Dosimetric characteristics of 6 and 10MV unflattened photon beams.
OBJECTIVE
To determine dosimetric properties of unflattened megavoltage photon beams.


METHODS
Dosimetric data including depth dose, profiles, output factors and phantom scatter factors from three different beam qualities provided by Elekta Precise linacs, operated with and without flattening filter were examined. Additional measurements of leaf transmission, leakage radiation and surface dose were performed. In flattening filter free (FFF) mode a 6-mm thick copper filter was placed into the beam to stabilize it.


RESULTS
Depths of dose maxima for flattened and unflattened beams did not deviate by more than 2mm and penumbral widths agreed within 1mm. In FFF mode the collimator exchange effect was found to be on average 0.3% for rectangular fields. Between maximum and minimum field size head scatter factors of unflattened beams showed on average 40% and 56% less variation for 6 and 10MV beams than conventional beams. Phantom scatter factors for FFF beams differed up to 4% from the published reference data. For field sizes smaller than 15cm, surface doses relative to the dose at d(max) increased for unflattened beams with maximum differences of 7% at 6MV and 25% at 10MV for a 5x5cm(2) field. For a 30x30cm(2) field, relative surface dose decreased by about 10% for FFF beams. Leaf transmission on the central axis was 0.3% and 0.4% lower for unflattened 6 and 10MV beams, respectively. Leakage radiation was reduced by 52% for 6MV and by 65% for 10MV unflattened beams.


CONCLUSIONS
The results of the study were independently confirmed at two radiotherapy centres. Phantom scatter reference data need to be reconsidered for medical accelerators operated without a flattening filter.
Kragl
Gabriele
G
Department of Radiotherapy, Medical University Vienna/AKH Vienna, Austria. gabriele.kragl@akhwien.at
af Wetterstedt
Sacha
S
Knäusl
Barbara
B
Lind
Mårten
M
McCavana
Patrick
P
Knöös
Tommy
T
McClean
Brendan
B
Georg
Dietmar
D
D004307:Dose-Response Relationship, Radiation; D005374:Filtration / Q000295:instrumentation; D006801:Humans; D007091:Image Processing, Computer-Assisted; D019047:Phantoms, Imaging; D017785:Photons / Q000627:therapeutic use; D011832:Radiation Injuries / Q000517:prevention & control; D011834:Radiation Monitoring; D011836:Radiation Tolerance; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity