"A major question in the field has been how reliably such a system could perform in the presence of any imperfections from the simulated ideal case," said first author John Eley, who carried out the research while at the University of Texas MD Anderson Cancer Centre in Houston. "This study aimed to answer many of the questions regarding the reliability of target dose coverage when motion uncertainties were accounted for during treatment simulation," added Eley, now at the University of Maryland School of Medicine in Baltimore.

Using simulations in phantoms and patient CT scans, the collaboration assessed the impact of several sources of uncertainty on a tracking technique developed by senior author Christoph Bert and colleagues at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt. The technology, which is not yet in clinical use, corrects for tumour motion during irradiation using offsets derived from a pre-treatment 4D CT scan. An external tumour motion surrogate, such as the signal from a respirometer, synchronises the beam with the tumour. The approach assumes the pre-treatment scan accurately represents tumour motion during treatment and that the correlation between the tumour's motion and the surrogate is fixed.

Dosimetric impact of motion uncertainty

Two liver and two lung phantoms, each containing a 3 cm spherical tumour that moved sinusoidally were used in the simulations. For each, 4D treatment plans using a single field were generated for 18 motion cycles with varying periods and starting phases. The researchers compared the dose distributions for a single fraction delivery when the tumour was perfectly tracked, and when several random and systematic errors were deliberately introduced.

These errors included phase delays, where the beam motion lags behind that of the tumour, and random errors due to the limits in precision of the tracking technology. The same uncertainties were introduced to a four-field plan generated using CT scans of a previously treated lung-cancer patient.

Technical uncertainties in beam delivery had little effect. An error of 0.5 mm resulted in a drop of less than 1% in V95, the volume receiving 95% of the prescribed dose. In contrast, phase delays caused dose coverage of the clinical target volume (CTV) to deteriorate significantly. In the phantoms, a 15° delay resulted in drops in V95 of up to 22% when averaged over the 18 motions, compared to when the beam tracked the tumour perfectly. Similar drops were observed in the lung-cancer patient.

In light of the findings, systems that predict tumour motion during treatment or real-time tracking systems with motion detection loops shorter than 50 ms are needed to limit the effects, Eley told medicalphysicsweb.

Inter-fraction changes in tumour motion

In another test, the researchers examined the effect of changes in tumour motion between fractions over a whole treatment course, using weekly 4D CT scans acquired in six lung-cancer patients. They observed significant deteriorations in V95 that increased with the range of tumour motion. Averaged over all six, V95 dropped from 94.3% to 85.9% when treatment was simulated on a 4D CT scan acquired one week after treatment planning.

At present, the beam tracking technology at Darmstadt can track tumours with complex 3D motions in anatomically realistic chest phantoms. With this latest research, it is moving closer to clinical implementation and potential commercialization.

"We believe the vast majority of the groundwork has been completed to enable a commercial vendor of proton or carbon-ion accelerators to deliver this technology into clinical practice in a relatively short time frame of, perhaps, two to three years," said Eley.

Related articles in PMB
Robustness of target dose coverage to motion uncertainties for scanned carbon ion beam tracking therapy of moving tumors
John Gordon Eley et al Phys. Med. Biol. 60 1717
4D optimization of scanned ion beam tracking therapy for moving tumors
John Gordon Eley et al Phys. Med. Biol. 59 3431
Tumor tracking based on correlation models in scanned ion beam therapy: an experimental study
M Seregni et al Phys. Med. Biol. 58 4659
Ion-optical studies for a range adaptation method in ion beam therapy using a static wedge degrader combined with magnetic beam deflection
Naved Chaudhri et al Phys. Med. Biol. 55 3499

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