Experimental evaluation of actual delivered dose using mega-voltage cone-beam CT and direct point dose measurement.

Radiation therapy in patients is planned by using computed tomography (CT) images acquired before start of the treatment course. Here, tumor shrinkage or weight loss or both, which are common during the treatment course for patients with head-and-neck (H&N) cancer, causes unexpected differences from the plan, as well as dose uncertainty with the daily positional error of patients. For accurate clinical evaluation, it is essential to identify these anatomical changes and daily positional errors, as well as consequent dosimetric changes. To evaluate the actual delivered dose, the authors proposed direct dose measurement and dose calculation with mega-voltage cone-beam CT (MVCBCT). The purpose of the present study was to experimentally evaluate dose calculation by MVCBCT. Furthermore, actual delivered dose was evaluated directly with accurate phantom setup. Because MVCBCT has CT-number variation, even when the analyzed object has a uniform density, a specific and simple CT-number correction method was developed and applied for the H&N site of a RANDO phantom. Dose distributions were calculated with the corrected MVCBCT images of a cylindrical polymethyl methacrylate phantom. Treatment processes from planning to beam delivery were performed for the H&N site of the RANDO phantom. The image-guided radiation therapy procedure was utilized for the phantom setup to improve measurement reliability. The calculated dose in the RANDO phantom was compared to the measured dose obtained by metal-oxide-semiconductor field-effect transistor detectors. In the polymethyl methacrylate phantom, the calculated and measured doses agreed within about +3%. In the RANDO phantom, the dose difference was less than +5%. The calculated dose based on simulation-CT agreed with the measured dose within±3%, even in the region with a high dose gradient. The actual delivered dose was successfully determined by dose calculation with MVCBCT, and the point dose measurement with the image-guided radiation therapy procedure.

An improved thyroid hormone reporter assay to determine the thyroid hormone-like activity of amiodarone, bithionol, closantel and rafoxanide.

A number of environmental chemicals have been reported to exhibit thyroid hormone-like activity. Since thyroid hormones play a crucial role in development, it is important to identify chemicals in the environment that are capable of endocrine disruption of thyroid hormone homeostasis. In order to detect thyroid hormone-like activity, the growth of pituitary cell lines has been commonly used as a sensitive marker, albeit with limited specificity to thyroid hormones. Reporter gene assays using the thyroid hormone responsive element (TRE) connected to the luciferase reporter gene have also been developed. Thus far however, this type of assay appears to have limited sensitivity compared to cell growth assays. In the present study, we developed a highly sensitive TRE reporter gene assay by using a pituitary cell line, MtT/E-2, and by culturing cells in a serum-free medium. Our assay was developed in order to detect T3 activity at a concentration of 10(-11)M. This assay identified thyroid hormone-like activity from the antiarrhythmic drug, amiodarone, and from three anti-parasitic drugs, bithionol, closantel and rafoxanide, all commonly used in veterinary medicine. Thyroid hormone-like activity of these compounds was further confirmed by the induction of BCL3 gene expression in MtT/E-2, which is known to be regulated by thyroid hormones. Our improved assay was proved to be a sensitive tool for assessing thyroid hormone-like activity of environmental chemicals.

Proton dose distribution measurements using a MOSFET detector with a simple dose-weighted correction method for LET effects.

We experimentally evaluated the proton beam dose reproducibility, sensitivity, angular dependence and depth-dose relationships for a new Metal Oxide Semiconductor Field Effect Transistor (MOSFET) detector. The detector was fabricated with a thinner oxide layer and was operated at high-bias voltages. In order to accurately measure dose distributions, we developed a practical method for correcting the MOSFET response to proton beams. The detector was tested by examining lateral dose profiles formed by protons passing through an L-shaped bolus. The dose reproducibility, angular dependence and depth-dose response were evaluated using a 190 MeV proton beam. Depth-output curves produced using the MOSFET detectors were compared with results obtained using an ionization chamber (IC). Since accurate measurements of proton dose distribution require correction for LET effects, we developed a simple dose-weighted correction method. The correction factors were determined as a function of proton penetration depth, or residual range. The residual proton range at each measurement point was calculated using the pencil beam algorithm. Lateral measurements in a phantom were obtained for pristine and SOBP beams. The reproducibility of the MOSFET detector was within 2%, and the angular dependence was less than 9%. The detector exhibited a good response at the Bragg peak (0.74 relative to the IC detector). For dose distributions resulting from protons passing through an L-shaped bolus, the corrected MOSFET dose agreed well with the IC results. Absolute proton dosimetry can be performed using MOSFET detectors to a precision of about 3% (1 sigma). A thinner oxide layer thickness improved the LET in proton dosimetry. By employing correction methods for LET dependence, it is possible to measure absolute proton dose using MOSFET detectors.

Evaluation of the usefulness of a MOSFET detector in an anthropomorphic phantom for 6-MV photon beam.

In order to evaluate the usefulness of a metal oxide-silicon field-effect transistor (MOSFET) detector as a in vivo dosimeter, we performed in vivo dosimetry using the MOSFET detector with an anthropomorphic phantom. We used the RANDO phantom as an anthropomorphic phantom, and dose measurements were carried out in the abdominal, thoracic, and head and neck regions for simple square field sizes of 10 x 10, 5 x 5, and 3 x 3 cm(2) with a 6-MV photon beam. The dose measured by the MOSFET detector was verified by the dose calculations of the superposition (SP) algorithm in the XiO radiotherapy treatment-planning system. In most cases, the measured doses agreed with the results of the SP algorithm within +/-3%. Our results demonstrated the utility of the MOSFET detector for in vivo dosimetry even in the presence of clinical tissue inhomogeneities.

[Evaluation of a method for correcting systematic setup error in external-beam radiation therapy].

BACKGROUND AND PURPOSE: We verified the propriety of our systematic error reduction strategy by means of a computer simulation based on our data of position error with a prone fixation device for prostate IMRT. MATERIALS AND METHODS: Computer simulations of the off-line correction method for systematic setup errors based on the portal imaging taken on the first several days of the treatment session were performed. Using the computer simulations, an optimal number of portal images were evaluated for the SD value, from 0.5 mm to 1.5 mm at a 0.25 mm interval, and the respective required setup margins were calculated. RESULTS: The value of systematic error was reduced as the frequency of data obtained increased. Moreover, the reduction rate was so remarkable that random error was large.

[Evaluation of setup error and adequate setup margins in patients with prostate cancer treated by IMRT and fixed in the prone position using a set of immobilization devices].

PURPOSE: Positional reproducibility in patients with prostate cancer fixed in the prone position with a set of immobilization devices for external-beam intensity-modulated radiation therapy (IMRT) was evaluated. In addition, the adequacy of our positional error reduction strategy and current planning target volume (PTV) margins was also evaluated. RESULTS: Systematic error was corrected by the positional correction that we executed at the first stage of irradiation. The setup margin that we had calculated was 1.1 mm in the L-R direction, 1.3 mm in the A-P direction, and 2.7 mm in the C-C direction. CONCLUSION: We determined that the effectiveness of the method of correcting the error margin and the setup accuracy of the fixed method were well maintained.