Out-of-beam artifact suppression in charged nuclear fragment based carbon-ion radiotherapy monitoring

Carbon-ion radiotherapy offers highly precise targeting of tumors while sparing healthy tissue compared to X-ray therapy. However, this precision comes at the cost of an increased sensitivity of the treatment to range uncertainties, which can arise from anatomical changes of the patient. Our group develops an in-vivo treatment monitoring method by tracking of charged nuclear fragments using hybrid silicon pixel detectors.

Anatomical changes outside of the region accessed by carbon-ion beams are clinically not relevant, as they do not affect the dose distribution. However, they can potentially influence the fragment data, producing artifacts, which might be interpreted as signals produced by clinically relevant anatomical changes. This misinterpretation would cause unnecessary clinical action, like performing a CT scan. This work proposes methods for the identification and suppression of clinically irrelevant artifacts with the aim of avoiding unnecessary clinical action.

A clinically relevant and an irrelevant anatomical change are emulated by introducing coin-sized air cavities at different positions in a homogeneous cylindrical plastic head phantom. Charged nuclear fragments are detected by a Timepix3-based mini-tracker during irradiations of this phantom with a clinically realistic treatment plan. All measurements are performed for two different positions of the mini-tracker. The reconstructed fragmentation vertex distributions are analyzed and compared to those of reference measurements.

A significant signal from the clinically irrelevant air cavity was observed. This artifact was found to differ from the signal of the clinically relevant cavity. Most importantly, the location of the artifact changes with the mini-tracker position, whereas the relevant signal remains unchanged. This facilitates identification of the artifact as well as its suppression by combining the data from several mini-trackers at different positions around the patient.

Clinically irrelevant changes were shown to potentially impede carbon-ion treatment monitoring by tracking of charged nuclear fragments. However, positioning several mini-trackers around the patient, which monitor the treatment from different perspectives, was found to be the key to the identification and suppression of artifacts from anatomical changes outside of the region accessed by carbon-ion beams. This is implemented in the detection system of an ongoing clinical trial.