Abstract
PurposeTo quantify the effects of combining layer‐based repainting and respiratory gating as a strategy to mitigate the dosimetric degradation caused by the interplay effect between a moving target and dynamic spot‐scanning proton delivery.MethodsAn analytic routine modeled three‐dimensional dose distributions of pencil‐beam proton plans delivered to a moving target. Spot positions and weights were established for a single field to deliver 100 cGy to a static, 15‐cm deep, 3‐cm radius spherical clinical target volume with a 1‐cm isotropic internal target volume expansion. The interplay effect was studied by modeling proton delivery from a clinical synchrotron‐based spot scanning system and respiratory target motion, patterned from surrogate patient breathing traces. Motion both parallel and orthogonal to the beam scanning direction was investigated. Repainting was modeled using a layer‐based technique. For each of 13 patient breathing traces, the dose from 20 distinct delivery schemes (combinations of four gate window amplitudes and five repainting techniques) was computed. Delivery strategies were inter‐compared based on target coverage, dose homogeneity, high dose spillage, and delivery time.ResultsNotable degradation and variability in plan quality were observed for ungated delivery. Decreasing the gate window reduced this variability and improved plan quality at the expense of longer delivery times. Dose deviations were substantially greater for motion orthogonal to the scan direction when compared with parallel motion. Repainting coupled with gating was effective at partially restoring dosimetric coverage at only a fraction of the delivery time increase associated with very small gate windows alone. Trends for orthogonal motion were similar, but more complicated, due to the increased severity of the interplay.ConclusionsLayer‐based repainting helps suppress the interplay effect from intra‐gate motion, with only a modest penalty in delivery time. The magnitude of the improvement in target coverage is strongly influenced by individual patient breathing patterns and the tumor motion trajectory.
Highlights
The potential of charged particle beams in providing highly conformal, targeted therapy has long been recognized.[1,2,3] Compared with photon beams, charged particles exhibit a well‐defined penetration range in a patient, reducing unnecessary radiation dose distal to the intended target
Pencil‐beam scanning introduces the potential for deleterious interplay effects between the highly modulated delivery and a mobile target,[7] whereas the time independence of passively scattered deliveries render these treatments much more robust to target motion
The deployment of clinical tools has been limited by complexities associated with irregular, patient‐specific target motion, temporal beam delivery dynamics, and the high degree of manual intervention required when performing deformable image registration and dose accumulation. 4D dose calculation techniques potentially suffer from insufficient time resolution, which is limited by the number of reconstructed phases in a breathing cycle
Summary
The potential of charged particle beams in providing highly conformal, targeted therapy has long been recognized.[1,2,3] Compared with photon beams, charged particles exhibit a well‐defined penetration range in a patient, reducing unnecessary radiation dose distal to the intended target. Technological advances have spurred the proliferation of proton therapy as an increasingly conventional treatment modality.[5] Modern facilities almost exclusively feature pencil‐beam scanning,[6] in which a small beamlet of fixed energy (corresponding to a specific depth) is scanned over the lateral extent of the target. Once the layer is completed, the system switches energy to a more proximal depth, and the target is again laterally scanned. This process is completed until the prescribed dose has been fully delivered. Pencil‐beam scanning introduces the potential for deleterious interplay effects between the highly modulated delivery and a mobile target,[7] whereas the time independence of passively scattered deliveries render these treatments much more robust to target motion
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