Use of EPID for leaf position accuracy QA of dynamic multi-leaf collimator (DMLC) treatment.

We describe in this paper an alternative method for routine dynamic multi-leaf collimator (DMLC) quality assurance (QA) using an electronic portal imaging device (EPID). Currently, this QA is done at our institution by filming an intensity-modulated radiotherapy (IMRT) test field producing a pattern of five 1-mm bands 2 cm apart and performing a visual spot-check for leaf alignment, motion lags, sticking and any other mechanical problems. In this study, we used an amorphous silicon aS500 EPID and films contemporaneously for the DMLC QA to test the practicality and efficacy of EPID vis-à-vis film. The EPID image was transformed to an integrated dose map by first converting the reading to dose using a calibration curve, and then multiplying by the number of averaged frames. The EPID dose map was then back-projected to the central axis plane and was compared to the film measurements which were scanned and converted to dose using a film dosimetry system. We determined the full-width half-maximum (FWHM) of each band for both images, and evaluated the dose to the valley between two peaks. We also simulated mechanical problems by increasing the band gap to 1.5 mm for some leaf pairs. Our results show that EPID is as good as the film in resolving the band pattern of the IMRT test field. Although the resolution of the EPID is lower than that of the film (0.78 mm/pixel vs 0.36 mm/pixel for the film), it is high enough to faithfully reproduce the band pattern without significant distortion. The FWHM of the EPID is 2.84 mm, slightly higher than the 2.01 mm for the film. The lowest dose to the valley is significantly lower for the EPID (15.5% of the peak value) than for the film (28.6%), indicating that EPID is less energy independent. The simulated leaf problem can be spotted by visual inspection of both images; however, it is more difficult for the film without being scanned and contrast-enhanced. EPID images have the advantage of being already digital and their analysis can easily be automated to flag leaf pairs outside tolerance limits of set parameters such as FWHM, peak dose values, peak location, and distance between peaks. This automation is a new feature that will help preempt MLC motion interlocks and decrease machine downtime during actual IMRT treatment. We conclude that since EPID images can be acquired, analyzed and stored much more conveniently than film, EPID is a good alternative to film for routine DMLC QA.

Atomic force microscopy examination of conformations of polynucleotides in response to platinum isomers: significance of GC content at broken ends.

Atomic force microscopy is a technique that enables visualization of macromolecular conformations of polynucleotides at nanometer resolution. We investigated the results of interactions of cisplatin, a DNA binding anticancer drug, and its inactive counterpart, transplatin isomer, on the molecular conformation of polynucleotides: poly d(G-C). poly d(G-C) (polyGC) and poly d(A-T). poly d(A-T) (polyAT). We observed that polyAT exhibited an increased number of enlarged ends of molecules, which we attribute to unwound and/or collapsed regions of polyAT. PolyGC molecules did not show such ends unless cisplatin was added to the PolyGC polymers. Transplatin had the apparent effect of causing overlapping or stacking of the polymer molecules. Addition of exonuclease-III to these polymers removed the visible enlarged ends. The effects of cisplatin as compared to transplatin on the polyGC duplex polymers provide support for the presence of intrastrand covalent linkages, consistent with known N7 guanine interaction of the cis isomer on molecular conformation. Furthermore, our results indicate that the mechanism of interactions of DNA with cisplatin may be dependent on the GC content of the molecules. Int. J. Cancer (Radiat. Oncol. Invest.) 90, 68-72, 2000.

Wedge factor dependence on depth and field size for various beam energies using symmetric and half-collimated asymmetric jaw settings.

The depth- and field-size dependence of the in-phantom wedge factor have been determined for a Cobalt-60 (Co-60) teletherapy unit and four medical linear accelerators with 4-, 6-, 10-, and 18-MV x-ray beams containing 15 degrees-60 degrees (nominal) lead, brass, and steel wedge filters. Measurements were made with ionization chambers in solid water or water with a source-skin distance of 80 or 100 cm. Field sizes varied from 4 x 4 cm up to a maximum allowable size for each wedge filter. Measurements were performed for symmetric and half-collimated asymmetric fields at depth of maximum dose, 5- and 10-cm depths. For half-collimated fields, wedge factor reference points were located at a fixed off-axis distance from the collimator's rotational axis. These systematic measurements on wedges indicate that the wedge factor dependence on depth and field size is a function of beam energy as well as the design of the treatment head and wedge filters. Significance of the results reported herein are discussed for the most commonly used treatment depths and field sizes with various beam energies and wedge filters.

Three-dimensional dose distribution of total body irradiation by a dual source total body irradiator.

This study describes the three-dimensional dosimetric characteristics of total body irradiation by our dedicated irradiation unit, which consists of two modified 4-MV linear accelerators mounted opposite each other, providing a field size of 220 cm x 80 cm at the midplane. Our dose calculation algorithm considers the three-dimensional contour of the patient to evaluate the primary and scatter doses. The data base for the calculation includes tissue-to-maximum ratios measured for the large fields. The lung dose correction was calculated using the methods of Batho or ratio of TMR. The accuracy of the calculated dose distributions was verified by measurements with ionization chambers in a humanoid phantom. We also describe and verified a technique to achieve desirable midline lung doses using lead shields. The flexibility and the accuracy of the planning system offers the potential in optimizing the therapeutic ratio for total body treatments.