Poster - Thur Eve - 70: Quantification of tumour dose enhancement at kilo-voltage energies due to the presence of gold nanoparticles during radiation therapy: EGSnrcMP Monte Carlo study.

One of the greatest challenges in radiation therapy is the ability to deliver a lethal dose of radiation to a tumour while sparing the surrounding normal tissues. In theory, the dose delivered to a tumour during photon-based radiation therapy can be enhanced by loading high atomic number (Z) materials into the tumour, which results in greater photoelectric absorption and hence increased photoelectron fluence within the tumour than in surrounding tissues. The EGSnrcMP Monte Carlo code, together with DOSXYZnrc, a three-dimensional voxel dose calculation module has been used to study the macroscopic dose enhancement factor (MDEF) in a tumour infused with gold nanoparticles at the kilo-voltage energies. We observed that gold nanoparticles infused in a tumour irradiated with kilo-voltage energies has the potential to enhance the tumour dose by a factor ranging from 0.25 to about 5 depending on the mean energy of the beam and the concentration of gold nanoparticles in the tumour. The increase in dose can be attributed to the significant increase in the photoelectron fluence within the tumour loaded with gold particles during the irradiation. Future studies will involve the characterization of the MDEF at megavoltage energies.

Poster - Thur Eve - 66: A planning comparison between RapidArc and intensity modulated radiotherapy for head and neck cancer.

Volumetric modulated arc therapy (VMAT) has recently been used to improve the dose distribution and efficiency of treatment delivery over the standard intensity-modulated radiotherapy (IMRT) technique. This study compares the dosimetry between RapidArc plan and standard IMRT plan for head and neck cancer. Three head and neck patients treated clinically with sliding window intensity-modulated radiotherapy (IMRT) technique at Grand River Regional Cancer Center were selected randomly and re-planned using RapidArc technique with 6 MV photon beams generated by a Varian 21EX linac with 120-leaf multileaf collimator. Three dose prescriptions were used to deliver 70 Gy, 63 Gy and 58.1 Gy to the regions of the primary tumors, intermediate-risk nodes and low-risk nodal level, respectively, in 35 fractions. Dosimetric comparison based on the dose-volume histogram, target coverage, organ at risk (OAR) dose sparing were studied between the RapidArc plan and IMRT plan. RapidArc technique from Varian Medical Systems showed superior target coverage, better OAR sparing, fewer monitor units per fraction with less treatment time over IMRT technique for head and neck cancers. The average homogeneity index, defined as the difference between the percentage dose covering 5% and 95% of the PTV, is 9.5 for RapidArc plan and 10.5 for IMRT plan. All RapidArc plans met the dose objectives for the primary OAR: spinal cord, brainstem, brain etc. Both parotid mean dose and D50% are lower for RapidArc plan than those of the IMRT plan. The technique is currently being used clinically at our cancer center.

The Jacobian as a measure of planar dose congruence.

We propose a new starting point for comparing planar dose distributions using a Jacobian-based measure. The measure is normalization independent, i.e., is sensitive to dose patterns while not being biased by global dose scaling. The measure is free of tunable parameters and is bounded between zero and one. We also propose an integrated version of the measure, normalized over the integrated area: the Jacobian index. We demonstrate analytically and verified numerically that the Jacobian index tends to a unique value of 2/pi when comparing unrelated dose distributions. We present a preliminary demonstration of the sensitivity and general characteristics of this measure. Examples of intensity modulated photon dose distributions from a linac as predicted with the convolution-superposition algorithm for various scenarios of irradiation mismatch and a film measured distribution are compared.

Enhanced bremsstrahlung spectrum reconstruction from depth-dose gradients.

We present a technique to significantly improve the accuracy of bremsstrahlung spectra reconstructed from central-axis depth-dose data using inverse methods. While typical approaches directly use the measured depth-dose data, we show the advantage of using the gradient of these data for reconstruction. The inverse problem in terms of gradients is shown to be markedly less ill-conditioned than the usual inverse problem. In each case, a regularization is introduced to alleviate the effects of noise due to measurement and computation. The error in taking derivatives of depth-dose data is demonstrated to be sufficiently controllable as not to overtake the improvements in the conditioning of the inverse problem in a pair of simulated examples.

Spectral sensitivity study of dose distributions for a commercial convolution/superposition algorithm.

The focus of this study is to validate whether the sensitivity of dose distribution following the interface of different media can be used to distinguish between small variations of photon energy spectra in the context of the convolution/superposition algorithm in the polyenergetic implementation (Philips Pinnacle3, ADAC Laboratories, Milpitas, CA). Calculations were performed in homogeneous water and heterogeneous lung/water phantoms. Spectra were generated, in which the weights of the low-, medium- and high-energy components were adjusted sequentially. The heterogeneity correction factor CFlung, the D20/D10 ratio for homogeneous water and logarithmic derivative in buildup region LDbuildup were assessed for their relative ability to discriminate between different spectra for various field sizes. In accordance with another study (Charland et al 2004), the superior discrimination ability of the CFlung and LDbuildup tests over the D20/D10 test was observed for changes in an energy component as small as 0.3% of the total weight in the energy spectrum. Furthermore, new tests utilizing transverse dose profile data for discriminating between spectra, Fringe Index (FI) and Penumbra Index (PI), were introduced. The discrimination ability of the PI and FI tests was superior when a medium containing interface effects was exploited to obtain the transverse profile data (water/lung phantom for PIhung and FIlung tests) as opposed to when a homogeneous water medium was used (PIwater and FIwater tests).

Commissioning, evaluation, quality assurance and clinical application of a virtual micro MLC technique.

Multileaf collimators (MLCs) are a valuable tool in modern radiation therapy, offering flexible and convenient field shaping. One disadvantage, however, is the undulation of the dose distribution at the edge shaped by the leaves due to the finite leaf width. An attempt to reduce the effect of this undulation is the objective of the commercial linear accelerator package HD270, which incorporates three-dimensional couch translation together with leaf adjustment to emulate finer leaf widths. In this paper we report on the commissioning and evaluation of this feature, together with the development of a process for quality assurance, as well as description of a clinical application of this technique. It is concluded that this technique could be applied reliably to situations currently utilizing MLC for shielding, with little added cost in treatment time, provided that a comprehensive quality assurance program is in place to monitor the performance of this complicated procedure.

The use of deconvolution and total least squares in recovering a radiation detector line spread function.

We present a method for obtaining the line spread function (LSF) of any radiation detector from measured data. The problem of finding a LSF is essentially a discrete deconvolution from known values of the input (Monte Carlo generated data) and the output (measured data) which can be put into matrix form. We applied the total least squares (TLS) method which is particularly useful when there are errors in both the input and output data. Results from computer simulation as well as from actual data are shown. In a practical application, however, our technique is currently limited by the ability of the Monte Carlo data to simulate correctly the inherent data from the head of the linear accelerator (linac). To overcome this difficulty we have solved by deconvolution and TLS for a more realistic inherent beam profile of our linac using the information from both profile data as measured with film and the film densitometer response function. The LSF of the densitometer was estimated with a simple method of direct measurement of a slit image and a full width at half maximum (FWHM) of 0.997 mm was recorded. Additionally, using the knowledge of this realistic inherent profile of the linac, a blurring function representing the finite source size effect missing in our current Monte Carlo profile simulation was determined. Finally, with the realistic inherent beam profile we have applied the deconvolution and TLS method to find a LSF for the Markus chamber and found a resulting FWHM of 5.39 mm. The TLS approach for deconvolving can find a useful application for both finding the LSF and correcting for the detector size effect once its LSF is known. This type of correction is required when a high spatial resolution is needed (e.g., in small field off-axis measurements). Convolved and measured profiles are also presented to illustrate the effect of the blurring due to different LSFs.

Optimal display conditions for quantitative analysis of stereoscopic cerebral angiograms.

For several years the authors have been using a stereoscopic display as a tool in the planning of stereotactic neurosurgical techniques. This PC-based workstation allows the surgeon to interact with and view vascular images in three dimensions, as well as to perform quantitative analysis of the three-dimensional (3-D) space. Some of the perceptual issues relevant to the presentation of medical images on this stereoscopic display were addressed in five experiments. The authors show that a number of parameters-namely the shape, color, and depth cue, associated with a cursor-as well as the image filtering and observer position, have a role in improving the observer's perception of a 3-D image and his ability to localize points within the stereoscopically presented 3-D image. However, an analysis of the results indicates that while varying these parameters can lead to an effect on the performance of individual observers, the effects are not consistent across observers, and the mean accuracy remains relatively constant under the different experimental conditions.