A system for three-dimensional dosimetric verification of treatment plans in intensity-modulated radiotherapy with heavy ions

The introduction of dynamic intensity modulation into radiotherapy using co nventional photon beams or scanning particle beams requires additional and efficient methods of dose verification. Dose measurements in dynamically ge nerated dose distributions with a single ionization chamber require a compl ete application of the treatment field for each single measurement. Therefo re measurements are performed by simultaneous use of multiple ionization ch ambers. The measurement is performed by a computer controlled system and is comprised of the following steps: (a) automated positioning of the ionizat ion chambers, (b) measurement at these points, (c) a comparison with the ca lculated dose from the treatment planning system, and (d) documentation of the measurement. The ionization chambers are read out by a multichannel ele ctrometer and are densely packed into a mounting of polymethylmetacrylate, which is attached to the arm of a three-dimensional motor-driven water phan tom. The measured and planned dose values are displayed numerically as well as graphically. The mean deviation between measured and planned doses as w ell as their standard deviation are calculated and displayed. Through print outs complete documentation of the measurement is obtained and a quick deci sion can be made whether the dose distribution is acceptable for the patien t. The system is now routinely used for dose verification at the heavy ion therapy project at the Gesellschaft fur Schwerionenforschung in Darmstadt. Up to now 242 measurements have been performed for heavy ion treatment of 3 0 patients. The system allows efficient verification and documentation of c arbon ion fields and is in principle also applicable to intensity-modulated photon beams. (C) 1999 American Association of Physicists in Medicine. [S0 094-2405(99)00810-X].