Testing the GLAaS algorithm for dose measurements on low- and high-energy photon beams using an amorphous silicon portal imager
- Medical Physics Unit, Oncology Institute of Southern Switzerland, Bellinzona (Switzerland)
- Switzerland
The GLAaS algorithm for pretreatment intensity modulation radiation therapy absolute dose verification based on the use of amorphous silicon detectors, as described in Nicolini et al. [G. Nicolini, A. Fogliata, E. Vanetti, A. Clivio, and L. Cozzi, Med. Phys. 33, 2839-2851 (2006)], was tested under a variety of experimental conditions to investigate its robustness, the possibility of using it in different clinics and its performance. GLAaS was therefore tested on a low-energy Varian Clinac (6 MV) equipped with an amorphous silicon Portal Vision PV-aS500 with electronic readout IAS2 and on a high-energy Clinac (6 and 15 MV) equipped with a PV-aS1000 and IAS3 electronics. Tests were performed for three calibration conditions: A: adding buildup on the top of the cassette such that SDD-SSD=d{sub max} and comparing measurements with corresponding doses computed at d{sub max}, B: without adding any buildup on the top of the cassette and considering only the intrinsic water-equivalent thickness of the electronic portal imaging devices device (0.8 cm), and C: without adding any buildup on the top of the cassette but comparing measurements against doses computed at d{sub max}. This procedure is similar to that usually applied when in vivo dosimetry is performed with solid state diodes without sufficient buildup material. Quantitatively, the gamma index ({gamma}), as described by Low et al. [D. A. Low, W. B. Harms, S. Mutic, and J. A. Purdy, Med. Phys. 25, 656-660 (1998)], was assessed. The {gamma} index was computed for a distance to agreement (DTA) of 3 mm. The dose difference {delta}D was considered as 2%, 3%, and 4%. As a measure of the quality of results, the fraction of field area with gamma larger than 1 (%FA) was scored. Results over a set of 50 test samples (including fields from head and neck, breast, prostate, anal canal, and brain cases) and from the long-term routine usage, demonstrated the robustness and stability of GLAaS. In general, the mean values of %FA remain below 3% for {delta}D equal or larger than 3%, while they are slightly larger for {delta}D=2% with %FA in the range from 3% to 8%. Since its introduction in routine practice, 1453 fields have been verified with GLAaS at the authors' institute (6 MV beam). Using a DTA of 3 mm and a {delta}D of 4% the authors obtained %FA=0.9{+-}1.1 for the entire data set while, stratifying according to the dose calculation algorithm, they observed: %FA=0.7{+-}0.9 for fields computed with the analytical anisotropic algorithm and %FA=2.4{+-}1.3 for pencil-beam based fields with a statistically significant difference between the two groups. If data are stratified according to field splitting, they observed %FA=0.8{+-}1.0 for split fields and 1.0{+-}1.2 for nonsplit fields without any significant difference.
- OSTI ID:
- 21036150
- Journal Information:
- Medical Physics, Vol. 35, Issue 2; Other Information: DOI: 10.1118/1.2828182; (c) 2008 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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