Transit dosimetry in dynamic IMRT with an a-Si EPID

Using an amorphous silicon (a-Si) EPID for transit dosimetry requires detailed characterization of its dosimetric response in a variety of conditions. In this study, a measurement-based model was developed to calibrate an a-Si EPID response to dose for transit dosimetry by comparison with a reference ionization chamber. The ionization chamber reference depth and the required additional buildup thickness for electronic portal imaging devices (EPID) transit dosimetry were determined. The combined effects of changes in radiation field size, phantom thickness, and the off-axis distance on EPID transit dosimetry were characterized. The effect of scattered radiation on out-of-field response was investigated for different field sizes and phantom thicknesses by evaluation of the differences in image profiles and in-water measured profiles. An algorithm was developed to automatically apply these corrections to EPID images based on the user-specified field size and phantom thickness. The average phantom thickness and an effective field size were used for IMRT fields, and images were acquired in cine mode in the presence of an anthropomorphic phantom. The effective field size was defined as the percentage of the jaw-defined field that was involved during the delivery. Nine head and neck dynamic IMRT fields were tested by comparison with a MatriXX two-dimensional array dosimeter using the Gamma (3 %, 3 mm) evaluation. A depth of 1.5 cm was selected as the ionization chamber reference depth. An additional 2.2 mm of copper buildup was added to the EPID. Comparison of EPID and MatriXX dose images for the tested fields showed that using a 10 % threshold, the average number of points with Gamma index <1 was 96.5 %. The agreement in the out-of field area was shown by selection of a 2 % threshold which on average resulted in 94.8 % of points with a Gamma index <1. The suggested method is less complicated than previously reported techniques and can be used for all a-Si EPIDs regardless of the manufacturer

Low-latitude Pi 1-2 magnetic pulsations of the August 28, 1986 substorm event

An isolated substorm occurred at 1153 UT on August 28,1986 provides an opportunity for studying a relation between low-latitude Pi 1-2 magnetic pulsations in the conjugate area around L=1.3-2.1 on the ground and a substorm-associated variation at geocentric distance of 8 R_E near the midplane of the magnetotail. The low-latitude Pi activities on the ground started &acd;60 s after the onset time of the substorm-associated dipolarization with a possible formation of an X-type neutral line at AMPTE/CCE. The ground H-component pulsation showed a discrete frequency and an amplitude of nearly the same magnitude in a wide area around L=1.3-2.1,and its phase propagation was poleward and westward (m ～1.7) during premidnight. The D-component odd-mode oscillation localized at L ～2.1 on the ground had the same quasi-monochromatic period ～13 s) as a compressional wave at AMPTE/CCE. These H-component global eigen mode and D-component localized mode may have been associated with a poloidal magnetic oscillation (including the "magnetosphere" cavity resonance mode) and a toroidal standing field-line oscillation excited by the compressional waves with impulsive and quasi-monochromatic components at AMPTE/CCE, respectively

A simple approach to using an amorphous silicon EPID to verify IMRT planar dose maps

A simplified method of verifying intensity modulated radiation therapy (IMRT) fields using a Varian aS500 amorphous silicon electronic portal imaging device (EPID) is demonstrated. Unlike previous approaches, it does not involve time consuming or complicated analytical processing of the data. The central axis pixel response of the EPID, as well as the profile characteristics obtained from images acquired with a 6 MV photon beam, was examined as a function of field size. Ion chamber measurements at various depths in a water phantom were then collected and it was found that at a specific depth dref, the dose response and profile characteristics closely matched the results of the EPID analysis. The only manipulation required to be performed on the EPID images was the multiplication of a matrix of off axis ratio values to remove the effect of the flood field calibration. Similarly, dref was found for 18 MV. Planar dose maps at dref in a water phantom for a bar pattern, a strip pattern, and 14 clinical IMRT fields from two patient cases each being from a separate anatomical region, i.e., head and neck as well as the pelvis, for both energies were generated by the Pinnacle planning system (V7.4). EPID images of these fields were acquired and converted to planar dose maps and compared directly with the Pinnacle planar dose maps. Radiographic film dosimetry and a MapCHECK dosimetry device (Sun Nuclear Corporation, Melbourne, FL) were used as an independent verification of the dose distribution. Gamma analysis of the EPID, film, and Pinnacle planar dose maps generated for the clinical IMRT fields showed that approximately 97% of all points passed using a 3% dose/3mm DTA tolerance test. Based on the range of fields studied, the author's results appear to justify using this approach as a method to verify dose distributions calculated on a treatment planning system, including complex intensity modulated fields

MRI-guided prostate radiation therapy planning: Investigation of dosimetric accuracy of MRI-based dose planning

Background and purpose: Dose planning requires a CT scan which provides the electron density distribution for dose calculation. MR provides superior soft tissue contrast compared to CT and the use of MR-alone for prostate planning would provide further benefits such as lower cost to the patient. This study compares the accuracy of MR-alone based dose calculations with bulk electron density assignment to CT-based dose calculations for prostate radiotherapy