Steep Dose Gradient Regions Research Articles

TU‐D‐M100F‐05: Pre‐Treatment Verification of Large‐Field IMRT Dose Painting Plans for Head and Neck Cancer Using a Commercial QA Device

Objective: To develop a practical procedure for routine pre‐treatment verification of large dose painting fields using a commercial IMRT QA device, MapCheck. Materials and Methods: Optimized dose painting intensity maps were applied to a 40×40×40 cm3 digital phantom. With a voxel size of 1 mm3, the dose distributions were re‐calculated for a coronal plane at a depth of 5 cm. The DMLC files were then transferred to a Varian Clinac for plan verification. We first delivered a beam with MapCheck at isocenter. The isocenter was then shifted superiorly and inferiorly by 4 cm, respectively. Two more exposures were made. The isocenter was shifted to the left and right by 4 cm, respectively. Two more exposures were made. To detect accurately the steep dose gradient regions, we doubled the effective detector surface density by shifting the isocenter by 5 mm along the X and Y axes. Two more exposures were made. Finally, we merged all these files to yield a desirable dose distribution. We used three different criteria for data analysis: percent difference, distance to agreement (DTA), and γ index. We set the percent difference threshold to 3% and the DTA to 3 mm. Results: Using these criteria, 99% ∼ 99.5% dose points passed the test for most of our IMRT dose painting plans. For a ten‐field plan, it took about 35 minutes to complete the data acquisition and post‐analysis. Conclusions: The proposed procedure not only improves the efficiency, but also enhances the accuracy of measured dose distribution in the steep dose gradient regions. MapCheck is a handy, fast, and practical tool for routine pretreatment verification of large‐field dose painting plans.

Performance of the first ΔOSI microstrip dosimeter prototype in the characterization of a clinical accelerator

As the technology for medical radiotherapy and radiosurgery evolves, there is a growing need for dosimeters capable of measuring dose distributions on-line with submillimetre spatial resolution, both for facility commissioning and patient-related quality assurance. We have designed and built a high spatial resolution dosimeter based on silicon microstrip technology for characterization of small radiotherapy and radiosurgery fields. The aim is to provide relative dosimetry measurement with film-like spatial resolution, and to be able to observe the temporal evolution of the dose distribution. Beam test results of a 250 μm pitch, 128-channel prototype with X-rays in a clinical 6 MV accelerator are presented in this paper. The device has demonstrated good dosimetric capabilities when compared to reference measurements made with ionization chambers. Here, we evaluate the performance of our prototype in the steep dose gradient region produced by the collimator edge with respect to radiographic film and characterize a Multi-Leaf Collimator system used for Intensity Modulated Radiotherapy at the Weston Park Hospital.

TU-D-224A-02: Intro: Particles and Intensity Modulation

In the past 15–20 years substantial research and development effort has been put into the optimization of photonbeamradiation therapy, primarily into the development of intensity‐modulation techniques (IMRT). As far as the development of photonIMRT delivery technology is concerned, we may have almost reached a point of diminishing returns. In recent years novel treatment modalities (other than photons) are gaining more attention, which goes hand in hand with the construction of a number of new proton therapy facilities in this country. The physical advantage of proton therapy is indisputable, even though it is not always used to its full extent in clinical treatments: The blessing of the finite range of protons goes along with some uncertainty in the position of the steep distal dose gradient region in the patient. This is because the position of the distal gradient is affected by setup errors, internal organ motion, metallic implants, and biological effects, among others. Due to these reasons, the distal gradient is not currently used for dose shaping. The full clinical utilization of the physical advantage of proton therapy also requires beam scanning and intensity‐modulated particle therapy (IMPT), which is in clinical use at only two centers. The clinical benefit of proton therapy has been demonstrated for a relatively limited number of cases, and will be further studied as more proton centers are being brought on line. Of course, the benefit comes at a price. Several investigators now look into novel and potentially cheaper proton acceleration techniques, including laser acceleration, which will be discussed by Dr. Ma, and dielectric wall acceleration. Outside of the USA the treatment with heavier charged particles, such as carbon ions, has become a topic of great interest, which will be reviewed by Dr. Jakel. Both the treatment with protons and carbon ions open new avenues for image‐guided radiation therapy: Those particle beams activate positron emitters in the patient, which can be visualized with PET scanners and serve as an in‐vivo dosimeter, as explained by Dr. Parodi. Last but not least, there has been renewed interest in lighter charged particles, namely electrons. The talks by Drs. Li and Papiez will show that electrons are a particularly attractive treatment option in combination with IMRT. Educational Objectives: 1. To give a brief update of the state of the art of protonradiation therapy. 2. To link the various presentations of this symposium.

Dose response characteristics and basic dose distribution data for a polymerization-based dosemeter gel evaluated using MR.

A safe and reproducible mixing procedure for the manufacture of a polymerization-based dosemeter gel evaluated using MRI (PoMRI) is presented. The dose response, obtained by irradiating gel-filled vials with absorbed doses in the interval 0-20 Gy and evaluated with respect to 1/T2, was found to be linear in the interval 0-8 Gy, with a sensitivity of 0.211 s-1Gy-1 (r2 = 0.998) at 1.5 T. Evaluation of the same set of vials with respect to 1/T1 gave a sensitivity of 0.018 s-1Gy-1 (r2 = 0.960). PoMRI and diode data were compared for standard photon and electron treatment beams. A deviation of less than 3% was found between the two methods for central depth dose curves as well as dose profiles (2 mm for electrons in the steep dose gradient regions). The importance of the method used for background correction for the reliability of the results was also evaluated. Barex (with a wall thickness of 1.5 mm) was investigated for use as phantom material and found to be favourable compared with glass. The results obtained in this study show that PoMRI has excellent potential as a 3D detector.