Sci-Thur AM: Planning - 02: Validation of XiO's eMC module using Gafchromic EBT3 films and triple channel dosimetry.

The aim of this study is to validate the electron Monte Carlo module implemented in XiO, a treatment planning system commercialized by Elekta CMS inc. Two types of phantoms were investigated: homogeneous water phantoms with irregular surfaces and phantoms containing slab and 3D heterogeneities. The phantoms were CT scanned, and dose to water calculations were performed in the eMC module using 2 ×2 × 2 mm2 voxels and a mean relative statistical uncertainty of 0.5%. Concurrently, Gafchromic EBT3 film measurements were performed in the same phantoms. To obtain reliable absolute dose readings from the films, a new method using triple channel dosimetry in the Film QA Pro software was developed. The accuracy of the proposed method was determined empirically and an uncertainty of ±1.5% was found over the range [75, 800] cGy. Dose comparisons between film and simulations were done using an in-house MATLAB program. XiO's eMC module provides accurate dose distributions in the presence of surface irregularities and slab heterogeneities for 12 MeV beams. In the presence of 3D heterogeneities, the percent dose difference comparisons highlighted the need to perform 3D gamma comparisons. In conclusion, the electron Monte Carlo module offered in the XiO treatment planning system is promising and could greatly improve the accuracy of clinical dose calculations. The validation of the software is ongoing, notably concerning more complex phantom geometries. Small field calculations, oblique incidences and cutout factors will also be investigated.

Poster - Thur Eve - 42: Dynamic delivery quality assurance on Elekta linacs.

Introduction Recently, Elekta linacs have gained the capability to deliver dynamic fields, such as sliding window IMRT or VMAT fields. Because of the potential complexity of such delivery modes, linac QA and patient-specific QA are of prime importance. The aim of this paper is to explain the fundamental concepts of this new mode of operation on Elekta linacs as well as to introduce the linear-α slit, a novel dynamic QA sequence which allows the performance of the delivery system to be objectively aassessed against dosimetric measurements. In the Elekta dynamic mode, dose rate can only take a limited number of discontinuous values. We have used the uniform slit, a simple slit moving at a constant speed across the field to verify how the linac selects leaf speed and dose rate for a given MU setting. Based on this, we expose the principles behind the linear-α slit, which exhibits two main characteristics: 1) it exercises the linac over a range of dose rates and 2) it produces a dose distribution which is theoretically equivalent to the uniform slit. Discrepancies between measurements of the linear-α slit and the uniform slit directly reflects problems with the delivery. The linear-α slit has been recently introduced in our routine monthly linac QA. We hope it will nicely complement patient-specific QA, and exising linac-QA.

Patient characteristics differently affect early cup and stem loosening in THA: a case-control study on 7,535 patients.

We postulated that certain patient characteristics have different effects on early THA component loosening. With two matched case-control studies we assessed 3,028 cups and 5,224 stems. Loosening was defined using signs of mechanical component failure on routine follow-up radiographs or revision for aseptic loosening. Women and men had similar cup-loosening odds, but women had lower odds for stem loosening (p < 0.0001). Odds for cup loosening decreased by 2.1% per additional year of age (p = 0.0004), those for stem loosening by 2.4% (p < 0.0001). Each additional kilogram of weight decreased cup loosening odds by 1.3% (p = 0.0051). Each additional unit of BMI increased stem loosening odds (p = 0.0109). Charnley classes B and C were protective factors against loosening of both components. There were no risk differences for the various main diagnoses. Certain patient characteristics differently affected early cup and stem loosening, although some characteristics had the same protective or harmful effect on component survival.

Does total knee joint replacement with the soft tissue balancing surgical technique maintain the natural joint line?

INTRODUCTION: The outcomes of 106 total knee arthroplasties implanted using a soft tissue balancing surgical technique at one surgical centre were used to assess the accuracy maintaining the knee's original joint line (JL). The aim of the study was to determine whether there is a shift of the presumed joint line after surgery. MATERIALS AND METHODS: Preoperative and post-operative radiographs were compared to determine any changes in the articulation height. The preoperative distance of the fibular head to the natural joint line was measured and compared with the post-operative measurement of the fibular head to the femoral articulation line (measured on the radiograph and defined as Rxmm). Based on the actual medio-lateral dimension of the tibial metal back, the measured difference (RXmm) could be converted into true distances (in mm). The Blackburn-Peel index was assessed as an additional outcome prior to and following surgery. RESULTS: Preoperatively, the average distance from the fibular head to the joint line was 15.1 Rxmm (SD 4.3) while the post-surgical distance was 15.5 Rxmm (SD 5.6). The average deviation of the post-surgical JL in relation to the original JL amounted to 0.4 Rxmm (SD 3.7). The average deviation of the joint line converted into the true distance was -0.3 mm (with a range of -5.9 mm in distal direction to + 8.3 mm in the proximal direction). Valgus position appeared to generate rather a shift in proximal direction whereas varus deformity favours a shift in distal direction. Seven patients exhibited a deviation of more than 5 mm in either the distal or proximal direction. All of the patients of this subgroup had a preoperative anatomical abnormality including a severe malalignment, serious bone destruction or had previously undergone a high tibial osteotomy. CONCLUSION: An exact reconstruction of the natural Joint Line is achievable when using the described soft tissue balancing surgical technique with the posterior cruciate ligament (PCL) retaining prosthesis design used in this series.

An inverse problem approach to the correction of distortion in EPI images.

Magnetic resonance imaging using the echo planar imaging (EPI) technique is particularly sensitive to main (B0) field inhomogeneities. The primary effect is geometrical distortion in the phase encoding direction. In this paper, we present a method based on the conjugate gradient algorithm to correct for this geometrical distortion, by solving the EPI imaging equation. Two versions are presented: one that attempts to solve the full four-dimensional (4-D) imaging equation, and one that independently solves for each profile along the blip encoding direction. Results are presented for both phantom and in vivo brain EPI images and compared with other proposed correction methods.

Perfusion-based functional magnetic resonance imaging with single-shot RARE and GRASE acquisitions.

For perfusion-based functional magnetic resonance imaging, the previously introduced flow-sensitive alternating inversion recovery (FAIR) technique is combined with single-shot RARE (rapid acquisition with relaxation enhancement) and GRASE (gradient and spin echo) imaging sequences. The advantages of these sequences compared to commonly used echo-planar imaging (EPI) are an increased signal-to-noise ratio and the absence of distortions and artifacts due to magnetic field inhomogeneities. RARE- and GRASE-FAIR are applied to functional brain mapping studies in humans during visual stimulation. Results demonstrate that the presented techniques allow for perfusion maps with higher spatial resolution compared to EPI-FAIR. Relative regional cerebral blood flow change in the occipital cortex during visual stimulation was measured to be 41+/-4% (n = 5). The comparison of FAIR data obtained with RARE and GRASE techniques shows that RARE yields images with the higher signal-to-noise ratio. However, the GRASE technique features a shorter acquisition time and less RF power deposition and is thus better suited for multi-slice acquisitions.

Three-dimensional multimodal image-guidance for neurosurgery.

The authors address the use of multimodality imaging as an aid to the planning and guidance of neurosurgical procedures, and discuss the integration of anatomical (CT and MRI), vascular (DSA), and functional (PET) data for presentation to the surgeon during surgery. The authors' workstation is an enhancement of a commercially available system, and in addition to the guidance offered via a hand-held probe, it incorporates the use of multimodality imaging and adds enhanced realism to the surgeon through the use of a stereoscopic three-dimensional (3-D) image display. The probe may be visualized stereoscopically in single or multimodality images. The integration of multimodality data in this manner provides the surgeon with a complete overview of brain structures on which he is performing surgery, or through which he is passing probes or cannulas, enabling him to avoid critical vessels and/or structures of functional significance.

Comparison of relative accuracy between a mechanical and an optical position tracker for image-guided neurosurgery.

An essential component in the execution of image-guided surgery is a hand-held probe whose spatial position is tracked during the procedure and displayed on a three-dimensional operative workstation. This paper describes an experiment performed in order to compare the accuracy of a mechanically linked pointing device (FARO surgical arm) and an optical position tracker (OPTOTRAK) against a "gold standard."

Integration of stereoscopic DSA and 3D MRI for image-guided neurosurgery.

We demonstrate the feasibility and utility of using anatomical/vascular correlation in image-guided surgery, by interfacing a PC-based stereoscopic Digital Subtraction Angiography (DSA) analysis system to a three-dimensional (3D) image based surgical workstation that has been modified to allow presentation of stereoscopic images. Numerical values representing the position and angulation of a hand-held probe are transmitted to both systems simultaneously, enabling the probe to be visualized stereoscopically in both anatomical and vascular images during the surgical procedure. The integration of the patient's vascular and anatomical data in this way provides the surgeon with a complete overview of brain structures through which he is passing the electrode-guiding cannulas, enabling him to avoid critical vessels en route to the targets.

Plasma catecholamine and lactate response during graded exercise with varied glycogen conditions.

The relationships between the lactate threshold (TLa), plasma catecholamines, and ventilatory threshold (TVE) were examined under normal and glycogen-depleted conditions. Nine male subjects performed a graded exercise test on a bicycle ergometer in a normal glycogen (NG) state and in a glycogen-depleted (GD) state to determine if manipulation of muscle glycogen content would affect their ventilatory, lactate, and catecholamine responses. High correlations were found between plasma lactate and the two catecholamines, epinephrine (r = 0.964) and norepinephrine (r = 0.965) under both conditions. The GD protocol resulted in a shift in the TLa to a later work rate; inflections in epinephrine and norepinephrine shifted in a coordinated manner. TVE and TLa occurred at similar work loads under NG conditions [67.2 +/- 1.5 and 65.6 +/- 2.3% maximal oxygen consumption (VO2max), respectively], but TLa occurred at a later work load (75.3 +/- 1.9% VO2max) compared with TVE (68.3 +/- 1.6% VO2max) under GD conditions. These results suggest a causal relationship between plasma lactate and epinephrine during a graded exercise test under the glycogen conditions studied. Although an association existed between ventilation and lactate, this relationship was not as strong.