Spatial distribution of the imaging dose and characterization of the scatter radiation contribution in CyberKnife radiosurgery.

PURPOSE: The imaging dose for intra- and extra-cranial CyberKnife radiosurgery applications was calculated and the scattered radiation reaching the digital detectors was quantified and analyzed with regard to its origin. METHODS: The image guidance subsystem of the CyberKnife was modeled based on vendor-provided information. The emitted X-ray energy spectrum for 120 kV was estimated using the SpekPy software tool. Monte Carlo (MC) image acquisition simulations were performed to calculate the total, primary and scattered photon fluences reaching each detector as a function of the imaged object dimensions. MC calculations of the imaging dose were performed for intra- and extra-cranial applications assuming 120 kV and 10 mAs acquisition settings. RESULTS: The amount of scattered radiation reaching each detector was found to depend on the dimensions of the imaged anatomical region, contributing more than 40 % to the total photon fluence for regions more than 20 cm thick. More than 20 % of this scattered radiation originates from the contralateral imaging field. A maximum organ dose of 1.5 mGy at the nasal bones and an average dose of 0.37 mGy to the eye lenses per image pair acquisition was calculated for head applications. An entrance imaging dose of 0.4 mGy was calculated for extracranial applications. CONCLUSIONS: Scattered radiation reaching each detector in the skull and spine tracking applications can be reduced by acquiring the pair of radiographs sequentially instead of simultaneously. A dose of 3.7 cGy to the eye lenses is estimated assuming 100 image pair exposures required for treatment completion.

MRI-Related Geometric Distortions in Stereotactic Radiotherapy Treatment Planning: Evaluation and Dosimetric Impact.

In view of their superior soft tissue contrast compared to computed tomography, magnetic resonance images are commonly involved in stereotactic radiosurgery/radiotherapy applications for target delineation purposes. It is known, however, that magnetic resonance images are geometrically distorted, thus deteriorating dose delivery accuracy. The present work focuses on the assessment of geometric distortion inherent in magnetic resonance images used in stereotactic radiosurgery/radiotherapy treatment planning and attempts to quantitively evaluate the consequent impact on dose delivery. The geometric distortions for 3 clinical magnetic resonance protocols (at both 1.5 and 3.0 T) used for stereotactic radiosurgery/radiotherapy treatment planning were evaluated using a recently proposed phantom and methodology. Areas of increased distortion were identified at the edges of the imaged volume which was comparable to a brain scan. Although mean absolute distortion did not exceed 0.5 mm on any spatial axis, maximum detected control point disposition reached 2 mm. In an effort to establish what could be considered as acceptable geometric uncertainty, highly conformal plans were utilized to irradiate targets of different diameters (5-50 mm). The targets were mispositioned by 0.5 up to 3 mm, and dose-volume histograms and plan quality indices clinically used for plan evaluation and acceptance were derived and used to investigate the effect of geometrical uncertainty (distortion) on dose delivery accuracy and plan quality. The latter was found to be strongly dependent on target size. For targets less than 20 mm in diameter, a spatial disposition of the order of 1 mm could significantly affect (>5%) plan acceptance/quality indices. For targets with diameter greater than 2 cm, the corresponding disposition was found greater than 1.5 mm. Overall results of this work suggest that efficacy of stereotactic radiosurgery/radiotherapy applications could be compromised in case of very small targets lying distant from the scanner's isocenter (eg, the periphery of the brain).

On the experimental validation of model-based dose calculation algorithms for 192Ir HDR brachytherapy treatment planning.

There is an acknowledged need for the design and implementation of physical phantoms appropriate for the experimental validation of model-based dose calculation algorithms (MBDCA) introduced recently in 192Ir brachytherapy treatment planning systems (TPS), and this work investigates whether it can be met. A PMMA phantom was prepared to accommodate material inhomogeneities (air and Teflon), four plastic brachytherapy catheters, as well as 84 LiF TLD dosimeters (MTS-100M 1 × 1 × 1 mm3 microcubes), two radiochromic films (Gafchromic EBT3) and a plastic 3D dosimeter (PRESAGE). An irradiation plan consisting of 53 source dwell positions was prepared on phantom CT images using a commercially available TPS and taking into account the calibration dose range of each detector. Irradiation was performed using an 192Ir high dose rate (HDR) source. Dose to medium in medium, [Formula: see text], was calculated using the MBDCA option of the same TPS as well as Monte Carlo (MC) simulation with the MCNP code and a benchmarked methodology. Measured and calculated dose distributions were spatially registered and compared. The total standard (k = 1) spatial uncertainties for TLD, film and PRESAGE were: 0.71, 1.58 and 2.55 mm. Corresponding percentage total dosimetric uncertainties were: 5.4-6.4, 2.5-6.4 and 4.85, owing mainly to the absorbed dose sensitivity correction and the relative energy dependence correction (position dependent) for TLD, the film sensitivity calibration (dose dependent) and the dependencies of PRESAGE sensitivity. Results imply a LiF over-response due to a relative intrinsic energy dependence between 192Ir and megavoltage calibration energies, and a dose rate dependence of PRESAGE sensitivity at low dose rates (<1 Gy min-1). Calculations were experimentally validated within uncertainties except for MBDCA results for points in the phantom periphery and dose levels <20%. Experimental MBDCA validation is laborious, yet feasible. Further work is required for the full characterization of dosimeter response for 192Ir and the reduction of experimental uncertainties.

Experimental determination of the Task Group-43 dosimetric parameters of the new I25.S17plus (125)I brachytherapy source.

PURPOSE: To present experimental dosimetry results for the new IsoSeed I25.S17plus (125)I brachytherapy source, in fulfillment of the American Association of Physicists in Medicine recommendation for, at least one, experimental dosimetry characterization of new low-energy seeds before their clinical implementation. METHODS AND MATERIALS: A batch of 100 LiF thermoluminescent dosimeter (TLD)-100 microcubes was used for the experimental determination of the dose-rate constant, radial dose, and anisotropy functions, in irradiations performed using two Solid Water phantoms. Monte Carlo (MC) simulations were used to determine appropriate correction factors that account for the use of Solid Water as a phantom material instead of liquid water and for the different energy response of the TLD dosimeters in the experimental (125)I photon energies relative to the 6 MV x-ray photon beam used for the TLD calibration. Measurements were performed for four I25.S17plus seeds; one with direct traceability of air-kerma strength calibration to National Institute of Standards and Technology and three with secondary National Institute of Standards and Technology traceability. RESULTS: A mean dose-rate constant, Λ, of 0.956 ± 0.043 cGy h(-1) U(-1) was experimentally determined for the I25.S17plus source, which agrees within uncertainties with the MC result of 0.925 ± 0.013 cGy h(-1) U(-1) calculated independently for the same seed model in a previous study. Agreement was also observed between the measured and the MC-calculated radial dose and anisotropy function values. CONCLUSIONS: Experimental dosimetry results for the I25.S17plus (125)I source verify corresponding independent MC results in the form of Task Group-43 dosimetry parameters. The latter are found in agreement within uncertainties with sources of similar design incorporating a silver marker, such as the Oncura OncoSeed Model 6711.

A simple and efficient methodology to improve geometric accuracy in gamma knife radiation surgery: implementation in multiple brain metastases.

PURPOSE: To propose, verify, and implement a simple and efficient methodology for the improvement of total geometric accuracy in multiple brain metastases gamma knife (GK) radiation surgery. METHODS AND MATERIALS: The proposed methodology exploits the directional dependence of magnetic resonance imaging (MRI)-related spatial distortions stemming from background field inhomogeneities, also known as sequence-dependent distortions, with respect to the read-gradient polarity during MRI acquisition. First, an extra MRI pulse sequence is acquired with the same imaging parameters as those used for routine patient imaging, aside from a reversal in the read-gradient polarity. Then, "average" image data are compounded from data acquired from the 2 MRI sequences and are used for treatment planning purposes. The method was applied and verified in a polymer gel phantom irradiated with multiple shots in an extended region of the GK stereotactic space. Its clinical impact in dose delivery accuracy was assessed in 15 patients with a total of 96 relatively small (<2 cm) metastases treated with GK radiation surgery. RESULTS: Phantom study results showed that use of average MR images eliminates the effect of sequence-dependent distortions, leading to a total spatial uncertainty of less than 0.3 mm, attributed mainly to gradient nonlinearities. In brain metastases patients, non-eliminated sequence-dependent distortions lead to target localization uncertainties of up to 1.3 mm (mean: 0.51 ± 0.37 mm) with respect to the corresponding target locations in the "average" MRI series. Due to these uncertainties, a considerable underdosage (5%-32% of the prescription dose) was found in 33% of the studied targets. CONCLUSIONS: The proposed methodology is simple and straightforward in its implementation. Regarding multiple brain metastases applications, the suggested approach may substantially improve total GK dose delivery accuracy in smaller, outlying targets.

A virtual reality simulation curriculum for intravenous cannulation training.

OBJECTIVES: Although virtual reality (VR) simulators play an important role in modern medical training, their efficacy is not often evaluated using learning curves. In this study, the learning curves of novice and intermediate users were elicited during a VR simulation-based curriculum for intravenous (IV) cannulation. METHODS: This was a prospective observational study of subjects undergoing training using a VR model of IV cannulation. Participants were divided into two groups: novices (third-year medical students with no prior practical experience in IV catheterization) and intermediates (recent graduates with limited experience). Performance was measured with two endpoints: time to completion and errors committed. Errors were categorized as critical or noncritical. Learning curves (error score and time completion vs. session number) were analyzed using the Friedman's test. Performance before and after training was compared using the Kruskal-Wallis test. The Spearman rank correlation coefficient (r(s)) was used to determine the correlation between time completion and error score estimates. The number of attempts required to complete the training phase was also measured and compared between the two groups. RESULTS: Thirty subjects were enrolled: 17 in the novice group and 13 in the intermediate group. Learning curve plateaus of intermediates were reached in the sixth case scenario (session), whereas novices reached a plateau in the eighth session. Performance comparison of time to completion and errors showed significant improvement for both groups. Less time and fewer attempts were required by all trainees to complete a scenario while progressing through the curriculum. The overall number of IV cannulation attempts of novices was significantly higher than that of the intermediates throughout the course. CONCLUSIONS: Significant learning curves for novice and intermediate students were demonstrated after following the VR simulation-based curriculum. Competencies acquired during this educational course may provide an important advantage for training prior to actual clinical practice.