gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

Image-guided particle therapy: Correction strategies for intrafractional motion

Meeting Abstract

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  • A. Trofimov - Department of Radiation Oncology, Massachusetts General Hospital, Boston, USA

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog207

DOI: 10.3205/09ptcog207, URN: urn:nbn:de:0183-09ptcog2075

Published: September 24, 2009

© 2009 Trofimov.
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Outline

Text

As a wide variety of imaging modalities and monitoring tools are introduced into the treatment room, the possibilities increase for observation of intrafractional variations, and management of their impact on the distribution of delivered dose. In particle therapy, due to the finite range in tissue, dose distributions are especially vulnerable to the changes in the anatomy in the beam path during delivery. Such changes may be due to relatively short-time-scale quasi-repeatable periodic motion (respiratory, cardiac cycles), fast aperiodic motion (peristalsis, catastalsis), or long-time-scale changes (drift in the diaphragm “baseline” position, tissue “settling” due to gravity, etc.) In this short review, we will consider examples of the effect of intrafaractional variations on the particle dose distributions, and discuss the strategies that may be helpful in mitigating such effects.

The choice of image data, margin definition, as well as irradiation directions for treatment planning is the essential first step in addressing intrafractional changes. Further, a number of planning techniques have been proposed, which employ probabilistic description of target motion and its variability to reduce the potential deterioration of the dose distribution during therapy delivery. In the treatment room, patient immobilization is another essential step to reduce the target motion, with either the external, whole body (casts, frames), or internal (rectal balloons, active breathing control) devices. With passive beam scattering delivery of spread-out Bragg peak fields, respiratory gating has been widely used to irradiate highly mobile tumors; additionally, dose-modulation within the SOBP can be used to reduce the distal gradient deterioration, and ensure the irradiation of the distal portion of the target. With the wider availability of beam scanning delivery, a range of possibilities for motion management broadens, to include fast (conformal) or slow (mean position) tracking of the tumor, as well as adaptive dose repainting to reduce the interplay between the target and beam motion. Finally, with increased accuracy of monitoring of delivery conditions, and dose accumulation on the variable anatomy, the “accumulated” or residual effect of intrafractional motion can be addressed with by making adjustments to the treatment plan or complete replanning, within the framework of the adaptive therapy course.

Corrective measures can potentially be applied at all stages of particle therapy, off-line and on-line: from treatment planning and hardware manufacturing, to motion-adaptive delivery. The best results in motion management are likely to be achieved with a combination of such off-line and on-line approaches.