Users Manual Part 2

Clinical User’s Guide IBA | 20-3|
Volume 1 - Treatment Session
|Part III - The Pencil Beam Scanning (PBS) Suite Pencil Beam Scanning Principles |
Longitudinal and Transversal Dose
Distribution in PBS
A very precise dose distribution can be obtained both longitudinally (in depth) and
transversally (on the plane perpendicular to the beam direction) using PBS.
Longitudinal Distribution
In order to spread the dose delivery along the longitudinal axis (that is to say, in
depth), the dose is delivered in successive single energy layers. A layer is therefore
defined as the set of spots for which the same beam energy is required.
In PBS, the energy is adjusted in the Energy Selection System (ESS) for each one of
the layers.
Layers
In the Pencil Beam Scanning (PBS) treatment mode, layers are defined: the tumor is
divided in a stack of layers, whereby each layer corresponds to one specific energy.
The division in layers takes the patient body outline, the target shape, and the
presence of heterogeneities into account. This means that the dose deposited in a
given layer is not necessarily maximal (Bragg peak) in the same transversal plane.
Every layer can be painted several times and the range modulation is done layer per
layer at the energy selection level. This is different from the scattering delivery
techniques, where the beam shape is passively broadened before entering the target
volume.
The spacing of the different layers and the weight of the beam at each position is
computed by the TPS in order to fulfill the required dose conformity.