Cherenkov imaging for linac beam shape analysis as a remote electronic quality assessment verification tool

Abstract

Purpose: A remote imaging system tracking Cherenkov emission was analyzed to verify that the linear accelerator (linac) beam shape could be quantitatively measured at the irradiation surface for Quality Audit (QA).

Methods: The Cherenkov camera recorded 2D dose images delivered on a solid acrylonitrile butadiene styrene (ABS) plastic phantom surface for a range of square beam sizes, and 6 MV photons. Imaging was done at source to surface distance (SSD) of 100 cm and compared to GaF film images and linac light fields of the same beam sizes, ranging over 5 × 5 cm² up to 20 × 20 cm² . Line profiles of each field were compared in both X and Y jaw directions. Each measurement was repeated on two different Clinac2100 machines. An interreader comparison of the beam width interpretation was completed using procedures commonly employed for beam to light field coincidence verification. Cherenkov measurements are also done for beams of complex treatment plan and isocenter QA.

Results: The Cherenkov image widths matched with the measured GaF images and light field images, with accuracy in the range of ±1 mm standard deviation. The differences between the measurements were minor and within tolerance of geometrical requirement of standard linac QA procedures conducted by human setup verification, which had a similar error range. The measurement made by the remote imaging system allowed for beam shape extraction of radiation fields at the SSD location of the beam.

Conclusions: The proposed Cherenkov image acquisition system provides a valid way to remotely confirm radiation field sizes and provides similar information to that obtained from the linac light field or GaF film estimates of the beam size. The major benefit of this approach is that with a fixed installation of the camera, testing could be done completely under software control with automated image analysis, potentially simplifying conventional QA procedures with appropriate calibration of boundary definitions, and the natural extension to capturing dynamic treatment beamlets at SSD could have future value, such as verification of beam plans with complex beam shapes, like IMRT or "star-shot" QA for the isocenter.

Keywords: QA; Cerenkov; dosimetry; light field; linear accelerator; radiotherapy.

Figure 1

The treatment room is shown with the ABS test plate placed at isocenter on the couch and camera mounted to the ceiling, shown in (a). A typical image of the linac light field on the board is shown from a 10 × 10 cm beam in (b), as captured by the Cherenkov camera in image sensor reading [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2

In (a) the square patterned board used for calibration is illustrated, with (b) a calibration image taken from camera showing the corners detected (red dots) using the OpenCV library, and remapped (c) to be projected as the undistorted board image. In (d) a Cherenkov image of a 10 × 10 cm² beam is shown, and (e) the remapped Cherenkov image is shown as a square [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

The sequence of image analysis is shown in (a) with a line profile from the Cherenkov image showing the standard of Full Width Half Maximum Illustration (c): Boundaries extracted using the standard of maximum derivatives [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Beam width measurement using GaF films (a): Film scan for 10 cm × 10 cm beam and 5 cm × 5 cm beam, with line profile region in 10 cm × 10 cm beam (b): Dose profile along the line, with red lines representing the boundaries extracted through maximum derivative method [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

A common method used in light field quality assessment is illustrated with (a) a pattern used for a 20 × 20 cm² square beam. In (b) one example processed phosphor film is shown after exposure, with the location of the circular bead markers visible on each of the four edges [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

Paper image patterns to verify the errors of image transformation: (a): Checkerboard patterns with 1 cm × 1 cm cells, (b): 15 cm × 15 cm black square pattern to simulate radiation and light field with the same size [Color figure can be viewed at wileyonlinelibrary.com]

Figure 7

Cherenkov images of IMRT QA plan on the isoplane, compared with MLC shapes projection, with leaves marked by red rectangles [Color figure can be viewed at wileyonlinelibrary.com]

Figure 8

Star‐shot analysis using measurement of Cherenkov and film, (a): the Cherenkov field measurement; and (b): the film measurement of 30 cm × 0.5 cm radiation beams for different collimator angles, the center axis of beams extracted in the Cherenkov profile (c) and film profile (e), with the comparison shown in (d) [Color figure can be viewed at wileyonlinelibrary.com]

Figure 9

Comparison between the 2D profiles of light field, Cherenkov, and dose of 10 × 10 cm², 6 MV, photon beam. (a): 2D profile of light field, (b): 2D profile of Cherenkov emission in an ABS board, (c): 2D profile of Cherenkov emission on a solid water phantom, (d): 2D profile of dose from GaF film measurement [Color figure can be viewed at wileyonlinelibrary.com]