Effects of respiratory motion on dose uniformity with a charged particle scanning method

Abstract

A three-dimensional spot-scanning technique for radiotherapy with protons is being developed at the Paul Scherrer Institute. As part of the effort to optimize the design and ensure clinically useful dose distributions, a computer simulation of the dose deposition in the presence of respiratory motion was performed. Preliminary experiments have characterized the proton beam and the scanning procedure. Using these parameters, the computer program calculated the dose within a uniform volume of water in the presence of respiratory motion. Respiration amplitude, respiration period, respiration direction, number of fractions, size and position of the beamspots and rescanning multiplicity were systematically varied and the effect on the dose distribution determined. The dose uniformity is very dependent on the direction of the respiration relative to the three independent beam scanning directions. The dose uniformity decreases with increasing respiration amplitude, but has little response to changes in respiration frequency. Rescanning the volume, such as with fractionation, improves the dose uniformity roughly as the square root of the number of fractions. Broad, Gaussian beams result in better dose uniformity than narrow, sharply delineated ones, but produce slower dose fall-off at the edges of the scanned volume. Results of this work are being incorporated into the design of the system.

In silico comparison of photons versus carbon ions in single fraction therapy of lung cancer

“…The beam delivery sequence and the target motion interfere with one another, resulting in over-and underdosages in patients. This effect is called interplay and it has been thoroughly reviewed elsewhere [Phillips et al, 1992, Bert et al, 2008. The effect of interplay depends on many factors, such as motion amplitude, beam direction, starting breathing phase etc.…”

“…Rescanning is a technique that uses statistical averaging of different interplay patterns [Phillips et al, 1992]. Instead of applying the whole dose D at once, the target is scanned N times, each time irradiated with D/N.…”

Proton pencil beam scanning for mediastinal lymphoma: treatment planning and robustness assessment

“…One commonly considered strategy to mitigate the interplay effect is repainting, i.e. scanning repetitively within one fraction [8]. A number of repainting schemes have been investigated previously [9][10][11].…”

The use of proton-beam therapy in the treatment of non-small-cell lung cancer

“…Pencil beam scanning has been used for static targets in some facilities, and expected to be beneficial for NSCLC [59]; however, it is not currently in practical use for NSCLC. This scanning technique is very sensitive to organ motion, and margin decisions for this technique are more complicated than those with use of a scattering beam [12,60,61]. A beam-specific margin is needed as well as a scattering beam, and furthermore, each scanned beam can overlap and separate due to respiratory displacement, which produces hot spots and cold spots.…”