ABSTRACT
PURPOSE: Film dosimetry as a part of patient specific intensity modulated radiation therapy quality assurance (IMRT QA) was performed to develop a new optimization method of film isocenter offset and to then suggest new quantitative criteria for film dosimetry. MATERIALS AND METHODS: Film dosimetry was performed on 14 IMRT patients with head and neck cancers. An optimization method for obtaining the local minimum was developed to adjust for the error in the film isocenter offset, which is the largest part of the systemic errors. RESULTS: The adjust value of the film isocenter offset under optimization was 1 mm in 12 patients, while only two patients showed 2 mm translation. The means of absolute average dose difference before and after optimization were 2.36 and 1.56%, respectively, and the mean ratios over a 5% tolerance were 9.67 and 2.88%. After optimization, the differences in the dose decreased dramatically. A low dose range cutoff (L-Cutoff) has been suggested for clinical application. New quantitative criteria of a ratio of over a 5%, but less than 10% tolerance, and for an absolute average dose difference less than 3% have been suggested for the verification of film dosimetry. CONCLUSION: The new optimization method was effective in adjusting for the film dosimetry error, and the newly quantitative criteria suggested in this research are believed to be sufficiently accurate and clinically useful.
Subject(s)
Humans , Film Dosimetry , Head , NeckABSTRACT
PURPOSE: Film dosimetry as a part of patient specific intensity modulated radiation therapy quality assurance (IMRT QA) was performed to develop a new optimization method of film isocenter offset and to then suggest new quantitative criteria for film dosimetry. MATERIALS AND METHODS: Film dosimetry was performed on 14 IMRT patients with head and neck cancers. An optimization method for obtaining the local minimum was developed to adjust for the error in the film isocenter offset, which is the largest part of the systemic errors. RESULTS: The adjust value of the film isocenter offset under optimization was 1 mm in 12 patients, while only two patients showed 2 mm translation. The means of absolute average dose difference before and after optimization were 2.36 and 1.56%, respectively, and the mean ratios over a 5% tolerance were 9.67 and 2.88%. After optimization, the differences in the dose decreased dramatically. A low dose range cutoff (L-Cutoff) has been suggested for clinical application. New quantitative criteria of a ratio of over a 5%, but less than 10% tolerance, and for an absolute average dose difference less than 3% have been suggested for the verification of film dosimetry. CONCLUSION: The new optimization method was effective in adjusting for the film dosimetry error, and the newly quantitative criteria suggested in this research are believed to be sufficiently accurate and clinically useful.
Subject(s)
Humans , Film Dosimetry , Head , NeckABSTRACT
PURPOSE: A new fiduciary plate and orientation marker have been devised to assist the quality assurance (QA) procedures for port films in radiation therapy department. The plate is used in conjunction with the film/cassette combination during weekly QA procedures, at Seoul National University Hospital (SNUH), in order to verify treatment fields in high energy radiation therapy. MATERIALS AND METHODS: A new fiduciary plate was fabricated using an acrylic plate, cerrobend, standard blocking tray and mercury. The acrylic plate had the dimension of 1x25x25 cm, with two fiduciary markers. The plate was rigidly attached onto the standard blocking tray, thus making it easier to set the fiduciary plate to the center of the radiation field of the linear accelerator. The plate had two 2-mm vertical and horizontal lines, with the minor scales in 2-cm steps. The orientation marker was a small mercury filled disk, which was inserted into the plate. RESULTS: The geometrical structure of the lines in the plate makes it easier to correlate two different images between the simulation and port films. The marker clearly indicated the orientation of the film, for example, the anterior, posterior, left, right and various oblique orientations, without the placement of a conventional orientation marker. Also, the new orientation marker could easily be applied to the simulator by placing the small orientation marker onto the image intensifier or in front of the film/cassette holder. CONCLUSIONSs: The new fiduciary plate appears to be useful in verifying the treatment fields, and the new orientation marker makes the film orientation simple, which is expected to lower the block fabrication errors.