Clinical beam test of inter- and intra-fraction relative range monitoring in carbon ion radiotherapy

TL;DR

The paper demonstrates a clinical beam test of the prototype filtered IVI (fIVI) Range Monitoring System using large-area silicon trackers to monitor relative Bragg peak shifts in carbon ion radiotherapy. It shows that BP-range differences can be detected with sub-millimetre to millimetre precision, depending on BP depth and collected statistics, and that the BP-depth vs fIVI translation is non-linear with a small quadratic component. In head-scale phantoms, millimetre-level precision is achieved at clinical ion counts, while abdominal-scale phantoms are statistics-limited, indicating the need for larger sensor areas or arrays for wider clinical applicability. The work supports the potential for online RM to improve treatment accuracy and margins, while outlining necessary optimization in sensor area, calibration, and data processing for clinical deployment.

Abstract

Interaction Vertex Imaging (IVI) is used for range monitoring (RM) in carbon ion radiotherapy. The purpose of RM is to measure the Bragg peak (BP) position for each contributing beam, and detect any changes. Currently, there is no consensus on a clinical RM method, the use of which would improve the safety and consistency of treatment. The prototype filtered IVI (fIVI) Range Monitoring System is the first system to apply large-area and high-rate-capable silicon detectors to IVI. Two layers of these detectors track prompt secondary fragments for use in RM. This device monitored 16 cm and 32 cm diameter cylindrical plastic phantoms irradiated by clinical carbon ion beams at the Heidelberg Ion Beam Therapy Center. Approximately 20 different BP depths were delivered to each phantom, with a minimum depth difference of 0.8 mm and a maximum depth difference of 51.9 mm and 82.5 mm respectively. For large BP range differences, the relationship between the true depth difference and that measured by fIVI is quadratic, although for small differences, the deviation from a linear relationship with a slope of 1 is negligible. RM performance is strongly dependent on the number of tracked particles, particularly in the clinically-relevant regime. Significant performance differences exist between the two phantoms, with millimetric precision at clinical doses being achieved only for the 16 cm phantom. The performance achieved by the prototype fIVI Range Monitoring System is consistent with previous investigations of IVI, despite measuring at more challenging shallow BP positions. Further significant improvements are possible through increasing the sensitive area of the tracking system beyond the prototype, which will both allow an improvement in precision for the most intense points of a scanned treatment plan and expand the number of points for which millimetric precision may be achieved.

Figures (9)

• Figure 1: The assembled tracker setup. (a) Sensor modules mounted, without enclosure installed. Front-end readout electronics are visible at the bottom of each module. The baseplate region exterior to the frame provides a mounting surface to install the tracker in an experimental setup. (b) Enclosure installed. A foil entrance window (left) sits in front of the sensor modules. Connections for power, data, and liquid cooling of each module pass through the enclosure (lower right).
• Figure 2: Experimental setup for fIVI measurement. (a) Schematic diagram. The primary ^12C^6+ beam (blue) was oriented along the $Z$ axis, travelling in the $+Z$ direction. The central axis of the tracker (magenta, dashed) was placed at a 40 angle from the beam axis (blue, dotted), with the intersection of the two being at the center of the phantom (white). Sensors (grey) were placed orthogonal to the tracker axis, 44.6 and 164.6 from the edge of the phantom. Secondary fragments (red) were produced by beam-patient interactions; those fragments passing through both sensor layers were tracked. Data were collected for phantom radii $r$ of 80 and 160. (b) Photograph of experimental setup. The beam exited the nozzle in the lower right, and travelled toward the setup (left), consisting of the phantom (clear plastic) and tracker (black). The smaller phantom, with a 16 diameter, is pictured.
• Figure 3: Tracker acceptance as a function of fragment angle from the $Z$ axis for (a) 16 phantom, and (b) 32 phantom. Acceptance was computed for particles originating from a point directly on the $Z$ axis (i.e. $x=0$, $y=0$).
• Figure 4: A typical interaction vertex distribution produced from one spill in the 32 phantom at a BP depth of 156.3. The proximal edge is visible as a sigmoidal curve beginning at approximately -200, while the distal edge is centered around -50. The distal plateau extends until approximately +50, before linearly dropping to zero at around +130, an effect of the field of view of the fIVI tracker.
• Figure 5: Relationship between BP depth and collected statistics per spill for the complete vertex distributions (blue, magenta) and the global maximum bin used for normalization (black, red). The number of counts in the global maximum bin is an important surrogate signal for the ability to perform range monitoring: when collected statistics are so low that this amplitude drops below 200, fitting the distal edge is no longer consistent. Data are presented for both the 16 phantom (blue, black), and the 32 phantom (magenta, red). Each data set is fit using a quadratic function.