Leksell Gamma Knife Perfexion Research Articles

Consistency of dose rates after applying machine-specific reference correction factors for the gamma knife 16mm collimator field.

The machine-specific reference (msr) correction factors ( ) were introduced in International Atomic Energy Agency (IAEA) Technical Report Series 483 (TRS-483) for reference dosimetry of small fields. Several correction factor sets exist for a Leksell Gamma Knife (GK) Perfexion or Icon. Nevertheless, experiments have not rigorously validated the correction factors from different studies. This study aimed to assess the role and accuracy of values in determining the absorbed dose rates to water in the reference dosimetry of Gamma Knife. The dose rates in the 16mm collimator field of a GK were determined following the international code of practices with three ionization chambers: PTW T31010, PTW T31016 (PTW Freiberg GmbH, New York, NY), and Exradin A16 (Standard Imaging, Inc., Middleton, WI). A chamber was placed at the center of a solid water phantom (Elekta AB, Stockholm, Sweden) using a detector-specific insert. The reference point of the ionization chamber was confirmed using cone-beam CT images. Consistency checks were repeated five times at a GK site and performed once at seven GK sites. Correction factors from six simulations reported in previous studies were employed. Variations in the dose rates and relative dose rates before and after applying the were statistically compared. The standard deviation of the dose rates measured by the three chambers decreased significantly after any correction method was applied (p=0.000). When the correction factors of all studies were averaged, the standard deviation was reduced significantly more than when any single correction method was applied (p≤0.030), except for the IAEA TRS-483 correction factors (p=0.148). Before any correction was applied, there were statistically significant differences among the relative dose rates measured by the three chambers (p=0.000). None of the single correction methods could remove the differences among the ionization chambers (p≤0.038). After TRS-483 correction, the dose rate of Exradin A16 differed from those of the other two chambers (p≤0.025). After the averaged factors were applied, there were no statistically significant differences between any pairs of chambers according to Scheffe's post hoc analyses (p≥0.051); however, PTW T31010 differed from PTW 31016 according to Tukey's HSD analyses (p=0.040). The significantly reduced variations in the dose rates measured by the three ionization chambers. The mean correction factors of the six simulations produced the most consistent results, but this finding was not explicitly proven in the statistical analyses.

Facial nerve preservation in stereotactic radiosurgery of tumors of the cerebellopontine cistern

Introduction. Surgery has traditionally remained the main treatment for tumors of the cerebellopontine cistern but is associated with high risks of dysfunction of the cranial nerves. Radiosurgery is usually performed both as an adjuvant treatment and as an independent option.The study objective is to assess the safety of routine use of radiosurgery to preserve facial nerve function in the treatment of tumors of different origins located in the cerebellopontine cistern.Materials and methods. Since March 2018 to March 2020 there were 145 patients with tumors of cerebellopontine cistern (CPC) at the Center for High-Precision Radiology. Vestibilar schwannomas were detected in 116 (80 %) patients, in 37 cases - relapses or remains after surgery. The 22 patients had meningiomas, 6 after open surgery (WHO I). Trigeminal schwannomas - in 3 patients, facial schwannomas - in 2, jugular schwannomas - in 1 and metastasis of prostate cancer - in 1. 31 patients had facial paresis of different degrees after previous surgery. Among non-operated patients, facial paresis before radiosurgery was observed in only 1 patient. Radiosurgery was performed using the Leksell Gamma Knife Perfexion, the mean marginal dose was 12.2 Gy (11-15 Gy), the mean tumor volume was 3.98 cm3 (0.06-17.47 cm3).Results. Follow-up was performed in 85 patients. The mean follow-up was 359.3 days (91-776), the median follow-up was 367 days. Reduction of the tumor volume was detected in 27 patients, stabilization of the process in 15. The average decrease was 23.9 % (95 % CI 16.8-31.0 %). Transient postirradiation increase was observed in 30 patients only in the group of schwannomas and mean tumor volume increase was 53.7 % (95 % CI 38.5-68.9 %). In patients with previous surgery there were not cases of decline or regression facial paresis. Among patients who had not been operated on before radiosurgery, only in 1 case was the development of facial paresis (House-Brackman III) 5 months after irradiation, which amounted to 1.8 %. It should be especially noted that facial nerve function remained unchanged (House-Brackman I) in both patients with facial schwannomas. Also, not a single case of hemifacial spasm was identified.Conclusion. Radiosurgery of CPC tumors with a marginal dose from 12 to 15 Gy has a high degree of safety in relation to the facial nerve. This makes it possible to justify radiosurgery as alternative to traditional neurosurgical interventions.

Radiosurgical treatment of medically refractory Parkinson’s tremor

Introduction. In spite of medication treatment being today the basic form of treatment for Parkinson’s disease, surgical methods may be preferable to improve symptoms of the disease. Radiosurgery is one of them, but in Russia, to date, there are no publications summarizing data about the experience of using this method throughout the country.The study objective is to evaluate the effect of radiosurgical thalamotomy (destruction of the intermediate ventral nucleus of the thalamus) on the severity of tremor in patients with Parkinson’s disease, as well as to estimate the frequency of complications of this intervention, the frequency of different variants of radiological and clinical-neurological response.Materials and methods. One hundred and one (101) patients with medically refractory Parkinson’s tremor received Gamma Knife radiosurgery in our centre over a 10‑year period, of whom 10 patients were treated bilaterally. Pre-treatment evaluation was done using tremor assessment scales and video recordings of tremor intensity. Gamma Knife radiosurgery was provided with Leksell Gamma Knife 4C and Leksell Gamma Knife Perfexion (Elekta AB, Sweden) preceded by magnetic resonance tractography the day before treatment. The same planning protocol was used for all patients, the target was ventral intermediate nucleus and the prescription dose of 130 Gy was delivered with a 4 mm isocenter. After treatment, patients were evaluated radiologically and neurologically at regular 6‑month intervals, as well as through telephone interviews and video recordings.Results. Ninety-two (92) patients were included in the analysis of the results of radiosurgical treatment. Median follow-up time after radiosurgery was 26 months, from 6 to 113 months. Tremor reduction was achieved in 76 % of the patients, 61 % of whom had almost complete tremor arrest. Decreased tremors were observed from 1 to 6 months after treatment, with a mean of 4 months. Three (3) patients experienced tremor resurgence after 3–7 years. Complications were observed in 6.5 % of patients and were transient in character. Some of the patients developed severe depression. Only 1 patient had a serious complication in the form of thalamic haemorrhage (at 22 months after treatment). Ten (10) patients who were treated bilaterally all showed considerable clinical improvement and absence of complications.Conclusion. Gamma Knife radiosurgery is a safe and effective functional neurosurgical procedure for tremor correction to improve patients’ quality of life, especially when deep brain stimulation is not accessible. Patients with severe medically refractory tremors are good candidates for Gamma Knife treatment, which showed high efficacy and a low risk of complications.

Experimental Validation of Monte Carlo Simulation for the Leksell Gamma Knife Perfexion Using Gafchromic EBT3 Dosimetry Film and Diamond Detector T60019 PTW

This work presents experimental measurements performed using GAFChromic EBT3 dosimetry film and diamond detector T60019 to validate penEasy Monte Carlo based model developed previously for the Leksell Gamma Knife Perfexion. The relative dose profiles along X and Z coordinate axes, for each collimator size 4, 8, and 16 mm, and the relative output factors have been investigated. The penEasy model has been validated comparing the calculated output factors and dose profiles with the corresponding experimental results. The differences between the calculated output factors and the corresponding data measured with diamond detector were found to be 1.8 and 2.1% for the 8 and 4 mm collimator, respectively. Excellent agreement was found between penEasy and EBT3 results. The differences in the output factors were found to be 0.4% and −0.7% for the 8 and 4 mm collimators, respectively. The relative dose profiles were found to be in good agreement for both detectors and penEasy data. The gamma index test returns values (3%, 1 mm) < 1 for all profiles comparison calculated between penEasy and EBT3 film.

Dose evaluation of organs at risk during treatment using gamma knife stereotactic radiosurgery (GKSRS): phantom study

Gamma Knife Stereotactic Radiosurgery (GKSRS) treatment requires the high accuracy of delivering single high dose without negative impact on the surrounding tissue. During radiosurgery treatments using GKSRS, the surrounding tissue will receive high scatter radiation because the single dose treatment will employed long irradiation time. Therefore, radiation doses in critical organs of patients during GKSRS treatment is important to be evaluated. In this study, we investigated the radiation dose to the lens, thyroid, breast, uterine fundus, ovaries and testes during treatment using Leksell Gamma Knife Perfexion (LGK; Elekta AB, Stockholm, Sweden). The dose was simulated using the variation of tumour target from 5 to 20 cc. The measurement were performed using variation of collimator size of 4 mm, 8 mm, 16 mm and employed thermoluminescent dosimeters (TLDs) to be placed at the surface of Rando anthropomorphic phantom. TLDs reader Harshaw model 3500 were employed to evaluate the measurement result. The result of this study explains that the dose of organs at risk decrease linearly depend on the distance of the target, target volume and the size of collimator.

Verification of dose distribution on the gamma knife perfexion radiosurgery using gafchromic EBT3 film: RANDO phantom study

Leksell gamma knife (LGK) is an advanced modality of radiation therapy sourced Co-60 radioactive for treating patient with intracranial lesion. Small field techniques with highly integrated radiation delivering to patients in single session must be calculated accurately and verified carefully. This study illustrates a procedure to verify the accuracy of dose distribution associated with Leksell Gamma Plan (LGP) using RANDO phantom and gafchromic EBT3 film dosimetry. First, we assessed the profile dose on LGK standard phantom with collimators size 4, 8 and 16 mm and compared the results with the profile dose based on RANDO to obtained Full Width Half Maximum (FWHM) and beam-symmetry. Absorbed-dose distributions on RANDO with various combinations of lesion volume, collimator size, location and number of shots assessed by EBT3 film using LGK Perfexion. Scanned images of the measured films were processed following standard EBT3 film-handling procedures. Dose distribution calculation were performed using ImageJ software and MATLAB in-house software. The study shows different FHWM and beam symmetry of collimator size of 16 mm on standard phantom between LGP and measurement is 1.83 % and 1.58 % respectively whereas the discrepancy in RANDO phantom is 2.15 %, and 1.64 % respectively. Verification of max dose shows the smallest discrepancy on collimator size 16 single shot is 1.9 % on 7.61 cc superior lesion volume, where 12.12 Gy obtained by measurement and 12.36 Gy from LGP. In conclusion, collimator size 16 mm have a highest accuracy of FWHM and beam-symmetry value. However, on smaller lesion volume, collimator 16 mm with single shot give higher deviation dose value.

Эффективный способ моделирования системы Leksell Gamma Knife Perfexion методом поворота в файле фазового пространства

Purpose: To develop an effective method of Monte Carlo simulation of the GammaKnife Perfexion system by rotating particles in the phase space file (PSF). This method does not require simulating of all 192 sources that are distributed in the conical form of the Perfexion collimator. The simulation was performed only for 5 out of 192 sources for each collimator size.&#x0D; Material and methods: Monte Carlo simulation of dose distribution for previous models of GammaKnife system requires phase space file for only one source, since this phase space is identical for all the 201 sources. The Perfexion model is more complex due to the non-coaxial positions of the sources and the complexity of the collimator system itself.&#x0D; In this work, we present an effective method to simulate the Perfexion model using a phase space file. Penelope Monte Carlo code was used to perform this simulation. In this method, the PSF was obtained for one source in each ring, resulting in five files for each collimator size. PSF for other sources were created by azimuthal redistribution of particles, in the obtained PSF, by rotation around the Z-axis. The phase space files of the same ring were then stored together in a single file.&#x0D; Results: The paper presented MC simulation using the azimuthal redistribution of particles in the phase space file by rotation around the Z-axis. The simulation has been validated comparing the dose profiles and output factors with the data of the algorithm TMR10 planning system Leksell Gamma Plan (LGP) in a homogeneous environment. The acceptance criterion between TMR10 and Monte Carlo calculations for the profiles was based on the gamma index (GI). Index values more than one were not detected in all cases, which indicates a good agreement of results. The differences between the output factors obtained in this work and the TMR10 data for collimators 8 mm and 4 mm are 0.74 and 0.73 %, respectively.&#x0D; Conclusion: In this work successfully implemented an effective method of simulating the Leksell Gamma knife Perfexion system. The presented method does not require modeling for all 192 sources distributed in the conical form of the Perfexion collimator. The simulation was performed for only five sources for each collimator and their files PSF were obtained. These files were used to create the PSF files for other sources by azimuthal redistribution of particles, in these files, by rotation around the Z-axis providing correct calculations of dose distributions in a homogeneous medium for 16, 8 and 4 mm collimators.

Gamma Knife radiosurgery for vestibular schwannomas: Evaluation of planning using the sphericity degree of the target volume.

IntroductionThis study explores the possibility of a relationship between the sphericity degree of a target volume with the dose distribution. This relationship is evaluated based on the ratio isodose volume / target volume (IV/TV) and the metrics coverage, i.e., selectivity, gradient index, conformity index and mean dose when planning radiosurgery for vestibular schwannoma.MethodsSphericity degree (φ) was calculated for each target volume (TV) of 64 patients who underwent stereotactic radiosurgery (SRS) for vestibular schwannoma. The calculation of this parameter was developed using the theoretical definition for operational sphericity φ = VP/VCS. The values found are evaluated considering the following metrics:—Coverage (C), selectivity (S), gradient index (GI), Paddick conformity index (CIPaddick) and dose distribution (IV/TV). The planning was also carried out considering a spherical target volume defined in a spherical phantom. The spherical volume is the same as the target used in the treatment plan. The planning of the spherical target was considered as a reference plan to evaluate the dose distribution inside and outside the volume.ResultsIt was possible to observe that the majority of target volumes has (ϕ) around 0,66–0,77, corresponding to 54,7% of the total. Considering the mean values for metrics, the results are: C = 0,98, S = 0,78, GI = 3,11 and CI = 0,81. The dose distribution was equivalent for treatment plans and reference plans. Quantitative analysis for IV/TV shows that these values are higher than 30% for treatment plans where shot density is large.ConclusionThis study demonstrates that de sphericity degree (φ) can be related to the dose distribution (IV/TV). Therefore the sphericity degree is a good parameter to evaluate the dose distribution of a plan for vestibular schwannoma treatment, considering the reference plan as being a spherical target using a leksell gamma knife® perfexion (LGKP). This study shows that the sphericity degree offers important information of the dose distribution outside and inside the target volume. This is not evaluated by the other parameters already implemented as metric to analyzing the GKP plans.

Radiomics-Based Prediction of Long-Term Treatment Response of Vestibular Schwannomas Following Stereotactic Radiosurgery.

Stereotactic radiosurgery (SRS) is one of the treatment modalities for vestibular schwannomas (VSs). However, tumor progression can still occur after treatment. Currently, it remains unknown how to predict long-term SRS treatment outcome. This study investigates possible magnetic resonance imaging (MRI)-based predictors of long-term tumor control following SRS. Retrospective cohort study. Tertiary referral center. Analysis was performed on a database containing 735 patients with unilateral VS, treated with SRS between June 2002 and December 2014. Using strict volumetric criteria for long-term tumor control and tumor progression, a total of 85 patients were included for tumor texture analysis. All patients underwent SRS and had at least 2 years of follow-up. Quantitative tumor texture features were extracted from conventional MRI scans. These features were supplied to a machine learning stage to train prediction models. Prediction accuracy, sensitivity, specificity, and area under the receiver operating curve (AUC) are evaluated. Gray-level co-occurrence matrices, which capture statistics from specific MRI tumor texture features, obtained the best prediction scores: 0.77 accuracy, 0.71 sensitivity, 0.83 specificity, and 0.93 AUC. These prediction scores further improved to 0.83, 0.83, 0.82, and 0.99, respectively, for tumors larger than 5 cm. Results of this study show the feasibility of predicting the long-term SRS treatment response of VS tumors on an individual basis, using MRI-based tumor texture features. These results can be exploited for further research into creating a clinical decision support system, facilitating physicians, and patients to select a personalized optimal treatment strategy.

Gamma Knife Radiosurgery and Immunotherapy as Primary Treatment for a Malignant Tumor of the Cranial Base Beginning as Lentigo Maligna: A Case Report

Metastatic spread of facial melanoma to cranial nerves causing neuropathy was described as early as 1989. 1 Jain S. Allen P.W. Desmoplastic malignant melanoma and its variants. A study of 45 cases. Am J Surg Pathol. 1989; 13: 358-373 Crossref PubMed Scopus (203) Google Scholar The trigeminal nerve is involved in several reported cases. 1 Jain S. Allen P.W. Desmoplastic malignant melanoma and its variants. A study of 45 cases. Am J Surg Pathol. 1989; 13: 358-373 Crossref PubMed Scopus (203) Google Scholar , 2 Kano H. Niranjan A. Novotny Jr., J. Bhatnagar J. Flickinger J.C. Lunsford L.D. Radiosurgery for desmoplastic melanoma of the head and neck using the Leksell Gamma Knife Perfexion technology: A case report. Stereotact Funct Neurosurg. 2009; 87: 61-65 Crossref PubMed Scopus (3) Google Scholar , 3 Majoie C.B. Hulsmans F.J. Castelijns J.A. Walter A. Bras J. Peeters F.L. Perineural tumor extension of facial malignant melanoma: CT and MRI. J Comput Assist Tomogr. 1993; 17: 973-975 Crossref PubMed Scopus (17) Google Scholar , 4 Peterson H.E. Larson E.W. Fairbanks R.K. et al. Gamma knife radiosurgery treatment for metastatic melanoma of the trigeminal nerve and brainstem: A case report and a review of the literature. Case Rep Neurol Med. 2013; 2013: 256962 PubMed Google Scholar , 5 Tritschler P. Rezazadeh Azar A. De Coene B. Maraite N. Michotte A. Trigeminal melanoma metastasis. Clin Neuroradiol. 2014; 24: 51-54 Crossref PubMed Scopus (5) Google Scholar , 6 Walters H. Lewis E. Wolper R. Yachnis A.T. Green J. Lewis S. Neurotropic melanoma of the trigeminal nerve: A case of atypical facial pain. J Oral Maxillofac Surg. 2008; 66: 547-550 Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar Presenting signs and symptoms may include hypoesthesia, paresthesia, and facial pain. Common magnetic resonance imaging (MRI) findings include abnormal nerve enlargement, masseter denervation atrophy, and enhancement within Meckel's cave. 7 Majoie C.B. Hulsmans F.J. Verbeeten Jr., B. et al. Trigeminal neuralgia: Comparison of two MR imaging techniques in the demonstration of neurovascular contact. Radiology. 1997; 204: 455-460 Crossref PubMed Scopus (79) Google Scholar , 8 Majoie C.B. Hulsmans F.J. Verbeeten Jr., B. et al. Perineural tumor extension along the trigeminal nerve: Magnetic resonance imaging findings. Eur J Radiol. 1997; 24: 191-205 Abstract Full Text PDF PubMed Scopus (40) Google Scholar It is difficult to diagnose metastatic disease without histologic confirmation, but metastases should be highly considered in a patient presenting with a cranial neuropathy, history of a facial lesion, and remarkable image findings.

Gamma Knife radiosurgery for intravestibular and intracochlear schwannomas.

Schwannomas of the VIIIth cranial nerve are benign tumours, with vast majority occurring in vestibular division. Rarely, they can also arise from distal branches of cochlear, superior or inferior vestibular. We review our experience with Gamma Knife radiosurgery (GKR), as first intention treatment for intracochlear (ICS) and intravestibular (IVS) schwannomas. A total number of five patients were analysed, treated over 8years, between June 2010 and September 2018, with Leksell Gamma Knife Perfexion or Icon (Elekta Instruments, AB, Sweden). The marginal dose prescribed was 12Gy at a mean prescription isodose line of 61.4% (range 50-70). Clinical evaluation included auditory and facial function. The mean age was 49.9 (range 34-63). The mean follow-up period was 52.8months (range 12-84). The mean target volume (TV) was 0.087ml (range 0.014-0.281). The mean maximal dose received by the cochlea was 11.2Gy (range 2.6-20.3). The mean marginal dose received by the vestibule (e.g. utricula) was 14.2Gy (range 3.8-17.5). No patient experienced an acute or subacute clinical adverse radiation effect after GKR. Four cases had overall symptom stability. In one patient (1/5), the vertigo, which was the main clinical complain, disappeared 1year after GKR. However, it reappeared 3years later, with same pretherapeutic characteristics and is currently fluctuating. One patient experienced hearing decrease after GKR, during the first 12months. This case received 11.2Gy to the cochlea. Follow-up MRI course showed a decrease in size in four patients, and stability in one. Gamma Knife radiosurgery is a valuable first intention treatment for ICS or IVS, in selected cases. Special attention should be paid for the dose delivered to the cochlea and the vestibular apparatus. Acute and subacute clinical effects are exceptional, while tumour control was achieved in all cases in our small series.

Improved Dose Conformity for Adjacent Targets: A Novel Planning Technique for Gamma Knife Stereotactic Radiosurgery

PurposeIn the current Gamma Knife (GK) planning system (GammaPlan, version 10.2, Elekta AB, Stockholm, Sweden), multiple adjacent brain metastasis (BMs) had to be planned sequentially if BMs were drawn separately, leading to less conformal target dose in the composite plan due to inter-target dose contribution and fine-tuning of the shots being quite tedious. We proposed a method to improve target dose conformality and planning efficiency for such cases.Methods and MaterialsFifteen patients with multiple BMs treated on the Leksell GK Perfexion system were retrospectively replanned in the Institutional Review Board (IRB) approved study. The recruitment criterion was all the BMs should be entirely encompassed within the maximum dose grid allowed in the GammaPlan. The BMs were first planned sequentially as routine clinic cases. The contours of the BMs were then exported to the VelocityAI (Varian, CA, USA) to generate a composite contour after a union operation, and all the BMs were planned again simultaneously using this composite contour in the GammaPlan. The inverse planning (IP) was employed in both methods with the same treatment time allowed for a fair plan comparison. Dose evaluation was performed in the VelocityAI with all planning magnetic resonance (MR) images, structure set and dose were exported to the VelocityAI. The dosimetery parameters, including conformality index (CI), V20Gy, V16Gy, V12Gy, and V5Gy, were compared between the two methods.ResultsThe planning results from both methods were reviewed qualitatively and quantitatively. The proposed method exhibited superior CI, except for an outlier case with very tiny BMs. The mean and standard deviation (std.) of the Paddick CI for all patients were 0.76±0.11 for the proposed method, comparing to 0.69±0.13 for the sequential method. The V20Gy, V16Gy, V12Gy, and V5Gy for the proposed method were 10.9±0.9%, 9.5±10.2%, 6.2±16.4% and 3.3±21.8%, all lower than those from the sequential method.ConclusionsThe proposed method showed improved target dose conformality for all cases except for very tiny BMs. Planning efficiency is considerably better with the combined target technique. The improved dose conformality will be beneficial to patients in long term with lowered risk of radiation necrosis after GK stereotactic radiosurgery (SRS).

STEREOTACTIC RADIOSURGICAL TREATMENT OF THE STALK CRANIOPHARYNGIOMA WITH APPARATUS LEKSELL GAMMA KNIFE PERFEXION

Craniopharyngioma is a benign tumor of the brain, developing from the epithelial cells of craniopharyngeal duct, from which are formed abiogenetically the stem and rear part of the pituitary gland. The peculiarity of stem craniopharyngiomas is dense accretion of the tumor with arterial and neural structures of supra-para-retrosellar area, which makes surgical treatment technically difficult, and sometimes — obviously impossible. In the world literature at this point does not contain messages about the isolated stem radiosurgical treatment of craniopharyngiomas with application installations, Gamma knife or CyberKnife. A clinical case of stereotactic radiosurgical treatment in the mode of hypofractionation on the Leksell Gamma Knife Perfexion apparatus with a positive result in the form of a significant regression of visual disturbances is presented. Index terms: stereotactic radiosurgery, hypofractionation, craniopharyngioma, sellar region.

Independent dose validation system for Gamma Knife radiosurgery, using a DICOM-RT interface and Geant4

Leksell GammaPlan was specifically designed for Gamma Knife (GK) radiosurgery planning, but it has limited accuracy for estimating the dose distribution in inhomogeneous areas, such as the embolization of arteriovenous malformations. We aimed to develop an independent patient dose validation system based on a patient-specific model, constructed using a DICOM-RT interface and the Geant4 toolkit. Leksell Gamma Knife Perfexion was designed in Geant4.10.00 and includes a DICOM-RT interface. Output factors for each collimator in a sector and dose distributions in a spherical water phantom calculated using a Monte Carlo (MC) algorithm were compared with the output factors calculated by the tissue maximum ratio (TMR) 10 algorithm and dose distributions measured using film, respectively. Studies using two types of water phantom and two patient simulation cases were evaluated by comparing the dose distributions calculated by the MC, the TMR and the convolution algorithms. The water phantom studies showed that if the beam size is small and the target is located in heterogeneous media, the dose difference could be up to 11%. In the two patient simulations, the TMR algorithm overestimated the dose by about 4% of the maximum dose if a complex and large bony structure was located on the beam path, whereas the convolution algorithm showed similar results to those of the MC algorithm. This study demonstrated that the in-house system could accurately verify the patient dose based on full MC simulation and so would be useful for patient cases where the dose differences are suspected.

Inhibitors of HIF-1α and CXCR4 Mitigate the Development of Radiation Necrosis in Mouse Brain

There is mounting evidence that, in addition to angiogenesis, hypoxia-induced inflammation via the hypoxia-inducible factor 1α (HIF-1α)-CXC chemokine receptor 4 (CXCR4) pathway may contribute to the pathogenesis of late-onset, irradiation-induced necrosis. This study investigates the mitigative efficacy of an HIF-1α inhibitor, topotecan, and a CXCR4 antagonist, AMD3100, on the development of radiation necrosis (RN) in an intracranial mouse model. Mice received a single-fraction, 50-Gy dose of hemispheric irradiation from the Leksell Gamma Knife Perfexion and were then treated with either topotecan, an HIF-1α inhibitor, from 1 to 12weeks after irradiation, or AMD3100, a CXCR4 antagonist, from 4 to 12weeks after irradiation. The onset and progression of RN were monitored longitudinally via noninvasive, invivo magnetic resonance imaging (MRI) from 4 to 12weeks after irradiation. Conventional hematoxylin-eosin staining and immunohistochemistry staining were performed to evaluate the treatment response. The progression of brain RN was significantly mitigated for mice treated with either topotecan or AMD3100 compared with control animals. MRI-derived lesion volumes were significantly smaller for both of the treated groups, and histologic findings correlated well with the MRI data. By hematoxylin-eosin staining, both treated groups demonstrated reduced irradiation-induced tissue damage compared with controls. Furthermore, immunohistochemistry results revealed that expression levels of vascular endothelial growth factor, CXC chemokine ligand 12, CD68, CD3, and tumor necrosis factor α in the lesion area were significantly lower in treated (topotecan or AMD3100) brains versus control brains, while ionized calcium-binding adapter molecule 1 (Iba1) and HIF-1α expression was similar, though somewhat reduced.CXCR4 expression was reduced only in topotecan-treated mice, while interleukin 6 expression was unaffected by either topotecan or AMD3100. By reducing inflammation, both topotecan and AMD3100 can, independently, mitigate the development of RN in the mouse brain. When combined with first-line, antiangiogenic treatment, anti-inflammation therapy may provide an adjuvant therapeutic strategy for clinical, postirradiation management of tumors, with additional benefits in the mitigation of RN development.

Determination of small field synthetic single-crystal diamond detector correction factors for CyberKnife, Leksell Gamma Knife Perfexion and linear accelerator

PurposeThe aim of this study was to determine small field correction factors for a synthetic single-crystal diamond detector (PTW microDiamond) for routine use in clinical dosimetric measurements. Materials and methodsCorrection factors following small field Alfonso formalism were calculated by comparison of PTW microDiamond measured ratio MQclinfclin/MQmsrfmsr with Monte Carlo (MC) based field output factors ΩQclin,Qmsrfclin,fmsr determined using Dosimetry Diode E or with MC simulation itself. Diode measurements were used for the CyberKnife and Varian Clinac 2100C/D linear accelerator. PTW microDiamond correction factors for Leksell Gamma Knife (LGK) were derived using MC simulated reference values from the manufacturer. ResultsPTW microDiamond correction factors for CyberKnife field sizes 25–5 mm were mostly smaller than 1% (except for 2.9% for 5 mm Iris field and 1.4% for 7.5 mm fixed cone field). The correction of 0.1% and 2.0% for 8 mm and 4 mm collimators, respectively, needed to be applied to PTW microDiamond measurements for LGK Perfexion. Finally, PTW microDiamond MQclinfclin/MQmsrfmsr for the linear accelerator varied from MC corrected Dosimetry Diode data by less than 0.5% (except for 1 × 1 cm2 field size with 1.3% deviation). ConclusionsRegarding low resulting correction factor values, the PTW microDiamond detector may be considered an almost ideal tool for relative small field dosimetry in a large variety of stereotactic and radiosurgery treatment devices.

Failure modes and effects analysis (FMEA) for Gamma Knife radiosurgery.

PurposeGamma Knife radiosurgery is a highly precise and accurate treatment technique for treating brain diseases with low risk of serious error that nevertheless could potentially be reduced. We applied the AAPM Task Group 100 recommended failure modes and effects analysis (FMEA) tool to develop a risk‐based quality management program for Gamma Knife radiosurgery.MethodsA team consisting of medical physicists, radiation oncologists, neurosurgeons, radiation safety officers, nurses, operating room technologists, and schedulers at our institution and an external physicist expert on Gamma Knife was formed for the FMEA study. A process tree and a failure mode table were created for the Gamma Knife radiosurgery procedures using the Leksell Gamma Knife Perfexion and 4C units. Three scores for the probability of occurrence (O), the severity (S), and the probability of no detection for failure mode (D) were assigned to each failure mode by 8 professionals on a scale from 1 to 10. An overall risk priority number (RPN) for each failure mode was then calculated from the averaged O, S, and D scores. The coefficient of variation for each O, S, or D score was also calculated. The failure modes identified were prioritized in terms of both the RPN scores and the severity scores.ResultsThe established process tree for Gamma Knife radiosurgery consists of 10 subprocesses and 53 steps, including a subprocess for frame placement and 11 steps that are directly related to the frame‐based nature of the Gamma Knife radiosurgery. Out of the 86 failure modes identified, 40 Gamma Knife specific failure modes were caused by the potential for inappropriate use of the radiosurgery head frame, the imaging fiducial boxes, the Gamma Knife helmets and plugs, the skull definition tools as well as other features of the GammaPlan treatment planning system. The other 46 failure modes are associated with the registration, imaging, image transfer, contouring processes that are common for all external beam radiation therapy techniques. The failure modes with the highest hazard scores are related to imperfect frame adaptor attachment, bad fiducial box assembly, unsecured plugs/inserts, overlooked target areas, and undetected machine mechanical failure during the morning QA process.ConclusionsThe implementation of the FMEA approach for Gamma Knife radiosurgery enabled deeper understanding of the overall process among all professionals involved in the care of the patient and helped identify potential weaknesses in the overall process. The results of the present study give us a basis for the development of a risk based quality management program for Gamma Knife radiosurgery.

Abstract ID: 157 Development of Geant4-based patient-specific QA system of gamma knife treatment plans using automated DICOM-RT interface

Gamma knife system which uses 192 small photon beams is widely considered as a very precise modality for intracranial radiosurgery. However, there are still limitations in current treatment planning system (TPS) for accurately estimating the absolute dose in the small volume, or the dose distribution in the inhomogeneous area such as the case of embolization of arteriovenous malformations and the located target volume behind an air cavity [1] . The purpose of this study is to develop quality assurance (QA) system of gamma knife treatment plans based on a patient-specific model constructed by an automated DICOM-RT interface in Geant4. The Leksell Gamma Knife Perfexion was designed in Geant4.10.00 based on the manufacturer’s detailed information. Dose distributions in the spherical water phantom with a diameter of 16 cm were compared between Geant4 and the film measurement for the validation system. For extracting the patient-dependent parameters, automated DICOM-RT interface was developed for constructing the patient-specific model. In-house developed system is able to manually define the physical property of a special volume using the region of interest (ROI) contour data stored in a DICOM file. The dose distributions in the water phantom were compared between Geant4, measurement data and manufacture’s values in terms of full width at half maximum (FWHM). FWHM of Geant4 and measurement were quite well matched with manufacture’s value within 0.3, 0.2 and 0.7 mm in both the x and y direction for 4, 8 and 16 mm, respectively. Furthermore, the dose distributions of Geant4 and commercial TPS were well matched for different beam size combinations for each of the eight sectors. The developed QA system shows the potential to precisely estimate the dose distribution in the patients. In the future, assessment of the absolute dose distribution calculated by Geant4 and commercial TPS for the clinical cases will be performed. Moreover, magnetic resonance (MR) images will be employed in our system for sophisticated modelling the heterogeneities in the patients.

A Monte Carlo model and its commissioning for the Leksell Gamma Knife Perfexion radiosurgery system.

To develop and commission a Monte Carlo (MC) simulation model for the Leksell Gamma Knife (LGK) Perfexion (PFX) radiosurgery system. We previously established a source model for MC simulations of the LGK PFX for the purpose of the treatment planning system (TPS) dose verification and plan evaluation. To make practical and effective use of the model in clinic, several issues need to be addressed. First, thorough commissioning procedures are needed to ensure the validity of the model parameters, such as the source-to-focus (STF) distance, the source solid angle. Second, an efficient source particle sampling method is required to facilitate dose calculations for multitarget and multishot configurations in patient treatment plans. Third, inseparably, it is interesting to know the dose difference between the two GK TPS algorithms (TMR and convolution) and the MC method in extreme heterogeneous cases resulting from the inhomogeneous effect. We report our recent development in addressing these issues. Phantoms with the frame fiducials were manually created in the format of DICOM CT image to eliminate the uncertainties associated with scanner artifacts and image registration. The created homogeneous phantom was used to calibrate the model parameters to match the output factors with the manufacturer provided data, and the heterogeneous phantom with multilayer materials was used to study the inhomogeneous effect. The agreement between the MC calculation and TPS was very good for the homogeneous spherical phantom. The difference of the full width at half maximum (FWHM) of the profiles was less than 1 mm except for the profile for 16 mm collimator along z-axis (less than 2 mm). For the extreme heterogeneous test case, it was shown that the TMR algorithm can overestimate the target dose by up to 22% using the measure of dose volume parameter D95. The agreement between the MC method and the TPS convolution method was better (within 3.6%) for the target near the center of phantom, however, discrepancy (up to 10.7%) existed for the target close to the skull. The difference between the two TPS dose algorithms was about 11%. Considerable dose difference may result from the effect of heterogeneity, such as in the regions of the air cavities and bones. As the MC method has been extensively used in conventional external beams, it is worthwhile for further investigation in applying the MC method to accurate dose planning in the new GK PFX radiosurgery platform.

Evaluation of Clinical Application and Dosimetric Comparison of Treatment Plans of Gamma Knife and CyberKnife in Treating Arteriovenous Malformations

Purpose: To analyze and compare the characteristics of dose distributions for Leksell Gamma Knife Perfexion (LGK-PFX) and CyberKnife (CK) in treating arteriovenous malformations (AVMs). Subjects and Methods: Twenty-four patients with AVMs who received CK radiosurgery at a prescribed dose (PD) of 16-25 Gy in a single fraction were selected. A LGK-PFX treatment plan with the same PD was designed for each patient. Dosimetric values for both systems were compared with respect to the conformity index (CI); selectivity index (SI); gradient index (GI) of 75, 50, and 25% of the PD; heterogeneity index; volume of the brain tissue covered by doses of 10 and 12 Gy; maximum dose delivered to the brainstem; and beam-on time. Results: The CIs of LGK-PFX and CK were 0.744 ± 0.075 and 0.759 ± 0.071 (p = 0.385), respectively. The SIs of LGK-PFX and CK were 0.764 ± 0.081 and 0.780 ± 0.076 (p = 0.424), respectively. The GI_75%, GI_50%, and GI_25% values of LGK-PFX and CK were 1.028 ± 0.123 and 2.439 ± 0.338 (p < 0.001), 3.169 ± 0.265 and 4.972 ± 0.852 (p < 0.001), and 8.650 ± 0.914 and 14.261 ± 2.476 (p < 0.001), respectively. Volumes of the brain tissue covered by 10 Gy and 12 Gy for LGK-PFX and CK (p < 0.001) exhibited a significant difference. Conclusions: LGK-PFX and CK exhibited similar dose conformity. LGK-PFX showed superior normal tissue sparing.