Flattening‐filter‐based empirical methods to parametrize the head scatter factor

Abstract

Parametrizing the collimator scatter factor, Sc (or head scatter factor), of a linear accelerator by the side of the equivalent square of the collimator setting at the isocenter does not accurately predict the change in Sc when the width and length of a rectangular field are exchanged. We have studied two methods based on measurements of square fields to predict Sc's of rectangular fields more accurately. The first method parametrizes Sc by the side of the equivalent square of the flattening filter region visible from the point of calculation. The Sc's of rectangular fields were predicted to an accuracy of 1% from measurements with square fields. The second method computes Sc of rectangular configurations by integrating radiation that can reach the point of calculation from a point source at the target and a second extended source at the flattering filter. The radial distribution of the extended source at the level of the flattering filter is computed from Sc of square fields measured at the isocenter. Effects of extended distance are modeled by separately performing inverse square law corrections for the two sources. This method also predicted the measured values to within 1% accuracy.

Influence of focal spot on characteristics of very small diameter radiosurgical beams

“…8,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] Various techniques reported in these studies include the following: ͑1͒ direct measurements, 8,22 ͑2͒ Monte Carlo simulations, 14,25,28 and ͑3͒ analytical and empirical methods. [17][18][19][20][21]23,24,26,27,[29][30][31][32] Figure 5 shows profiles of the net measured diode signal ͑shown by data points͒ for four scanning directions carried out with our sliding collimator-slit device. Each profile was fitted with our two-source model consisting of two characteristic peak functions, one representing the focal spot distribution and the other the extra-focal source distribution.…”

Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71

“…The change in incident fluence can be modeled by the collimation of a primary source at the target and a radially symmetric planar extended scattered-radiation source close to the target. [14][15][16][17][18][19][20] The volumes of these two sources that are not blocked by the jaws and the field shaping collimator (e.g., blocks and MLC) from the point of view of the point of calculation (point'seye-view or PEV) determine S c . Because this exposed region from PEV depends on both the jaw settings and the field-shaping collimator that are at different distances from the sources, an accurate formalism will involve a method to combine the effects of different collimators into an equivalent field size.…”

Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique

“…The change of the backscatter depends on the accelerator structure and the monitor chamber construction, as discussed by other research groups as well. [3][4][5][6][7][8][9][10][11] Although the effect of the backscatter is small compared to that of the head scatter and phantom scatter, to achieve more accurate dose calculation, it is important to include the correction for the backscatter in using the convolution algorithm or any other model-based methods. 12 However, how to determine the effect of the backscatter poses a challenging problem, because the change in photon output caused by the backscatter alone is often shadowed, and thus, difficult to assess experimentally.…”

“…12 However, how to determine the effect of the backscatter poses a challenging problem, because the change in photon output caused by the backscatter alone is often shadowed, and thus, difficult to assess experimentally. In the past, a number of methods were investigated, including those using ionization chambers and phantoms in telescopic settings, [3][4][5][6][7] as well as techniques based on measuring electron pulses, current, or charge from the electron target of linear accelerators. 5,[8][9][10][11] In general, these published data were limited to symmetrical field settings used for the measurements.…”