gms | German Medical Science

Background: We hypothesise that a low-dose, narrow proton pencil beam of sufficient energy such as to pass through a patient can be used as a range probe (RP) for assessing range accuracy in proton therapy. A Monte Carlo (MC) study looking into the feasibility of this approach is presented. Furthermore some preliminary measurements of this concept have been performed.

Materials and methods: Using a patients planning CT, MC simulations (VMCpro) have been made for single pencil beams (177MeV) laterally traversing the head and stopping in a simulated range telescope (RT) behind the patient. RPs have been calculated for different locations and the residual range from the integrated Bragg peak (BP) signal in the RT has been assessed for different assumed detector thicknesses. The sensitivity of this approach to changes in CT values, calibration curves and positional shifts of the CT have been investigated. Initial measurements have been performed using a multi layer,large area ionization detector to perform measurements on a homogeneous phantom and an anthropomorphic head phantom.

Results: From our analysis, range resolutions of 1mm may be possible with a detector thickness of 4 mm for homogeneous regions. Additionally, for heterogeneous regions, changes of the BP shape due to spatial shifts of the CT could be a sensitive measure for detecting patient set-up errors directly in treatment position. For the measurements a multi layer ionization chamber (MLIC) with an in-plane width of 10x10 cm and a water equivalent thickness of 2.3 mm between each layer has been used. The results of the comparison between MC calculations and measurements will be shown at the presentation (they are currently being analysed).

Conclusion: The concept of the proton range-probe appears to be feasible for high resolution range verification (~1 mm). First measurements indicate that the MLIC could be a suitable device for this kind of measurement.