Energy constancy checks on a linear accelerator: comparison of different procedures
M.G. Brambilla 1 , M. Ciocca 2* , P. Lattuada 3 , A.F. Monti 4 , M. Princivalli 5 , L. Raffaele 6
1. Department of Medical Physics, Ospedale
Niguarda Cą Granda, piazza Ospedale Maggiore 3, Milano (Italy)
2. Department of Medical Physics, European Institute of
Oncology, via Ripamonti 435, 20141 Milano (Italy)
3. Department of Radiation Oncology, Casa di Cura S. Pio
X, via F. Nava 31, 20159 Milano (Italy)
4. Department of Medical Physics, Ospedale S. Anna, via
Napoleona 60, 22100 Como (Italy)
5. Department of Medical Physics, Ospedale
Ca’Foncello, piazza Ospedale 1, 31100 Treviso (Italy)
6. IST Genova, Istituto di Radiologia, Universitą di
Catania, viale Santa Sofia 5, 95100 Catania (Italy)
Manuscript received: May 30, 2000; revised: January 26 and May 28, 2001.
Accepted for publication: May 30, 20001
Abstract
Within a program of Quality Assurance on a medical linear accelerator, periodic tests of radiation energy constancy should be performed. In this work, different procedures adopted in Italy for energy constancy checks have been tested, in order to show relative differences in terms of practicability, adequacy and sensitivity to forced beam energy variations. Energy constancy checks were performed in photon and electron beams, using an automatic water phantom, a solid slab phantom and a commercial phantom for quality control. Procedures suggested or adapted from five different technical reports were tested. Known variations of radiation beam energies were obtained using a 60° motorized wedge and by modifying beam bending magnet currents via software. Depth dose measurements in the water phantom seemed to be the most reliable procedure, because several parameters related to both dose and depth (i.e., D10, D20, d80 , d50 and so on) could be analysed, but it was not as practical as other procedures. The sensitivity to beam energy variations strongly depended on the analysed parameter (depth dose values, dose ratios and depths) and beam quality. Methods based on measurements of depth dose ratios in a slab phantom appeared faster and more practical. The sensitivity of these procedures was very high for electron beams, while for photons it did not seem to depend significantly on the adopted geometry nor on the analysed parameter (TPRs and dose ratios). The method involving the use of a commercial phantom appeared to be quick enough to allow daily checks, although beam energy characterization using an off-axis detector did not seem reliable enough, due to sensitivity to fluctuations in beam flatness.
KEYWORDS: Linear accelerators, quality control, radiation energy checks.