Abstract:
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Purpose: To study the influence of the field setup on the dosimetry at the junction in singleisocenter
half-beam techniques.
Methods: The dosimetry at the junction for a two-field setup with the gantry at zero was first
evaluated with radiochromic films. A three-field setup, with an anterior field and two opposed
lateral fields, was also analyzed for two different relative positions of the fields involved. In all
cases, the dose increase at the central axis, called the junction dose, was measured.
Results: Junction doses varied greatly with the setup. For the three-field setup, the junction dose
differed from that obtained with the two-field setup, and it greatly depended on the relative position
of the fields. When the anterior field was closer to the gantry than the lateral fields, a field gap
occurred and the junction dose was negative. When the anterior field was farther from the gantry
than the lateral fields, a field overlap was obtained and the junction dose was positive. The difference
in the junction dose between the three-field setups was around 18% for the three accelerators
evaluated.
Conclusions: Having a uniform dose distribution for two fields at gantry 0° does not guarantee a
uniform distribution at other gantry angles. Junction doses are largely affected by the relative
position of the radiation fields, which may have an impact in clinical practice. Therefore, any
method aiming to assess or to optimize the dose homogeneity at the junction should take this effect
into account. |