Recent developments in radiation therapy planning and treatment optimization.

Abstract

Radiation therapy of cancer is today going through a very dynamic development with the introduction of a large number of new treatment principles, new types of treatment units and new radiobiologically based optimization algorithms for treatment planning. All of these make use of the recent developments in three dimensional tumor diagnostics, molecular biology of cancer, the fractionation sensitivity of different tissues and most recently predictive assays of radiation sensitivity. The most efficient but also least developed area of treatment optimization is to use a few (approximately 3) non uniform radiation beams directed towards the tumor. Today patient individual collimation with beam blocks or multileaf collimators protect organs at risk laterally outside the tumor volume. Non uniform dose delivery also allows protection of normal tissues anterior, posterior and even inside the target volume by shaping the isodoses tightly around the tumor tissues and thereby also allowing longitudinal protection of normal tissues. Some of the most advanced new algorithms are even treating therapy optimization as an inverse problem where the optimal incident beam shapes are determined directly from the location of gross disease, presumed microscopic tumor spread and organs at risk. The optimization is then performed such that the probability, P+, to eradicate all clonogenic tumor cells without severely damaging healthy normal tissues is as high as possible. Already with a few non uniform beams the treatment outcome is within a few percent of what can be achieved with infinitely many co-planar beams in a dynamic mood. With such optimized non uniform treatments it should be possible to improve the treatment outcome by as much as 20% and more, particularly in patients with a local complex spread of the disease or several organs at risk.