Stereotactic Linear Accelerator Research Articles

Integrating Deep Learning-Based Dose Distribution Prediction with Bayesian Networks for Decision Support in Radiotherapy for Upper Gastrointestinal cancer.

Selecting the better techniques to harbor optimal motion management, either a stereotactic linear accelerator delivery using TrueBeam (TBX) or Magnetic Resonance (MR)-guided gated delivery using MRIdian (MRG), is time-consuming and costly. To address this challenge, we aimed to develop a decision-supporting algorithm based on a combination of deep learning-generated dose distributions and clinical data. We retrospectively analyzed 65 patients with liver or pancreatic cancer who underwent both TBX and MRG simulations and planning process. We trained three-dimensional U-Net deep learning models to predict dose distributions and generated dose volume histograms (DVHs) for each system. We integrated predicted DVH metrics into a Bayesian network (BN) model incorporating clinical data. The MRG prediction model outperformed the TBX model, demonstrating statistically significant superiorities in predicting normalized dose to the PTV and liver. We developed a final BN prediction model integrating the predictive DVH metrics with patient factors like age, PTV size, and tumor location. This BN model an area under the receiver operating characteristic curve index of 83.56%. The decision tree derived from the BN model showed that the tumor location (abutting vs. apart of PTV to hollow viscus organs) was the most important factor to determine TBX or MRG. We demonstrated a decision-supporting algorithm for selecting optimal RT plans in upper gastrointestinal cancers, incorporating both deep learning-based dose prediction and BN-based treatment selection. This approach might streamline the decision-making process, saving resources and improving treatment outcomes for patients undergoing RT.

Leveraging intelligent optimization for automated, cardiac-sparing accelerated partial breast treatment planning.

Accelerated partial breast irradiation (APBI) yields similar rates of recurrence and cosmetic outcomes as compared to whole breast radiation therapy (RT) when patients and treatment techniques are appropriately selected. APBI combined with stereotactic body radiation therapy (SBRT) is a promising technique for precisely delivering high levels of radiation while avoiding uninvolved breast tissue. Here we investigate the feasibility of automatically generating high quality APBI plans in the Ethos adaptive workspace with a specific emphasis on sparing the heart. Nine patients (10 target volumes) were utilized to iteratively tune an Ethos APBI planning template for automatic plan generation. Twenty patients previously treated on a TrueBeam Edge accelerator were then automatically replanned using this template without manual intervention or reoptimization. The unbiased validation cohort Ethos plans were benchmarked via adherence to planning objectives, a comparison of DVH and quality indices against the clinical Edge plans, and qualitative reviews by two board-certified radiation oncologists. 85% (17/20) of automated validation cohort plans met all planning objectives; three plans did not achieve the contralateral lung V1.5Gy objective, but all other objectives were achieved. Compared to the Eclipse generated plans, the proposed Ethos template generated plans with greater evaluation planning target volume (PTV_Eval) V100% coverage (p = 0.01), significantly decreased heart V1.5Gy (p< 0.001), and increased contralateral breast V5Gy, skin D0.01cc, and RTOG conformity index (p = 0.03, p = 0.03, and p = 0.01, respectively). However, only the reduction in heart dose was significant after correcting for multiple testing. Physicist-selected plans were deemed clinically acceptable without modification for 75% and 90% of plans by physicians A and B, respectively. Physicians A and B scored at least one automatically generated plan as clinically acceptable for 100% and 95% of planning intents, respectively. Standard left- and right-sided planning templates automatically generated APBI plans of comparable quality to manually generated plans treated on a stereotactic linear accelerator, with a significant reduction in heart dose compared to Eclipse generated plans. The methods presented in this work elucidate an approach for generating automated, cardiac-sparing APBI treatment plans for daily adaptive RT with high efficiency.

Dosimetric Study of the Effect of Calypso-Compatible Couch Top for Spine Stereotactic Body Radiation Therapy

Purpose: To study the effect of the Qfix kVue Calypso-compatible couch top on the dosimetry of Spine Stereotactic Body Radiation Therapy (SBRT). Methods and Materials: The computed tomography (CT) data set for Qfix kVue Calypso-compatible couch top with rails were imported into the treatment planning system (TPS). Nine patients who underwent spine SBRT were selected for this study. The inclusion criteria included patients who were treated on a stereotactic linear accelerator with 5 fractions or less from 2016 to 2017 without the couch model. Seven patients were treated with static intensity-modulated radiation therapy (IMRT) fields and two patients were treated using volumetric modulated arc therapy (VMAT) technique. The dose was recalculated for 1) couch top and rails setup (CR) 2) couch-top no rails setup (CNR), and then compared to 3) no couch no-rails setup (NCNR). Dose to 100% of the target volume (D100%), Dose to cover 99% of the target volume (D99%), Dose to cover 95% of the target volume (D95%), Dose to cover 90% of the target volume (D90%), volume receiving 100% of the prescription dose (V100%), conformity index (CI), dose gradient index (DGI), and spinal cord threshold and maximum dose were compared to the plan with NCNR. Results: The average D100% was 77.89% ± 11.78%, 74.51% ± 12.24%, and 75.83 ± 12.67% for NCNR, CR, CNR (р = 0.84), respectively. The average D99% was 91.64% ± 9.57%, 89.93% ± 9.48%, and 91.15% ± 9.55% for NCNR, CR, CNR (р = 0.98), respectively. The average D95% was 99.14% ± 9.96%, 95.23% ± 9.76, and 96.78% ± 9.84% for NCNR, CR, CNR (р = 0.047), respectively. The average D90% was 101.3% ± 0.65%, 97.11% ± 2.48%, and 98.75% ± 2.12% for NCNR, CR, CNR (р = 0.0004), respectively. The maximum dose to the spinal cord was 1750.79 ± 41.84, 1672.90 ± 40.90, and 1709.91 ± 41.35 (cGy) for NCNR, CR, CNR (р = 0.97), respectively. In all cases, the spinal cord threshold dose was far below the tolerances and the differences were insignificant. Average CI was 1.18 ± 0.16, 0.53 ± 0.39, and 0.86 ± 0.24 for NCNR, CR, CNR (р = 0.0002), respectively. Conclusions: The study investigated the dosimetric impact of Qfix kVue Calypso-compatible couch top on the quality of the spinal SBRT treatment using static IMRT or VMAT techniques. IMRT plans showed more sensitivity to the couch being in the plan than the VMAT plans.

Clinical Outcomes in Patients With NFPAs Treated With Stereotactic Linear Accelerator Radiation Therapy

Abstract Background: Non-functioning pituitary adenoma (NFPAs) are benign tumors of the pituitary gland characterized by the absence of hormonal hypersecretion. Surgical treatment and fractionated stereotactic radiotherapy (FSRT) are necessary in majority of patients. NFPAs appear to cause higher mortality compared to the general population. Objective: To assess outcomes in patients with NFPAs treated with stereotactic linear accelerator radiation therapy. Material and Methods: Comparative, observational, longitudinal study, with prolective evaluation, from January 1987 to August 2019. Results: 264 patients (65.5% women and 34.5% men, mean age 43 ± 16 years) with a diagnosis of non-functional adenomas were evaluated. 30 patients received FSRT (14%), 93% were macroadenomas, and 80% of the cases were operated as a first line of treatment. The mean number of surgeries before radiotherapy was 1.2+-.847 (0-4). FSRT were used in 91.7% (n=21) because of post-surgical recurrence. Diabetes insipidus after surgery was transitory in 5.4%, and persistent in 4.0%. Hypopituitarism was identified in 59.4% before radiation (n=19), and in 13 patients (7.7%) after mean of 6.5+-5.5 years of FSRT. Dyslipidemia (62.0% vs. 45.9%), high blood pressure (16.5% vs. 23%), type 2 diabetes mellitus (10% vs. 77%) and heart failure (10% vs. 8.4%) were observed in the irradiated vs. non-irradiated groups, respectively (p&amp;lt;0.05). Conclusions: FSRT is a useful and safe strategy as an adjuvant treatment in patients with NFPA.

The individual risk of symptomatic radionecrosis after brain metastasis radiosurgery is predicted by a continuous function of the V12Gy

IntroductionBrain metastases are frequently treated with stereotactic radiosurgery (SRS). Radionecrosis (RN) is the late side effect in up to 24% of patients, being symptomatic in 8–10%. Fixed values of the radiosurgical volume receiving 12 Gy or more (V12Gy) are used to roughly predict the global risk. The aim of this retrospective study is to fine-tune the model of individual risk prediction for symptomatic radionecrosis and identify modulating factors. Materials and methodsData of patients treated with SRS for ≤3 BM of solid tumours at CHU-UCL-Namur were retrospectively reviewed. Doses ranging from 15 to 24 Gy were prescribed to the 70% isodose in function of the lesion diameter. Treatment was administered with a stereotactic linear accelerator. Follow-up magnetic resonance imaging was performed 3-monthly for 18 months and 6-monthly thereafter. RN was prospectively diagnosed and confirmed by the tumour board. V12Gy, previous or salvage whole-brain radiotherapy (WBRT), smoking history, diabetes, postoperative SRS, diagnosis-specific graded prognostic assessment score, cerebral lobe location and relative location (superficial versus deep) were retrieved. Univariate and multivariate analyses were performed to assess their predictive values and derive a model. Results128 patients with 220 lesions were analysed. The risk of RN was predicted by a continuous function of the V12Gy (p = 0.005). No other factor had a significant impact, particularly WBRT that did not increase the risk. ConclusionThe risk of symptomatic RN is predicted on an individual basis by a model in function of the V12Gy and must be confirmed in a prospective study.

Pituitary adenoma treatment plan quality comparison between linear accelerator volumetric modulated arc therapy and Leksell Gamma Knife® radiosurgery.

The aim of this study was to compare radiosurgical treatment plan quality of a linear accelerator with Leksell Gamma Knife (LGK) for pituitary adenoma irradiation. Thirty pituitary adenoma patients were evaluated in this study. Treatment plans were prepared on LGK and stereotactic linear accelerator Varian TrueBeam STx. Volumetric Modulated Arc Therapy (VMAT) plans (21 plans with 2 coplanar arcs and 9 plans with 4 non-coplanar arcs) were calculated for linear accelerator. All the plans were evaluated in terms of conformity, selectivity, gradient index and organ at risk (OAR) sparing. VMAT produced dosimetrically comparable treatment plans to LGK regarding conformity and selectivity (New Conformity Index (NCI): 1.76 ± 0.65 for 4 arc VMAT, 2.33 ± 1,16 for 2 arc VMAT and 1.96 ± 0.71 for LGK; Selectivity Index (SI): 0.63 ± 0.16 for 4 arc VMAT, 0.51 ± 0.16 for 2 arc VMAT and 0.58 ± 0.17 for LGK). Gradient index (GI) was superior for LGK plans (GI: 2.74 ± 0.20 for LGK and 5.28 ± 2.29 for 4 arc VMAT). OAR sparing for optics, brainstem, and hypophysis was similar for both modalities while target volume coverage was maintained the same. Finally, treatment time resulted in favor of VMAT plans (in this study VMAT plans were almost 5 times faster than LGK treatment regarding beam on time). According to the results of this study stereotactic linear accelerator with VMAT treatment could be used as a reasonable alternative to LGK for pituitary adenoma radiosurgery but only if the same head fixation method accuracy and target volume delineation are maintained for both modalities.

SU‐F‐T‐557: Evaluation of Detector Response in Rectangular Small Field Dosimetry

Purpose:As stereotactic treatment modalities grow towards becoming the standard of care, the need for accurate dose computation in small fields is becoming increasingly essential. The purpose of this study is to evaluate the response of different detectors, intended for small field dosimetry, in jaw defined small rectangular fields by analyzing output factors from a stereotactic clinical accelerator.Methods:Two Dosimeters, the Exradin A26 Microionization Chamber (Standard Imaging) and Edge Diode Detector (Sun Nuclear) were used to measure output factors taken on the Varian Edge Stereotactic Linear accelerator. Measurements were taken at 6MV and 6FFF at 10cm depth, 100cm SSD in a 48×48×40cm3 Welhoffer BluePhantom2 (IBA) with X and Y jaws set from 0.6 to 2.0cm. Output factors were normalized to a 5×5cm2 machine‐specific reference field. Measurements were made in the vertical orientation for the A26 and horizontal orientation for both the A26 and Edge. Output factors were measured as: OFFS = MFS/Mref where MFS and Mref are the measured signals for the clinical field and the reference field, respectively. Measured output factors were then analyzed to establish relative responses of the detectors in small fields.Results:At 6MV the Edge detector exhibited a variation in output factors dependent on jaw positioning (X‐by‐Y vs Y‐by‐X) of 5.7% of the 5×5cm reference output and a variation of 3.33% at 6FFF. The A26 exhibited variation of output factor dependent on jaw positioning of upto 7.7% of the 5×5cm reference field at 6MV and upto 5.33% at 6FFF.Conclusion:Both the Edge detector and A26 responded as expected at small fields however a dependence on the jaw positioning was noted. At 6MV and 6FFF the detector response showed an increased dependence on the positioning of the X jaws as compared to the positioning of the Y jaws.

Investigation of two linear accelerator head designs for treating brain metastases with hypofractionated volumetric-modulated arc radiotherapy.

To investigate brain radiation dose in complex cases arising from two stereotactic linear accelerator designs and to present a method for comparing brain dose to published data. Two head designs were considered: Beam Modulator (BM) and Agility (AG). 12 patients treated on BM were replanned with AG. Planning objectives were: minimize brain dose and satisfy target coverage and organs at risk dose constraints. Each of the 36 targets was analyzed for conformality index (CI Paddick), gradient index (GI) and homogeneity index (HI). Total volume of tissue receiving 80% (V80) of the prescription dose down to 25% (V25) was evaluated. Similarly the volume of brain minus planning target volume receiving 80% (BMP80) down to 25% (BMP25) was evaluated. The mean brain dose and BMP dose were also evaluated. System differences were statistically evaluated using Wilcoxon signed-rank test. Power-law models for total volume (V) and brain minus planning target volumes (BMP) were generated based on BM data. The median CI Paddick was 0.74 and 0.76 for BM and AG, respectively (p = 0.04). The median GI was 5.5 and 6.1 (p < 0.01) and the median HI was 1.17 and 1.16 for BM and AG, respectively (p < 0.01). Neither V or BMP receiving doses of 80% down to 40% exhibited statistically significant difference between the two systems, whereas the volume of brain minus PTV receiving 25% (BMP25) was weakly different (p = 0.02). AG exhibited a lower mean BMP dose (4.1 Gy) than BM (4.6 Gy) (p < 0.01). Power-law models for V/BMP showed excellent (R(2) > 0.80) agreement for the dose levels studied and comparable results with published data. Treatment plans of equivalent quality were attained with AG compared with BM. The AG system involves a novel collimation design. The present article demonstrates equivalent or improved brain dose for complex, multitarget cases using AG vs an older stereotactic system.

Stereotactic LINAC radiosurgery for the treatment of typical intracranial meningiomas. Efficacy and safety after a follow-up of over 12 years.

The efficacy and safety of stereotactic radiosurgery (SRS) for treatment of intracranial meningiomas has been demonstrated in numerous studies with short- and intermediate-term follow-up. In this retrospective single-center study, we present long-term outcomes of SRS performed with a linear accelerator (LINAC) for typical intracranial meningiomas. Between August 1990 and December 2007, 148 patients with 168 typical intracranial meningiomas were treated with stereotactic LINAC-SRS, either as primary treatment or after microsurgical resection. A median tumor surface dose of 12 Gy (range 7-20 Gy) and a median maximum dose of 24.1 Gy (range 11.3-58.6 Gy) was applied. The median target volume was 4.7 ml (range 0.2-32.8 ml, SD ± 4.8 ml). Overall mean radiological follow-up was 12.6 years. Tumor shrinkage was seen in 75 (44.6 %) and stable disease in 85 (50.6 %) cases. Eight of 168 meningiomas (4.8 %) showed local tumor progression. The tumor control rate (TCR) after 5, 10, and 15 years was 93.6 % at each time point, and the progression-free survival (PSF) rates were 92, 89, and 89 %, respectively. The neurological symptoms existing prior to LINAC-SRS improved in 77 patients (59.7 %), remained unchanged in 42 (32.6 %), and deteriorated in 10 (7.8 %) patients. Our study emphasizes the efficacy of LINAC-SRS for de novo, residual and recurrent typical intracranial meningiomas. A high long-term local TCR with a low morbidity rate could be achieved. LINAC-SRS should thus be considered as a primary treatment option, as one arm of a combined treatment approach for incompletely resected meningiomas, or as a salvage therapy for recurrences.

Generation and verification of QFix kVue Calypso-compatible couch top model for a dedicated stereotactic linear accelerator with FFF beams.

This study details the generation, verification, and implementation of a treatment planning system (TPS) couch top model for patient support system used in conjunction with a dedicated stereotactic linear accelerator. Couch top model was created within the TPS using CT simulation images of the kVue Calpyso‐compatible couchtop (with rails). Verification measurements were compared to TPS dose prediction for different energies (6 MV FFF and 10 MV FFF) and rail configurations (rails in and rails out) using: 1) central axis point‐dose measurements with pinpoint chamber in water‐equivalent phantom at 42 gantry angles for various field sizes (2×2 cm2,4×4 cm2,10×10 cm2); and 2) Gafchromic EBT3 film parallel to beam in acrylic slab to assess changes in surface and percent depth doses in PA geometry. To assess sensitivity of delivered dose to variations in patient lateral position, measurements at central axis using the pinpoint chamber geometry were taken at lateral couch displacements of 2, 5, and 10 mm for 6 MV FFF. The maximum percent difference for point‐dose measurements was 3.24% (6 MV FFF) and 2.30% (10 MV FFF). The average percent difference for point‐dose measurements was less than 1.10% for all beam energies and rail geometries. The maximum percent difference between calculated and measured dose can be as large as 13.0% if no couch model is used for dose calculation. The presence of the couch structures also impacts surface dose and PDD, which was evaluated with Gafchromic film measurements. The upstream shift in the depth of dose maximum (dmax) was found to be 10.5 mm for 6 MV FFF and 5.5 mm for 10 MV FFF for ‘Rails In’ configuration. Transmission of the treatment beam through the couch results in an increase in surface dose (absolute percentage) of approximately 50% for both photon energies (6 MV FFF and 10 MV FFF). The largest sensitivity to lateral shifts occurred at the lateral boundary of the rail structures. The mean magnitude (standard deviation) of the deviation between shifted and centered measurements over all field sizes tested was 0.61% (0.61%) for 2 mm shifts, 0.46% (0.67%) for 5 mm shifts, and 0.86% (1.46%) for 10 mm shifts.PACS numbers: 87.56.‐v, 87.56.Da, 87.56.Fc

SU‐E‐T‐738: The Limit of Detectability and Resolution of a Cylindrical Diode Array Phantom for VMAT and SRS Quality Assurance

Purpose:To determine the limit of detectability and resolution of a cylindrical QA phantom for quality assurance of VMAT and SRS procedures in small field sizes.Methods:Eight different square field sizes were delivered onto a cylindrical QA phantom with 1386 diodes at gantry angles of 0, 90, 180 and 270 degrees, using a 6 MV beam at a maximum dose rate of 600 MU/min on a dedicated stereotactic linear accelerator with 2.5mm and 5mm leaves. After verifying pass rates of normal square field sizes, errors were introduced into the quality‐assurance analysis, including an expansion of the field size by 1 mm for all jaws on all sides, an expansion of the field size by 1 mm on one side of the X and Y jaws,MLC errors, and a table shift by 1 mm vertically and longitudinally at 270 and 90 degrees and a table shift of 1mm laterally and longitudinally for angles of 0 and 180 degrees. Treatment planning system calculated dose distributions for measured square field size dose data was imported into the QA phantom software and compared against a treatment planning system calculated 3D dose distribution. Percent pass rates were computed using gamma analysis with 2mm/2% analysis criteria with a background threshold value of 30%.Results:QA software comparison between measured fields and treatment planning system calculations indicate that the passing rate of square fields decreases significantly when errors are induced in the quality assurance analysis. Adding two opposing MLC leaves on opposing sides of each square field in the treatment planning system produced a 31.2% average difference from normally measured square field sizes compared between the treatment planning system and what was measured by the QA software.Conclusion:This work indicates the potential for detecting errors in VMAT quality assurance in small field sizes using a cylindrical QA phantom.

Evaluation of linear accelerator (LINAC)-based stereotactic radiosurgery (SRS) for cerebral cavernous malformations: a 15-year single-center experience

BACKGROUND AND OBJECTIVESSurgery is the principal treatment for safely accessible hemorrhagic and symptomatic cavernous malformations. Nevertheless, the role of linear accelerator (LINAC)-based stereotactic radiosurgery (SRS) in the management of high-risk, symptomatic cavernoma lesions warrants further refinement. In this study, we evaluate the use of LINAC-based SRS for cerebral cavernous malformations (CMs) and report our 15-year single-center experience.DESIGN AND SETTINGSA retrospective study from the Department of Radiation Oncology and the Department of Neurosurgery at Gulhane Military Medical Academy and Medical Faculty, Ankara from April 1998 to June 2013.PATIENTS AND METHODSFifty-two patients (22 females and 30 males) with cerebral CM referred to our department underwent high-precision single-dose SRS using a LINAC with 6-MV photons. All patients had at least 1 bleeding episode prior to radiosurgery along with related symptoms. Median dose prescribed to the 85% to 95% isodose line encompassing the target volume was 15 Gy (range, 10–20).RESULTSOut of the total 52 patients, follow-up data were available for 47 patients (90.4%). Median age was 35 years (range, 19–63). Median follow-up time was 5.17 years (range, 0.08–9.5) after SRS. Three hemorrhages were identified in the post-SRS period. Statistically significant decrease was observed in the annual hemorrhage rate after radiosurgical treatment (pre-SRS 39% vs post-SRS 1.21, P<.0001). Overall, there were no radiosurgery-related complications resulting in mortality.CONCLUSIONLINAC-based SRS may be considered as a treatment option for high-risk, symptomatic cerebral CM of selected patients with prior bleeding from lesions located at surgically inaccessible or eloquent brain areas.

Adjuvant therapy after resection of brain metastases

Tumor bed stereotactic radiosurgery (SRS) after resection of brain metastases is a new strategy to delay or avoid whole-brain irradiation (WBRT) and its associated toxicities. This retrospective study analyzes results of frameless image-guided linear accelerator (LINAC)-based SRS and stereotactic hypofractionated radiotherapy (SHRT) as adjuvant treatment without WBRT. Between March 2009 and February 2012, 44resection cavities in 42patients were treated with SRS (23cavities) or SHRT (21cavities). All treatments were delivered using a stereotactic LINAC. All cavities were expanded by ≥ 2mm in all directions to create the clinical target volume (CTV). The median planning target volume (PTV) for SRS was 11.1cm(3). The median dose prescribed to the PTV margin for SRS was 17Gy. Median PTV for SHRT was 22.3cm(3). The fractionation schemes applied were: 4fractions of 6Gy (5patients), 6fractions of 4Gy (6patients) and 10fractions of 4Gy (10patients). Median follow-up was 9.6months. Local control (LC) rates after 6and 12months were 91and 77 %, respectively. No statistically significant differences in LC rates between SRS and SHRT treatments were observed. Distant brain control (DBC) rates at 6and 12months were 61and 33 %, respectively. Overall survival (OS) at 6and 12months was 87and 63.5 %, respectively, with a median OS of 15.9months. One patient treated by SRS showed symptoms of radionecrosis, which was confirmed histologically. Frameless image-guided LINAC-based adjuvant SRS and SHRT are effective and well tolerated local treatment strategies after resection of brain metastases in patients with oligometastatic disease.

Stereotactic radiosurgery for the treatment of Glomus Jugulare Tumors

Background:The glomus jugulare tumor is a slowly growing benign neoplasm originating from neural crest. There is a high morbidity associated with surgical resection of glomus jugulare. Radiosurgery play a relevant role as a therapeutic option in these tumors and its use has grown in popularity. The authors describe a retrospective series of 15 patients and reviewed the literature about the glomus jugulare tumors.Methods:We reviewed retrospectively the data of 15 patients treated with stereotactic linear accelerator stereotactic radiosurgery (LINAC) radiosurgery between 2006 and 2011.Results:The average tumor volume was 18.5 cm3. The radiation dose to the tumor margin ranged between 12 and 20 Gy. The neurological status improved in three patients and remained unchanged in 12 patients. One patient developed a transient 7th nerve palsy that improved after clinical treatment. All tumors remained stable in size on follow-up with resonance magnetic images.Conclusions:The radiosurgery is a safe and effective therapy for patients with glomus jugulare tumor. Despite the short follow-up period and the limited number of patients analyzed, we can infer that radiosurgery produce a tumor growth control with low morbidity, and may be used as a good option to surgical resection in selected cases.

Stereotactic LINAC radiosurgery for incompletely resected or recurrent atypical and anaplastic meningiomas

The optimal management of subtotally resected or recurrent malignant meningiomas remains controversial. We evaluated the efficacy of linear accelerator (LINAC) radiosurgery for atypical and anaplastic meningiomas after incomplete resection or treatment of recurrences. Between August 1990 and December 2003, 16 patients with 28 meningiomas WHO II and III were treated by stereotactic LINAC radiosurgery at our institution. The median radiological follow-up was 60.3 months, respectively (range: 7.2-173.9 months). Fourteen tumors in nine patients were classified as WHO II and 14 tumors in seven patients as WHO III. The median surface dose was 14 Gy (range: 10-15 Gy) with a median tumor volume of 4.8 ml (range: 0.51-51.4 ml). Clinical condition improved in four patients, remained unchanged in nine and deteriorated in one. Tumor shrinkage was seen in eight of 28 meningiomas and a stable disease in 12. Eight of 28 meningiomas showed local tumor progression. The overall tumor control rate (TCR) was 84%, 70%, 70% after 3, 5, 10 years. According to grading the corresponding TCR after 3, 5, 10 years was 91%, 81%, 81% for grade II and 77%, 60%, 60% for grade III meningiomas. Overall progression-free survival (PFS) was 74%, 67%, 58% after 3, 5, 10 years. According to grading the PFS after 3, 5, 10 years was 88%, 75%, 75% for grade II meningiomas and 57%, 57%, 43% for grade III meningiomas. Our results show the efficacy and safety of LINAC radiosurgery for incompletely resected or recurrent malignant meningiomas with a relatively high local tumor control and low morbidity.

Stereotactic Radiosurgery for Trigeminal Neuralgia Utilizing the BrainLAB Novalis System

Stereotactic radiosurgery (SRS) is one of the least invasive treatments for trigeminal neuralgia (TN). To date, most reports have been about Cobalt-based treatments (i.e., Gamma Knife) with limited data on image-guided stereotactic linear accelerator treatments. We describe our initial experience of using BrainLAB Novalis stereotactic system for the radiosurgical treatment of TN. A total of 20 patients were treated between July 2004 and February 2007. Each SRS procedure was performed using the BrainLAB Novalis System. Thin cuts MRI images of 1.5 mm thickness were acquired and fused with the simulation CT of each patient. Majority of the patients received a maximum dose of 90 Gy. The median brainstem dose to 1.0 cc and 0.1 cc was 2.3 Gy and 13.5 Gy, respectively. In addition, specially acquired three-dimensional fast imaging sequence employing steady-state acquisition (FIESTA) MRI was utilized to improve target delineation of the trigeminal proximal nerve root entry zone. Barrow Neurological Index (BNI) pain scale for TN was used for assessing treatment outcome. At a median follow-up time of 14.2 months, 19 patients (95%) reported at least some improvement in pain. Eight (40%) patients were completely pain-free and stopped all medications (BNI Grade I) while another 2 (10%) patients also stopped medications but reported occasional pain (BNI Grade II). Another 2 (10%) patients reported no pain and 7 (35%) patients only occasional pain while continuing medications, BNI Grade IIIA and IIIB, respectively. Median time to pain control was 8.5 days (range: 1-70 days). No patient reported severe pain, worsening pain or any pain not controlled on their previously taken medication. Intermittent or persistent facial numbness following treatments occurred in 35% of patients. No other complications were reported. Stereotactic radiosurgery using the BrainLAB Novalis system is a safe and effective treatment for TN. This information is important as more centers are obtaining image-guided stereotactic-based linear accelerators capable of performing radiosurgery.

Dosimetric evaluation of a dedicated stereotactic linear accelerator using measurement and Monte Carlo simulation

Dosimetric parameters of a dedicated stereotactic linear accelerator have been investigated using measurements and Monte Carlo simulations. This linac has a unique built in multileaf collimation (MLC) system with the maximum opening of 16 x 21 cm2 and 4 mm leaf width at the isocenter and has successfully been modeled for the first time using the Monte Carlo simulation. The high resolution MLC, combined with its relatively large maximum field size, opens up a new opportunity for expanding stereotactic radiation treatment techniques from traditionally treating smaller targets to larger ones for both cranial and extracranial lesions. Dosimetric parameters of this linac such as accuracy of leaf positioning and field shaping, leakage and transmission, percentage depth doses, off-axes dose profiles, and dose penumbras were measured and calculated for different field sizes, depths, and source to surface distances. In addition, the ability of the linac in accurate dose delivery of several treatment plans, including intensity modulated radiation therapy (IMRT), performed on phantom and patients was determined. Ionization chamber, photon diode detector, films, several solid water phantoms, and a water tank were used for the measurements. The MLC leaf positioning to any particular point in the maximum aperture was accurate with a standard deviation of 0.29 mm. Maximum and average leakages were 1.7% and 1.1% for the reference field of 10.4 x 9.6 cm2. Measured penumbra widths (80%-20%) for this field at source axis distance (SAD) of 100 cm at a depth of 1.5 cm (dmax) were 3.2 and 4 mm for the leaf-sides and leaf-ends, respectively. The corresponding results at 10 cm depth and SAD =100 cm were 5.4 and 6.3 mm. Monte Carlo results generally agreed with the measurements to within 1% and or 1 mm, with respective uncertainties of 0.5% and 0.2 mm. The linac accuracy in delivering non-IMRT treatment plans was better than 1%. Ionization chamber dosimetry results for a phantom IMRT plan in the high dose and low dose regions were -0.5% and +3.6%, respectively. Dosimetry results at isocenter for three patients' IMRT plans were measured to be within 3% of their corresponding treatment plans. Film dosimetry was also used to compare dose distributions of IMRT treatment plans and delivered cumulative doses at different cross sectional planes.

Verification procedure for isocentric alignment of proton beams

We present a technique—based on the Lutz, Winston, and Maleki test used in stereotactic linear accelerator radiosurgery—for verifying whether proton beams are being delivered within the required spatial coincidence with the gantry mechanical isocenter. Our procedure uses a proton beam that is collimated by a circular aperture at its central axis and is then intercepted by a small steel sphere rigidly supported by the patient couch. A laser tracker measurement system and a correction algorithm for couch position assures precise positioning of the steel sphere at the mechanical isocenter of the gantry. A film‐based radiation dosimetry technique, chosen for the good spatial resolution it achieves, records the proton dose distribution for optical image analysis. The optical image obtained presents a circular high‐dose region surrounding a lower‐dose area corresponding to the proton beam absorption by the steel sphere, thereby providing a measure of the beam alignment with the mechanical isocenter. We found the self‐developing Gafchromic EBT film (International Specialty Products, Wayne, NJ) and commercial Epson 10000 XL flatbed scanner (Epson America, Long Beach, CA) to be accurate and efficient tools. The positions of the gantry mechanical and proton beam isocenters, as recorded on film, were clearly identifiable within the scanning resolution used for routine alignment testing (0.17 mm per pixel). The mean displacement of the collimated proton beam from the gantry mechanical isocenter was 0.22±0.1 mm for the gantry positions tested, which was well within the maximum deviation of 0.50 mm accepted at the Proton Therapy Center in Houston.PACS numbers: 87.53.Xd, 87.53.Oc, 87.56.‐v, 87.66.‐a, 87.56.Fc

Radiation-induced changes of brain tissue after radiosurgery in patients with arteriovenous malformations: correlation with dose distribution parameters

purpose To investigate the correlation of radiation-induced changes of brain tissue after radiosurgery in patients with cerebral arteriovenous malformations (AVMs) with treatment planning and dose distribution parameters. Methods and materials The data from 73 AVM patients with complete follow-up information who underwent stereotactic linear accelerator radiosurgery at our institution between 1993 and 1998 were analyzed. Patients were treated with 11–14 noncoplanar fields shaped by a micromultileaf collimator. A median dose of 19 Gy (range, 13.3–22 Gy) was prescribed to the 80% isodose, which completely encompassed the target. Patients were followed at 3-month intervals the first year and then every 6 months with MRI and neurologic examinations. No patient developed radiation necrosis. The end point of radiation-induced tissue changes on follow-up neuroimaging (i.e., edema, blood–brain barrier breakdown [BBBB], and edema and/or BBBB combined) was evaluated. Each end point was further differentiated into four levels with respect to the extent of the image change (i.e., small, intermediate, large, and very large). The correlation of each end point was investigated for several treatment planning parameters, including prescribed dose and the absolute size of the AVM target volume. In addition, a number of dose–volume variables were calculated from each patient's dose distribution in the brain, including the mean dose to a specified volume of 16 and 20 cm 3 that was given the highest dose (Dmean16 and Dmean20, respectively), and the absolute and percentage of brain volume (including the AVM target) receiving a dose of at least 8, 10, and 12 Gy (V8–V12, and V8 rel–V12 rel, respectively). These parameters were also determined excluding the AVM target volume from the considered volume (subscript “excl”). The correlation of all treatment planning and dose–volume parameters with outcome was assessed in univariate Cox proportional hazards models. The results were assessed by p values (statistical significance for p ≤0.05), residual deviance (ResDev) of the fits, and odds ratios. Results The prescribed dose was not predictive of outcome ( p >0.05 for all end points). The AVM target volume correlated significantly with large edema, as well as large edema and/or BBBB. V12 and Dmean20 were significantly associated with all end points, except very large edema and large BBBB. Patients with V12 of 27.6 cm 3 (Dmean20 of 18.9 Gy) had a 2.8-fold (fourfold) higher risk of developing edema and/or BBBB with large extent than those with V12 of 4.2 cm 3 (Dmean20 of 8.4 Gy). For all end points, V12 rel correlated worse with outcome compared with V12 (e.g., end point of large edema and/or BBBB: ResDev = 85.8 and 86.5 for V12 and V12 rel, respectively). Excluding the AVM target volume from the considered irradiated volume led to only small changes in the resulting correlations (e.g., end point of small edema and/or BBBB: ResDev = 99.0 and 98.7 for V12 and V12 excl, respectively, and ResDev = 96.1 and 96.1 for Dmean20 and Dmean20 excl, respectively). Throughout the analysis, V8-V12, Dmean20, and Dmean16 yielded similar results and none of these parameters could be favored over the others. Conclusion Radiation-induced changes of brain tissue after AVM radiosurgery can be well predicted by single dose distribution parameters that are a function of both dose and volume. These can be used to quantify dose–volume response relations. Studies of this nature will eventually help to improve our current understanding of the mechanisms leading to radiation-induced tissue changes after AVM radiosurgery and to optimize radiosurgery treatment planning.

Stereotactic Linear Accelerator Radiotherapy for Pituitary Tumors

Last decade has seen important advances in radiotherapy technology which combine precise tumor localization with accurate targeted delivery of radiation. This technique of high precision conformal radiotherapy, described as stereotactic radiotherapy or radiosurgery, uses modern linear accelerators available in most radiation oncology departments. The article describes the new technique as applied to the treatment of pituitary adenoma and reviews published clinical results.