COMPARATIVE STUDY OF INTENSITY-MODULATED RADIATION THERAPY VERIFICATION USING TWO­ DIMENSIONAL IONIZATION CHAMBER ARRAY AND I'MRT VERIFICATION PHANTOM

Abstract

In the traditional external beam photon radiation therapy, most treatments are delivered with radiation beams that are of uniform intensity across the field (within the flatness specification limits). Occasionally, wedges or compensators are used to modify the intensity profile to offset contour irregularities and/or produce more uniform composite dose distributions. This process of changing beam intensity profiles to meet the goals of a composite plan is called intensity modulation. Thus, the compensators and wedges may be called intensity modulators, albeit much simpler than the modem computer-controlled intensity modulation systems such as dynamic multileaf collimators (I.2>.

Intensity-Modulated Radiation Therapy (IMRT) refers to a radiation therapy technique in which non-uniform fluence is delivered to the patient from any given position of the treatment beam to optimize the composite dose distribution. The treatment criteria for plan optimization are specified by the planner and the optimal fluence profiles for a given set of beam directions are determined through "inverse planning." The fluence files thus generated are electronically transmitted to the linear accelerator (Linac), which is computer controlled, equipped with the required software and hardware to deliver the calculated intensity-modulated beams (IMBs) (1•!).

The clinical implementation of IMRT requires at least two systems; (a) a treatment­ planning computer system that can calculate non-uniform fluence maps for multiple beams directed from different directions to maximize dose to the target volume while minimizing dose to the critical normal structures, and (b) a system of delivering the non-uniform planned fluences. Each of these systems must be appropriately tested and commissioned before actual clinical use.

The principle of!MRT is to treat a patient from a number of different directions (or continuous arcs) with beams of non-uniform fluences, which have been optimized to deliver a high dose to the target volume and an acceptably very low dose to the surrounding normal structures. The treatment-planning program divides each beam into a large number ofbeamlets and determines optimum setting of their fluences or weights. The optimization process involves inverse planning in which beamlet weights or intensities are adjusted to satisfy predefined dose distribution criteria for the composite plan (l.ll.