PURPOSE

Arguably, intensity-modulated radiotherapy (IMRT) is one of the most important contributions by physicists in medicine to the treatment of cancer. It enabled the achievement of dose distributions that allowed treatment of targets to high doses and yet spared normal tissues to degrees previously considered virtually impossible. The concept underlying IMRT for photons, originally called "inverse planning," was introduced by Anders Brahme in the early 1980s. Since then, many physicists have been conducting research to advance the state of the art, to overcome obstacles discovered and to develop tools and techniques to translate it clinically. IMRT was first implemented clinically the early 1990's. Soon thereafter, it was also incorporated into intensity modulated particle therapy (IMPT), initially for protons and eventually for ions heavier than protons.

METHODS

In IMRT, intensities of small segments or beamlets of multiple incident beams are optimized to produce the best approximation of desired dose distributions that deliver the requisite tumor dose and maximally spare normal tissues. Such dose distributions are delivered using dynamic multi-leaf collimators whose leaves move continuously while the radiation is on or in a step-and-shoot fashion. For IMPT, there is an extra degree of freedom, that of energies of particle beamlets, which are part of the optimization process. IMPT dose distributions allow substantial additional sparing of normal tissues. They are delivered using scanning beams.

RESULTS

IMRT and IMPT represents a true paradigm shift in the way cancer patients are treated. Their evolution, especially that of IMPT, is still ongoing, with new advancements being added continually. With the improving understanding of the immunomodulatory effects of radiotherapy, it is quite plausible that a combination of IMPT and immunotherapy will mature into one of the most effective ways of treating not just localized cancers but also systemic disease.