Technical note: Visual, rapid, scintillation point dosimetry for in vivo MV photon beam radiotherapy treatments

Abstract

Background: While careful planning and pre-treatment checks are performed to ensure patient safety during external beam radiation therapy (EBRT), inevitable daily variations mean that in vivo dosimetry (IVD) is the only way to attain the true delivered dose. Several countries outside the US require daily IVD for quality assurance. However, elsewhere, the manual labor and time considerations of traditional in vivo dosimeters may be preventing frequent use of IVD in the clinic.

Purpose: This study expands upon previous research using plastic scintillator discs for optical dosimetry for electron therapy treatments. We present the characterization of scintillator discs for in vivo x-ray dosimetry and describe additional considerations due to geometric complexities.

Methods: Plastic scintillator discs were coated with reflective white paint on all sides but the front surface. An anti-reflective, matte coating was applied to the transparent face to minimize specular reflection. A time-gated iCMOS camera imaged the discs under various irradiation conditions. In post-processing, background-subtracted images of the scintillators were fit with Gaussian-convolved ellipses to extract several parameters, including integral output, and observation angle.

Results: Dose linearity and x-ray energy independence were observed, consistent with ideal characteristics for a dosimeter. Dose measurements exhibited less than 5% variation for incident beam angles between 0° and 75° at the anterior surface and 0-60 ${}^{\circ }$ at the posterior surface for exit beam dosimetry. Varying the angle between the disc surface and the camera lens did not impact the integral output for the same dose up to 55°. Past this point, up to 75°, there is a sharp falloff in response; however, a correction can be used based on the detected width of the disc. The reproducibility of the integral output for a single disc is 2%, and combined with variations from the gantry angle, we report the accuracy of the proposed scintillator disc dosimeters as ±5.4%.

Conclusions: Plastic scintillator discs have characteristics that are well-suited for in vivo optical dosimetry for x-ray radiotherapy treatments. Unlike typical point dosimeters, there is no inherent readout time delay, and an optical recording of the measurement is saved after treatment for future reference. While several factors influence the integral output for the same dose, they have been quantified here and may be corrected in post-processing.

Keywords: in vivo dosimetry; photon therapy; scintillation imaging.

Figure 1:

In (a), the iCMOS camera is mounted to the ceiling in a treatment bunker, ~2 m away from isocenter. In (b), a scintillator disc is placed in the photon EBRT treatment field on an anthropomorphic phantom. In (c), a 50×50 pixel cumulative image of an irradiated scintillator disc is shown, with the corresponding Gaussian-convolved ellipse in in (d). Figure (e) shows a difference plot of the images in (c) and (d), revealing an average difference of 1.67% in the disc area (indicated by the dotted circle).

Figure 2:

In (a), integral output is plotted as a function of machine output in MU. The relationship between the same output measurements and dose readings from OSLDs at the same point is shown in (b), revealing a perfect linear relationship and affirming strong dose linearity. The 300 MU data point was excluded from the dose plot due to errors with the OSLD readout. In (c), the stability of a single scintillator disc is assessed for 10 identical measurements. Additionally, the measurement repeatability was explored and graphed in (d). In (e), a batch of 20 scintillators was tested to quantify the reproducibility of the measurements.

Figure 3:

In (a), the geometry of the scintillator relative to the camera is shown. The white arrow indicates the normal of the disc surface and the blue arrow is the camera vector, defining the angle $\theta$. In (b-d), three images of the scintillator at 0°, 50°, and 75° are shown, respectively. The corresponding Gaussian-convolved ellipses are shown in (e-g), and the width, b, is extracted from the ellipses.

Figure 4:

In (a), the integral output is plotted as a function of theta, the angle between the normal of the scintillator face and the camera line. In (b), the normalized width of the scintillator Gaussian-convolved ellipses is plotted as a function of theta and compared to the theoretical prediction. The power fit of the experimental data is used to estimate unknown theta from calculated b values. With this, a correction factor for angles $55°$ and above is derived from the power fit in (a), and in (c), the corrected data aligns within 2%, the expected variation from repeatability. Integral output is also plotted as a function of gantry angle for entrance (d) and exit (e) beams. On the same figures, average dose as measured by film is plotted. In (f), the average dose is used to normalize the integral output and the results are shown to deviate no more than +/−5% of the mean for gantry angles between $0°$ and $75°$ and $120°$ and $180°$.