Purpose: According to the literature radiochromic film GafChromic EBT® (ISP Corp., USA) is an excellent detector to be used for small beam dosimetry. By using the fact that these films can be immersed in water for radiation measurements, this work investigates its performance in a water phantom for small beam measurements. Method and Materials: Tissue maximum ratios (TMR), total output factors (OF) and off axis ratios (OARs) were measured in a water phantom (MP3XS, PTW‐Freiburg, Germany) using GafChromic EBT® film and a shielded solid state detector (PFD3G, IBA‐dosimetry, Germany). Circular collimators (4 to 20 mm in diameter) coupled to a dedicated 6 MV linear accelerator (Novalis®, BrainLAB, Germany) were used. Films were calibrated against an ionization chamber at dmax covering a dose range from 0.01 to 450 cGy. Analysis was performed using a commercial flat bed scanner (Microtek 9600XL) in transmission mode at 100 dpi and 48 bit color depth (RGB format). The films were carefully handled and analyzed following the recommendations of the TG55 report and those proposed by others authors. The measurements using both detectors were compared to evaluate accuracy and reproducibility. Results: The reproducibility of the EBT film is reported to be within 1–2% using a commercial flat bed scanner. The highest differences between GafChromic EBT film and diode measurements for all circular collimators were 0.87%, 2.69% and 7.61% for OAR, OF and TMR, respectively. Conclusion: The use of GafChromic EBT immersed in a water phantom allowed reliable dose measurements. Although differences are present in the TMR values, the fact that the diode is not tissue equivalent as the film may explain these differences. The use of Monte Carlo simulations may be helpful to enlighten this issue.
Among all vascular malformations, the intracranial arteriovenous malformations (AVM's) have the most powerful impact from the clinical point of view. The manifestations include hemorrhage, seizures, headacheh, but sometimes they are incidentally found during the diagnostic approach of patients with head trauma or chronic headache. There are three different types of treatments: microsurgery, endovascular treatment and radiosurgery. The actual role of the endovascular treatment is as an adjuvant therapy before microsurgery or radiosurgery just to diminish the nidus size. The goal of all treatments is complete nidus obliteration without causing a new neurological deficit. The overall obliteration index with LINAC based radiosurgery is about 80% and the result is dose, volume and time dependent. The mean dose reported in the literature fluctuates between 15 and 25 Gy, and the isodose coverage curve for the AVM with LINAC is generally the one of the 80%. There can be a treatment failure defined as the necessity to retreat the patient after three years from the first radiosurgical treatment in about 26% of the patients. There is a lack of evidence, principally from randomized trials, to point out the role of each of the modalities in the treatment of the AVM.
Purpose:
To correct for the over‐response of mini‐ionization chambers with high‐Z central electrodes. The hypothesis is that by applying a negative/reverse voltage, it is possible to suppress the signal generated in the high‐Z central electrode by low‐energy photons.
Methods:
The mini‐ionization chambers used in the experiments were a PTW‐31014, PTW‐31006 and IBA‐CC01. The PTW‐31014 has an aluminum central electrode while the PTW‐31006 and IBA‐CC01 have a steel one. Total scatter factors (Scp) were measured for a 6 MV photon beam down to a square field size of 0.5 cm. The measurements were performed in water at 10 cm depth with SAD of 100 cm. The Scp were measured with the dosimeters with +400V bias voltage. In the case of the PTW‐31006 and IBA‐CC01, the measurements were repeated with −400V bias voltage. Also, the field factors in water were calculated with Monte Carlo simulations for comparison.
Results:
The measured Scp at +400V with the PTW‐31006 and IBA‐CC01 detectors were in agreement within 0.2% down to a field size of 1.5 cm. Both dosimeters shown a systematic difference about 2.5% with the Scp measured with the PTW‐31014 and the Monte Carlo calculated field factors. The measured Scp at −400V with the PTW‐31006 and IBA‐CC01 detectors were in close agreement with the PTW‐31014 measured Scp and the field factors within 0.3 and 1.0%, respectively. In the case of the IBA‐CC01 it was found a good agreement (1%) down to field size of 1.0 cm. All the dosimeters shown differences up to 17% between the measured Scp and the field factor for the 0.5 cm field size.
Conclusion:
By applying a negative/reverse voltage to the mini‐ionization chambers with high‐Z central electrode it was possible to correct for their over‐response to low energy photons.
Purpose: Characterize a medical linear accelerator using Monte Carlo methods to investigate the precision and exactitude of dose delivery on small animal irradiation, particularly Wistar rat species. Method and Materials: A dedicated 6.0 MV linear accelerator (Novalis®, BrainLAB, Germany) for stereotactic radiosurgery (SRS) was simulated using BEAMnrc. A phase space (PS) data file was generated. The Monte Carlo (MC) calculations were tuned and validated to match depth and off‐axis dose profile data measured using a shielded diode detector (PFD3G, IBA‐dosimetry, Germany) for a 15.0 mm circular collimator. The animal model was based on a computed tomography (CT) scan of a Wistar rat for medullar trauma lesion model. An in‐house mask fixation system compatible with the treatment planning system (TPS) was developed to immobilize the rat. The CT scan images were imported to DOSXYZnrc using ctcreate tool and to the TPS via DICOM network. Dose distribution was calculated by the TPS using 2 non‐coplanar circular (NCAs). The NCAs were simulated using 20 static beams. The MC dose calculations were exported for analysis and compared with TPS dose calculations. On the other hand, absolute dose measurements were performed using the PFD3G inter calibrated using a Farmer type ionization chamber. Results: Absolute dose measurements showed that dose difference between TPS and treatment delivery is less than 4.3% on the selected points. Difference between TPS and MC dose calculations showed an over‐estimation by the TPS up to 15.0%. However, in the spinal cord lesion there is a good agreement between TPS and MC calculated data. Conclusion: Dose delivered by the dedicated linear accelerator is reliable to perform spinal cord irradiation in trauma lesion models. These preliminary results must be improved including an accurate source model of an arc radiation beam instead of the 20 discrete static beams.
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