning platforms for volume calculations and comparisons. This procedure was repeated for the case of a patient with 12 metastatic lesions distributed throughout the brain. Both the phantom and patient datasets were exported to a stand-alone workstation for an independent volume-calculation analysis using a series of 10 algorithms that included approaches such as slice stacking, surface meshing, point-cloud filling, and so forth. Contour data-rendered volumes exhibited large variations across the current SRS platforms investigated for both the phantom (-3.6% to 22%) and patient case (1.0%-10.2%). The majority of the clinical SRS systems and algorithms overestimated the volumes of the spheres, compared with their known physical volumes. An independent algorithm analysis found a similar trend in variability, and large variations were typically associated with small objects whose volumes were < 0.4 cm(3) and with those objects located near the end-slice of the scan limits. Significant variations in volume calculation were observed based on data obtained from the SRS systems that were investigated. This observation highlights the need for strict quality assurance and benchmarking efforts when commissioning SRS systems for clinical use and, moreover, when conducting multiinstitutional cross-SRS platform clinical studies.

Dose linearity and monitor unit stability of a G4 type cyberknife robotic stereotactic radiosurgery system

Dose linearity studies on conventional linear accelerators show a linearity error at low monitor units (MUs). The purpose of this study was to establish the dose linearity and MU stability characteristics of a cyberknife (Accuray Inc., USA) stereotactic radiosurgery system. Measurements were done at a depth of 5 cm in a stereotactic dose verification phantom with a source to surface distance of 75 cm in a Generation 4 (G4) type cyberknife system. All the 12 fixed-type collimators starting from 5 to 60 mm were used for the dose linearity study. The dose linearity was examined in small (1–10), medium (15–100) and large (125–1000) MU ranges. The MU stability test was performed with 60 mm collimator for 10 MU and 20 MU with different combinations. The maximum dose linearity error of –38.8% was observed for 1 MU with 5 mm collimator. Dose linearity error in the small MU range was considerably higher than in the medium and large MU ranges. The maximum error in the medium range was –2.4%. In the large MU range, the linearity error varied between –0.7% and 1.2%. The maximum deviation in the MU stability was –3.03%.

Dosimetric comparison of 2.5 mm vs. 3.0 mm leaf width micro-multileaf collimator-based treatment systems for intracranial stereotactic radiosurgery using dynamic conformal arcs: implications for treatment planning

The objective of our study was to explore any significant dosimetric differences between different leaf width (3.0 mm vs. 2.5 mm) micro-multileaf collimator (mMLC)-based treatment systems for intracranial stereotactic radiosurgery using dynamic conformal arcs (DCAs). The systems included a 3 mm leaf width mMLC (m3) mounted on a nondedicated linac, and the Novalis Tx system with an integrated 2.5 mm width mMLC (HD120). Thirty plans for brain metastases were replanned for both systems using a uniform method for target definition and treatment planning for baseline comparison. The target coverage values for the 80% isodose surface (IDS) and the D95 values in the HD120 plans were significantly lower than those for the m3 plans. The ratios of lower isodose volumes to the target for the HD120 were smaller than those for the m3. When a 1 mm leaf margin was added to the HD120 plans, these differences were reversed, but statistically significant differences were still observed. Significant dosimetric differences were observed between these systems. Different planning methods are required for the two systems to attain similar target coverage values with selected IDS, which can be achieved by adjusting the leaf margin with 0.1 mm increments or isocenter dose settings.

Stereotactic radiosurgery for early-stage breast cancer: A new paradigm.

120 Background: Stereotactic radiosurgery (SRS) has not been widely used for breast cancer partly due to: 1) the lack of a suitable immobilization device; and 2) the lack of a delivery system that can concentrate a high radiation dose noninvasively. We have developed a SRS system that effectively addresses these obstacles and demonstrated the dosimetric feasibility of simultaneously ablating the intact tumor and sterilizing the tumor bed. Methods: A breast immobilization device consists of a solid outer cup with embedded stereotactic frame and a porous inner cup; the two cups join at the chest wall side by a soft goggle-like lip for comfort. A negative pressure is applied to the space between the two cups to immobilize the breast tissues. 15 patients with surgical clips in their breast underwent a geometric reproducibility trial, in which the patients received two CT scans spaced 10-60 minutes while wearing the cup. A SRS delivery system uses 36 Co-60 sources that rotate around a focal point to produce 36 non-coplanar arcs. The breast cup is locked on the treatment couch with the patient in prone position. The couch moves dynamically during treatment, allowing the focal spots to “paint” the desired 3D dose distribution. Results: The reproducibility of the 42 available clips was 1.83 mm ± 1.08 mm (2 std) and patients reported good comfort. 8 treatment plans simultaneously delivering an ablative dose of 18Gy to an intact tumor plus a 3mm margin (CTV1) and 10Gy to the tumor bed (CTV2 = CTV1+1cm) have been developed and verified with measurements. The dose-volume histograms showed very uniform dose coverage with 98% of CTV1 receiving 18Gy and 100% of tumor bed receiving 10Gy. On average, only 21% of the normal breast receives greater than 5Gy, safe to deliver 3 fractions of such doses. Conclusions: A breast-specific SRS system has been developed. Dosimetric studies illustrated the feasibility of managing early-stage breast cancer with SRS. Clinical studies that will identify patients in whom current forms of surgery and postoperative radiation may be eliminated are planned, allowing completion of local therapies noninvasively within a few days rather than months.