Dosimetric characteristics of 6 and 10 MV unflattened photon beams

Purpose

To determine dosimetric properties of unflattened megavoltage photon beams.

Materials and 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 2 mm and penumbral widths agreed within 1 mm. 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 10 MV beams than conventional beams. Phantom scatter factors for FFF beams differed up to 4% from the published reference data. For field sizes smaller than 15 cm, surface doses relative to the dose at d_max increased for unflattened beams with maximum differences of 7% at 6 MV and 25% at 10 MV for a 5 × 5 cm² field. For a 30 × 30 cm² 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 10 MV beams, respectively. Leakage radiation was reduced by 52% for 6 MV and by 65% for 10 MV 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.