Table 2 Motion-managed and PET/CT-guided radiotherapy components

Abdominal displacement markers

Clinical feasibility

Insensitive to small abdominal displacements

Indicated for most patients. Use patient-specific block position, camera aperture and brightness to maximize detectable abdominal displacement

Lung volume spirometer

Stronger correlation to internal target motion

Patient coaching complexity

Indicated in patients with small abdominal displacements

Fiducial implants

Direct image of internal target motion

Invasive procedure and subsequent migration

Indicated in patients with accessible lesions when other respiratory signal surrogates not indicated

Image segmentation of diaphragm ROI

Non-invasive measure of respiratory motion

Challenges associated with deformable registration across phases

Ensure phase-sorted images not undersampled through sufficient projections or reliable undersampled image reconstruction algorithms

Deep inspiration breath hold

Clinical feasibility

Lack of reproducibility and temporal inefficiency

Indicated in patients with sufficient lung function to allow for reliable breath hold under audiovisual coaching

Active Breathing Control

Reduction of motion envelope

Lung function requirement to permit forced breath hold

Determine patient-specific lung volume for breath hold (50–80% of max)

Abdominal compression

Reduction of abdominal displacement

Upper lobe lesions subject to motion in non-diaphragmatic breathers

Indicated in diaphragmatic breathers with additional measurement of residual motion when possible to enact tolerance criteria

Static PET/CT

Reproducibility

Motion-blurred image

Indicated for low amplitude motion lesions (e.g. upper lobe, chest wall attached)

Static prospectively gated PET/CT

Suppression of motion blurring without loss of SNR

Temporal inefficiency

Use in conjunction with ABC for patients with random breathing pattern that can achieve sufficient lung volume

Dynamic motion-tracked PET/CT

Better representation of target motion

Challenge to reproduce correlation at treatment

Use in conjuction with RF block, spirometer, fiducials, or image segmentation over all phases of breathing cycle for patients with periodic breathing

Phase-averaged PET/CT

Robust low noise image

Reduced contrast and quantitative accuracy without motion information

Evaluate helical CT to determine whether to use phase-averaged PET or motion-compensated PET/CT

Maximum Intensity Projection PET/CT

Represents high confidence interval of motion envelope

PET image SNR reduced to equivalent counts for single phase

Weight intensity projection distribution across respiratory phases to improve SNR while maintaining motion envelope confidence interval

Quiescent period gated PET/CT

Variance reduction from motion over reproducible phase bin

Image quality dependent on fractional counts within quiescent window

Patient-specific gating window based on either relative displacement amplitude or absolute phase

Multiphase PET/CT

Motion compensated images with little information loss

Requires sufficient correlation between respiratory signal and target motion

Optimize number of phases and phase bin sizes as function of lesion size, location, motion amplitude

Manual contour

Patient-specific target delineation

Inter-observer variability in target definition

Useful as higher order correction to target definition following automated techniques

Absolute/relative threshold

Clinical feasibility

Uncertainty in threshold due to noise or variation in backround uptake

Validate threshold-defined targets as prognostic factors of treatment outcome in abdominothoracic cancer patients

Confidence interval

Target motion margins weighted by spatiotemporal likelihood map

Limited to single target envelope by ignoring phase-specific information

Establish relevant confidence interval criteria based on MIP or motion-weighted intensity projection to build dose volume relationship for fixed normal tissue integral dose

Phase adaptive threshold

ROI specific to different phases of target motion

Complexity of threshold determination for all phases

Validate phase-adapted threshold-defined targets against known target parameters in motion phantoms

Phase adaptive stochastic segmentation

Robust to image noise and heterogeneities

Dependent on initialization conditions and susceptible to statistical variation

Validate in motion phantoms followed by comparison of prognostic value to phase-averaged targets

Single plan from ROI

Clinical feasibility

Single plan may require frequent adaptation during treatment course

Indicated in patients with fewer normal tissue tolerance constraints that allow for sufficient target dose

Single plan from optimal margin target definition

Single plan feasibility with motion-compensated target definition

Reduced delivery degrees of freedom compared to phase-adapted plan

Indicated in patients whose single plan normal tissue constraints do not allow for sufficient target dose

Phase-adapted plan

Physical/biological advantages to differential delivery across phases

No consensus on weighting scheme for phase fluence maps

Indicated in patients whose single motion-compensated plan normal tissue constraints do not allow for sufficient target dose

Single plan to static phantom

Clinical feasibility

Ignores impact of motion on clinical deliverability of treatment plan

Baseline measure of plan deliverability prior to motion uncertainties

Single plan to patient-specific motion phantom

Accounts for realistic motion trajectories

Plan deliverability limited by motion

Plans that fail QA due to motion should be replanned on individual phases

Phase-adapted plan to patient-specific motion phantom

Characterize deliverability of phase-correlated plan

Higher sensitivity to phantom setup and dosimeter measurement uncertainties

Ensure precise and accurate setup of phantom and sufficient spatiotemporal resolution of dosimeters

IGRT

Clinical feasibility

Reliant on motion control or static lesion to maximize delivery efficacy

Daily imaging to verify target motion envelope within PTV

Respiratory-gated IGRT

Compromise between delivery reproducibility and treatment efficacy

Temporal inefficiency

Ensure gating window provides sufficient target coverage to phase gate-matched PTV through daily imaging and respiratory signal measurement

Respiratory-tracked IGRT

Advanced delivery optimized to complete target motion trajectory

Requires accurate and precise motion prediction algorithm to account for delivery system latency

Ensure correlation between imaged target trajectory and planned phase-correlated target trajectory

Planned adaptive treatment

Adapt to morphological and biological changes during RT

Adapted plan does not account for changes in image signal due to motion

Establish criteria for adapting plan that include uncertainties in imaging signal change due to motion

Planned phase-adaptive treatment

Adapt to motion-compensated morphological and biological changes during RT

Challenge of re-planning from mid Tx motion-compensated PET/CT or from on-board imager alone

Determine disease and site-specific criteria for adapting plan based on PET/CT or on-board imager