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Annual Scientific Meeting of the Canadian Organization of Medical Physicists — July 9–12, 2014, The Banff Centre, Banff, Alberta, Canada

Managing patient motion in proton therapy delivery

Proton therapy offers precise dose distributions that spare healthy tissue more effectively than many photon-based approaches, yet the physical advantage of the Bragg peak can be undermined by something as ordinary as a patient shifting position during treatment. Even sub-millimetre movements alter the range of the beam in tissue, potentially shifting dose away from the target and into organs at risk. For clinics across Australia and internationally, controlling patient motion has become a central pillar of safe and effective proton therapy delivery, particularly for thoracic, abdominal, and paediatric cases where target motion is most pronounced.

Discussions at the 2014 COMP Annual Scientific Meeting in Banff brought together medical physicists working on these exact challenges, sharing approaches that range from rigid immobilisation to advanced surface-guided and respiration-managed techniques. The sessions remain relevant because motion management is not a solved problem; it requires constant refinement, interdisciplinary coordination, and a willingness to learn from centres that have pioneered different solutions. With proton therapy capacity in Australia still developing and many patients historically travelling overseas for treatment, getting the fundamentals right carries real clinical and economic weight.

Sources and consequences of intrafraction movement

Patient motion during proton therapy falls into three broad categories: physiological motion such as respiration and cardiac cycles, peristalsis in abdominal treatments, and baseline shifts caused by muscle relaxation, discomfort, or gradual repositioning. Each contributes differently to dose uncertainty. Respiratory motion in lung or liver lesions can shift the tumour by more than a centimetre, while cardiac motion introduces rapid, high-frequency perturbations that are harder to track with conventional imaging.

In Australian clinics, where hypofractionated protocols are increasingly common for prostate and lung cases, these uncertainties carry particular weight. A 2 mm shift may not drastically change photon dose coverage, but in proton therapy it can pull the distal edge of the beam entirely off the target. Clinics in Sydney and Melbourne have reported that without robust motion management, plan robustness drops noticeably.

The clinical consequences of unmitigated motion are not abstract. Dosimetric studies have shown underdosing of target volumes and overdosing of adjacent normal tissue when motion is not accounted for, along the beam path where the Bragg peak sits. For paediatric patients, who form a significant proportion of proton therapy referrals in Australia due to the long-term tissue-sparing benefits, the stakes are especially high because developing organs are more sensitive to stray dose.

Immobilisation, positioning, and comfort

Effective motion control begins long before the beam is switched on. Immobilisation devices such as thermoplastic masks, vacuum bags, and knee-foot supports establish a reproducible baseline, but their success depends on patient cooperation and physical comfort. A device that causes discomfort leads to involuntary shifts mid-fraction, defeating the purpose of rigid fixation. Australian centres have increasingly adopted open-face masks for head and neck patients, which reduce claustrophobia and allow surface-guided monitoring without compromising positional accuracy.

Setup verification using in-room CT, cone-beam CT, or orthogonal kV imaging is now standard practice. The goal is to align the patient to within the tolerance specified by the treatment plan, which for proton therapy often demands sub-2 mm accuracy. Surface-guided systems, which track the patient's external contour in real time, have proven valuable for catching baseline drifts before they translate into internal anatomical shifts. A key lesson shared during the diagnostic x-ray quality control session at COMP 2014 was that quality assurance for imaging and positioning is inseparable from motion management; one cannot function reliably without the other.

Equipment selection also requires regulatory awareness. In Australia, immobilisation devices and surface-guided systems must comply with Therapeutic Goods Administration requirements, and clinics must ensure that any new technology for motion management has appropriate TGA clearance and meets state-level radiation safety standards overseen by ARPANSA-aligned frameworks. This administrative layer adds time to the adoption of new tools but ultimately protects patients and staff.

Respiratory management and gating

For tumours in the thorax and upper abdomen, breathing is the dominant source of intrafraction motion. Several strategies address it: abdominal compression to reduce excursion, breath-hold techniques (both deep inspiration and end-expiration), and respiratory gating that delivers the beam only during a specified phase of the breathing cycle. Each approach has trade-offs. Compression improves reproducibility but may be uncomfortable for elderly patients or those with respiratory comorbidities. Breath-hold offers excellent reproducibility but requires active patient participation and longer treatment times.

Gating, particularly when combined with internal fiducial markers or implanted electromagnetic transponders, allows free breathing while limiting dose delivery to specific phases. The challenge lies in the latency between beam hold-off and beam-on, and in verifying that the internal target position actually corresponds to the external surrogate being tracked. In Brisbane and Adelaide, where proton therapy capacity is limited and patient throughput must be optimised, gating has been used selectively rather than universally, reserved for cases where the dosimetric benefit clearly justifies the added time and complexity.

Australian clinicians have also explored audiovisual biofeedback to train patients towards regular breathing patterns, reducing motion amplitude without mechanical constraints, aligning with the local preference for less invasive interventions.

Image guidance and real-time adaptation

Modern proton therapy rooms are equipped with imaging systems capable of verifying position immediately before and, in some cases, during treatment. Periodic volumetric imaging catches slow drifts, but truly real-time monitoring remains an active area of development. Magnetic resonance-guided proton therapy is on the horizon internationally, though Australian access is constrained by capital cost and regulatory pathways under the Therapeutic Goods Administration and state health authorities.

Adaptive planning offers another layer of protection. If daily imaging reveals that the target has shifted or anatomy has deformed, the plan can be recalculated using deformable registration and Monte Carlo dose engines. Some Australian centres have implemented offline adaptive workflows that re-optimise plans after a set number of fractions, balancing workload against dosimetric benefit within Medicare funding constraints.

Workflow integration and training

Technology alone does not reduce motion; protocols do. Effective motion management depends on clear procedures for setup, imaging, beam hold criteria, and staff response to deviations. Training frontline therapists is particularly important, as they are often the first to notice a patient struggling with a mask or becoming restless mid-fraction. The COMP 2014 programme highlighted how interdisciplinary discussions strengthen these workflows, and delegates appreciated the chance to decompress with colleagues while exploring mountain biking trails between sessions.

Regular audits and multidisciplinary reviews help maintain standards. The shift in Australian healthcare towards value-based care aligns well with these practices. Documentation of motion events and near-misses builds institutional memory and meets the expectations of the Australian Commission on Safety and Quality in Health Care.

Practical recommendations for reducing motion

The following strategies have proven effective across multiple proton therapy programmes.

  • Perform a motion assessment during simulation for every patient, using 4D CT to characterise target excursion.
  • Select immobilisation devices based on comfort and reproducibility, not habit; fit-check before CT simulation.
  • Use surface guidance or optical tracking to monitor baseline shifts in real time, with intervention thresholds.
  • For thoracic and abdominal sites, establish criteria for choosing breath-hold, compression, or gating based on patient fitness and motion amplitude.
  • Integrate volumetric imaging review into the workflow, with protocols for triggering adaptive replanning.
  • Train all team members on motion procedures, including therapists, physicists, and oncologists, with annual competency checks.
  • Document motion events and audit outcomes regularly to identify trends and refine local protocols.

For delegates and readers who wish to revisit the foundational work presented in Banff, the conference registration portal provides ongoing access to archived abstracts and session materials. These lessons continue to inform Australian practice as proton therapy expands locally, and the connections made between sessions and shared experiences remain a valuable part of professional development in this demanding field.