Considering marker visibility during leaf sequencing for segmental intensity-modulated radiation therapy

Abstract

Segmental intensity-modulated radiation therapy (IMRT) delivers a sequence of segments to obtain a desired intensity distribution. Many leaf sequencing algorithms for segmental IMRT have been developed with the aim to reduce delivered monitor units (MUs) and (or) number of segments, and consequently to reduce the total treatment delivery time. With the development of real-time detection technology, it is useful to develop leaf sequencing algorithms that consider the detecting probability of markers implanted into or near target volume. In this study, we defined the concept of marker visibility to denote the marker's detecting probability, and proposed a new leaf sequencing algorithm based on the algorithm of Kamath et al (Phys. Med. Biol. 48: 307-324, 2003). The new algorithm firstly uses Kamath algorithm to generate an initial leaf sequence and then performs a series of column transformations to obtain a new leaf sequence that is optimal in terms of MU efficiency and marker visibility. We evaluated the performance of the new algorithm with six test fields that had randomly-generated intensity matrices. Results show that, compared with Kamath algorithm, the new algorithm does not increase MUs, but improves marker visibility. When the radiation field contains multiple (3) markers, the marker visibility increased from 66.67% to 91.67% for small size (5×5) radiation field; from 39.29% to 42.86% for medium size (10×10 ) field; and from 31.48% to 37.04% for large size ( 20× 20 ) field. We also rigorously proved the optimality of the new algorithm. In conclusion, a new leaf sequencing algorithm was developed for optimal MU efficiency and marker visibility. © 2009 Springer-Verlag.