content_image/326426.jpg content_image/326427.jpg
<img src='content_image/326426.jpg'>

<img src='content_image/326427.jpg'>

towards the Bragg peak. The difference in the absolute Bragg peak dose was consistent with the findings of previous studies [19, 24, 25], and was not increased by the magnetic field. Thus, the introduction of a magnetic field did not compromise the dose prediction accuracy. 

To quantify the accuracy of the calculated Bragg peak position, its retraction and deflection were com- pared (see Figure 6). Both quantities agreed within measurement and simulation uncertainties (0 . 8 mm) for all studied energies. There was no systematic difference in Bragg peak retraction and deflection between the simulation and measurement data. 

## Conclusions 

For the first time, magnetic field induced dose distortions of a slowing-down therapeutic proton beam in a tissue-like medium have been measured and compared against particle tracking simulations. In a transverse magnetic field of 0 . 95 T, it was shown that lateral Bragg peak displacement ranges between 1 mm and 10 mm for proton energies between 80 and 180 MeV in PMMA. Range retraction was found to be ≤ 0 . 5 mm, which 

