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. 2012 Jul 5;13(4):3701.
doi: 10.1120/jacmp.v13i4.3701.

Dosimetric properties of a beam quality-matched 6 MV unflattened photon beam

Yunfei Huang  1 , R Alfredo Siochi, John E Bayouth
Affiliations

Dosimetric properties of a beam quality-matched 6 MV unflattened photon beam

Yunfei Huang et al. J Appl Clin Med Phys. .

Abstract

The purpose of this study was to report the characteristics of an equivalent quality unflattened (eqUF) photon beam in clinical implementation and to provide a generalized method to describe unflattened (UF) photon beam profiles. An unflattened photon beam with a beam quality equivalent to the corresponding flat 6 MV photon beam (WF) was obtained by removing the flattening filter from a Siemens ONCOR Avant-Garde linear accelerator and adjusting the photon energy. A method independent from the WF beam profile was presented to describe UF beam profiles and other selected beam characteristics were examined. The short-term beam stability was examined by dynamic beam profiles, recorded every 0.072 s in static and gated delivery, and the long-term stability was evidenced by the five-year clinical quality assurance records. The dose rate was raised fivefold using the eqUF beam. The depth of maximum dose (d(max)) shifted 3 mm deeper, but the percent depth dose beyond d(max) was very similar to that of the WF beam. The surface dose and out-of-field dose were lower, but the penumbra was slightly wider. The variation in head scatter and phantom scatter with changes in field size was smaller; the variation in the profile shape with change in depth was also smaller. The eqUF beam is stable 0.072 s after the beam is turned on, and the five-year beam stability was comparable to that of the WF beam. A fivefold dose rate increase was observed in the eqUF beam with similar beam characteristics to other reported UF beam data except for a deeper dmax and a slightly wider penumbra. The initial and long-term stability of the eqUF beam profile is on parity with the WF beam. The UF beam profile can be described in the generalized method independently without relying on the WF beam profile.

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Figures

Figure 1
Figure 1
Comparison (a) of %dd measured with 0.125 cc ion chamber in water with a field size of 10×10 cm2 between WF, UF and eqUF photon beams; (b) between WF and eqUF photon beams; and (c) %dd for depth 2–20 mm measured with Markus plane‐parallel ion chamber.
Figure 2
Figure 2
Comparison of %dd at depth of 0.5, 10, and 20 cm between WF and eqUF photon beams
Figure 3
Figure 3
Comparison of collimator scatter factors (Sc) and phantom scatter factor (Sp) between the WF and eqUF photon beams.
Figure 4
Figure 4
Comparison of cross‐axis profiles of field size 4, 10, 20, 30, and 40 cm of WF and eqUF photon beams at depth of 10 cm.
Figure 5
Figure 5
Comparison (a) of beam profiles at various depths with/without the beam divergence removed between the WF and eqUF beams. Variation in OAR (b) associated with the same change in depth is 2.6 times larger in WF beams than in eqUF beams at 3 cm lateral distance.
Figure 6
Figure 6
Results of daily QA on output constancy (top) for WF and eqUF beams and (bottom) on symmetry of WF beams for five years and eqUF beam for 1.5 years.
Figure 7
Figure 7
Profiles of eqUF beam with 0.5 cm spatial resolution during a delivery of 100 MU in the static or 500 ms gated mode.
Figure 8
Figure 8
Percent differences in the profile of each frame in static or 500 ms gated mode compared to the average profile.
See this image and copyright information in PMC

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