Miguel Angel Celis

Radiation transmission, leakage and beam penumbra measurements of a micro-multileaf collimator using GafChromic EBT film

Micro‐multileaf collimator systems coupled to linear accelerators for radioneurosurgery treatments require a rigorous dosimetric characterization in order to be used in 3D conformal and intensity modulated stereotactic radiosurgery and radiotherapy applications. This characterization involves high precision measurements of leaf transmission, leakage and beam penumbra through the collimation system and requires the use of detectors with high spatial resolution, high sensitivity and practically no energy dependence. In this work the use of GafChromic EBT radiochromic film to measure the basic dosimetric properties of the m3‐mMLC (BrainLAB, Germany) micro‐multileaf collimator system integrated to a 6 MV linear accelerator, is reported. Results show that average values of transmission and leakage radiation are 0.93±0.05% and 1.08±0.08%, respectively. The 80–20% beam penumbra were found to be 2.26±0.11 mm along the leaf side (perpendicular to leaf motion) and 2.31±0.11 mm along the leaf end (parallel to leaf motion) using square field sizes ranging from 9.1 to 1.8 cm. These measurements are in agreement with values reported in the literature for the same type of mMLC using different radiation detectors. PACS number: 87.56.N‐

Small photon beam measurements using radiochromic film and Monte Carlo simulations in a water phantom

This work reports the use of both GafChromic EBT film immersed in a water phantom and Monte Carlo (MC) simulations for small photon beam stereotactic radiosurgery dosimetry. Circularly collimated photon beams with diameters in the 4-20 mm range of a dedicated 6 MV linear accelerator (Novalis, BrainLAB, Germany) were used to perform off-axis ratios, tissue maximum ratios and total scatter factors measurements, and MC simulations. GafChromic EBT film data show an excellent agreement with MC results (<2.7%) for all measured quantities.

Diffusion tensor imaging in radiosurgical callosotomy

Callosotomy by radioneurosurgery induces slow and progressive axonal degeneration of white matter fibers, a key consequence of neuronal or axonal injury (radionecrosis). However, the acute effects are not apparent when using conventional MRI techniques. Diffusion tensor imaging (DTI) during the first week following radioneurosurgical callosotomy allowed evaluation of these microstructural changes. The present report details that the use of sequential DTI to evaluate axonal degeneration following radioneurosurgical callosotomy in a patient normalized with the data of six healthy subjects. We describe a 25-year old woman with symptomatic generalized epilepsy who underwent a radioneurosurgical callosotomy using LINAC (Novalis® BrainLAB). DTI was acquired at the baseline, 3 and 9 months and showed a progressive decrease of the fractional anisotropy values in the irradiated areas compared to the controls that could be interpreted as a progressive disconnection of callosal fibers related to the outcome.

Intracranial arteriovenous malformations treated with LINAC-based conformal radiosurgery: validation of the radiosurgery-based arteriovenous malformation score as a predictor of outcome.

Abstract Objective: To validate the radiosurgery-based arteriovenous malformation score (RBAS) as a predictor of outcome in patients with arteriovenous malformations (AVMs) treated with LINAC-based conformal radiosurgery. Methods: We analysed 40 patients with a mean follow-up of 22 months. One patient (2.5%) pertained to Spetzler–Martin Grade I, 11 (27.5%) to Grade II, 19 (47.5%) to Grade III, eight (20%) to Grade IV and one (2.5%) to grade V. The mean RBAS was 2.0 (range: 0.76–5.56). The mean obliteration prediction index (OPI) and the Karlsson index (KI) were 0.74 (range: 0.2–2.86) and 109.48 (range: 3.0–350.7) respectively. Outcomes were analysed according to the OPI, KI and RBAS. Results: We analysed different cutoff points in the RBAS and found a significant difference to predict the outcome in four scores: 1.2, 1.6, 1.7 and 1.8. In the group with RBAS ≤ 1.8, 13 (68%) had an excellent outcome and six (33%) did not, while with RBAS>1.8, seven (32%) had an excellent outcome and 14 (67%) did not (p = 0.02). We did not find any correlation between Spetzler–Martin grading scale or OPI and outcome (p = 0.7 and p = 0.3 respectively). The KI predicted the excellent outcome in 8/9 patients (89%) with KI ≤ 27 and in 12/31 patients (39%) with KI>27 (p = 0.08). Conclusion: The RBAS seems to be a good predictor of outcome in patients with AVMs treated with LINAC-based conformal radiosurgery as in patients treated with Gamma Knife. It remains only to find the best cutoff point based on a larger series and longer follow-up.

Stereotactic radiosurgery for pediatric patients with intracranial arteriovenous malformations: Variables that may affect obliteration time and probability

INTRODUCTION It is debatable whether pediatric patients diagnosed with arteriovenous malformations (AVMs) should be treated as adults. Several indexes to classify AVMs have been proposed in the literature, and most try to predict the outcome for each specific treatment. The indexes differ in the variables considered, but they are all based in adult populations. In this study, we analyzed the variables that influence the obliteration time and probability of occurrence in a Mexican pediatric population diagnosed with an AVM and treated with stereotactic radiosurgery (SRS). METHODS We analyzed 45 pediatric patients (<18 years) with a minimum follow-up of 10 months and a maximum of 112 months. We used logistic regression analysis and Kaplan-Meier curves to evaluate the influence of age, AVM volume, prescribed dose, minimum dose, maximum dose, time of follow-up, sex, previous hemorrhage, venous drainage, treatment technique, previous treatment and location. We also evaluated the predictive power of the following indexes: Spetzler-Martin, RBAS, or K index dose deviation. RESULTS We found that the radiation technique used may influence the obliteration occurrence (p=0.057). The data suggests that circular arcs are a more efficient treatment technique than dynamic arcs. However, no relationship of dose or volume with treatment technique could be found. Obliteration was also dependent on follow-up time and after three years of follow-up, the obliteration probability decreases (p=0.024). According to Kaplan-Meier analysis, the nidus obliteration time was related with the location according to the Spetzler-Martin index. If the nidus was located in a non-eloquent region, there was a tendency of a shorter obliteration time (p=0.071). CONCLUSION None of the previously proposed indexes for adults predict obliteration in this pediatric population. Treatment technique, eloquence and follow up time were the only variables that showed influence in obliteration. Since the highest probability of obliteration occurs during the first three years, if the nidus has not been obliterated after this time then another treatment option could be considered.

A Novel Stereotactic Device for Spinal Irradiation in Rats Designed for a Linear Accelerator

Objective: Our purpose was to report the design and positioning accuracy testing of a stereotactic device designed for a linear accelerator to perform spinal radiosurgery in rats. Methods: To define the spatial and repositioning accuracy of the device, we measured the 3-dimensional (3D) translation of a paraspinal fiducial mark implanted by microsurgery in 5 Wistar rats during a sequence of setups and treatment simulations, thus obtaining final 3D translation vectors and maximum displacements. Results: For spatial accuracy, the differential coordinate translations were 0.8 ± 0.3, 0.6 ± 0.2 and 0.5 ± 0.1 mm in the x-, y- and z-directions, respectively. The median magnitude of the 3D vector was 1.3 mm (σ = 0.2 mm), with a maximum error of 2.2 mm. The differential coordinate translation for the repositioning accuracy showed values of 1.4 ± 0.3, 1.3 ± 0.3 and 0.8 ± 0.1 mm for the x-, y- and z-coordinates, resulting in a 3D displacement vector of 2.2 mm (σ = 0.2 mm) and a maximum displacement error of 3.6 mm. Conclusions: Using a linear accelerator, our novel stereotactic device provides accurate immobilization and repositioning of paraspinal structures under experimental conditions in rats.

Effect of the embolization material in the dose calculation for stereotactic radiosurgery of arteriovenous malformations

It is reported in the literature that the material used in an embolization of an arteriovenous malformation (AVM) can attenuate the radiation beams used in stereotactic radiosurgery (SRS) up to 10% to 15%. The purpose of this work is to assess the dosimetric impact of this attenuating material in the SRS treatment of embolized AVMs, using Monte Carlo simulations assuming clinical conditions. A commercial Monte Carlo dose calculation engine was used to recalculate the dose distribution of 20 AVMs previously planned with a pencil beam dose calculation algorithm. Dose distributions were compared using the following metrics: average, minimal and maximum dose of AVM, and 2D gamma index. The effect in the obliteration rate was investigated using radiobiological models. It was found that the dosimetric impact of the embolization material is less than 1.0 Gy in the prescription dose to the AVM for the 20 cases studied. The impact in the obliteration rate is less than 4.0%. There is reported evidence in the literature that embolized AVMs treated with SRS have low obliteration rates. This work shows that there are dosimetric implications that should be considered in the final treatment decisions for embolized AVMs.

Assessment of an image-guided neurosurgery system using a head phantom

Abstract Objective: To acknowledge the challenges and limitations of image-guided neurosurgery systems, we compared the application accuracy of two different image registration methods for one commercial system. (VectorVision, BrainLab, Germany). Methods: We used an anthropomorphic head phantom for radiosurgery and a custom built add-on to simulate surgical targets inside the brain during an image-guided neurosurgery. We used two image registration methods, fiducial registration using attachable surface markers for computed tomography (CT) and surface registration using infrared laser face scanning. After simulation, we calculated the three-dimensional (3D) distance between the predicted position of a target, and its actual position using a registered pointer and an infrared camera. Deviations were measured for both superficial fiducial markers and internal surgical targets by five different users. Results: Deviations from the location of fiducial markers after each registration method were 2.15 ± 0.93 mm after CT surface marker registration and 1.25 ± 0.64 mm after infrared face scanner registration. The mean target registration errors were 2.95 ± 1.4 mm using fiducial registration and 2.90 ± 1.3 mm using surface registration. The largest deviations (6.2 mm) were found for the targets in the skull base and posterior cranial fossa. Fiducial deviations and target registration errors were statistically uncorrelated. The total application accuracy was 4.87 ± 0.97 mm after CT surface marker registration and 4.14 ± 0.64 mm after infrared face scanner registration. Conclusions: Despite others have reported differences, we did not find significant variations between both registration methods for the target registration error, although application accuracy was slightly better after surface face registration. Superficial registration errors, but not the target registration error, can be routinely evaluated in the operating room. Since both errors were uncorrelated, surgeons may neglect the achievable accuracy of the procedure. The described method is recommended to assess application accuracy in the operating room.

Radiosurgical third ventriculostomy: Technical note

Background: We describe a minimally invasive technique to perform a radiosurgical third ventriculostomy in a patient with mild obstructive hydrocephalus secondary to malignant pathology. Methods: A 42 years old woman with diagnosis of clear cells renal carcinoma and with right nefrectomy performed last year. Cranial Magnetic Resonance Imaging showed two brain metastasis: one right temporal, and other in the pons with Sylvian aqueduct partial obliteration and mild ventricular enlargement. The patient received radiosurgical treatment for brain metastasis; after this procedure a new target was defined on the floor of the third ventricle, in the midpoint between the mamillary bodies and the infundibular recess where we delivered 100 Gy delivered by an isocentric multiple noncoplanar arcs technique, with a 6 MV Novalis® dedicated LINAC. A series of 21 arcs was arranged with a radiation field generated by a 4 mm circular collimator. Results: One week pos-irradiation in the head CT we did not find significant changes in the metastatic lesions; however the VSI diminished 4%, despite of persistent aqueduct obliteration. At three months we perform 3.0 T MRI where we confirmed the presence of the third ventriculostomy (2.63 mm diameter). Conclusion: This report demonstrates, for the first time, the ability of a dedicated LINAC to perform a precise third ventriculostomy without associate morbility in short term.