Commissioning of a fluoroscopic-based real-time markerless tumor tracking system in a superconducting rotating gantry for carbon-ion pencil beam scanning treatment.

PURPOSE: To perform the final quality assurance of our fluoroscopic-based markerless tumor tracking for gated carbon-ion pencil beam scanning (C-PBS) radiotherapy using a rotating gantry system, we evaluated the geometrical accuracy and tumor tracking accuracy using a moving chest phantom with simulated respiration. METHODS: The positions of the dynamic flat panel detector (DFPD) and x-ray tube are subject to changes due to gantry sag. To compensate for this, we generated a geometrical calibration table (gantry flex map) in 15° gantry angle steps by the bundle adjustment method. We evaluated five metrics: (a) Geometrical calibration was evaluated by calculating chest phantom positional error using 2D/3D registration software for each 5° step of the gantry angle. (b) Moving phantom displacement accuracy was measured (±10 mm in 1-mm steps) with a laser sensor. (c) Tracking accuracy was evaluated with machine learning (ML) and multi-template matching (MTM) algorithms, which used fluoroscopic images and digitally reconstructed radiographic (DRR) images as training data. The chest phantom was continuously moved ±10 mm in a sinusoidal path with a moving cycle of 4 s and respiration was simulated with ±5 mm expansion/contraction with a cycle of 2 s. This was performed with the gantry angle set at 0°, 45°, 120°, and 240°. (d) Four types of interlock function were evaluated: tumor velocity, DFPD image brightness variation, tracking anomaly detection, and tracking positional inconsistency in between the two corresponding rays. (e) Gate on/off latency, gating control system latency, and beam irradiation latency were measured using a laser sensor and an oscilloscope. RESULTS: By applying the gantry flex map, phantom positional accuracy was improved from 1.03 mm/0.33° to <0.45 mm/0.27° for all gantry angles. The moving phantom displacement error was 0.1 mm. Due to long computation time, the tracking accuracy achieved with ML was <0.49 mm (=95% confidence interval [CI]) for imaging rates of 15 and 7.5 fps; those at 30 fps were decreased to 1.84 mm (95% CI: 1.79 mm-1.92 mm). The tracking positional accuracy with MTM was <0.52 mm (=95% CI) for all gantry angles and imaging frame rates. The tumor velocity interlock signal delay time was 44.7 ms (=1.3 frame). DFPD image brightness interlock latency was 34 ms (=1.0 frame). The tracking positional error was improved from 2.27 ± 2.67 mm to 0.25 ± 0.24 mm by the tracking anomaly detection interlock function. Tracking positional inconsistency interlock signal was output within 5.0 ms. The gate on/off latency was <82.7 ± 7.6 ms. The gating control system latency was <3.1 ± 1.0 ms. The beam irradiation latency was <8.7 ± 1.2 ms. CONCLUSIONS: Our markerless tracking system is now ready for clinical use. We hope to shorten the computation time needed by the ML algorithm at 30 fps in the future.

Range verification system using positron emitting beams for heavy-ion radiotherapy.

It is desirable to reduce range ambiguities in treatment planning for making full use of the major advantage of heavy-ion radiotherapy, that is, good dose localization. A range verification system using positron emitting beams has been developed to verify the ranges in patients directly. The performance of the system was evaluated in beam experiments to confirm the designed properties. It was shown that a 10C beam could be used as a probing beam for range verification when measuring beam properties. Parametric measurements indicated the beam size and the momentum acceptance and the target volume did not influence range verification significantly. It was found that the range could be measured within an analysis uncertainty of +/-0.3 mm under the condition of 2.7 x 10(5) particle irradiation, corresponding to a peak dose of 96 mGyE (gray-equivalent dose), in a 150 mm diameter spherical polymethyl methacrylate phantom which simulated a human head.

[The examination of optimal positron emitting nuclide in the estimation of attainment depth within the body of RI beams by positron camera].

The (10)C and (11)C beam stop position in a homogeneous phantom was measured using the range verification system in HIMAC. This system was developed to clear uncertainty of beam range within the patient body in heavy ion radiotherapy. In this system, a target is irradiated with RI beams ((11)C or (10)C) and the distribution of the beam end-points are measured by a positron camera. To inspect the precision of the measurement, three experiments were done, simple PMMA phantom irradiation, empirical beam stop position measurements using a range shifter and boundary irradiation using PMMA and lung phantom. Results of the first two experiments were consistent. Consequently, a 0.2 mm standard deviation of statistical error measurement was possible with 250 determinations. For the third experiment, we compared the precision using (10)C and (11)C beams. The boundary of the PMMA and lung phantom was irradiated with both beams to maximize the positron range effect in the beam range measurement. Consequently, no significant difference was observed between the two beams in spite of the different positron range. Thus, we conclude that the (10)C beam was useful for clinical application because of its good statistics owing to the short half-life.

[Experimental study on treatment planning verification using a positron emitting (10)C beam for heavy-ion radiotherapy.].

An advantage of heavy-ion therapy is its good dose concentration. A limit for full use of this desirable feature comes from range ambiguities in treatment planning. The treatment planning is based on X-ray CT measurements, and the range ambiguities are mainly due to an error in calibration of the CT number. The heavy-ion ranges are related to electron density of the medium while the CT numbers are defined using the X-ray attenuation coefficient. The range verification method using positron emitter beams has been developed to reduce the range ambiguities. In this verification, probing beams of positron emitters are implanted into the tumor, and pairs of annihilation gamma rays are detected with a positron camera. This paper demonstrates an application to verify treatment planning. Here the treatment planning was made on a head phantom and the ranges estimated from the CT-number were compared with the ranges measured with the positron camera. As a result, disagreements were detected between the planned ranges and the measured ones; there were 1.6 mm at maximum. The disagreements were due to an error of transformation of CT-number to range for the phantom material in the water column depth-dose measurement. The disagreements could be lowered to 0.4 mm by using the calibrated water-equivalent lengths. It was confirmed that the range verification system has a designed measurement accuracy of 1 mm and is useful for verifying irradiation fields on heavy-ion radiotherapy.