A practical three-dimensional dosimetry system for radiation therapy

Abstract

There is a pressing need for a practical three-dimensional (3D) dosimetry system, convenient for clinical use, and with the accuracy and resolution to enable comprehensive verification of the complex dose distributions typical of modern radiation therapy. Here we introduce a dosimetry system that can achieve this challenge, consisting of a radiochromic dosimeter (PRESAGE) and a commercial optical computed tomography (CT) scanning system (OCTOPUS). PRESAGE is a transparent material with compelling properties for dosimetry, including insensitivity of the dose response to atmospheric exposure, a solid texture negating the need for an external container (reducing edge effects), and amenability to accurate optical CT scanning due to radiochromic optical contrast as opposed to light-scattering contrast. An evaluation of the performance and viability of the PRESAGE/OCTOPUS, combination for routine clinical 3D dosimetry is presented. The performance of the two components (scanner and dosimeter) was investigated separately prior to full system test. The optical CT scanner has a spatial resolution of < or = 1 mm, geometric accuracy within 1 mm, and high reconstruction linearity (with a R2 value of 0.9979 and a standard error of estimation of approximately 1%) relative to independent measurement. The overall performance of the PRESAGE/OCTOPUS system was evaluated with respect to a simple known 3D dose distribution, by comparison with GAFCHROMIC EBT film and the calculated dose from a commissioned planning system. The "measured" dose distribution in a cylindrical PRESAGE dosimeter (16 cm diameter and 11 cm height) was determined by optical-CT, using a filtered backprojection reconstruction algorithm. A three-way Gamma map comparison (4% dose difference and 4 mm distance to agreement), between the PRESAGE, EBT and calculated dose distributions, showed full agreement in measurable region of PRESAGE dosimeter (approximately 90% of radius). The EBT and PRESAGE distributions agreed more closely with each other than with the calculated plan, consistent with penumbral blurring in the planning data which was acquired with an ion chamber. In summary, our results support the conclusion that the PRESAGE optical-CT combination represents a significant step forward in 3D dosimetry, and provides a robust, clinically effective and viable high-resolution relative 3D dosimetry system for radiation therapy.

Fig. 1

Picture of the OCTOPUS™ optical-CT scanner. The laser beam from a He-Ne laser is guided by translation components to linearly scan the object mounted on a turn plate in the water tank. Tomographic scan of the object is achieved by repeatedly rotating and scanning the object.

Fig. 2

Irradiation of the PRESAGE™ dosimeter by a Varian^® 21EX linear accelerator. The irradiation was performed according to a 3D treatment plan with five coplanar (6 MV photons) beams.

Fig. 3

Calibration curve of the GAFCHROMIC^® EBT film. The calibration curve was obtained from the measurements on the next day (Day 2) after irradiation and was represented by the equation y =4.975x⁵−126.83x⁴ +1264.8x³−6408.5x²+19 086x+535.06, where x and y represent the irradiation dose and the corresponding pixel value change from the scanning measurements pre and post the irradiation.

Fig. 4

Imaging performance of the optical-CT scanner. (a) Reconstruction of the cross section of the thin wire (diameter 0.2 mm). (b) Profile of the reconstructed wire along the dotted line in (a). Pixel size is 0.5 mm. (c) Reconstruction of the uniform gel. (d) Profile along the dotted line in image (c). (e) Registered images of multiple wires (diameter 0.2 mm). The white dots represent the cross sections of the wires reconstructed by the x-ray CT scanner and the black dots are the reconstructions by the optical-CT scanner. (f) The detailed look of picture (e).

Fig. 5

Optical-CT scan and reconstructions of the PRESAGE™ dosimeter. (a) A projection of the dosimeter before (dotted line) and after (solid line) it was irradiated. (b) Reconstructed axial slice of the cylindrical dosimeter before it was irradiated. (c) The corresponding reconstruction after the dosimeter was irradiated. (d) Profiles of the reconstructions (b) and (c) along the dotted line in (c). The dotted line corresponds to the profile of the reconstruction pre irradiation and the solid line corresponds to that after the irradiation.

Fig. 6

Comparison of dose distributions between PRESAGE™ measurement (a), ECLIPSE calculation (b), and EBT film measurement (c). Percent isodose lines (95, 90, 80, 70, 60, 50, 40, 30, and 20%) were superimposed onto the dose distributions. (d) Profiles of figures (a), (b), and (c) along the solid line in figure (c).

Fig. 7

Comparison of percent isodose lines (95, 80, 60, 50, 40, and 30%) between ECLIPSE planning system, EBT film, and PRESAGE™ dosimeter. (a) Overlay of the percent isodose lines from PRESAGE™ (blue) and EBT film (red) measurements. (b) Overlay of the percent isodose lines from PRESAGE™ measurement (blue) and ECLIPSE calculation (red). (c) Overlay of the percent isodose lines from EBT film measurement (blue) and ECLIPSE calculation (red).

Fig. 8

Gamma comparison of dose distributions between ECLIPSE planning system, EBT film, and PRESAGE™ dosimeter. (a) Gamma map for PRESAGE™ measurement and ECLIPSE calculation. (b) Profile along the solid line in figure (a). (c) Gamma map for EBT film and PRESAGE™ measurements. (d) Profile along the solid line in figure (c). (e) Gamma map for EBT film measurement and ECLIPSE calculation. (f) Profile along the solid line in figure (e).

Fig. 9

Comparison of the dose distributions between the ECLIPSE calculation (a) and the PRESAGE™ measurement (b) in a coronal view. Percent isodose lines (95, 90, 70, 50, 35, and 30% were superimposed on both images (a) and (b), respectively. (c) Profiles for the calculated dose distribution (solid line) and the PRESAGE™ measurement (dotted line) along the solid (black) line in figure (b). The artifacts produced by the marks on the surface of the dosimeter are shown in both figures (b) and (c) by the arrows. (d) Overlay of the percent isodose lines from the calculation (dark solid line) and the PRESAGE™ measurement (light solid line).

Fig. 10

Comparison of the dose distributions between the ECLIPSE calculation (a) and the PRESAGE™ measurement (b) in a sagital view. The artifact produced by the marks on the surface of the dosimeter is shown by the arrow. Percent isodose lines (95, 90, 70, 50, 35, and 30%) were superimposed on both images (a) and (b), respectively. (c) Profiles for the calculated dose distribution (solid line) and the PRESAGE™ measurement (dotted line) along the solid (black) line in figure (b). (d) Overlay of the percent isodose lines from the calculation (dark solid line) and the PRESAGE™ measurement (light solid line).