Polymer gel dosimeters are fabricated from radiation sensitive chemicals which, upon irradiation, polymerize as a function of the absorbed radiation dose. These gel dosimeters, with the capacity to uniquely record the radiation dose distribution in three-dimensions (3D), have specific advantages when compared to one-dimensional dosimeters, such as ion chambers, and two-dimensional dosimeters, such as film. These advantages are particularly significant in dosimetry situations where steep dose gradients exist such as in intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery. Polymer gel dosimeters also have specific advantages for brachytherapy dosimetry. Potential dosimetry applications include those for low-energy x-rays, high-linear energy transfer (LET) and proton therapy, radionuclide and boron capture neutron therapy dosimetries. These 3D dosimeters are radiologically soft-tissue equivalent with properties that may be modified depending on the application. The 3D radiation dose distribution in polymer gel dosimeters may be imaged using magnetic resonance imaging (MRI), optical-computerized tomography (optical-CT), x-ray CT or ultrasound. The fundamental science underpinning polymer gel dosimetry is reviewed along with the various evaluation techniques. Clinical dosimetry applications of polymer gel dosimetry are also presented.

† De ´partement de chimie. ‡ De ´partement de me ´decine nucle ´aire et de radiobiologie and Centre d'imagerie mole ´culaire de Sherbrooke.

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Purpose-The objective of this study was to assess changes in the water apparent diffusion coefficient (ADC) and in pharmacokinetic parameters obtained from the fast-exchange regime (FXR) modeling of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) during neoadjuvant chemotherapy in breast cancer.Materials and Methods-Eleven patients with locally advanced breast cancer underwent MRI examination prior to and after chemotherapy but prior to surgery. A 1.5-T scanner was used to obtain T 1 , ADC and DCE-MRI data. DCE-MRI data were analyzed by the FXR model returning estimates of K trans (volume transfer constant), ν e (extravascular extracellular volume fraction) and τs i (average intracellular water lifetime). Histogram and correlation analyses assessed parameter changes posttreatment.Results-Significant ( P <.05) changes or trends towards significance ( P <.10) were seen in all parameters except τ i , although there was qualitative reduction in τ i values post-treatment. In particular, there was reduction ( P <.035) in voxels with K trans values in the range 0.2-0.5 min -1 and a decrease ( P <.05) in voxels with ADC values in the range 0.99×10 -3 to 1.35×10 -3 mm 2 /s. ADC and ν e were negatively correlated (r = -.60, P <.02). Parameters sensitive to water distribution and geometry (T 1 , ν e ,τs i and ADC) correlated with a multivariable linear regression model. Conclusion-The analysis presented here is sensitive to longitudinal changes in breast tumor status; K trans and ADC are most sensitive to these changes. Relationships between parameters provide information on water distribution and geometry in the tumor environment.

Easily disrupted: Micelles of a new amphiphilic block copolymer that bear coumarin groups are sensitive to near infrared light by two-photon absorption of the chromophore. Disruption of the micelles under irradiation at 794 nm results in release of both photocleaved coumarin and encapsulated nile red from the hydrophobic core of micelle into aqueous solution, which results in opposing changes in fluorescence emission intensity.