Optimization of the step-and-shoot leaf sequence for delivery of intensity modulated radiation therapy using a variable division scheme.

To deliver an intensity modulated radiation therapy (IMRT) plan by multileaf collimator (MLC) it is necessary to convert beam profiles, generated from the inverse treatment planning algorithm, into a series of instructions that the MLC control system can execute. An idealized IMRT beam profile can be regarded as a continuously varying two-dimensional function and is usually represented by an intensity map, i.e., a discretized description in space and in intensity of the beam profile. It is common to assume that the intensity map be defined over a regular grid with N steps and equal increments of intensity levels. In reality, this may not be the optimal representation of the beam profile and may introduce unnecessary discrepancies between the intensity pattern delivered and that ideally required. We have implemented an algorithm capable of minimizing the difference between the two patterns on a beam specific basis. In other words, it can produce optimized intensity maps, individually produced to suit the (continuous function) intensity profile they are intended to approximate. This enhancement in conformation is achieved by allowing variable step size and unconstrained intensity levels in the final leaf sequence.

Radiotherapy treatment planning of brain tumours using MRI alone.

MRI of the brain can provide images of very high quality revealing detailed information especially concerning the extent of abnormalities. As such MRI has great potential in the radiotherapy (RT) planning of brain tumours. However MRI has rarely been used alone as the basis for treatment planning primarily due to concern over potential geometric distortions. Treatment planning using MRI has therefore usually been carried out in conjunction with CT images. This work demonstrates that geometric distortions can be minimized by using a relatively small field-of-view, an increased receiver bandwidth, and a fast spin echo acquisition sequence, and that it is thus possible to perform RT planning using MRI.

Implementation of the Varian EDW into a commercial RTP system.

An enhanced dynamic wedge system (EDW) has been installed onto our Clinac 600C (6 MV) linear accelerator. This paper addresses and describes the measurements taken and subsequent analysis required to enable the planning of EDW using our commercial radiotherapy treatment planning (RTP) system. This implementation into a 'closed' commercial system is performed without the use of specialized measurement equipment or the necessity of introducing new calculation algorithms and is therefore independent of the RTP manufacturer. We consider that, for incorporation of techniques such as EDW into routine clinical use, a simple verifiable method of inclusion into the RTP system must be achieved. This paper also presents a methodology for quality control of dynamic fields chosen for clinical use which is quick and easy to use by virtue of its use of film dosimetry. We present the method of film calibration used in this work.