Transmission and dose perturbations with high-Z materials in clinical electron beams

An investigation of high-Z material for bolus in electron beam therapy

Highlight. Electron beam treatment often requires bolus to augment surface dose to nearly 100%. There are no optimum bolus materials and hence a high-Z based clothlike material is investigated to reduce air column in treatment that provides optimum surface dose. This material is well suited as it can be used multiple times and can be sanitized. Characteristics of W-Si material is provided.Purpose /Objective(s). Electron beams are frequently used for superficial tumors. However, due to electron beam characteristics the surface dose is 75-95% of the prescribed dose depending on beam energy thus requiring placement of bolus to augment surface dose. Various types of boluses are commonly used in clinics, each having it's own unique limitation. Most bolus devices do not conform to the skin contour and create airgaps that are known to produce dose perturbations creating hot and cold spots. A cloth-like high-Z materials; Tungsten, (Z = 74) and Bismuth, (Z = 83) impregnated in silicone gel is investigated for electron bolus.Materials/Methods. Super soft silicone-gel based submillimeter thin tungsten and bismuth sheets were investigated for bolus for 6-12 MeV. Parallel plate ion chamber measurements were performed in a solid water phantom on a Varian machine. Depth dose characteristics were measured to optimize the thickness for surface dose to be 100% for selected electron therapy and validated with Monte Carlo simulations.Results. Silicone-gel tungsten and bismuth sheets produce significant electrons thus increasing surface dose. Based on measured depth dose, our data showed that tungsten sheets of 0.14 mm, 0.18 mm and 0.2 mm and Bismuth sheets of 0.42 mm, 0.18 mm and 0.2 mm provide 100% surface dose for 6, 9 and 12 MeV beams, respectively without any significant changes in depth dose except increasing surface dose.Conclusions. The new high-Z clothlike sheets are extremely soft but high tensile metallic bolus materials that can fit flawlessly on any skin contour. Only 0.2 mm thick sheets are needed for 100% surface dose without degradation of the depth dose characteristics. These materials are reusable and ideal for bolus in electron beam treatment. This investigation opens a new frontier in designing new bolus materials optimum for patient treatment.

Dose calculation of 3D printing lead shield covered by biocompatible silicone for electron beam therapy

This study aims to calculate the dose delivered to the upstream surface of a biocompatible flexible absorber covering lead for electron beam treatment of skin and subcutaneous tumour lesions for head and neck. Silicone (Ecoflex™ 00-30, Smooth-On, Easton, PA, USA) was used to cover the lead to absorb backscattered electrons from lead. A 3D printer (Zortrax M300, Zortrax, Olsztyn, Poland) was used to fabricate the lead shield. Analytic calculation, simplified Monte Carlo (MC) simulation, and detailed MC simulation which includes a modeling of metal-oxide-semiconductor field-effect transistor (MOSFET) detector were performed to determine the electron backscatter factor (EBF) for 6 MeV and 9 MeV electron beams of a Varian iX Silhouette. MCNP6.2 was used to calculate the EBF and corresponding measurements were carried out by using MOSFET detectors. The EBF was experimentally measured by the ratio of dose at the upstream surface of the silicone to the same point without the presence of the lead shield. The results derived by all four methods agreed within 2.8% for 6 MeV and 3.4% for 9 MeV beams. In detailed MC simulations, for 6 MeV, dose to the surface of 7-mm-thick absorber was 103.7 [Formula: see text] 1.9% compared to dose maximum (D_max) without lead. For 9 MeV, the dose to the surface of the 10-mm-thick absorber was 104.1 [Formula: see text] 2.1% compared to D_max without lead. The simplified MC simulation was recommended for practical treatment planning due to its acceptable calculation accuracy and efficiency. The simplified MC simulation was completed within 20 min using parallel processing with 80 CPUs, while the detailed MC simulation required 40 h to be done. In this study, we outline the procedures to use the lead shield covered by silicone in clinical practice from fabrication to dose calculation.

Preradiosurgery embolization in reducing the postoperative hemorrhage rate for patients with cerebral arteriovenous malformations: a systematic review and meta-analysis

Few studies have examined the postoperative hemorrhage rate of cerebral arteriovenous malformations (AVMs) treated by embolization prior to stereotactic radiosurgery. The objective of this analysis was to compare the postoperative hemorrhage rate between AVMs treated with and those treated without preradiosurgery embolization. A systematic search of the PubMed and Embase databases was performed with no restriction on the publication period. Based on Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, we included studies with sufficient baseline and outcome data. The analysis was performed using Comprehensive Meta-Analysis (CMA) 2.0. Eleven studies comprising 2591 patients were eligible for analysis. There was no significant difference in the postoperative hemorrhage rate between patients who had undergone embolization followed by SRS and those who had undergone SRS alone (OR 1.140, 95% CI 0.851-1.526, p = 0.38). The obliteration rate was significantly lower in the E + SRS group than in the SRS group (OR 0.586, 95% CI 0.398-0.863, p = 0.007). No significant difference in permanent neurological deficits was identified between patients who had undergone embolization followed by SRS and those who had undergone SRS alone (OR 1.175, 95% CI 0.626-2.206, p = 0.616). Available data suggested that preradiosurgery embolization did not reduce the postoperative hemorrhage rate and resulted in a significantly lower obliteration rate than treatment with SRS alone.

Evaluation of backscatter dose from internal lead shielding in clinical electron beams using EGSnrc Monte Carlo simulations

Internal lead shielding is utilized during superficial electron beam treatments of the head and neck, such as lip carcinoma. Methods for predicting backscattered dose include the use of empirical equations or performing physical measurements. The accuracy of these empirical equations required verification for the local electron beams. In this study, a Monte Carlo model of a Siemens Artiste linac was developed for 6, 9, 12, and 15 MeV electron beams using the EGSnrc MC package. The model was verified against physical measurements to an accuracy of better than 2% and 2 mm. Multiple MC simulations of lead interfaces at different depths, corresponding to mean electron energies in the range of 0.2–14 MeV at the interfaces, were performed to calculate electron backscatter values. The simulated electron backscatter was compared with current empirical equations to ascertain their accuracy. The major finding was that the current set of backscatter equations does not accurately predict electron backscatter, particularly in the lower energies region. A new equation was derived which enables estimation of electron backscatter factor at any depth upstream from the interface for the local treatment machines. The derived equation agreed to within 1.5% of the MC simulated electron backscatter at the lead interface and upstream positions. Verification of the equation was performed by comparing to measurements of the electron backscatter factor using Gafchromic EBT2 film. These results show a mean value of 0.997±0.022 to 1σ of the predicted values of electron backscatter. The new empirical equation presented can accurately estimate electron backscatter factor from lead shielding in the range of 0.2 to 14 MeV for the local linacs.

PACS numbers: 87.53.Bn, 87.55.K‐, 87.56.bd

Stereotactic radiosurgery for arteriovenous malformations after Onyx embolization: a case-control study

OBJECT

Onyx, an ethylene-vinyl alcohol copolymer mixed in a dimethyl sulfoxide solvent, is currently one of the most widely used liquid materials for embolization of intracranial arteriovenous malformations (AVMs). The goal of this study was to define the risks and benefits of stereotactic radiosurgery (SRS) for patients who have previously undergone partial AVM embolization with Onyx.

METHODS

Among a consecutive series of 199 patients who underwent SRS between January 2007 and December 2012 at the University of Virginia, 25 patients had Onyx embolization prior to SRS (the embolization group). To analyze the obliteration rates and complications, 50 patients who underwent SRS without prior embolization (the no-embolization group) were matched by propensity score method. The matched variables included age, sex, nidus volume before SRS, margin dose, Spetzler-Martin grade, Virginia Radiosurgery AVM Scale score, and median imaging follow-up period.

RESULTS

After Onyx embolization, 18 AVMs were reduced in size. Total obliteration was achieved in 6 cases (24%) at a median of 27.5 months after SRS. In the no-embolization group, total obliteration was achieved in 20 patients (40%) at a median of 22.4 months after SRS. Kaplan-Meier analysis demonstrated obliteration rates of 17.7% and 34.1% in the embolization group at 2 and 4 years, respectively. In the no-embolization group, the corresponding obliteration rates were 27.0% and 55.9%. The between-groups difference in obliteration rates after SRS did not achieve statistical significance. The difference in complications, including adverse radiation effects, hemorrhage episodes, seizure control, and patient mortality also did not reach statistical significance.

CONCLUSIONS

Onyx embolization can effectively reduce the size of many AVMs. This case-control study did not show any statistically significant difference in the rates of embolization or complications after SRS in patients who had previously undergone Onyx embolization and those who had not.

Liquid embolization material reduces the delivered radiation dose: clinical myth or reality?

BACKGROUND AND PURPOSE

To be radiopaque, BAVM embolization products must contain high-atomic-number materials, which may also attenuate photon beams delivered with radiosurgery. This "shielding effect" has been invoked to explain why radiation therapy may be less effective for previously embolized BAVMs. To evaluate the impact of embolization material on radiation dose, we measured and compared the dose delivered to the center of an AVM model, before and following embolization with various materials in a LINAC.

MATERIALS AND METHODS

Two in vitro AVM models were constructed by drilling interconnected tubular perforations in plastic water phantoms to simulate nidal vessels. Phantoms were designed to allow the positioning of a radiation detector at their center. One model was embolized with Onyx 18 and a second model, with a combination of Indermil, Lipiodol, tungsten powder, and Onyx 18. The radiation delivered was compared between embolized and nonembolized controls following irradiation with a standard 250-cGy dose.

RESULTS

The mean dose of radiation delivered to the model embolized with Onyx alone was 244 ± 5 cGy before and 246 ± 5 cGy following embolization. The mean dose of radiation delivered to the model embolized with various agents was 242 ± 5 cGy before, and 254 ± 5 cGy after embolization.

CONCLUSIONS

Embolic material did not reduce the radiation dose delivered by a LINAC to the center of our experimental BAVM models. The shielding effect may be compensated by scattered and reflected radiation.

Dosimetric dependence of the dimensional characteristics on a lead shield in electron radiotherapy: a Monte Carlo study

This study investigates the dosimetric dependence of the dimension of a lead (Pb) layer for shielding using clinical electron beams with different energies. Monte Carlo simulations were used to generate phase space files for the 4, 9 and 16 MeV electron beams produced by a Varian 21 EX linear accelerator using the EGSnrc‐based BEAMnrc code, and validated by measurements using films. Pb layers with different thicknesses (2, 4, 6 and 8 mm) and diameters (2.5, 3, 3.5 and 4 cm) were placed at the center of an electron field on a solid water phantom. Beam profiles were determined at the depth of maximum dose (dm) using Monte Carlo simulations. The dose profiles under the Pb layer at dm, including the penumbra at the edge of the layer and relative dose at the central beam axis (CAX), were studied with varying thicknesses and diameters of Pb. It is found that 2 mm of Pb is adequate to provide 5 half value layer (HVL) attenuation for the 4 MeV electron beams, and the beam profiles at dm are dependent on the diameter but not the thickness of the Pb. However, for the 9 and 16 MeV electron beams, the relative dose at the CAX and dm depends on both the thickness and diameter of the Pb layer. For 8 mm thickness of Pb, 4 and 5 HVL attenuation of electron beams with energies of 9 and 16 MeV can be achieved at dm, respectively. Moreover, the beam profile under the Pb layer at dm depends on: (1) the penumbra region at the edge of the Pb layer; (2) the beam attenuation varying with the thickness of the Pb layer; (3) the electron side scatter contributing to the CAX under the Pb layer; and (4) the photon contamination produced by the Pb layer. A parameter called “shielding area factor” (defined as the ratio of the length between two points of 50% relative doses in the beam profile at dm to the diameter of the Pb layer) is suggested to predict the required size and thickness of Pb for shielding a target with known dimension at dm. The dosimetric data calculated by Monte Carlo simulations in this study are useful to select the suitable thickness and size of Pb for the protection of critical tissue in electron radiotherapy.

PACS number: 87.53.Bn; 87.55.kh and 87.55.km.

Increased apoptotic potential and dose-enhancing effect of gold nanoparticles in combination with single-dose clinical electron beams on tumor-bearing mice

High atomic number material, such as gold, may be used in conjunction with radiation to provide dose enhancement in tumors. In the current study, we investigated the dose-enhancing effect and apoptotic potential of gold nanoparticles in combination with single-dose clinical electron beams on B16F10 melanoma tumor-bearing mice. We revealed that the accumulation of gold nanoparticles was detected inside B16F10 culture cells after 18 h of incubation, and moreover, the gold nanoparticles were shown to be colocalized with endoplasmic reticulum and Golgi apparatus in cells. Furthermore, gold nanoparticles radiosensitized melanoma cells in the colony formation assay (P = 0.02). Using a B16F10 tumor-bearing mouse model, we further demonstrated that gold nanoparticles in conjunction with ionizing radiation significantly retarded tumor growth and prolonged survival compared to the radiation alone controls (P < 0.05). Importantly, an increase of apoptotic signals was detected inside tumors in the combined treatment group (P < 0.05). Knowing that radiation-induced apoptosis has been considered a determinant of tumor responses to radiation therapy, and the length of tumor regrowth delay correlated with the extent of apoptosis after single-dose radiotherapy, these results may suggest the clinical potential of gold nanoparticles in improving the outcome of melanoma radiotherapy.