CT gel dosimetry technique: comparison of a planned and measured 3D stereotactic dose volume

Abstract

This study presents a 3D dose mapping of complex dose distributions using an x-ray computed tomography (CT) polymer gel dosimetry technique. Two polyacrylamide gels (PAGs) of identical composition were irradiated with the same four arc stereotactic treatment to maximum doses of 15 Gy (PAG1) and 8 Gy (PAG2). The PAGs were CT imaged using a previously defined protocol that involves image averaging and background subtraction to improve image quality. For comparison with the planned isodose distribution, the PAG images were converted to relative dose maps using a CT number-dose calibration curve or simple division. The PAG images were then co-registered with the planning CT images in the BrainLab treatment planning software which automatically provides reconstructed sagittal and coronal images for 3D evaluation of measured and planned dose. The hypo-intense high dose region in both sets of gel images agreed with the planned 80% isodose contour and was shifted by up to 1.5 and 3.0 mm in the axial and reconstructed planes, respectively. This demonstrates the ability of the CT gel technique to accurately localize the high dose region produced by the stereotactic treatment. The resulting agreement of the measured relative dose volume for PAG1 was within 3.0 mm for the 50% and 80% isodose surfaces. However, the dose contrast was too low in PAG2 to allow for accurate definition of measured relative dose surfaces. Thus, a PAG should be irradiated to higher doses if quantitative relative dose information is required. Unfortunately, this implies use of an additional PAG and its CT number dose response since doses greater than 8-10 Gy fall outside the linear regions of the response.

Figure 1

(Color) (a) The four arc SRS treatment used to irradiate both PAG1 and PAG2. (b) A PAG immobilized in the BrainLab® SRT mask and frame system.

Figure 2

(Color) Axial images at isocenter illustrating the difference between the “co‐registration” and “dose analysis” CT image sets obtained for each gel. (a) The co‐registration image sets are characterized by high noise and artifacts caused by the fiducial markers (not shown) on the localizer box. (b) The final $\Delta N_{\mathit{\text{CT}}}$ images sets used for dose analysis cover solely the high dose region of the PAGs and have improved image quality due to omission of the localizer box, image averaging, and background subtraction.

Figure 3

The $\Delta N_{\mathit{\text{CT}}}$‐dose response curve for a PAG calibration gel, reproduced here from Hilts et al. ¹⁹ $\Delta N_{\mathit{\text{CT}}}$ increases approximately linearly with dose up to $\sim 10-12$ Gy and then increases more slowly at higher doses.

Figure 4

(Color) (a) Axial, (b) coronal, and (c) sagittal CT images of PAG1 (irradiated to a maximum dose of 15 Gy) at isocenter. The coronal and sagittal images were reconstructed from the set of axial gel images (3 mm slice spacing). The planned 80% isodose contour and a cross indicating the isocenter are marked in each image.

Figure 5

(Color) (a) Axial, (b) coronal, and (c) sagittal CT images of PAG2 (irradiated to a maximum dose of 8 Gy) at isocenter. The coronal and sagittal images were reconstructed from a set of axial gel images (3 mm slice spacing). The planned 80% isodose contour and a cross indicating the isocentre are marked in each image.

Figure 6

(Color) (a) Axial, (b) coronal, and (c) sagittal relative dose images of PAG1 at isocenter. These images were obtained by binning the grayscale images shown in Fig. 4 into four relative dose regions: $<30\%,30–50\%,50–80\%$, and $>80\%$. The planned 30%, 50%, and 80% isodose contours are shown in each image. The cross indicates the isocenter.