On the feasibility of 4D offline PET-based treatment verification in ion beam therapy

Aim: Due to the accessible sharp dose gradients, ion beam therapy is prone to uncertainties introduced by organ motion. Hence, in-vivo treatment verification is highly desirable. At the Heidelberg Ion-Beam Therapy Center, this is realised by comparing the irradiation-induced β+-activity within the patient, measured by a commercial full-ring PET/CT scanner installed next to the treatment site, with a corresponding Monte-Carlo (MC) simulation based on the treatment plan. While 3D PET-based treatment verification is used in clinical practice, the feasibility of 4D PET-based treatment monitoring, accounting for tumour motion during the irradiation and the subsequent PET, still needs to be demonstrated.

Methods: PMMA phantoms of different geometries have been irradiated under stationary and moving conditions using a dedicated motion platform. Target movement has been monitored by a pressure sensor motion surrogate, enabling a 4D analysis of the ion beam delivery and the post-irradiation PET. Similarly, 4D datasets of the irradiation and the subsequent PET scans of patients with moving tumours have been collected. The acquired PET images were compared to activity distributions calculated within a dedicated 4D MC simulation framework.

Results: It could be shown that, in the case of moving phantoms, motion induced blurring in the acquired offline PET data can significantly be reduced by a gated 4D PET reconstruction, yielding results comparable to static reference measurements and thus enable the verification of the actual beam delivery.
The analysis of measured irradiation-induced activities within patients is however limited by the very low counting statistics, hindering a reliable verification of the applied treatment.

Conclusion: First moving-phantom studies showed the feasibility of 4D offline PET-based treatment verification. For clinical cases the method is currently limited by the low level of the measured activity.

Acknowledgement: FP7 EU project ENVISION