A study of the complexity, necessary for the delivery of Intensity‐Modulated Radiotherapy

Abstract

Intensity-modulated radiotherapy (IMRT) beam profiles tend to be highly modulated, which presents certain practical delivery and verification difficulties, as well as enhancing the susceptibility to patient setup errors, and patient and organ movement. The main goal of this work was therefore to investigate the form of intensity-modulated beams (IMBs) and to find different approaches to resolve some of the problems associated with these complex IMBs. First of all, a practical solution was sought to make the total fluence distribution less noisy and more deliverable. This investigation led to the development of what is referred to as hybrid planning. It consists of replacing some IMBs by unmodulated, geometrically conformal beams. It was shown that, out of a standard five-field-IMRT plan of the prostate, up to two beams could be replaced by unmodulated beams without greatly compromising the advantages of IMRT. To further optimize IMRT the origins of the structure and complexity of IMBs were investigated. A technique was established to analyze and quantify stochastic noise in IMBs. This provided the means to investigate the impact of optimization techniques on this noise and evaluate how the resulting information can be used to improve IMRT planning and delivery. Results indicate three possible sources of stochastic noise in IMBs, i.e., the optimization technique, the cost-function, and the definition of convergence of the cost-function. Additionally, attention was drawn to the relationship between beam structure and motion. The sensitivity of IMBs to deformable organ motion was investigated and it was assessed whether smoothing of the IMBs can reduce this susceptibility. Results show a systematic shift when planning treatment on only one CT scan. Smoothing beam profiles either based on a single scan or throughout breathing did not seem beneficial to reducing this shift for this patient. Finally, in the light of future motion compensation methods, a model was developed that calculates dynamic internal margins based on the variability of breathing motion.