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of the dose distribution in the patient have been predicted; particularly, the Bragg peak is expected to be displaced by a few millimeters up to several centimeters, depending on beam energy and magnetic flux density [12, 13, 14, 15, 16, 17, 18]. 

For the design of dose distributions during treatment planning, the dose simulation models predicting these effects need to be validated with measurements. Hence experimental benchmark data is required. However, such data does not exist, thereby inhibiting the employment of the models used in simulation studies for potential clinical use. 

For this purpose, measurements are needed involving a clinical proton pencil beam (beam energy E$_{0}$ between 70 MeV and 230 MeV), a magnet whose flux density is in the clinical range ( B$_{0}$ = 0 . 35 ... 3 T), a tissue-like phantom (density ρ ≈ 1 gcm − $^{3}$), and a dosimeter having an adequate spatial resolution (Δ x < 1 mm) as well as a magnetic field-independent dose response. 

Here we describe the first measurement of magnetic field induced dose distortion of a slowing-down therapeutic proton pencil beam within a tissue-like medium, and a comparison to state-of-the-art dose simulations based on Monte Carlo particle tracking. 

## Proton dose measurement within a magnetic field 

The measurement setup is depicted in Figure 1. It consists of a collimated proton beam, a slab phantom containing a horizontally placed film dosimeter, and a permanent magnet assembly. 

The proton beam was generated by an isochronous cyclotron (C230, IBA, Louvain-La-Neuve, Belgium) at the proton therapy facility of our institution. The horizontal static beam line was used and defined the 

