Speaker
Description
Compared to other radiotherapy modalities for cancer treatment, carbon-ion radiotherapy permits a higher dose concentration in targeted tumor volumes while better sparing patient’s healthy tissues. However, this benefit comes with the price of a higher sensitivity to any changes of the patient internal geometry due to movements, or tumor swelling and shrinkage. Monitoring methods of the patient radio-treatment are therefore of great importance to visualize carbon-ion beam range in-vivo and, thus, to detect possible under- or over-dosage in the patient.
In this contribution, we focus on a carbon-ion beam monitoring method based on secondary-ions detection. This method uses the fact that when a carbon-ion beam enters a patient during radiotherapy, the carbon ions undergo nuclear reactions (also called "fragmentation") with the targeted patient tissue ‘s nuclei. These nuclear reactions produce detectable secondary-ion fragments along the path of the carbon-ion beam. In this work, we investigate a possible application of pixelated silicon detectors for carbon-ion beam range monitoring, by developing a monitoring method based on the detection of secondary charged nuclear-fragments emerging from an irradiated target. A correlation between the measured depth of the fragments’ origin (i.e. where primary carbon-ion beam fragmentation occurs) and the planned primary carbon-ion-beam range in the target was studied. Moreover, we also explore additional benefits of the knowledge of both fragments’ position and energy-loss information on the precision of our method.
In order to mimic a clinical treatment, a typical 12Carbon-ion treatment plan fraction of a head tumor was applied on an anthropomorphic head model at the Heidelberg Ion-Beam Therapy Center (HIT), Germany. Several carbon ion beam energies ranging from 165,09 MeV/n to 246,57 MeV/n were used to treat the whole volume of the targeted tumor. The directions and energy deposition of the emerging secondary-ions were measured by a pair of parallel pixelated silicon detectors Timepix3 (TPX3), developed at CERN and commercialized by ADVACAM s.r.o. (Prague) placed behind the targeted head. Each of the detectors we used in this work contains a 300 µm thick sensor, with a sensitive area of 1,4 x 1,4 cm² divided into 256 x 256 pixels (55 µm pitch). Compared to Timepix detectors, the new generation of Timepix3 used in this contribution offers the advantage of a data-driven dead-time free flow of pixel information containing simultaneously: (1) a precise time of arrival (TOA with time resolution of 1,56 ns) and (2) deposited energy in sensitive layer for each detected secondary-ion. To reconstruct a secondary-ion-track, detected clusters in both our detectors are matched based on coincident ToA information. This measured track is then extrapolated back in space to the carbon-ion beam axis, allowing us to estimate the carbon-ion fragmentation position.
We have shown, that the deducted fragmentation position in the targeted head varies in depth for different incident carbon-ion beam energies, which could be related to the carbon-ion beam range in the patient head (cf. Figure 1.a). Moreover, we have found that there are significant differences in the ions’ track origins position for different energy depositing ions (cf. Figure 1.b). Indeed, detected secondary-ions with high a deposited energy dE in our detector are related to tracks that originated more deeply in the head phantom (cf. light blue distribution in Figure 1.b). In the end, we have shown that the energy deposition of single detected secondary ions in TPX3 detectors is linked to the secondary ion’s velocity. This enables us to identify slower secondary ions increasingly suffering from scattering in the patient. Therefore, this additional deposited energy information, together with the almost dead-time-free secondary-ions’ time-of-arrival given by Timepix3 detectors, can be used in the future to improve the precision of the carbon-ion beam range monitoring method based on secondary charged ions.
