Combining the advantages of step-and-shoot and dynamic delivery of intensity-modulated radiotherapy by interrupted dynamic sequences.

Abstract

A hybrid between step-and-shoot and dynamic operation, called interrupted dynamic sequences, was investigated for prostate intensity-modulated radiotherapy (IMRT) delivered by a multisegment close-in technique. The new delivery mode was compared to the step-and-shoot mode concerning dose distribution. Segments suitable for dynamic transition were selected using a system of segment classes. Transitions were only allowed between two segments of the same class, keeping intended sharp in-field dose gradients unchanged. Delivery was performed by an Elekta SLiplus (Crawley, UK) linear accelerator equipped with a dynamic multileaf collimator (MLC). Because no modeling of the dose during the transitions is made, accurate dose measurements were performed. Dose profiles were measured using a linear ion chamber array (LA48, PTW-Freiburg). The suitability of this detector for measurements in sharp dose gradients was investigated first. In addition, field flatness was examined for segments with a low monitor unit (MU) count. Uncertainties in dose output were investigated using an ionization chamber (30001, PTW-Freiburg). Because linear array measured penumbrae are only slightly broader (< or = 0.4 mm for MLC collimated field) than those obtained using a diamond detector, the array is a good device for profile measurements. Uncertainties related with the use of low MU beam segments are very small (< 1% for segments of minimum 3 MU), giving no contra-evidence for the step-and-shoot mode. Interrupted dynamic sequences are shown to introduce only small dosimetric differences as compared to the step-and-shoot delivery. Both delivery modes, step-and-shoot and interrupted dynamic sequences, result in similar dose distributions for the forward planned prostate class solution.