Tumour tracking with scanned proton beams: assessing the accuracy and practicalities

Abstract

The potential of tumour tracking for active spot-scanned proton therapy was assessed. Using a 4D-dose calculation and simulated target motion, a tumour tracking algorithm has been implemented and applied to a simple target volume in both homogenous and heterogeneous in silico phantoms. For tracking and retracking (a hybrid solution combining tumour tracking and rescanning), three tracking modes were analysed: 'no tracking' (uncorrected irradiation of a moving target), 'perfect tracking' (no time delays and exact knowledge of target position) and 'imperfect tracking' (simulated time delays or position prediction errors). For all plans, dose homogeneity in the target volume was assessed as the difference between D5 and D95 in the CTV. For the homogeneous phantom, perfect tracking could retrieve nominal dose homogeneity for all motion phases and amplitudes while severe deterioration of treatment outcomes was found for imperfect tracking. The use of retracking reduced the sensitivity to position errors significantly in the homogeneous phantom. In the heterogeneous phantoms (simulated rib proximal to target), the nominal dose homogeneity could not be obtained with perfect tracking. Adjustments in pencil beam positions could cause pencil beams to deform under the influence of the bone, resulting in loss of dose homogeneity. As retracking was not capable of reducing these effects, rescanning provided the best treatment outcomes for moving heterogeneous targets in this study.

Robustness of target dose coverage to motion uncertainties for scanned carbon ion beam tracking therapy of moving tumors

“…Even when the pencil beam Bragg peaks perfectly tracked the moving target, off-axis dose contributions from individual pencil beams were affected by variation in tissue upstream of the moving target and dose was degraded. This finding confirms the finding of van de Water et al (2009).…”

“…Our rationale for these multiple motion scenarios was that, even when pencil beams perfectly track target motion and depth changes, some deviations in off-axis dose contributions for individual pencil beams are expected (van de Water et al , 2009). By sampling these 18 scenarios of motion, we avoided the chance that our findings would be confounded by a single “lucky” or “unlucky” choice of τ or ϕ.…”

“…For example, van de Water et al (2009) studied beam tracking with scanned proton beams and reported deterioration of target dose coverage when simulated time delays or position errors were introduced to tracking simulations, which was improved by combining the principles of rescanning and beam tracking. To our knowledge, the robustness of target dose coverage to uncertainties in scanned carbon ion beam tracking therapy for moving tumors has not been reported in the literature.…”

4D optimization of scanned ion beam tracking therapy for moving tumors

“…For 3D-optimized beam tracking for the lung patient, this finding of non-uniform target dose for “perfect” tracking agrees with previous studies of Bert and Rietzel (2007) and van de Water et al (2009), which reported that dose degradation can result from issues that are not considered in 3D-optimized beam tracking, namely rotations, deformations, and changes in scattering properties of tissue upstream to the target. For example, if the target rotates, the entrance channel dose for individual pencil beams can overlap in irregular patterns.…”

“…To elucidate, in their approach, particle numbers were optimized using a single 3D reference phase of the 4DCT, e.g., end-exhale. Consequently, 4D dose was deteriorated somewhat, even with perfect geometric tracking of target motion, since off-axis dose contributions from each ion beam can distort in the presence of complex tissue motion (van de Water et al , 2009). To address this problem, Lüchtenborg et al (2011) proposed a real-time 4D dose compensation method for scanned ion beam tracking that improves target dose coverage by adapting particle numbers in real-time based on the motion status of the target during the delivery of each ion beam.…”