Linac Stereotactic Radiosurgery Research Articles

RADT-22. STEREOTACTIC RADIOSURGERY FOR VESTIBULAR SCHWANNOMA WITH RADIOGRAPHIC BRAINSTEM COMPRESSION

Abstract BACKGROUND The safety of single treatment stereotactic radiosurgery (SRS) for vestibular schwannoma (VS) with radiographic evidence of brainstem compression- but without motor deficit- is controversial. Data on linac-based SRS in this setting are scarce. We address this deficit in the literature with a report of outcome of an unselected series of patients with VS with radiographic brainstem compression treated with linac SRS our institution. METHODS We included patients with unilateral VS of any size with radiographic brainstem compression at the time of SRS. All patients were without serious brainstem neurologic deficits at the time of SRS. The SRS prescription dose was 12.5 Gy in a single fraction using 6MV photon beams produced by a linear accelerator and delivered with a multiple arc technique. Inclusion in this study required at least 1 year of radiographic follow-up with magnetic resonance imaging (MRI). The primary endpoint was freedom from serious brainstem toxicity (CTCAEv5 grade >3); secondary endpoint was freedom from tumor enlargement considered to represent tumor progression, or any type of enlargement requiring surgical intervention. We also assessed serious cranial nerve complications, excluding hearing loss, defined as CTCAEv5 grade 3 toxicity. RESULTS Among 139 patients, the median MRI follow-up time was 5 years and the median tumor size was 2.5 cm greatest axial dimension and 5cc in volume. The median brainstem D0.03cc = 12.6 Gy, and the median brainstem V10Gy = 0.4cc. At 5 years, the actuarial freedom from serious brainstem toxicity was 100% (zero cases), and freedom from tumor enlargement, either requiring surgery or due to progression, was 90%. The rate of grade 3 facial nerve damage in patients without tumor enlargement was 0.9% (1/113). CONCLUSION Linac-based SRS as delivered in our series for VS with radiographic brainstem compression is safe and effective.

Dosimetric Analysis of Intra-Fraction Motion Detected by Surface-Guided Radiation Therapy During Linac Stereotactic Radiosurgery

Stereotactic radiosurgery (SRS) immobilization with an open face mask is more comfortable and less invasive than frame based, but concerns about intrafraction motion must be addressed. Surface-guided radiation therapy (SGRT) is an attractive option for intrafraction patient monitoring because it is continuous, has submillimeter accuracy, and uses no ionizing radiation. The purpose of this study was to investigate the dosimetric consequences of uncorrected intrafraction patient motion detected during frameless linac-based SRS. Fifty-five SRS patients were monitored during treatment using SGRT between January 1, 2017, and September 30, 2020. If SGRT detected motion >1 mm, imaging was repeated and the necessary shifts were made before continuing treatment. For the 25 patients with intrafraction 3-dimensional vector shifts of ≥1 mm, we moved the isocenter in the planning system using the translational shifts from the repeat imaging and recalculated the plans to determine the dosimetric effect of the shifts. Planning target volume (PTV) coverage, minimum gross tumor volume (GTV) dose (relative and absolute), and normal brain V12 were evaluated. Wilcoxon signed rank tests were used to compare planned and simulated dosimetric parameters and median 2 sample tests were used to investigate these differences between cone and multileaf collimator (MLC) plans. For simulated plans, V12 increased by a median of 0.01 cc (P=.006) and relative GTV minimum dose and PTV coverage decreased by a median of 15.8% (P < .001) and 10.2 % (P < .001), respectively. Absolute minimum GTV dose was found to be significantly lower in the simulated plans (P < .001). PTV coverage decreased more for simulated cone plans than for simulated MLC plans (11.6% vs 4.7%, P=.011) but median V12 differences were found to be significantly larger for MLC plans (-0.34 cc vs -0.01 cc, P=.011). Differences in GTV minimum dose between cone and MLC plans were not statistically significant. SGRT detected clinically meaningful intrafraction motion during frameless SRS, which could lead to large underdoses and increased normal brain dose if uncorrected.

Image-guided LINAC radiosurgery in hypothalamic hamartomas.

IntroductionHypothalamic hamartomas (HH) are developmental malformations that are associated with mild to severe drug-refractory epilepsy. Stereotactic radiosurgery (SRS) is an emerging non-invasive option for the treatment of small and medium-sized HH, providing good seizure outcomes without neurological complications. Here, we report our experience treating HH with frameless LINAC SRS.Materials and methodsWe retrospectively collected clinical and neuroradiological data of ten subjects with HH-related epilepsy that underwent frameless image-guided SRS.ResultsAll patients underwent single-fraction SRS using a mean prescribed dose of 16.27 Gy (range 16–18 Gy). The median prescription isodose was 79% (range 65–81 Gy). The mean target volume was 0.64 cc (range 0.26–1.16 cc). Eight patients experienced complete or near complete seizure freedom (Engel class I and II). Five patients achieved complete seizure control within 4 to 18 months after the treatment. Four patients achieved Engel class II outcome, with stable results. One patient had a reduction of seizure burden superior to 50% (Engel class III). One patient had no benefit at all (Engel class IV) and refused further treatments. Overall, at the last follow-up, three patients experience class I, five class II, one class III and one class IV outcome. No neurological complications were reported.ConclusionsFrameless LINAC SRS provides good seizure and long-term neuropsychosocial outcome, without the risks of neurological complications inherently associated with microsurgical resection.

LINAC stereotactic radiosurgery for brain arteriovenous malformations: An updated single centre analysis of outcomes

Arteriovenous malformations (AVM) of the brain are congenital, high pressure vascular malformations, which are at risk of haemorrhage. Stereotactic radiosurgery (SRS) can obliterate the nidus by delivering a precise high dose of ionising radiation in a single fraction. This paper updates long term AVM obliteration rates, time to obliteration and retreatment outcomes in LINAC delivered SRS treatment at the Royal Adelaide Hospital. A retrospective review of a prospectively maintained AVM SRS database supplemented by clinical case notes, patient correspondence and electronic medical records was performed. 89 AVMs received primary SRS treatment for which the crude obliteration rate was 61% (68% for 79 patients with adequate follow up). Higher marginal dose, smaller nidus size and lower Pollock-Flickinger (PF) score were significantly associated with AVM obliteration. The crude obliteration rates for patients with adequate follow-up and AVM diameter < 3 cm vs ≥ 3 cm were 76% vs 48%, respectively, and 93% with PF score < 1.0. Median time to obliteration was 36 months. Higher dose and lower PF score were associated with earlier obliteration. The crude obliteration rate after second SRS was 56% (9/16 patients) and no significant associations were found. These obliteration rates after primary and retreatment LINAC SRS are comparable to other studies. Marginal dose and PF score were the main predictors of obliteration overall as well as early (<36 months) obliteration.

Dosimetric Analysis of Intra-Fraction Motion Detected by Surface Guided Radiation Therapy During Linac Stereotactic Radiosurgery

Stereotactic radiosurgery (SRS) requires very sharp dose gradients, high conformality and typically uses minimal expansion margins. Immobilization with an open face mask is more comfortable and less invasive than frame-based immobilization but concerns about intra-fraction motion must be addressed. Surface guided radiation therapy (SGRT) is an attractive option for intra-fraction patient monitoring because it is continuous, has sub-millimeter accuracy and uses no ionizing radiation. The purpose of this study was to investigate the dosimetric consequences of uncorrected intra-fraction patient motion detected during frameless Linac-based SRS.All SRS patients (pts) were immobilized in an SRS-specific open face thermoplastic mask with bite plate and were monitored during treatment using SGRT with tolerances of 1 mm translations and 1° rotations; the treatment beam automatically turned off if these tolerances were exceeded. SRS pts received 18 or 21 Gy in 1 fraction or 27 Gy in 3 fractions based on tumor size and location. GTV to PTV margins ranged from 0 to 3 mm. Twelve pts were treated with circular cones and 13 were treated with MLC-based plans with the technique chosen based on PTV size. If the SGRT system detected motion greater than tolerance during treatment, conebeam CT(CBCT) was repeated and the necessary shifts were made prior to continuing treatment. For the 25 pts with intra-fraction 3D vector shifts of at least 1 mm, we offset the isocenter in the treatment planning system using the translational shifts from the repeat CBCT and re-calculated the plans with the planned MU to determine the dosimetric effect the shift would have caused had it not been detected. The GTV and PTV coverage and normal brain V12 for the shifted plans were compared to the planned values. Wilcoxon signed rank tests were used to compare planned and shifted dosimetric indices and median 2 sample tests were used to investigate these differences between cone and MLC plans.From the recalculated plans, the resulting median V12 increased by 0.24 cc (P = 0.006) and the median GTV and PTV coverage decreased 9% (P < 0.001) and 10.2 % (P < 0.001), respectively. PTV coverage decreased more for shifted cone plans than for shifted MLC plans (11.6% vs 4.7%, P = 0.011) but the median V12 differences were found to be significantly larger for MLC plans (-0.34 cc vs -0.01 cc, P = 0.011), due to the sharper dose gradients and higher conformality for the cone plans compared to the MLC plans. Differences in GTV coverage between cone and MLC plans were not statistically significant.SGRT detected clinically meaningful intra-fraction motion during frameless SRS, which could potentially lead to large underdoses and increased normal brain dose if not corrected. Dosimetric differences between planned and shifted plans were statistically significant.R.D. Foster: None. B.J. Moeller: Independent Contractor; Novant Health. Partner; SERO. Stock; Moderna. M. Robinson: None. M. Bright: SEAAPM.J.L. Ruiz: Stock; Obseva USA, Inc. C.J. Hampton: None. J.H. Heinzerling: Research Grant; Vision RT, Inc, AstraZeneca. Honoraria; Vision RT, Inc, AstraZeneca. Consultant; Vision RT, Inc.

Gamma knife radiosurgery of epidermoid tumors: an analysis of treatment results of eight patients.

Epidermoid tumors (ETs) of the central nervous system (CNS) are rare tumors that typically occur in the 4th decade. They typically grow around vital neurovascular structures which makes surgical treatment difficult. The objective of this paper is to report on the effectiveness and safety in the management of epidermoid tumors with gamma knife surgery (GKS). This is a retrospective study of the medical records of 8 patients treated with GKS for epidermoid tumors between July 2010 to June 2019. The median prescription dose was 11 Gy, ranging from 10 to 12 Gy, 5 patients received the total dose target to the 50% line and 3 to the 55% isodose line. The mean tumor volume was 12.4 cc ranging from 4.4 to 24.8cc. The median follow-up time was 33.7 months and ranged from 0.9 to 58.8 months. At follow-up, patients were evaluated for neurological signs and symptoms and radiographic evidence of progression of disease. Two patients were treated after failure of linac stereotactic radiosurgery. One patient underwent stereotactic radiosurgery prior to GKS, and the other had failed surgical resection prior to GKS. The median age was 33 years old. There were two males and six females. The most common presenting manifestation was headaches followed by vision and hearing problems. Symptoms were resolved in all cases, except for one who had partial control of trigeminal neuralgia. All patients were locally controlled by imaging and neurological examination at first follow-up. Gamma knife surgery is a safe and effective alternative treatment in patients with CNS epidermoid tumors and should be included in the initial recommendation.

Evaluation of surgery and radiosurgery for acromegaly: A review of efficacy, complications, and follow-up

Background: Acromegaly is a rare disease caused by an over-increase in growth hormone (GH) and insulin-like growth factors. If left untreated, acromegaly is associated with many complications and increased mortality. The three modalities of treatment for this disease are surgery, pharmacotherapy, and radiotherapy. Another treatment option is stereotactic radiosurgery (SRS), which is used as an adjunct and alternative treatment in patients with acromegaly who are not suitable options for surgery. Methods: The present study is a review study conducted by searching the databases of Elsevier, PubMed, Springer, and Wiley, and using the keywords of acromegaly, treatment, transsphenoidal surgery, and radiosurgery. Fifteen studies, which had been performed between 2010 and 2021, were selected for review. Results: The results of these studies indicated that the use of SRS (LINAC SRS and GKRS) after surgery and medical treatment, before surgery and during radiotherapy improve biochemical and endocrine control and the quality of life of patients. However, due to some side effects of these treatments, it is necessary to conduct further studies in this field. Conclusion: All three modalities of treatment would be effective in acromegaly if used with appropriate indication in right sequence. Keywords: Pituitary Neoplasms, Pituitary Gland, Radiotherapy, Radiosurgery, Acromegaly

PO-1465: Automated versus manual LINAC stereotactic radiosurgery plans in patients with 4-10 brain metastases

PO-1465: Automated versus manual LINAC stereotactic radiosurgery plans in patients with 4-10 brain metastases

Radiation-Induced Malignancy following Stereotactic Radiosurgery for Benign Intracranial Pathology

Radiation-induced malignant transformation is a risk associated with stereotactic radiosurgery (SRS); however, prevalence is uncertain. We sought to characterize the cumulative incidence of radiation-induced malignancy at our institution. Based on other retrospective studies, we hypothesized that the five-year incidence of malignant transformation and radiation-associated secondary central nervous system (CNS) cancer after SRS is less than 1%. After IRB approval, we undertook a retrospective consecutive case series evaluating patients with at least 5 years of event-free clinical follow-up treated with non-invasive SRS or Linac-based SRS between 1990 and 2014 for benign CNS pathology including: arteriovenous malformation (AVM), pituitary adenoma, WHO grade I-II meningiomas, vestibular schwannoma, paraganglioma, angiofibroma, chordoma, craniopharyngioma, epilepsy, ganglioglioma, hemangioblastoma, hemangiopericytoma, oligodendroglioma, papilloma, parotid adenoma, tremor, and trigeminal neuralgia. Radiation-associated secondary intracranial cancer was defined as a new primary CNS malignancy within the plan’s 2 Gy isodose line. Biologically effective dose (BED) was calculated using an alpha/beta ratio of 2. Baseline characteristics and treatment details were recorded and summarized with descriptive statistics. After exclusion of 362 patients lacking follow-up, a total of 273 patients were enrolled. Median age at treatment was 44 years (range 2-82) including pediatric (n=33) and adult (n=240) patients. SRS indications included AVM (n=100, 37%), pituitary adenoma (n=67, 25%), WHO grade I-II meningioma (n=39, 14%), vestibular schwannoma (n=32, 12%), paraganglioma (n=13, 4.8%), and other (n=22, 8%). Median prescription dose was 18 Gy (range 6-140 Gy), median number of fractions was 1 (range 1-5), and median BED was 131 Gy2(range 24-9940 Gy2). Median follow-up after SRS was 10.5 years (range 4.7-27). Over 3,216 patient-years following SRS, two cases of radiation-induced malignant transformation occurred for a per-patient rate of 0.73% or case rate of 0.62 per 1,000 patient-years. The first patient was treated with Linac SRS for vestibular schwannoma with 21 Gy in 3 fractions, and 16 years later developed an ipsilateral malignant peripheral nerve sheath tumor in the cerebellopontine angle. The second patient was treated with non-invasive stereotactic radiosurgery for vestibular schwannoma and 3 years later developed a malignant peripheral nerve sheath tumor. The five-year incidence of radiation-associated malignant transformation and radiation-induced secondary malignancy following SRS for benign CNS pathology at our institution is reassuringly low at 0.62 cases per 1,000 patient-years. These data support the safety of SRS for benign intracranial pathology.

Frameless Image-Guided Radiosurgery for Multiple Brain Metastasis Using VMAT: A Review and an Institutional Experience.

We undertook a structured review of stereotactic radiosurgery (SRS) using linear particle accelerator (linac) equipment, focusing on volumetric modulated arc therapy (VMAT) technology, and frameless image-guided radiotherapy (IGRT), for the treatment of brain metastases. We analyzed the role of linac SRS and its clinical applications, exploring stereotactic localization. Historically, there was a shift from fixed frames to frameless approaches, moving toward less invasive treatments. Thus, we reviewed the concepts of VMAT for multiple-target applications, comparing its dosimetric and technical features to those of other available techniques. We evaluated relevant technical issues and discussed the planning parameters that have gained worldwide acceptance to date. Thus, we reviewed the current literature on the clinical aspects of SRS, especially its main indications and how the advantages of VMAT may achieve clinical benefits in such scenarios. Finally, we reported our institutional results on IGRT-VMAT for SRS treatments for patients with multiple brain metastases.

Analysis of morphological measurements of the trigeminal nerve in the linac stereotactic radiosurgery simulation targeting the root entry zone in trigeminal neuralgia

PurposeTo describe the anatomical measurements of the trigeminal nerve in patients with trigeminal neuralgia (TN) during Linac (linear accelerator)-based stereotactic radiosurgery (SRS) simulation, targeting the root entry zone (REZ), with a 30% isodose line tangential to the pons, using 4-mm and 6-mm collimators. MethodsIn this retrospective study, 53 TN patients, who underwent Fiesta sequence scanning prior to any treatment modality, were assessed. Bilateral measurements were obtained from the cisternal segment of the trigeminal nerve, the trigeminal-pontine angle, and the lateral width of the pontine cistern on the Fiesta MRI sequence. Linac-based SRS simulations were estimated with a radiation dosage of 90Gy to 30% isodose line tangential to the pons, with both 4- and 6-mm collimators. Distances from the calculated targets to the pons and the Gasserian ganglion were measured for later analysis. The statistical analysis was performed comparing the affected side against the unaffected side. ResultsRight trigeminal nerve was affected in 36 patients (67.9%), and left one in 17 (32.1%) patients. The mean length of the trigeminal nerve was 9.8mm (range: 4.6–16.8mm) on the affected side, and 10.5mm (range: 5.6–18.4mm) on the unaffected side (p=.02). The mean trigeminal-pontine angle was 12.5° (range: 5.4° to 19.5°) on the affected side, and 10.2° (range: 5.0° to 30.5°) on the unaffected side (p=.01). In the simulations, the distances from the estimated targets to the pons and the Gasserian ganglion were not statistically different between sides. The variation of target-pons and target-ganglion distances was statistically significant on the affected side with the change of collimators (p<.001). ConclusionsIn this anatomical study, significant differences were identified in the length of the affected trigeminal nerve and trigeminal-pontine angle compared to the unaffected side in TN patients in Fiesta sequences prior to surgery or radiosurgery. Significant variation of the target location was found on the REZ between the 4- and 6-collimators during the Linac-based SRS simulations with the estimated radiation dosage of 90Gy and 30% isodose line tangential to the pons.

Analysis of morphological measurements of the trigeminal nerve in the linac stereotactic radiosurgery simulation targeting the root entry zone in trigeminal neuralgia.

To describe the anatomical measurements of the trigeminal nerve in patients with trigeminal neuralgia (TN) during Linac (linear accelerator)-based stereotactic radiosurgery (SRS) simulation, targeting the root entry zone (REZ), with a 30% isodose line tangential to the pons, using 4-mm and 6-mm collimators. In this retrospective study, 53 TN patients, who underwent Fiesta sequence scanning prior to any treatment modality, were assessed. Bilateral measurements were obtained from the cisternal segment of the trigeminal nerve, the trigeminal-pontine angle, and the lateral width of the pontine cistern on the Fiesta MRI sequence. Linac-based SRS simulations were estimated with a radiation dosage of 90Gy to 30% isodose line tangential to the pons, with both 4- and 6-mm collimators. Distances from the calculated targets to the pons and the Gasserian ganglion were measured for later analysis. The statistical analysis was performed comparing the affected side against the unaffected side. Right trigeminal nerve was affected in 36 patients (67.9%), and left one in 17 (32.1%) patients. The mean length of the trigeminal nerve was 9.8mm (range: 4.6-16.8mm) on the affected side, and 10.5mm (range: 5.6-18.4mm) on the unaffected side (p=.02). The mean trigeminal-pontine angle was 12.5° (range: 5.4° to 19.5°) on the affected side, and 10.2° (range: 5.0° to 30.5°) on the unaffected side (p=.01). In the simulations, the distances from the estimated targets to the pons and the Gasserian ganglion were not statistically different between sides. The variation of target-pons and target-ganglion distances was statistically significant on the affected side with the change of collimators (p<.001). In this anatomical study, significant differences were identified in the length of the affected trigeminal nerve and trigeminal-pontine angle compared to the unaffected side in TN patients in Fiesta sequences prior to surgery or radiosurgery. Significant variation of the target location was found on the REZ between the 4- and 6-collimators during the Linac-based SRS simulations with the estimated radiation dosage of 90Gy and 30% isodose line tangential to the pons.

Re-Irradiation of Trigeminal Neuralgia with LINAC-Based Stereotactic Radiosurgery

Stereotactic radiosurgery (SRS) is an effective non-invasive treatment option for trigeminal neuralgia (TGN). If TGN pain recurs after SRS, re-irradiation with SRS can be considered. While there is published data regarding re-treatment with gamma-knife radiosurgery, little has been published regarding the safety and efficacy of retreatment with LINAC-based SRS. Between July 2007 and July 2017, there were 22 cases of repeat SRS for recurrent TGN to 21 patients (one patient had bilateral re-irradiation) available for analysis. Initial treatment consisted of 90 Gy to the isocenter using frameless LINAC SRS equipped with a 6D robotic table and for re-treatment 60 Gy was used to the root entry zone of CN V. Outcomes were categorized as excellent (minimal to no pain while still taking smaller amounts or fewer medications), good (pain controlled with medical management), and refractory (uncontrolled pain). Toxicity data was also collected and scored using CTCAE v4.0. The median follow up after re-irradiation was 7.5 months with a range of 1-72 months. The median time between SRS sessions was 16 months with a range of 5 to 76 months. Nine out of the twenty-two cases (40.9%) experienced an excellent outcome, six out of the twenty-two (27.3%) cases experienced a good outcome, and seven out twenty-two cases (31.8%) had a refractory outcome. Regarding toxicity from re-irradiation, there was only one episode of mild and non-bothersome unilateral facial numbness. For those 15 cases who had an excellent/good response to SRS, three (20%) had further recurrence of the TGN pain at a median interval of sixteen months from the second SRS (8-18 months). Re-irradiation of TGN with frameless LINAC based SRS appears to be both safe and effective with 68% of cases in this series responding to re-treatment along with minimal toxicity.

P05.63 The best treatment for Vestibular Schwannoma: Systematic Review of Clinical Trials and Case Series

Background Vestibular Schwannoma (VS) causes loss of hearing, loss of balance, tinnitus and even may lead to facial numbness. Various treatment choices have been increasing such as surgery, fractionated stereotactic radiotherapy (FSRT), Gamma Knife radiosurgery (GKRS), single fraction linac stereotactic radiosurgery (SFSRT) and conservative treatment. However, there is no conclusive evidence about the best treatment of VS. In this study, we aimed to systematically review and meta-analyze all possible strategies for management of VS.

LINAC stereotactic radiosurgery for trigeminal neuralgia \u2013retrospective two-institutional examination of treatment outcomes

BackgroundIn this pooled 2-center series LINAC radiosurgery (SRS) has been applied as a treatment option for a subset of refractory trigeminal neuralgia (TN) patients. This study approached to retrospectively assess the efficacy and safety of LINAC SRS and to provide a brief overview addressed to the technical development from frame-based towards frameless robotic SRS.MethodsFrom 2001 to 2017 n = 55 patients (pts) were treated, n = 28 were female (51%), mean age: 66 years (range 36–93 years); TN etiology: 37 classic TN, 15 multiple sclerosis (MS)-related TN, 2 symptomatic TN, and 1 atypical TN. Previous treatment was present in n = 35 (63.6%) pts. (some multiple or combined) with n = 23 microsurgical vascular decompression and n = 17 percutaneous retrogasserian rhizotomy. A 6 MV LINAC (4–5 mm collimators) was applied in all pts. (n = 26 framebased - n = 29 frameless robotic). The dorsal root entry zone (DREZ) was targeted in n = 35 cases and the retrogasserian target in n = 20 pts. with a homogeneous dose for the entire study cohort (90 Gy). SRS outcome was measured using the Barrow Neurological Institute (BNI) score for pain and hypaesthesia and statistically evaluated by univariate and multivariate analyzes.ResultsMean follow-up (FU) was 30 months (2 lost FU); the total rate of post SRS BNI pain I-IIIa (=painfree w or w/o medication) was 69% (88% for the classic TN pts), 29% (38.8% classic TN) were classified as BNI pain I-II (=painfree w/o medication). A BNI hypaesthesia II-III was present in 9.4% (n = 5) and BNI hypaesthesia IV in n = 2. Between groups analysis demonstrated no correlation of SRS responsiveness with age, gender, MS- or not MS-associated TN, previous surgery, framebased/frameless robotic SRS. DREZ targeting significantly better suppressed TN compared to RG targeting (p = 0.01). Additionally, a statistical trend for a better BNI pain outcome (p = 0.07) along with a significant increase in BNI hypaesthesia (p = 0.01) was found when using a larger partial trigeminal 70 Gy volume.ConclusionOur retrospective analysis support LINAC SRS as an effective and safe treatment option in TN. Frameless robotic SRS of TN is safe when using a dedicated LINAC system. A target definition closer to the brainstem and tendencially a larger target volume were associated with a better outcome for pain.

Hippocampal-Sparing and Target Volume Coverage in Treating 3-10 Brain Metastases: A Comparison of 192-Source Cobalt Radiosurgery, Single-Isocenter VMAT, and Nonisocentric Robotic Arm-Mounted Linac Stereotactic Radiosurgery

Hippocampal-Sparing and Target Volume Coverage in Treating 3-10 Brain Metastases: A Comparison of 192-Source Cobalt Radiosurgery, Single-Isocenter VMAT, and Nonisocentric Robotic Arm-Mounted Linac Stereotactic Radiosurgery

Vestibular Schwannoma: The Evolution of Hearing and Tumor Size in Natural Course and after Treatment by LINAC Stereotactic Radiosurgery

Objective: To review the natural course of tumor size and hearing during conservative management of 151 patients with unilateral vestibular schwannoma (VS), and to evaluate the same parameters for the part of the group (n = 84) who were treated by LINAC stereotactic radiosurgery (SRS). Methods: In prospectively collected data, patients underwent MRI and complete audiovestibular tests at inclusion, during the conservative management period and after SRS. Hearing was graded according to the Gardner-Robertson (GR) scale and tumor size according to Koos. Statistics were performed using Kaplan-Meier survival analysis and multivariate analyses including linear and logistic regression. Specific insight was given to patients with serviceable hearing. Results: During the conservative management period (mean follow-up time: 24 months, range: 6-96), the annual risk of GR class degradation was 6% for GRI and 15% for GR II patients. Hearing loss as an initial symptom was highly predictive of further hearing loss (p = 0.003). Tumor growth reached 25%. For SRS patients, functional hearing preservation was 51% at 1 year and 36% at 3 years. Tumor control was 94 and 91%, respectively. Conclusion: In VS patients, hearing loss at the time of diagnosis is a predictor of poorer hearing outcome. LINAC SRS is efficient for tumor control. Patients who preserved their pretreatment hearing presented less hearing loss per year after SRS than before treatment, suggesting a protective effect of SRS when cochlear function can be preserved.

SU‐E‐T‐674: Relationship Between Conformity Indices and Treatment Characteristics in Cone Based Linac Stereotactic Radiosurgery

Purpose:To investigate the correlation between conformity indices and dose prescription methodologies for LINAC based stereotactic surgery (SRS) cone plans.Methods:A vestibular schwannoma Linac based SRS cone plan was selected for this study. The target volume was 0.49cc and its equivalent sphere diameter is 1.0cm. Single 360 degree ARC Plans with different sizes of cones were done in Eclipse Cone planning System from Varian Medical. The plans were prescribed to satisfy to be normalized to 50% isodose line, 80% isodose line, 100% isodose line covering 95% of the target volume, at target mean dose and target minimum dose. The cones were select to be 10mm, 12mm, 14mm, 16mm, 18mm, 20mm, and 22mm in diameter. The indices for this study were conformity index(CI), which is defined to be the ratio between prescription isodose volume (PIV) and target volume (TV), and integrated conformity index (ICI), which is defined to be (TVpiv)2/(PIVxTV), where TVpiv is the intersection between PIV and TV.Results:When the plans were normalized to 50% of maximum dose, 80% of maximum dose, 100% isodose line covering 95% of the target volume, at target mean dose and target minimum dose, the corresponding values of ICI ranged from 0.45 to 0.04, 0.68 to 0.08, 0.58 to 0.54, 0.65 to 0.58, and 0.37 to 0.26; and the corresponding values of CI ranged from 2.2 to 22.8, 0.9 to 11.4, 1.5 to 1.7, 0.7 to 0.5, and 2.7 to 3.9.Conclusion:The values of indices used to describe SRS plans vary with different prescription conditions for single 360 degree ARC cone plans. For plans satisfied 100 % isodose covering 95% of target volume, the values of CI were between 1 and 2. A dynamic sensitive analysis system should be developed to correlate the plan evaluation index such as CI and ICI with clinical parameters.

Tumor volume threshold for achieving improved conformity in VMAT and Gamma Knife stereotactic radiosurgery for vestibular schwannoma

Background and purposeRecent advances in multileaf collimator field shaping technology and inverse planning software have resulted in highly conformal LINAC based stereotactic radiosurgery (SRS) plans with minimal dose to critical structures. This modeling study compares Gamma Knife (GK) and LINAC SRS for vestibular schwannoma (VS). Materials and methods76 treatment plans from nineteen patients with VS were planned using GK forward planning and volumetric arc therapy (VMAT) inverse planning software. VMAT plans were generated with 1 coplanar, 3 and 5 non-coplanar arcs. Dose to normal structures and beam-on time (dose rate 600MU/min) were compared using Kruskal–Wallis and Dunn’s post hoc test. ResultsMedian tumor volume was 1.2cm3 (range 0.1–4.8cm3). A peripheral tumor dose of 12Gy was prescribed. Tumor coverage was >99.8%. VMAT plans had lower target D2% and mean dose, as well as decreased beam-on time, compared to GK plans (p<0.0001). Paddick conformity index in VMAT 5 arc plans was superior to that of GK plans for targets >0.5cm3 (p=0.002). Similar dose to cochlea, normal brain tissue and brainstem was observed. ConclusionVMAT should be considered as a safe, alternative modality to GK for VS SRS treatment, especially for tumors larger than 0.5cm3.

Radiosurgery for brain metastases and cerebral edema

The objective of this study was to assess reduction in cerebral edema following linear accelerator radiosurgery (LINAC) as first line therapy for brain metastasis. We reviewed the medical records of all patients who underwent LINAC radiosurgery for brain metastasis at our institution during 2010–2012, and who had not previously undergone either surgery or whole brain radiotherapy. Data were analyzed for 55 brain metastases from 46 patients (24 males), mean age 59.9years. During the 2months following LINAC radiosurgery, the mean steroid dose decreased from 4.8 to 2.6mg/day, the mean metastasis volume decreased from 3.79±4.12cc to 2.8±4.48cc (p=0.001), and the mean edema volume decreased from 16.91±30.15cc to 12.85±24.47cc (p=0.23). The 17 patients with reductions of more than 50% in brain edema volume had single metastases. Edema volume in the nine patients with two brain metastases remained stable in five patients (volume change <10%, 0–2cc) and increased in four patients (by >10%, 2–14cc). In a subanalysis of eight metastases with baseline edema volume greater than 40cc, edema volume decreased from 77.27±37.21cc to 24.84±35.6cc (p=0.034). Reductions in brain edema were greater in metastases for which non-small-cell lung carcinoma and breast cancers were the primary diseases. Overall, symptoms improved in most patients. No patients who were without symptoms or who had no signs of increased intracranial pressure at baseline developed signs of intracranial pressure following LINAC radiosurgery. In this series, LINAC stereotactic radiosurgery for metastatic brain lesions resulted in early reduction in brain edema volume in single metastasis patients and those with large edema volumes, and reduced the need for steroids.