José M. Lárraga-Gutiérrez

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.

Evaluation of the Gafchromic® EBT2 film for the dosimetry of radiosurgical beams

PURPOSE Radiosurgery uses small fields and high-radiation doses to treat intra- and extracranial lesions in a single session. The lack of a lateral electronic equilibrium and the presence of high-dose gradients in these fields are challenges for adequate measurements. The availability of radiation detectors with the high spatial resolution required is restricted to only a few. Stereotactic diodes and EBT radiochromic films have been demonstrated to be good detectors for small-beam dosimetry. Because the stereotactic diode is the standard measurement for the dosimetry of radiosurgical beams, the goal of this work was to perform measurements with the radiochromic film Gafchromic(®) EBT2 and compare its results with a stereotactic diode. METHODS Total scatter factors, tissue maximum, and off-axis ratios from a 6 MV small photon beams were measured using EBT2 radiochromic film in a water phantom. The film-measured data were evaluated by comparing it with the data measured with a stereotactic field diode (IBA-Dosimetry). RESULTS The film and diode measurements had excellent agreement. The differences between the detectors were less than or equal to 2.0% for the tissue maximum and the off-axis ratios. However, for the total scatter factors, there were significant differences, up to 4.9% (relative to the reference field), for field sizes less than 1.0 cm. CONCLUSIONS This work found that the Gafchromic(®) EBT2 film is adequate for small photon beam measurements, particularly for tissue maximum and off-axis ratios. However, careful attention must be taken when measuring output factors of small beams below 1.0 cm due to the films energy dependence. The measurement differences may be attributable to the films active layer composition because EBT2 incorporates higher Z elements (i.e., bromide and potassium), hence revealing a potential energy dependence for the dosimetry of small photon beams.

Evaluation of the Gafchromic{sup Registered-Sign} EBT2 film for the dosimetry of radiosurgical beams

PURPOSE Radiosurgery uses small fields and high-radiation doses to treat intra- and extracranial lesions in a single session. The lack of a lateral electronic equilibrium and the presence of high-dose gradients in these fields are challenges for adequate measurements. The availability of radiation detectors with the high spatial resolution required is restricted to only a few. Stereotactic diodes and EBT radiochromic films have been demonstrated to be good detectors for small-beam dosimetry. Because the stereotactic diode is the standard measurement for the dosimetry of radiosurgical beams, the goal of this work was to perform measurements with the radiochromic film Gafchromic(®) EBT2 and compare its results with a stereotactic diode. METHODS Total scatter factors, tissue maximum, and off-axis ratios from a 6 MV small photon beams were measured using EBT2 radiochromic film in a water phantom. The film-measured data were evaluated by comparing it with the data measured with a stereotactic field diode (IBA-Dosimetry). RESULTS The film and diode measurements had excellent agreement. The differences between the detectors were less than or equal to 2.0% for the tissue maximum and the off-axis ratios. However, for the total scatter factors, there were significant differences, up to 4.9% (relative to the reference field), for field sizes less than 1.0 cm. CONCLUSIONS This work found that the Gafchromic(®) EBT2 film is adequate for small photon beam measurements, particularly for tissue maximum and off-axis ratios. However, careful attention must be taken when measuring output factors of small beams below 1.0 cm due to the films energy dependence. The measurement differences may be attributable to the films active layer composition because EBT2 incorporates higher Z elements (i.e., bromide and potassium), hence revealing a potential energy dependence for the dosimetry of small photon beams.

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.

Experimental determination of field factors (

Recently, Alfonso et al proposed a new formalism for the dosimetry of small and non-standard fields. The proposed new formalism is strongly based on the calculation of detector-specific beam correction factors by Monte Carlo simulation methods, which accounts for the difference in the response of the detector between the small and the machine specific reference field. The correct calculation of the detector-specific beam correction factors demands an accurate knowledge of the linear accelerator, detector geometry and composition materials. The present work shows that the field factors in water may be determined experimentally using the daisy chain correction method down to a field size of 1 cm × 1 cm for a specific set of detectors. The detectors studied were: three mini-ionization chambers (PTW-31014, PTW-31006, IBA-CC01), three silicon-based diodes (PTW-60018, IBA-SFD and IBA-PFD) and one synthetic diamond detector (PTW-60019). Monte Carlo simulations and experimental measurements were performed for a 6 MV photon beam at 10 cm depth in water with a source-to-axis distance of 100 cm. The results show that the differences between the experimental and Monte Carlo calculated field factors are less than 0.5%-with the exception of the IBA-PFD-for field sizes between 1.5 cm × 1.5 cm and 5 cm × 5 cm. For the 1 cm × 1 cm field size, the differences are within 2%. By using the daisy chain correction method, it is possible to determine measured field factors in water. The results suggest that the daisy chain correction method is not suitable for measurements performed with the IBA-PFD detector. The latter is due to the presence of tungsten powder in the detector encapsulation material. The use of Monte Carlo calculated [Formula: see text] is encouraged for field sizes less than or equal to 1 cm × 1 cm for the dosimeters used in this work.

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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.

) for small radiotherapy beams using the daisy chain correction method

The multileaf collimator (MLC) is the standard device used to shape radiation beams for 3-d conformal and intensity-modulated radiation therapy (IMRT). Due to the inherent properties of MLC, there is a small amount of radiation transmitted through the leaves, called radiation transmission (RT). Accurate measurements of this radiation are required to commission and validate IMRT-capable treatment planning systems because this radiation may impact the dosimetry of IMRT-calculated dose distributions. This work compares several detectors in the measurement of RT for a micro-multileaf collimation system. The results show that there are statistically significant differences in the measured RT values between detectors from 3.5 to 12.5% for the same MLC model and less than 0.2% relative to the isocentre dose for an open reference field. However, although small in magnitude, these differences may impact the dosimetry of IMRT treatment planning by up to 1.78 Gy to the healthy tissue surrounding the target for a treatment of 60 Gy in 30 fractions. By the later, these differences must be included as a source of uncertainty in IMRT dose delivery. Also, it must be established which detector offers the most reliable results in the measurement of the RT by using Monte Carlo simulation methods.

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

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.

Comparative analysis of several detectors for the measurement of radiation transmission and leakage from a multileaf collimator

Trigeminal neuralgia (TN) is a chronic, episodic facial pain syndrome that can be extremely intense, and it occurs within the regions of the face that are innervated by the three branches of the trigeminal nerve. Stereotactic radiosurgery (SRS) is the least invasive procedure to treat TN. SRS uses narrow photon beams that require high spatial resolution techniques for their measurement. The use of radiographic or radiochromic films for small‐field dosimetry is advantageous because high spatial resolution and two‐dimensional dose measurements can be performed. Because these films have different properties, it is expected that the calculated dose distributions for TN patients will behave differently, depending on the detector used for the commissioning of the small photon beams. This work is based on two sets of commissioned data: one commissioned with X‐OMAT V2 film and one commissioned with EBT2 film. The calculated dose distributions for 23 TN patients were compared between the commissioning datasets. The variables observed were the differences in the half widths of the 35 and 40 Gy isodose lines (related to the entrance distance to the brainstem) and the volume of the brainstem that received a dose of 12 Gy or more (V12). The results of this comparison showed that there were statistically significant differences between the two calculated dose distributions. The magnitudes of these differences were up to 0.33 mm and 0.38 mm for the 35 and 40 Gy isodose lines. The corresponding difference for the V12 was up to 2.1 cc. It is clear that these differences may impact the treatment of TN patients, and then it must be important to perform this type of analysis when observing complication rates. Clinical reports on irradiation techniques for trigeminal neuralgia should consider that different detectors used for commissioning treatment planning systems might result in small but significant differences in dose distributions. PACS number: 87.55.km