Online image guided tumour tracking with scanned proton beams: a comprehensive simulation study

Abstract

Tumour tracking with scanned particle beams potentially requires accurate 3D information on both tumour motion and related density variations. We have previously developed a model-based motion reconstruction method, which allows for the prediction of deformable motions from sparsely sampled surrogate motions tracked via an on-board imaging system (Zhang et al (2013 Phys. Med. Biol. 58 8621)). Here, we investigate the potential effectiveness of tumour tracking for scanned proton beam therapy using such an approach to guide scanned beam tracking, together with the effectiveness of 're-tracking' for reducing residual motion effects due to tracking uncertainties. Three different beam tracking strategies (2D, 2D deformable and 3D) have been applied to three different liver motion cases, with mean magnitudes ranging from 10-20 mm. All simulations have been performed using simulated 4DCTs derived from 4DMRI datasets, whereby inter-breath-cycle motion variability is taken into account. The results show that, without beam tracking, large interplay effects are observed for all motion cases, resulting in CTV D5-95 values of 34.9/58.5/79.4% for the three cases, respectively. These can be reduced to 16.9/18.8/29.1% with 2D tracking, to 15.5/17.9/23.3% with 2D deformable tracking and to 15.1/17.8/21.0% with 3D tracking. Clear 'inverse interplay' effects have also been observed in the proximal portion of the field. However, with three-times re-tracking, D5-95 for the largest motions (20 mm) can be reduced to 13.0/12.8% for 2D and 3D tracking, respectively, and 'hot spots' resulting from the 'inverse interplay' effect can be substantially reduced. In summary, we have found that, for motions over 10 mm, tracking alone cannot fully mitigate motion effects, and can lead to substantially increased doses to normal tissues in the entrance path of the field. However, three-times re-tracking substantially improves the effectiveness of all types of beam tracking, with substantial advantages of 3D over 2D re-tracking only being observed for the largest motion scenario investigated.

Future of medical physics: Real‐time MRI‐guided proton therapy

“…31,32,33 These effects, i.e., possible significant hot or cold spots within the treatment volume accumulated over the course of a single fraction, are shown to be mitigated by fractionation, 32 multiple rescanning, 33 and retracking. 34 In the case of MRPT, the real-time MR images at the rate of 4-8 frames per second may in fact be available to assist in the management of interplay effects. It may be possible to track in real-time a tumor margin and correspondingly alter or translate the pencil beam scanning pattern.…”

What will the medical physics of proton therapy look like 10 yr from now? A personal view

“…Using organ meshing methods to ensure geometric consistency between the selected CT and the MR extracted motion library, multiple 4DCT's can be generated for any patient using motion from this library, such as to model a wide variety of potential motions. Such data sets can then be used to test, a priori, the robustness of a plan, or a given motion mitigation approach, to a wide variety of motion scenarios . Indeed, by combining this approach with a surrogate driven motion model, it may even be possible, in principle at least, to generate a patient and breath‐cycle specific computational phantom on which to plan treatments.…”

Motion management in particle therapy

“…A dose simulation study for a tracked delivery with a scanned proton beam to a phantom geometry was performed by van de Water et al . A clinical simulation of online image guided tumor tracking with scanned proton beams was performed by Zhang et al . It was shown that interplay effects could be significantly reduced applying tracking.…”

“…Comparison of 4D dose distributions on an example slice using different motion mitigation strategies …”