Dose modification factors for 192Ir high-dose-rate irradiation using Monte Carlo simulation.

A recently introduced brachytherapy system for partial breast irradiation, MammoSite, consists of a balloon applicator filled with contrast solution and a catheter for insertion of an 192Ir high-dose-rate (HDR) source. In using this system, the treatment dose is typically prescribed to be delivered 1 cm from the balloon's surface. Most treatment-planning systems currently in use for brachytherapy procedures use water-based dosimetry with no correction for heterogeneity. Therefore, these systems assume that full scatter exists regardless of the amount of tissue beyond the prescription line. This assumption might not be a reasonable one, especially when the tissue beyond the prescription line is thin. In such a case, the resulting limited scatter could cause an underdose to be delivered along the prescription line. We used Monte Carlo simulations to investigate how the thickness of the tissue between the surface of the balloon and the skin or lung affected the treatment dose delivery. Calculations were based on a spherical water phantom with a diameter of 30 cm and balloons with diameters of 4 cm, 5 cm, and 6 cm. The dose modification factor is defined as the ratio of the dose rate at the typical prescription distance of 1 cm from the balloon's surface with full scatter obtained using the water phantom to the dose rate with a finite tissue thickness (from 0 cm to 10 cm) beyond the prescription line. The dose modification factor was found to be dependent on the balloon diameter and was 1.098 for the 4-cm balloon and 1.132 for the 6-cm balloon with no tissue beyond the prescription distance at the breast-skin interface. The dose modification factor at the breast-lung interface was 1.067 for the 4-cm balloon and 1.096 for the 6-cm balloon. Even 5 cm of tissue beyond the prescription distance could not result in full scatter. Thus, we found that considering the effect of diminished scatter is important to accurate dosimetry. Not accounting for the dose modification factor may result in delivering a lower dose than is prescribed.

Wave-front sensing from subdivision of the focal plane with a lenslet array.

A wave-front sensing scheme based on placing a lenslet array at the focal plane of the telescope with each lenslet reimaging the aperture is analyzed. This wave-front sensing arrangement is the dual of the Shack-Hartmann sensor, with the wave front partitioned in the focal plane rather than in the aperture plane. This arrangement can be viewed as the generalization of the pyramid sensor and allows direct comparisons of this sensor with the Shack-Hartmann sensor. We show that, as with the Shack-Hartmann sensor, when subdividing in the focal plane, the quality of the wave-front estimate is a trade-off between the quality of the slope measurements over each region in the aperture and the resolution to which the slope measurements are obtained. Open-loop simulation results demonstrate that the performance of the lenslet array at the focal plane is equivalent to that of the Shack-Hartmann sensor when no modulation is applied to the lenslet array. However, when the array is modulated in a manner akin to that of the pyramid sensor, subdivision at the focal plane provides advantages when compared with the Shack-Hartmann sensor.

Contrast effects on dosimetry of a partial breast irradiation system.

MammoSite is a high-dose rate brachytherapy procedure for partial breast irradiation, which uses a balloon filled with radiopaque iodine-based contrast solution and catheter for insertion of 192Ir high-dose-rate source. The radiopaque material helps visualizing the balloon contour, catheter, and source position within the balloon, which is essential for computerized tomography-based treatment planning and for daily QA using x-ray radiographs. Because of the high content of iodine in contrast media, increased absorption and attenuation of photons may take place within the balloon, which would affect the resultant dose rates outside the balloon. The impact of the concentration of the radiopaque solution on the physical dosimetry of this brachytherapy procedure is investigated in this study using MCNPX (version 2.4) Monte Carlo simulation. Calculations were based on a 30 cm diameter water sphere phantom. The source geometry was that of the Nucletron microSelectron HDR v2 192Ir source. Concentration of the iodine-based radiopaque solution was varied from 5% to 25% by volume, a range recommended by the balloon's manufacturer. Balloon diameters of 4, 5, and 6 cm were simulated. Dose rate per unit air-kerma strength was calculated in 1 mm scoring bin steps. The dose rate reduction at the typical prescription line of 1 cm away from the balloon surface ranged from - 0.8% for the smallest balloon diameter and contrast concentration to a maximum of - 5.7% for the largest balloon diameter and contrast concentration, relative to a water-filled balloon. Limiting the contrast concentration to 10% would insure less than 3% reduction in the prescription dose, regardless of balloon diameter.

Phase retrieval from subdivision of the focal plane with a lenslet array.

A phase retrieval algorithm derived from subdivision of the complex field at the focal plane is proposed. This subdivision is achieved with a lenslet array at the focal plane in a manner similar to the pyramid wave-front sensor. The phase retrieval algorithm significantly improves the wave-front estimate that can be attained as a linear combination of the aperture images. This phase retrieval algorithm also avoids the twin-image stagnation problem inherent in phase retrieval and phase retrieval in conjunction with the Shack-Hartmann sensor.

Generalized scintillation detection and ranging results obtained by use of a modified inversion technique.

The measurement of the strength of atmospheric optical turbulence by use of a modified generalized SCIDAR (scintillation detection and ranging) inversion technique is outlined and demonstrated. This new method for normalizing and inverting scintillation covariances incorporates the geometry specific to generalized SCIDAR. Examples of profiles from two astronomical observation sites are presented.

Wave-front sensing from defocused images by use of wave-front slopes.

We describe a novel technique for deriving wave-front aberrations from two defocused intensity measurements. The intensity defines a probability density function, and the method is based on the evolution of the cumulative density function of the intensity with light propagation. In one dimension, the problem is easily solved with a histogram specification procedure, with a linear relationship between the wave-front slope and the difference in the abscissas of the histograms. In two dimensions, the method requires use of a Radon transform. Simulation results demonstrate that good reconstructions can be attained down to 100 photons in each detector. In addition, the method is insensitive to scintillation at the aperture.

Extended analysis of curvature sensing.

Curvature sensors are used in adaptive optics to measure the wave-front aberrations. In practice, their performance is limited by their nonlinear behavior, which we characterize by solving simultaneously the irradiance transport equation and the accompanying wave-front transport equation. We show how the presence of nonlinear geometric terms limits the accuracy of the sensor and how diffraction effects limit the spatial resolution. The effect of photon noise on the sensor is also quantified.

Tip/tilt estimation from defocused images.

In astronomical imaging, the errors in the wave-front slope are a significant cause of aberrations in the detected image. We investigate how the slope can be estimated optimally using an intensity measurement of the propagated wave front. We show that the optimal location for detection of wave-front tilt is the focal plane, and we quantify the error in using defocused images, such as would be obtained from a curvature sensor, for estimating the wave-front tilt. The effect of using broadband light is also quantified.

Passive navigation from image sequences by use of a volumetric approach.

The paper describes a volumetric approach to depth estimation for robot navigation with use of only an approximately calibrated translating camera. Our approach is related to techniques for photo-realistic object reconstruction but with the emphasis on issues associated with navigation. The technique performs three-dimensional matching by a process of image interpolation and can adjust for errors in camera position. The reconstruction is achieved from a small angular range of scene views, and the technique is demonstrated to be insensitive to large errors in the camera positions. The ability to correct for more critical errors such as the camera orientation is shown to significantly improve the algorithm's performance. Our technique is demonstrated on real image sequences and compares favorably with techniques based on optical flow.