Tumor motion during irradiation reduces target coverage and increases dose to healthy tissues.
The standard clinical approaches using margins are overly conservative because they only take into account the maximum amplitude of the movement. Comparable coverage and reduced dose to healthy organs appear achievable with robust motion-adaptive treatment planning, based on the nominal expected probability distribution of target position, and considering the uncertainty of its realization duringtreatment. We test a robust approach to IMRT using patient data, to evaluate the effect of target motion variability and the MLC motion interplay on the outcome.

Method and Materials

We built our robust framework using external marker motion data (traces),acquired during patient treatments. These included 251 traces from two lung, one cardiac and one liver patient. Long-term shifts (such as exhale baseline drift) were removed from the data by band pass filtering. IMRT plans with varying grades of robustness were tested on 15 traces of a liver patient and on 37 traces of a cardiac patient. A computer-controlled motion phantom reproduced traces during plan delivery on a linac. The dose was measured with the Scanditronix IMRT MatriXX, a two dimensional array of 1020 chambers with a resolution of 7.6 mm.


Although tumor coverage differed for each delivery fraction due to MLC interplay and motion variability, it converged to the expected level after multiple fractions. A considerable sparing of non-target volume was achieved without underdosing the target.


Dosimetric tests largely confirmed that the robust motion-adaptive approach to IMRT planning allows one to better manage the effect of motion, and can be extended to any uncertainty described by probability density functions. With target monitoring during the treatments, motion-adaptive
plans and the underlying robust framework could be verified and re-adjusted if necessary.

Personnel involved

  • Christian Vrancic
  • Alexei Trofimov, PhD
  • Timothy C Y Chan
  • Thomas Bortfeld, PhD
  • Gregory C Sharp, PhD


Vrančić C, Trofimov A, Chan T C Y, Sharp G C & Bortfeld T
Experimental evaluation of a robust optimization method for IMRT of moving targets.
Phys. Med. Biol. 54 (2009) 2901-2914