Measurement of a photon penumbra‐generating kernel for a convolution‐adapted ratio‐TAR algorithm for 3D treatment planning

Abstract

A method has been developed to measure a photon penumbra-generating kernel using dosimetry equipment available in most radiation therapy departments. The kernel is used in a convolution-adapted ratio-TAR algorithm in our three-dimensional treatment planning system. The kernel is assumed to be invariant with respect to off-axis position, axially symmetric, and is divided into short- and long-range components, with a different measurement technique for each. The data required to obtain the short-range component are measured by scanning across a split-field geometry incident on a water phantom. The derivative of the measured profile is proportional to one-dimensional projections across the kernel. Because the kernel is axially symmetric, only one profile measurement is required for each depth. A CT reconstruction technique is used to extract the radial dependence of the kernel from the strip integrals. Electronic noise in the acquisition system yields significant uncertainties in the kernel shape for distances beyond 3 cm. The long-range portion of the kernel is obtained by examining tissue-air ratios (TARs). The derivative of the TAR at the center of a circular field is proportional to the kernel value at the distance corresponding to the radius of the field. The kernel measurement method was tested by comparing measured and calculated square-field profiles at a variety of depths. Agreement was within 1% within the field boundary and 3% outside the field boundary for all depths.

Scatter kernel estimation with an edge-spread function method for cone-beam computed tomography imaging

“…These special cases are utilized to characterize imaging systems. The full details of kernel derivation can be found in Low et al (1995) and Padgett and Kotre (2005).…”

“…Monte Carlo methods have been utilized to generate the scatter kernel (Hansen et al 1997, Jarry et al 2006, but these techniques require explicit modeling of the CBCT imaging system and are usually computationally demanding. In this study, we report the performance of an alternative method utilizing the edge-spread function (ESF) (Zhu and Gillin 2005, Low et al 1995, Padgett and Kotre 2005, Samei and Flynn 1998 to determine the magnitude and distribution of scatter of a Varian Trilogy CBCT system (Varian Medical Systems, Palo Alto, CA). We modeled the scatter as a depth-dependent convolution kernel that was derived for ten different waterequivalent thicknesses (WETs; ranging from 0 cm to 41 cm) from the measured data without assuming any empirical trial functions.…”

Initial evaluation of commercial optical CT‐based 3D gel dosimeter

“…The width of the laser beam as it enters the tank of the scanner was measured using a split-field-resolution technique. 24 With this technique, the beam intensity profile was determined by moving the beam across the interface between an opaque and transparent field while taking intensity data as the beam crossed the edge. In this measurement, an opaque (ODϾ4) radiographic film was used as the opaque field and taped to the glass tank wall.…”

Real-time auto-adaptive margin generation for MLC-tracked radiotherapy

“…To account for the effect in the dose model, the penumbra has to be described and estimated. Similar to convolution-adapted ratio-tissue-air ratio (CARTAR) of Low et al ( 1995 ), the penumbra due to scatter of beam limiting devices is estimated as a penumbra-generating kernel (PGK). It is assumed to be invariant to shifts with respect to the isocenter and radially symmetric.…”