IMRT delivery verification using a spiral phantom

Abstract

In this paper we report on the testing and verification of a system for IMRT delivery quality assurance that uses a cylindrical solid water phantom with a spiral trajectory for radiographic film placement. This spiral film technique provides more complete dosimetric verification of the entire IMRT treatment than perpendicular film methods, since it samples a three-dimensional dose subspace rather than using measurements at only one or two depths. As an example, the complete analysis of the predicted and measured spiral films is described for an intracranial IMRT treatment case. The results of this analysis are compared to those of a single field perpendicular film technique that is typically used for IMRT QA. The comparison demonstrates that both methods result in a dosimetric error within a clinical tolerance of 5%, however the spiral phantom QA technique provides a more complete dosimetric verification while being less time consuming. To independently verify the dosimetry obtained with the spiral film, the same IMRT treatment was delivered to a similar phantom in which LiF thermoluminescent dosimeters were arranged along the spiral trajectory. The maximum difference between the predicted and measured TLD data for the 1.8 Gy fraction was 0.06 Gy for a TLD located in a high dose gradient region. This further validates the ability of the spiral phantom QA process to accurately verify delivery of an IMRT plan.

Intensity-modulated radiation therapy: emerging cancer treatment technology

“…Moreover, a CT detector array diametrically opposed to the energy source allows for image and dose reconstruction capabilities during treatment. Each of these systems shares commonality of need for intensive physics support, precise anatomical target definition, and rigorous quality assurance (Richardson et al, 2003;Fenwick et al, 2004).…”

Dosimetry tools and techniques for IMRT

“…Silver halide based radiographic films, often referred to as radiographic films ͑although all dosimetry films are, in fact, radiographic͒, have been widely used for validation of the relative dosimetry of IMRT treatment phantom plans. 56,65,68,90,106,[117][118][119][120][121][122][123][124][125][126][127] In principle, such measurements allow for the verification of the appropriate shape and registration of the IMRT dose distribution in a selected twodimensional plane ͑film plane͒. Appropriate selection of the orientation and locations of the film planes is important to assure that the measured dose is useful for the desired QA function.…”

“…153 A novel cylin-drical phantom that places radiographic film in a spiral slot and an ionization chamber at the phantom center was developed for tomotherapy verification. 68,151 The advantage of this phantom is that it samples the volumetric dose distribution with a single film, although film dosimetry analysis software vendors have not provided software that interpolates the calculated dose distributions to the coordinates of the measured film. Another cylindrical water-equivalent phantom is provided by Tomotherapy, Inc., for the quality assurance of Tomotherapy treatments.…”

“…[36][37][38][39][40] Therefore, it is rec- Another type of small-field dosimeter that has been used in IMRT is the thermoluminescence dosimeter ͑TLD͒. [55][56][57][58][59][60][61][62][63][64][65][66][67][68] TLD dosimetry has been used since the 1970s to quantify megavoltage x-ray dosimetry. [69][70][71][72][73] A TLD is an integrating dosimeter, which is usually small in size, often in the form of a cubical or cylindrical chip, and has a nearly tissueequivalent atomic composition ͑Z = 8.1͒ and a typical physical density of 2.6 g cm −3 .…”

“…Phantoms are typically constructed using either water or water-equivalent plastic. 65,68,[149][150][151][152] Open water phantoms can be used when the beam is perpendicular to the phantom surface, and where great flexibility in detector positioning is desired. With the proper procedures and design, water-equivalent plastic phantoms can support multiple detectors, radiographic film, and rapid and efficient setup reproducibility.…”