Experimental investigations on in-beam PET for proton therapy

Following the promising results of [1], further phantom experiments have been performed at GSI in order to address the feasibility and accuracy of in-beam positron emission tomography (PET) for proton therapy. For the most challenging issue of range monitoring it has been demonstrated for mono-energetic as well as for spread-out Bragg peak (SOBP) irradiation that range differences of about 0.9mm can be resolved (figure 1). The activation study of an inhomogenous phantom also supported the usefulness of PET for assessment of local tissue modifications or field misalignments leading to strong density gradients in the beam path during fractionated therapy. Despite the limitations of the presently used calculation model for other materials than lucite [1] [2], a fairly good correspondence especially with respect to the range spike after the lateral density gradient polyethylene/lung was observed in measured and predicted images (figure 2). Finally, the value of PET for lateral position verification was assessed by irradiation of a specially designed lucite phantom allowing insertion of radiographic films for the simultaneous measurement of the lateral dose distribution (figure 3). All these results, more deeply addressed in [2], strongly support the value of in-beam PET for proton therapy. However, the extension of such millimetre precision to real clinical cases at least in low perfused tissues requires a detailed description of all reaction channels leading to βemitters in combination with an accurate knowledge of the tissue stoichiometry for a realistic prediction to be compared with the PET measurement. Investigations on the influence of the C/O ratio reported in [2] indicate also the sensitivity of the β-activity distal edge to the time