Determining optimal gel sensitivity in optical CT scanning of gel dosimeters

Abstract

A method for determining the gel sensitivity that is necessary for obtaining optimal image contrast in optical CT scanning of gel dosimeters is presented. The effective dynamic range of the OCTOPUS-ONE research scanner (MGS Research, Inc., Madison, CT) is analyzed. Optical density increments for selected straight-line paths across a gel cylinder to be scanned are calculated based on the optical properties of the polymer gel and the dose distribution from a commercial treatment planning system (Cadplan, Varian Corporation, Palo Alto, CA). Maximum optical density increment across the entire gel is obtained by searching the gel cylinder over a set of transverse planes at different rotational angles. The application of this quantity as a criterion for optimizing the quality of the optical CT scanning is demonstrated through dose verification of two representative treatment plans. When the MU dependence of the dose distribution for a treatment plan is linear, as is the case for static field irradiation, it is possible to scale the treatment plan such that the intensity variation of the signals received by the photodetector spans its entire dynamic range. For treatment plans that are possibly nonlinear, IMRT plans, for example, modification of the sensitivity of the gel material is necessary for the high-dose signals to be collected at a certain signal-to-noise ratio. Results obtained using the optimized CT scanning approach are compared with those from the treatment planning system and the film measurement.

3D dosimetry by optical-CT scanning

“…(From [2]). a, b reconstructed optical-CT images from Xu et al [9] for 6 x 6 cm 2 square field irradiation. The cross artifact is not readily apparent at low doses (a) but is clearly present at higher doses (b).…”

“…Scatter related artifacts are more severe than beam-hardening artifacts, and include cupping, streaking and inaccurate reconstructed attenuation coefficients [7]. Very little is known however of the specifics of how scattered light might perturb 3D dose maps obtained from optical-CT. Xu et al [9] and Islam et al [10] both noted a cross shaped artifact that appeared in optical-CT images of a high-dose square field irradiations (figure 4). The cross shaped depression of reconstructed attenuation values was observed at higher optical-density values of the gel.…”

“…Techniques have been developed to address most of these issues (see section 2 and references [2,[11][12][13]17]). Xu et al [9] demonstrated the importance of matching the sensitivity of the dosimeter and the delivered dose to yield optical-density changes in the dosimeter that match the dynamic range of the scanner. This simple technique can increase the signal to noise in the projections and hence in the final reconstructed image.…”

Dosimetric assessment of the PRESAGE dosimeter for a proton pencil beam

“…The dosimeter sample was scanned with 1mm pixel resolution using the optical CT scanner, OCTOPUS TM . Details of the operating principles, 3-D dose reconstruction, and instrument architecture of the optical scanner were described previously [13,14]. For comparative dosimetry, Gafchromic films, placed at various depths in a solid water phantom, were irradiated with a proton pencil beam using the same beam parameters.…”

Dosimetry tools and techniques for IMRT

“…228 For MR measurements requiring high accuracy and highspatial resolution, long imaging times ͑several hours͒ may be required to achieve the low uncertainty and high spatial resolution associated with standard dosimeters. An alternative readout technique, optical-CT, 166,203,204,209,227,[231][232][233][234][235] has been proposed for imaging PAG gels; an approach analogous to first-generation x-ray CT. In optical-CT the x-ray source is replaced by a visible laser and the x-ray detector replaced with a light-sensitive photodiode.…”