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An automated tool for determining output factor for electron cutouts

An automated tool for determining output factor for electron cutouts. L Brewster Mallalieu, F Diaz Molina, J Yuen, G Gill, A Kapur, Y Cao, A Jamshidi North Shore – Long Island Jewish Health System. Electron output factor calculation.

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An automated tool for determining output factor for electron cutouts

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  1. An automated tool for determining output factor for electron cutouts L Brewster Mallalieu, F Diaz Molina, J Yuen, G Gill, A Kapur, Y Cao, A Jamshidi North Shore – Long Island Jewish Health System

  2. Electron output factor calculation • Measurement on a linear accelerator of output factor for electron cutouts is a time consuming task for physics staff, often requiring linear accelerator use after hours. • Existing methods that calculate output factor for shaped apertures are approximations based on cutout length and width, or require a printed version of a cutout shape for either manual measurement or scanning and digitizing [1],[2]. • In this implementation, an integrated spreadsheet and database provides several options for output factor determination: • a measured value may be used; • an image based database containing previously measured cutouts can be searched; • or the electron cutout shape contour can be imported electronically from the treatment planning system, and the output factor calculated without further user interaction. [1] Chow JCL, Grigorov GN, MacGregor C. A graphical user interface for an electron monitor unit calculator using a sector-integration algorithm and exponential curve-fitting method. J of Applied Clinical Medical Physics 2006 7(1): 52-64. [2] Jursinic PA, Mueller R. A sector-integration method for calculating the output factors of irregularly shaped electron fields. Med Phys November 1997 24(11):1765-1769.

  3. Image based database of electron cutout output factors • Scanned images of cutout measurement records entered into Microsoft Access database with associated parameters • Cutouts can be searched by linac, energy, cone size

  4. Image based database of electron cutout output factors • Cutout image can be zoomed to increase resolution • Large database available for further analysis

  5. Modified sector integration algorithm • Modifications: • A weighted sector integration based algorithm employs polar coordinates and does not require equispaced block contour points; • calculation of intersection of equispaced vectors with the block contour or the repeated rotation of the contour not needed. • Block contour definition is exported by the treatment planning system and imported to a spreadsheet using Visual Basic code. • N contour points (xi,yi) converted to polar coordinates (ri,θi).

  6. xi,yi ri θi Modified sector integration algorithm • The output factor OF is calculated by a weighted summation of sectors associated with N block contour points: • wi denotes the weight of the sector created by block contour point i. Weight is obtained from the sector size, based on the adjacent θ angles: Wi = ½ (θi-1 – θi+1) • OF(ri) is the output factor for a circular cutout of radius ri.

  7. Output factor data table creation: circular cutouts • Output factors for 10x10 cm2cone size circular cutouts ranging in diameter from 2cm to 10cm measured on Linac 2300 CD • Measurements in water phantom using Markus chamber at 100cmSSD • Scanned CAX to find maximum ionization point for open 10x10 cm2 field and circular cutout field; OF calculated from the ratio of cutout to open field ionization readings.

  8. Standard circular insert database: Measured electron output factors for 10x10 cm cone size

  9. Modified sector integration model verification procedure • Verification of interpolation on circular cutout output factor tables • Verification of model on range of circular cutout sizes and electron energies • Comparison of model output with measurements and “standard” sector integration model using manually digitized block contours with equispaced points, for range of electron energies

  10. Comparison of measured and calculated cutout output factors for standard (using manually digitized contour) and modified sector integration models:

  11. Challenges • Concavities that create intervening blocked regions: algorithm handles these points because Δθ’s are signed and produce signed weight values: in clockwise contours, positively weighted sectors add sector area; negatively weighted sectors subtract area (sum of all angles remains 360). Algorithm accuracy in these cases is being investigated. Positive weight Negative weight Positive weight

  12. Challenges • Long, narrow cutout shapes: may require measurement due to electron equilibrium issues • Additional contour points sometimes required to improve model accuracy: may be added automatically by spreadsheet based on a minimum line segment length between consecutive points

  13. Contact information Linda Brewster Mallalieu Department of Radiation Medicine Long Island Jewish Medical Center 270-05 76th Ave New Hyde Park, NY 11040 lmallali@lij.edu

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